PDA

View Full Version : Single phase Vs 3 phase motor ratings.



KBs PensNmore
10th Feb 2017, 11:50 PM
Am looking at getting some machinery, unfortunately most are 3 phase, I don't really want to go the VFD route, as I'm not electronically minded in doing the settings etc. My thoughts were to fit single phase motors to them, I read somewhere that a replacement single phase motor should be twice the capacity of the 3 phase? Is this correct? I could get 3 phase on but it's a rental place.
Kryn

jhovel
11th Feb 2017, 12:15 AM
Power is power - however many phases. 3hp 3-phase is the same speed and torque as a 3hp single phase. Getting 3-phase connected is not related - other than not having to find a suitable single phase motor. There are things like a 3-phase motor inherently runs MUCH smoother since there are 12 windings pushing and pulling the armature around, compared to the four in a single phase 4-pole motor.
Fitting a VFD is a once-off setting the parameters to what you ask of the motor, that's it. Nothing to do with electronics at all.
The benefits are you get the running characteristics of a 3-phase motor, PLUS the options of a gently accelerating and decelerating motor (and none of the huge start-up currents without a VFD) and the opportunity to vary the speed to suit the machine or the job without pulley changes.
So don't be put off by the more adventurous of us trying to stretch the capabilities of VFDs with more complexity than you are comfortable with.... go for it.

caskwarrior
11th Feb 2017, 08:49 AM
And Kryn athough i am too far away i am sure a forum member would help you get out of any pickles or confusion you might get into, as Joe says they arent too bad to set up at all.

Michael G
11th Feb 2017, 09:31 AM
... I don't really want to go the VFD route, as I'm not electronically minded in doing the settings etc

I say that every time I have am confronted with 3 phase gear and I manage. The documentation available for VFD's is usually pretty good and if you only want basic off/ on it is not that difficult. Probably the hardest part is stuffing the wires into the ridiculously small space in the housing.

Michael

BobL
11th Feb 2017, 10:41 AM
I'm a bit of a latecomer to the VFD game mainly because I the few machines I did have were all single phase and running fine as they were. It was only when I was given a Hercus with a 3P motor on it that I started looking into VFD.

What I am surprised about is how little electricians know about VFDs.
The young sparky I bought my 3P compressor from said he was selling it because he had moved into a place with no 3P. I asked him if he had heard about VFDs and he said he had but did not think he could run his compressor that way.
At the men's shed there are 3 retired electricians and only one has heard of VFDs and he told me he wouldn't know how to hook one up but I'm sure he's smart enough that if he read the manual he could do it.
At a country mens shed I visited recently there was a electrical fitter who had been retired for about 15 years and even he had never heard of VFDs.

Back to the OP. Last year I had a length email exchange with forum member sossity about a 3P - SP motor conversion of a used car hoist which was rated at 2.5t. When sossity put an SP motor with the same HP on the hoist he found it could only lift about 1.2t. Unfortunately he could not put the 3P motor back on and test the hoist because he did not have 3P power. I don't recall him solving that problem - would be interesting to find out if he sorted it.

KBs PensNmore
11th Feb 2017, 03:46 PM
Thanks everyone for their comments.:2tsup: I do have a small VFD here, what capacity it is I do not know, but to me, it's the scariest looking thing in my sheds, even the mess in there, isn't as scary. I think it's a fear of the unknown, probably once I know how to play with it, it won't be so bad.
Kryn

jhovel
11th Feb 2017, 04:58 PM
Kryn,
could you please post a photo of the VFD, and a readable photo of any labels you find on it? I'm sure we can help identufy it and give you at least the basic paramerters to get you going. Oh, the label of the motor you may want to drive with it would help too.

russ57
11th Feb 2017, 05:32 PM
I agree with general comments that a vfd is not so hard and probably the easiest way to go.
But the original question I think was asking about comparisons - as far as I understand, a single phase motor of equal power will be larger than a 3 phase. (but I guess a modern sp may be no bigger than an older 3ph.)
Russell


-russ

sossity
11th Feb 2017, 08:56 PM
I have also read the thing about going to 2x's power for single phase compared to 3phase.
I've been saving this up to ask for help but I wanted to do some more tests first but since it has come up…
I must admit I felt sure that power was power and in fact that's saying rpm x torque is the same on both motors rated the same.
My car hoist was originally 2.2kw 3ph delta 415v for max torque so I couldn't rewire it easily to 240v. I put my money where my mouth was and bought a 2.2kw single phase motor. The hoist can lift 2.5ton and I only want 1.5ton after all. It didn't work. With respect to power being power, I have gone over this for 6 months trying to think what's wrong.
This graphs shows a comparison of real time torque of single vs 3phase
366541

If power = torque x rpm, and both motors are the same power and both have the same rpm, does that must mean the single phase motor has a far higher peak torque, because its dropping to 10% of peak, while 3phase only drops to 50%. Is that right?
Or, do they cheat and use peak torque for the power calculations?
When I talked to BobL last, I had tried to lift a 1.5ton van and it would lift it up on its tiptoes, but not lift it off the floor. I could wind it up manually and I guessed it was lifting about 1.2 tons. Over xmas, I had to change a clutch on a 1 ton car and the B***** thing wouldn't lift even that and I had to wind it manually to get the job done. it would lift it off the ground but slow down and stop after about 6". If I let the belts slip it would lift it but the belts were wearing and that's not the answer.

I can lift the hoist by winding it manually. its a screw post hoist just like a big leadscrew with a 6mm pitch. As I lift it there is no sudden weight comes on, its all gradual. Once its up, its fairly hard to wind so I am not so surprised the motor struggles. I can wind it back down dead easy, which I take to mean the hoist is functioning easily without too much friction and all the force needed is that required to lift the weight.

The thing I wonder is the hoist requires large torque and has bugger all momentum for the torque involved and so if torque is dropping to 10% of peak 50xs a sec, does that mean it never over comes stiction. As the load comes on the motor it stops very quickly after it loses a bit of speed. It stops in less than a turn and just can’t start, I reckon.
I am even scared to buy a 5hp motor in case that is not enough. However, I notice that tecalemit now sell a screw hoist with a 5hp single phase motor and I think its got the same pitch, going by its lift time. Does anyone really know for sure, should a 5hp 1440rpm single phase motor do the same job as an asea 1440rpm 3ph delta 415V 2.2Kw when you got to extreme torque? I have been looking for a cheap hp single phase on ebay but nothing yet.
Having said all that, I am not trying to lift 2.5 ton, I’d be happy with 1 ton tbh and I can’t get past the thing that power is power, although perhaps not in real time as the graphs shows. I intended to strip the 2.2kw motor before posting for help in case there is a fault. In my mill, a 3hp 3phase start, I was able to bring out the start point and swap it to delta. I think (from an unreliable memory) that each phase had a pair of wires to make four pole. They had welded the copper wire, no solder. I have practiced a lot with tiny welds and did the same but its not straightforward to weld thin copper wire. I noticed they had not been so careful when they built it, and one of the phases only had one pair of wires connected at one end. The other wire had burnt back and was open circuit. I may be mixing up my story there, but I was thinking well at the time and came to the conclusion that mine was a 2 pole motor on one phase and four pole on the other two and had been like that from new. I am still hopeful if I open up the new motor I will find a fault like that.

Michael G
11th Feb 2017, 09:26 PM
Just looking at your graphs I wonder if in a situation like this which is basically demands a constant torque whether the problem is not the maximum torque but the minimum (as you say in your post).
As something else to consider, do you think it would help to gear down the motor - that is, reduce the lifting speed to get more torque. A chain drive or low ratio gear box (say 2:1 or 3:1) would be slower to hoist but at least should hoist...

Michael

Oldneweng
11th Feb 2017, 09:49 PM
Sossity, I had the same thought as Michael. In all the discussions we have had I never thought to ask how the motor is connected. Maybe you already mentioned it.

Dean

Oldneweng
11th Feb 2017, 09:53 PM
Kryn, When I got the VFD for my lathe I was a total VFD newby and asked some stupid questions. Getting it connected and setup was purely a matter of asking questions and following the replies. It may not be perfect, even now, but I had no real problems. I say go for it.

Dean

Theberylbloke
12th Feb 2017, 10:48 AM
Hi Sossity,

I'm not sure but I think that the difference between single phase and three phase torque will be proportional to the areas under the curve rather than a straight forward 10% vs 50%. Compare the torque value of the single phase motor when the voltage passes through 0 volts, 10% of the motors torque, compared to torque on the three phase motor, one leg would be at 100% torque. The fact that the single phase motor stops in less than a turn i.e. when the torque drops to 10% and stalls is something I'd take as a bit of a hint!

It's true that the torque produced by one leg of the three phase motor will be less than the torque produced by the winding of a single phase motor where they are both of the same output kW.

For most of us here, whether the motor is connected delta or star relates pretty much only to our ability to drive it from a 240V VFD supply.

All A.C. power calculations use Root Mean Square (RMS) values. The result of such calculations is a value that is the same as if you were using D.C. power.

Some smaller A.C. motors are designed to be swapped between star 415v and delta 240V quite easily. Their name plate will show the voltages and the star \delta symbols.

Cheers

The Beryl Bloke

BobL
12th Feb 2017, 12:36 PM
If power = torque x rpm, and both motors are the same power and both have the same rpm, does that must mean the single phase motor has a far higher peak torque, because its dropping to 10% of peak, while 3phase only drops to 50%. Is that right?
Or, do they cheat and use peak torque for the power calculations?

The torque will be that measured by a dyno which is average torque.
On my HP rig it measures the same thing - average torque so it doesn't matter what it's doing instantaneously.

I have thought a few times about your hoist and am wondering if there is a difference between what the hoist is physically rated at, and the actual motor they put on it.

Something like a hydraulic press being available with two different rams - so the press itself is engineering wise capable of X but when fitted with a smaller ram is only capable of something less than X?

Did you ever actually see the hoist lifting its rated 2.5t with the 2.2kW 3P motor?

sossity
12th Feb 2017, 05:54 PM
Thanks for the comments everyone.
Micheal G/Oldnegeng,
I have thought about lowering the gearing as it would seem a simple solution in one way.
366585
The picture shows the motor to shaft gearing. If I could make the motor pulley smaller that would be a simple fix, but its at its minimum. Likewise, the pulley on the shaft is 250mm and I couldn’t make anything bigger myself. I thought I could lower the motor, drive a pulley on a short shaft, which has another pulley on the top of the shaft in line with the existing big pulley. I think no less than 3:1 because this is a tool to be used which takes 90 secs to rise and I found when doing the clutch its very handy to move it up and down to get to the top and bottom. I don’t want to wait 10 minutes for it to rise for instance. I felt this would end up being quite a big project which I don’t really have time for but what’s the feeling, should this be easy? I can’t do keyways for the new pulleys so that kind of put me off.

I can buy a 4kw single phase motor for $290 new, which I’d pay for a quick fix if I was sure it would work. That seems cheap after lifting the car manually in the heat (I have to go to the top of one pillar and wind a big nut by hand, at the hottest part of the shed. 10 minutes to raise it 4ft)

TheBerylBloke/BobL
I searched a long time to come up with that graph. Ideally I wanted a comparison of the same hp for the motors. You’d think the internet would be full of torque comparison between 3ph and single ph but maybe I’m just no use at searching. With what you are all saying, the single phase must have higher peak torque right enough, but with such a high torque load, I think the lowest instantaneous figure becomes important. Of course the 3phase has a much higher low figure and starting torque, which might be the limiting factor in this application. The 290 dollar question is if I double the kw of the single phase, will that new low torque figure be high enough. It still won’t be as high as the 3phase motor, but momentum in the motor itself must contribute at least something to the equation. I don't want to have to lower and lift again to raise it a few inches because its likely the wheels will be off.
Here’s the plate on hoist which shows the kw, plus I have seen the same hoist on the internet. It all looks standard with its asea motor and pulleys and was advertised as 2.5Ton 2.2kw.
366586
I’ve got a photocopy of the manual and it implies the swl is an actual working load, so the motor should lift it. The way its built, I think the frame would be rated at 10 tons compared to those from china, its a heavy beast.
I’ve read all the posts on screw hoists I could find (not many, in fact there’s very little (no) info on actually refurbishing one of these) It seems common for people who change to single phase to be able to raise their car but then have to lower it to the ground if they want to move it higher. I am as certain as I can be that I saw an advert for new tecalemit hoists 5 months ago, which had 2.2kw 415V 3ph motors or as an option a 4kw single phase. That kind of gave me confidence to risk a 4kw single ph motor but I haven’t yet because I’m only lifting 1 ton and I feel sure my motor must be faulty. Its pretty hard lifting down the motor from the top of the hoist but I’ll get round to it because I’ve got another job for the hoist waiting to be done already.
I couldn’t find that same tecalemit advert now, in fact they have swapped to 4Kw 3phase now which is different to when I was looking just 5 months ago. They seem to have upped the rise time too to 45 secs. Hydraulic hoists were faster so they must have decided to compete on rise time just in the last 5 months since I was looking. On the delta thing, I thought I read that delta 415 would give higher torque than star 415. Since its delta 415, you can’t convert to delta 240 by just reconfiguring it. That was where I was going with that comment, the best I could hope for out the original motor at 240v 3 ph is half power.

Oldneweng
12th Feb 2017, 06:09 PM
Gearing the motor down 3:1 seems suitable to me. If you can lift x kg as it is then you should be able to get 3x after. It is a matter of time/money. Your choice. If it normally takes 90secs to raise (original motor) then it will take 4.5 mins to raise after conversion. I guess you have to hold your finger on the button all this time? That would probably annoy me pretty quickly, depending on how often it needs to be used.

Dean

Theberylbloke
12th Feb 2017, 08:17 PM
Hi Sossity,

I'd look at gearing the existing single phase motor down. I have looked at a 4kW single phase motor for an air compressor I bought. 4kw single phase motor has a starting current of around 125 amps! I doubt you want to go there given the likely 60 amp fuse in the service from the street.

Cheers

The Beryl Bloke

BobL
12th Feb 2017, 09:40 PM
Hi Sossity,

I'd look at gearing the existing single phase motor down. I have looked at a 4kW single phase motor for an air compressor I bought. 4kw single phase motor has a starting current of around 125 amps! I doubt you want to go there given the likely 60 amp fuse in the service from the street.

Cheers

The Beryl Bloke

You fuse should easily cope with that starting current because it should be specifically designed to cope with high start up current for a short period of time. My 2.2kW dust collector registers a start up current of over 60A but it is only connected to a 16A breaker.

Theberylbloke
13th Feb 2017, 09:41 PM
You fuse should easily cope with that starting current because it should be specifically designed to cope with high start up current for a short period of time. My 2.2kW dust collector registers a start up current of over 60A but it is only connected to a 16A breaker.

The difference being that the service fuse already has all the electrical load of your property on it when you start your 4kw motor, you might get away with it, you might not. Do you know what the average load is in your house at the times you want to run your hoist? How well will you and your family cope if it's 42 degrees outside with the air conditioner & beer fridge not working until someone arrives to replace the fuse? how long will it take for them to get there and what is it likely to cost you in charges? Not to mention that the electricty distribution system in S.A. and NSW really needs extra load just at the moment.

Cheers

The Beryl Bloke

BobL
14th Feb 2017, 09:55 AM
The difference being that the service fuse already has all the electrical load of your property on it when you start your 4kw motor, you might get away with it, you might not. Do you know what the average load is in your house at the times you want to run your hoist? How well will you and your family cope if it's 42 degrees outside with the air conditioner & beer fridge not working until someone arrives to replace the fuse? how long will it take for them to get there and what is it likely to cost you in charges? Not to mention that the electricty distribution system in S.A. and NSW really needs extra load just at the moment.

Cheers

The Beryl Bloke

If the correct breakers are installed they should easily be able to cope with this

Here's an example of some breaker curves.
A C type breaker can hold continuity for 7 times its rated current for up to 4 seconds before tripping out.
At 4 times its rating it can hold continuity for over 10 seconds.

So a 32A C type breaker should hold 128A for up to 4 seconds, which should cope with start up loads from large motors.

366597

Theberylbloke
14th Feb 2017, 08:24 PM
Hi Bob,

I agree whole heartedly that a 32A Type C circuit breaker would hold 128A for up to 7 seconds assuming there is no derating of the breaker. This will likely require the circuit to be wired in 4mm conductor rather than 2.5mm depending again on any derating factors that may crop up from the installation of said cable. 2.5mm conductor has a current rating of 28 amps (off the top of my head) if run on cable tray or other fully exposed to air situations. The use of a 32A breaker to protect 2.5mm cable would be defectable due to the fire risk involved, i.e. in the event of a fault within the cable or attached equipment, the cable my overheat and start a fire before the circuit breaker trips. Note that 32A is about 10% above the cable rating, this is a minor defect. At 20% above the cable rating it is a serious defect that requires disconnection of supply. The method of installation chosen for the cable run can be critical.

Non the less, I was talking about the service fuse that is at the point of supply, i.e. out on the street, not the circuit breaker that is protecting the final sub circuit. It is possible that where Sossity lives his distribution entity uses circuit breakers instead of fuses for overload protection. It is possible that Sossity has a 63 amp Type D circuit breaker on his main switch board protecting the distribution entities service and Sossity's mains. It is possible that the distribution entity has used a 45, 50, 63 or 80 amp fuse to protect the service and mains. The service fuse could be on a pole in the street, the fascia or on the main switchboard. I have not been to Sossity's house and don't know which of these scenarios exist. I don't know what size or how long the service is. I don't know what size or how long his mains are. I don't know what sub mains are involved. Nor am I aware of any derating factors that should be applied because of installation methods. I do not know what the loop impedances are.

Finally, I don't know what load is likely to be drawn by the other electrical items on his property when he turns on his hoist, it could be next to nothing or it could be 40 amps. This load has to be added to the starting current of the hoist for the purposes of circuit protection, voltage drop and cable sizing. It's this load added to the starting current of the 4kw motor, that will determine whether the service fuse blows or not.

Given my ignorance of all of the above, I am happy to recommend Sossity gears down his 2.2kW single phase motor by say 3:1. I, personally, am not comfortable suggesting he install a 4kw motor. If you have done the necessary homework on this installation, having in place the prerequisite licenses and appropriate insurance, i.e. putting your money and livelyhood on the line, then I am more than happy to agree with your recommendation of a 4kw motor.

Cheers

The Beryl Bloke

sossity
15th Feb 2017, 10:41 AM
Hi Theberylbloke
The 4kw motor was my idea and based on the equipment I already use, I thought it would be fine.
I have a 40amp supply to my shed, which was installed years ago specifically to run a panel beating business until it got too big and he had to move out to a proper business area. It all looks professional and I remember being surprised at the gauge of wire they had used to the shed and it must have been in the days before copper got so expensive. The house itself has an 80 amp switch/breaker connected to the meter, which feeds the distribution board and subsequently the shed. Would that be the power board fuse you refer to?
I have a 3kw (4hp) compressor which switches on as it pleases. I’ve had more than once where I just turn on the lathe and the compressor fires up too, so that would be a very high startup current combined. You know how these million to one chances crop up 9 times out of ten… Anyway, I always assumed I could do what I want and the 40 amp breaker would trip with no problem elsewhere, certainly not in the power companies side of things? I once tripped the 40 amp RCBO that feeds the shed at the house, but I traced that to the fact the welder torch was touching an earth when I powered up the welder, and its never happened again. God knows what the welder startup current is, I know it sometimes causes the lights to flicker when I switch it on. It’s a cheap Chinese 250amp/60amp plasma with no inrush current protection. Oh, and if the house is using 40 amps, there’s arses going to get kicked!
We already have so many outages here, up to 15 hours at a time, I have a generator for the aircon and fridges etc. They charge me $3.50 a day for the power connection and I guess at least 10 outages a year - and this is a Melbourne suburb. I thought that was normal for Australia but people at work say they never get an outage.
The thing I still don't know is, will a 4kw single phase motor have as high a start up torque as a 2.2kw 3phase motor wired in delta? I've not been able to find any figures on that side of motors. I'm also suspicious why my single phase motor can run at full speed as it lifts the wheels clear, then slow down over several inches until it stops suddenly. If it has higher peak torque than 3phase, and its already running at speed, shouldn't it be able to lift the thing? I understand why it ultimately stops quickly, but not why it also slows down slowly from full speed, if you know what I mean. That's my first port of call, to check out that motor physically because I don't think that's right.

DSEL74
15th Feb 2017, 10:55 AM
Kryn, The other point to consider with fitting the VFD is if the motor is suitable?
Is it an induction motor?
Is it wired star or delta and can it be changed at the terminal box or will it require rewiring?

This is all before connecting the VFD which could be considered the easy bit.




Sent from my iPad using Tapatalk

Keith_W
15th Feb 2017, 02:22 PM
Hi sossity,
Here is a link to a site that has some info on starting torque.Difference Between Single Phase and Three Phase Induction Motor - Circuit Globe (http://circuitglobe.com/difference-between-single-phase-and-three-phase-induction-motor.html)
Another site that may be of help. https://www.google.com.au/search?sourceid=navclient&ie=UTF-8&rlz=1T4GUEA_enAU709AU709&q=Use+of+Induction+Motors+in+the+Home+Workshop+%e2%80%93+issue+3.0+29th+December+2014
Where you say that it lifts for a bit, slows then stops, is it that the mechanical side, the screw nut and thread is binding causing the motor to draw more current and the overload trips.
Could be mechanical as well as electrical concerns here.

Keith.

Keith_W
15th Feb 2017, 02:48 PM
Another good read is found athttp://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en012135
Download the AN887 pdf.

Keith.

sossity
15th Feb 2017, 03:28 PM
Thanks for the links keith, there looks to be lots of useful info in there.

What i was really hoping for was an actual comparison of typical motor real time torque so i could guess if a 4kw 1ph starting torque was anywhere near a 2.2kw 3ph. I had a quick look at your links but couldn't find anything specific, but did you see something in there? I have looked a lot over 5 months and have lost the will tbh. I skimmed that pdf link and I think there is just about every graph except 3ph vs 1ph torque!

I know exactly what you mean it could be the hoist and I worry about that. I refurbished the whole thing and even made new brass nuts for example. Everything is cleaned and greased. I am as confident there is nothing wrong as I can be I guess. When i manually wind it, the load comes on gradually as the suspension lifts - there is no sudden increase. The fact that its hard to wind up but easy to turn down, i take as a good sign that there is nothing binding more than it should.

What it does, is with arms below the car on the ground, it will start and the motor picks up to full speed more or less straight away with no load. it will lift the car off the ground, and change pitch as it takes the load. it begins to sound like a car hoist if that doesn't sound daft. But once the wheels are clear of the ground (perhaps even before), it starts to slow audibly, but not a very fast rate of slowing, and it will do this while traveling 2 or 3 inches more. Then it stops with a bang, no slowing down gradually. An induction motor can only stand so much slipping, so no surprises in one way, but still, if it can run the load for several seconds while slowing just gradually, why is it doing that if it has the same average torque (and indeed higher peak torque) than a 3phase motor of the same power? The fact that it can complete even just one revolution but slow down just a bit, to me that means its not the same power as the 3phase one. That's the bit that makes me suspicious of that actual motor. It could be a mechanical problem too, but I can wind it down easily plus spent hours carefully checking everything. Although everything is heavy duty, its a pretty simple thing.

 

Keith_W
15th Feb 2017, 04:04 PM
I don't know if there is a graph that shows both single and 3 phase on the same graph.
Single phase motor is a two pole motor so to get the rotor turning you have two magnetic fields being generated where in three phase you have four poles so four magnetic fields being generated, this I am sure you have read several times.
In the current situation you have do you access to a Tong Tester to see the amount of Current Draw you have, at startup, initial raising of the car then as you get higher. This may indicate any binding you get mechanically eg high spots on the tread on the shaft, shaft out of round etc that would cause the motor to stall.
There then has been a lot said on the 4kW single phase motor option, and you will pull a lot of amps at startup typically 6 to 8 time FLC and because its a two pole the current will still be high if the motor is working under a big load. Remember the Circuit Breaker is only to monitor the Cable and if its a C Type limited inrush capability as show in previous posting, a D Curve Breaker is designed more for Motor Type applications.
My option would be to look at a VSD as you can get ones with adjustable torque settings, has inbuilt Breaker so to speak and can be programed so you get a Soft-Start Type operation.

Keith.

sossity
16th Feb 2017, 08:24 AM
Hi Keith,
The current behaves as expected except when it stalls it drops to 4.5amps. The manufacturer said it has a built in overload protection.
I have mostly ruled out anything mechanical because I can lower the hoist under load fairly easily, but it’s hard to raise. There is no binding. I don’t have anything to compare it to though. At full load I can only just turn the 250mm pulley at the top by using both hands at the top of a ladder. I can lower it easily by just pushing the rim with one hand. Every component has been apart, cleaned and greased.
I don’t want to use a vfd because my interpretation of AS1418:9 is that it would not meet australian standards and would therefore be non compliant. I can’t remember the details now, but I thought at the time it was pretty clear I wouldn’t be able to use a vfd and keep it within the regulations. I’m not saying it wouldn’t be safe, just that I couldn’t make it meet regulations on technicalities.
I’ll start a post on this in the future once I get it sorted as I don’t have time right now. It’s a major job to take anything apart because of the weight (750kg) so it has over 600kg on me in a fight. You’ve made me rethink about things though and I can disconnect the chain and load up one side at a time to make sure they both have the same resistance. Other than that, it becomes a major project to check anything.

Keith_W
16th Feb 2017, 09:38 AM
Hi sossity,
Understand that your busy and you will be looking at the control issues and motors latter when you have time.
The Standards for Cranes and Hoists and in particular AS2550.9 for Electrical refers to AS1418.1 and I believe it allows for control where a VSD can be used as long as the circuit is configured so it fails to a safe state.
For Cranes and Hoists the Standards get a bit messy with a lot of referring to other Standards so can get a bit confusing.

Regards,
Keith.

sossity
16th Feb 2017, 10:07 AM
Sorry, I had quoted the incorrect standard, but you are right, you end up having to read various other standards. The very reference you mention is one of the points I consider make it non-compliant. How do you ensure a vfd will fail in a safe state? I could not consider a cheap device with logic in it incapable of failing such that it remains driving a motor for instance. In expensive electronics you can have any device monitored by other devices so overall the circuit will fail in a safe state, but as it stands, a vfd does not meet the requirement in my opinion. There are also other standards which dictate the buttons required, and buttons on a vfd do not conform. you can wire out to proper buttons but I don't think its safe to have two sets of buttons for instance. it goes on and on... I've got at least 5 months on you with this project, but you are on the same lines as I was :D Eventually you will end up going round and round same as me!

NCArcher
16th Feb 2017, 03:08 PM
I know you don't want to get into this now but surely the safe state refers to the hoist rather than the VFD. It just means that the motor can fail but the hoist will not change position. The hoist cannot rely on the motor/VFD to maintain it's position. No different than the motor that is on there now. The VFD has nothing to do with the operation of the hoist. It is solely there to supply the correct voltage to the motor. The soft start/stop are just bonuses. External buttons can be connected to the VFD and the VFD key pad can be disabled. You can stick the VFD in a cupboard away from the hoist if you want. You never have to see it.
I think a VFD with a 3 phase motor is still a good, reasonably inexpensive option. I've wired up a few car hoists and I don't recall any of them having huge motors. But then I didn't pay much attention to the mechanics of the hoist at the time. They may have been heavily geared.

sossity
16th Feb 2017, 05:26 PM
Hi NCarcher,
I don’t want to spend too much time on this aspect and it does come down to my own interpretation of the standards, and if I even want to try to keep the hoist compliant to as 1418.9. I’m pretty sure the words are along the line that the vfd or circuit has to be fail-safe as this was written in 1418.x which is actually the electrical standard for hoists/cranes etc. A screw post hoist frame is mechanically fail-safe because it goes nowhere if you remove the power, as you say. Hydraulic hoists are more common (and cheaper) and get away with smaller motors but require more mechanical safety features because they can fail catastrophically if there is a failure in the hydraulics.

Did you wire your vfd’s to the input of the hoist and use all the original mechanical interlocks and buttons as intended, ie, the vfd output can get disconnected at full load? To do that you’d need to change the contactors and thermal overload to suit 240V (unless you were using a 415V vfd of course), so it’s a bit more expensive option than wiring direct to the motor. Until recently, I thought that could damage the vfd, if you disconnected the motor under load but I read someone who said its OK (on another application). I’d be interested to know if you had a system that broke power from the VFD as its driving and its been working for a long time.
Anyway, I didn’t want to be a guinea pig. Also, my vfd makes a big point not to switch off power to the vfd if its driving a motor because that will cause damage to the vfd. The kill switch would have to remove power from the hoist to be compliant and I’d say the vfd is part of the hoist because it won’t hoist anything without it. There is also a requirement that if the mechanical interlock trips at the top or bottom of travel, (or if the chain is loose) that the hoist cannot be operated again until a reset button is pressed. All these things would make a vfd wired directly to a motor, a non compliant system in my opinion. When you go through the standards, there’s loads of things that make using a vfd difficult, again in my opinion. I’d recommend everyone not to believe me but to make their own decision on it if they have an interest.

eskimo
17th Feb 2017, 08:26 AM
Hi Kryn

DSEL74 above mentioned it but I dont think he stressed it enough to you.

A single phase Variable Frequency Drive (VFD), Variable Speed Drive (VSD) or just an Inverter drive as they could be called, will need to be connected to a 3phase delta wound motor. (a few can be connected to 415v Star but are not as common)

Some motors are convertible within the terminal cover plate from star to delta and vice verca, but some are not without pulling them apart and reconnect as delta.

So Ideally you need
1.a single Ph to 240V 3 phase inverter drive, matched to the motor size ie HP or KW.
2. A three ph motor 240v delta wound or a 415V star wound Which Can Be Converted to Delta.

The Inverter drive could also have Vector control ( torque booster for you) if required.

and we are only in Adelaide if you need help.......not even a packed lunch away.

eskimo
17th Feb 2017, 08:30 AM
If the correct breakers are installed they should easily be able to cope with this

Here's an example of some breaker curves.
A C type breaker can hold continuity for 7 times its rated current for up to 4 seconds before tripping out.
At 4 times its rating it can hold continuity for over 10 seconds.

So a 32A C type breaker should hold 128A for up to 4 seconds, which should cope with start up loads from large motors.

366597

Commonly called Motor Start circuit breakers and are a good idea for motors without overload protection.

Keith_W
17th Feb 2017, 03:16 PM
Hi sossity,
The standard AS 1418.1 is a broad based standard to cover cranes, winches and hoists so as such has a lot in it that is not relevant to a car hoist.
In a circuit where you want to put contactors to open circuit a drive they normally go line side. I attach a simple circuit I have drawn to show how I would configure your application to meet what AS1418.1 and AS4024.1501 standards, the Safety Control Category is at Category 3 type circuit.

Keith.366622

sossity
18th Feb 2017, 09:40 AM
Hi Keith,

Thanks for going to the trouble but sorry, I still don't agree your circuit meets the requirements of 1418.9 (the standard for vehicle hoists) which says "Electrical equipment and controls incorporated in vehicle hoists shall comply with the requirements for electrical equipment and controls given in AS 1418.1."
However, I'm sure most of hoists converted to use vfd will use the circuit you show, without the logic even, because most people don't know or care about type approval. I make no claims on the validity of my opinion or on overall safety. In my opinion there are technicalities which can't be met meet simply, on hoists which are converted to use a vfd. AS 1418.1 is a 179 page document, it would take a long time to argue over it and we still wouldn't agree. If I retain the control system exactly as it is without introducing a vfd, then there is no argument - it has been type approved already. I actually bought a vfd intending to do exactly what you suggest, but after reading the vfd manual and the standards, I bought a single phase motor and have retained exactly the same control circuit features as original, something I couldn't do if I used the vfd.

Keith_W
18th Feb 2017, 10:16 PM
Hi sossity,
I don't want to get into a argument with you over this, you don't agree with what I have said that's fine.
BUT.... I have over 25yr's experience in designing safety circuits for Australian Industry and have involvement with Standards Australia, WorkCover and been part of the Safe Design Project as administrated by the Federal Government, and as such do have a very good understanding of Regulations and Australian Standards.
To end this I hope you find a suitable solution to your application.
Keith.

sossity
19th Feb 2017, 06:55 AM
Hi Keith,
I don't think this is about safety at all, what worries me is the approval as detailed on the hoist plate. I'm certain in Europe you can't change the design at all without having the whole thing approved again. Everyone talks about that when people do modifications to their cars etc. I notice here that for cars, people bolt or weld on all sorts of non approved things they have made but in UK everyone knows that voids your insurance. Are the rules in Australia not so bad? I confess to not knowing Australian laws. I can't get past thinking it comes down more to what a lawyer says than an engineer because that is who will saying your life insurance is void.

Your experience is certainly relevant and I'd genuinely like to know more.
If I do go ahead and use a vfd, what is your take on this,
AS / NZS 1418.9:1996
2.11 ELECTRICA L EQUIPMENT AND CONTROLS
Electrical equipment and controls incorporated in vehicle hoists shall comply with the requirements for electrical equipment and controls given in AS 1418.1.

from 1418.1

Section 8 Electrical equipment and controls
8.7 CONTROLLERS
8.7.4 Electronic control
Each electronic control circuit shall be designed and installed so that it complies with the
following requirements:
(a) The system shall be fail-safe.

APPENDIX C
FAILURE TO SAFETY (FAIL-SAFE SYSTEMS)
(Informative)
C1 GENERAL
It is self evident that a single component cannot fail-safe. To satisfy the concept of failure to safety, a single component is replaced by a system of components or by re-configuring the whole so that the failure of the component will be inconsequential.

A vfd could lock up while driving a motor for example. I don't think any unintended movement of the hoist could be considered inconsequential, that is the whole vibe of 1418.9. Surely you would need a watchdog circuit (capable of disconnecting the vfd) to monitor the button positions to make sure the vfd is not driving when it shouldn't, and also vfd phase, because I think 1418.9 says the buttons can't make the hoist go the wrong way.

It would be good to hear your take on failsafe and the whole type approval thing after changing the design once approval was granted.

Keith_W
21st Feb 2017, 06:37 AM
Hi sossity,
As you said the standard AS/NZS 1418.9:1996 is the specificpart of the 1418 series of standards that defines the requirements for VehicleHoists and in part 2.11 where the type of hoist uses electrical equipment itrefers to AS 1418.1 – 2002.
AS 1418.1 - 2002 Cranes, hoists and winches - General requirements is a broad basedstandard that covers all types of cranes, hoists and winches. When reading thisstandard you need to use the parts of the standard that would be relevant tothe electrical control of the Hoist in question.
You have asked me to give my thoughts on specific parts ofthe standards, this is my thoughts;
We are looking at various control methods of raising andlowering the hoist being
a. Standard three phase control, here the controlcomes standard with the hoist and no modifications needed. Not an option inyour application as you don’t have three phase.
b. Replacement of three phase motor with a singlephase motor and use much of the existing control wiring. A lot has been alreadysaid on this in the postings and there is some torque problems as has beenstated.
c. Use the original three phase arrangement and inserta drive to allow the use of single phase supply to run the three phase motor.
For option C you have raised issues in using this method as whetherit would be failsafe as outlined in section AS 1418.1 section 8.7 and in particular8.7.4. That particular part of section 8.7 is referring to custom orproprietary electronic control circuit that has been designed by the cranemanufacturer where it is used to control the crane. This is where the design andlayout of components of that circuit needs to be such that a single fault cannotlead to the failure of the control, that’s where the term failsafe comes intoit. The whole of section 8.7 is referring to several different types of methodsof control trying to account for existing and emerging technologies.
You have raised concerns that a Drive could fail ormalfunction or send the Hoist in the wrong direction. A fault like that couldhappen but highly unlikely as drives have demonstrated a high degree ofintegrity in their design and have been used very successfully in industry fora long time. If you look where a drive has failed in industry it has mainlybeen where it was not configured or used in keeping with the recommendations ofthe drive manufacturer. The type of drive also has a bearing, a drive that hasthe STO type design (safe torque output) could be used and the control systemdesigned so that feature is employed each time the drive is used to raise andlower the hoist.
In conclusion a hoist has locking recommendations etc. to beused when using the hoist and this is also outlined in the standard for vehiclehoists. Raising and lowering of the hoist is done away from the hoist where theoperator has full view of the area to see if there is any danger to themselvesand other people.
Legislation states that people conducting a business orundertaking PCBU are to do what is reasonably practicable to ensure the safetyof people in the workplace, and explanation of this is attached in a pdf, thisis what the lawyers will use.
http://www.safeworkaustralia.gov.au/sites/SWA/about/Publications/Documents/607/Interpretive%20guideline%20-%20reasonably%20practicable.pdf
Keith.

sossity
21st Feb 2017, 11:11 AM
Hi Keith,
As an aside, a screw post hoist has no physical safety locking mechanism because unlike hydraulics, it is inherently safe when at rest. It’s the big advantage of that type of hoist as you can move it up and down just a few mm as you need, where I think hydraulic ones have to be lowered into physical locks at certain positions. There really is a requirement for the control circuit to be unable to malfunction and I even have concerns about those button wires you take to the vfd wearing through and shorting out causing the hoist to move. The original electro mechanical control system is really bullet proof and I can’t see how it could fail in any way
I think you are agreeing that AS 1418.9 says the control system shall be failsafe, but your linked document says that safe work Australia allows businesses to take risks if they are small risks and so instead of failsafe, very reliable is OK. I had been under the impression that AS standards were a legal requirement but I just found out that’s not necessarily the case. It depends on whether the government has created legislation to make it a legal requirement. I couldn’t see anything to say if AS1418.9 is covered by legislation or not.
That may get me out of conforming to AS1418.9 but what about type approval? https://www.safework.sa.gov.au/uploaded_files/ha_vehicle_hoists.pdf
Vehicle hoists must have valid design registration numbers issued by a work health and safety authority in Australia. If in doubt, you must contact the supplier. It is illegal to operate a vehicle hoist without a valid design registration number.
This is basically what I was saying about type approval which all countries have some form of. My hoist was approved and has a different design registration number for each state. If you change the design of the control circuit in such a major way as introducing electronics into it as well as running cables where they were not intended to be, don’t you need to get it inspected and get a new design registration number?

BobL
21st Feb 2017, 11:54 AM
Hi Keith,
As an aside, a screw post hoist has no physical safety locking mechanism because unlike hydraulics, it is inherently safe when at rest. It’s the big advantage of that type of hoist as you can move it up and down just a few mm as you need, where I think hydraulic ones have to be lowered into physical locks at certain positions. There really is a requirement for the control circuit to be unable to malfunction and I even have concerns about those button wires you take to the vfd wearing through and shorting out causing the hoist to move.

This won't solve the compliance problem but a physical switch that isolates the VFD from the motor is easy to install.
Then the VFD and any switching circuits can short/open all the likes and the motor just won't start.
Of course both switches and wiring can fail but then so can the one in the original hoist.

This is sort of what I have to switch between two 3P grinders
The topmost black switch inside the transparent box switches between the two grinders, the centre switch position of which is a complete disconnect of both grinder motors.
To access the switch the transparent cover has to be opened and a microswitch on the door stops the VFD and throws a relay in the box under the VFD.
The VFD wont start a motor until the black switch in not in the middle position, the red/green switch is manually triggered to reset the relay and the toggle switch on the box under the VFD is its "start" position
The toggle switch is the normal start/stop switch used when working on the same grinder.

When the black switch is in the middle position the other switches and the VFD can do anything they like but nothing will happen because there's not physical connection to any motor.
If you try to start the VFD it will report an error.


366732

Michael G
21st Feb 2017, 12:38 PM
If a piece of equipment must have a design registration number for it to be legally used/operated then you can pretty well assume that one of the requirements will be that it conforms to or exceeds the appropriate standard(s). Without getting into the specifics of the discussion, if you modify an item away from a standard and something happens, there is a whole heap of trouble that can come your way.
Standards are usually not written to be prescriptive but rather set out what characteristics or performance is expected from an item covered by it. So a standard will generally not say 'use widget X' but instead have a performance requirement that effectively requires the use of widget X to achieve.

The idea of 'fail safe' is exactly that - if an item fails it must revert to a safe condition. This may be through multiple redundancies, significant over design of key components, mechanical interlocks, devices to remove/ deplete stored energy and so on. I have even worked in factories where there are written procedures on how automatic work cells are 'made safe' before anyone is allowed to enter (so robots are electrically isolated, counterweights are secured, pneumatics are exhausted (or locked) and any other steps to ensure that if a control system failed in either an on or off condition, nothing unexpected would happen), and written evidence is required to show a responsible person has done this.

In this particular case, if the VFD failed either in the on or off state would the hoist

stay in a safe condition, or would it
drive up/down in an uncontrolled manner, or would it
collapse/ settle into a lower energy state that has potential to endanger life or property?


This is why cable and hydraulic hoists have mechanical travel locks - so if a cable breaks or a line springs a leak, the platform will not descend without being arrested. As you have noticed, a screw hoist will self lock and so does not need those devices. When wiring E-stops, hard switching is preferred (required?) because when they go open all electrics have power cut, whether that is because the E-stop is tripped or a connection has broken. If you ran travel limit switches through a VFD, could you guarantee that the hoist would stop at the limit switch, or could a hardware or software issue cause a malfunction?
I suspect that if you said to a regulator that you wanted to use a VFD on a hoist their approval would be conditional having a set of independent switches, such that power would be cut to the drive motor on reaching travel limits or an E-stop trip, regardless of what the drive may do. That is, you are not reliant on the software or electronics in the drive to provide a safe system.

Michael

sossity
21st Feb 2017, 03:31 PM
This won't solve the compliance problem but a physical switch that isolates the VFD from the motor is easy to install.

Hi BobL,
I like that and I’d definitely put a switch in as you say. It would make it very safe, the hoist just can’t move anywhere unless you drive the post.

They do take the failsafe side of the hoist quite seriously though. The nut which runs on the screw post, is also failsafe. The load is taken on a brass nut, which has 6~9mm gap then a steel nut below it. The steel nut takes no load and is just along for the ride unless the brass nut fails. On my hoist, the brass nut was worn to a ridiculous amount, completely unsafe, but the steel nut is still like brand new with zero wear. You must check the gap every day and measure how much wear has occurred and replace the brass nut when required.

I know you said it wouldn't help with compliance and I like the idea as it stands, but just for completeness I'll mention the standard has words to the effect that the design of the switch must be such that every effort has been put into making the switch unable to fail. It also specifically says it should not be possible for it to weld together for instance. I remember thinking how can you make sure of that? You could make it unlikely but impossible is something else.


In this particular case, if the VFD failed either in the on or off state would the hoist


stay in a safe condition, or would it
drive up/down in an uncontrolled manner, or would it
collapse/ settle into a lower energy state that has potential to endanger life


Hi Michael G
If the motor drive fails in the off state, then there is no risk to anyone. The hoist goes nowhere and can be wound down manually quite easily.
The vfd could fail on, which would drive the hoist uncontrolled. I felt that could not be considered inconsequential. Even if you are at the switches, what if there was a transmission jack under the vehicle, it could tip the car in seconds.
The interlock switches as standard do indeed carry the current for the motor making the interlock a good safe guard to the end stop. Although it would be quite dangerous to have the hoist drive too far at the end, the standard says something like it should be designed to handle such an accident without collapsing to the ground. I think it might say it is ok for the hoist to be damaged but not to the extent that it collapses but I am relying on my memory for that.
It sounds like you are thinking the same as I was Michael, and that to modify at all will make it illegal. I kept all the interlocks the same and could argue the control system is the same design but I did rewire for single phase.
The interlocks on mine were burst and I couldn’t find the same type of switch anywhere so I substituted a higher spec lever switch. I had to modify the mount (drilled another hole in the mounting plate) I’d say changing the switch was inconsequential, but is it still illegal because it was not what was approved? I wonder if there is any legislation which covers what a modification is as against maintenance.

Michael G
21st Feb 2017, 06:20 PM
I doubt that replacing the switch would be considered illegal as such - the important thing is that it is there and functions as the designer (and standard) intends. The standard would not be concerned with the transmission jack possibility as that should be covered by your own hazard assessment. It would however be concerned with the idea of the hoist driving up or down and no way to stop it.

Michael

Oldneweng
22nd Feb 2017, 12:44 AM
Sossity, to prevent any chance of the VFD operating as a result of wire/switch damage etc, having a 2 button operation where one of the switches is normally open and cuts the DCM wire and the other one is the up or down button as required. A normally closed emergency stop type switch, also cutting the DCM wire will act as a lock switch. No external switches will operate while the DCM wire is cut.

These will not prevent any issues internal to the VFD, but will prevent any external problem from causing the hoist to move.

Dean

Keith_W
22nd Feb 2017, 06:15 AM
Hi sossity,
I think you need to talk to Tecalemit about their approval and your proposed modification as to whether it would be still valid to their approval. If not then do you want to go through the process of getting a new approval because of a design change?
One main point in the discussion has been the failure of the drive during the operation of either going up or down, in particular the drive not responding to the input signal and continuing to operate. Some of the other comments have given suggestions on the input to the drive but not addressed the drive not responding to the signal. End limit switches have been discussed but they don’t address the situation of faults occurring between the end limits.
You have raised the question of the system being failsafe and the drive being failsafe, look at this link where a Schneider drive has the STO features that I mentioned before. These type drives have functions and redundancy inbuilt to allow the drive to be configured into a circuit to achieve the desired safety level. http://www2.schneider-electric.com/resources/sites/SCHNEIDER_ELECTRIC/content/live/FAQS/237000/FA237443/en_US/S1A45606.pdf (http://www2.schneider-electric.com/resources/sites/SCHNEIDER_ELECTRIC/content/live/FAQS/237000/FA237443/en_US/S1A45606.pdf)
I have not seen the control circuit of your hoist so it’s hard to totally comment on its integrity. Let’s say in its original design for three phase it has two interlocked contactors being up and down with the raise and lower pushbuttons, end limit switches wired into the coils of the contactors and an emergency stop pushbutton wired so it drops power to both coils when activated. This arrangement is not failsafe, either of the contactors can weld and the input switches will have no effect. To be failsafe two contactors or even more would be needed for both the up and down contactors as well as a need for failsafe controllers wired to a control category 4 configuration to achieve this. I doubt that the circuit is configured this way.
Is all of this doing your head in yet……………..
Regards,
Keith.

sossity
22nd Feb 2017, 08:35 AM
Hi Keith,
yes, the whole thing goes round in circles in my head and has done for the last 6 months. That Schneider drive looks ideal but being Schneider will cost more than my hoist did. I always said this was not about safety though, only compliance, and this design approval number seems to make it illegal to do any modification. An obvious modification would definitely attract attention, a single phase motor perhaps less so. I'd like to think it would never come to that. I gave it a go last weekend again and my existing motor is now making a clattering noise, not happy at all. I still say a single phase motor already at full speed should raise the hoist especially at less than half load. If I can fix the motor, then the choice is obvious. Tecalemit is an expensive manufacturer. They do have a single phase 4kw motor option, I'm sure they would love to retrofit it for me and then there would be no compliance issue, because they will already have the design approval number.
On the contactor welding issue, I'm sure I saw a bit in the standard that said switches should be designed to make it impossible for them to weld together. I seem to remember that because I thought, how can something be impossible. Everyone knows a million to one chance crops up nine times out of ten. Anyway, it all comes down to that approval number, they must have convinced the authorities it was safe, and it may be that AS1418 is only a recommendation and not actually a legal requirement. I never found that out.
I have probably made my hoist illegal by fitting a single phase motor, but if it ever came to it, it might be worth a lawyers time to argue if it was safe. Probably not, I'm not worth very much, I think they would pay out of sympathy.

To be honest, if the single phase thing doesn't workout, I will use the original motor and a vfd (try for half power full torque at 30Hz, see if that theory works). The thing is just so handy. I did a clutch over xmas - I might have been tempted to try that without the hoist if I didn't already have it, but it would have killed me. Too old, too fat, can't even fit under the car!

Keith_W
22nd Feb 2017, 09:33 AM
Hi sossity,
I think the phone call to Tecalemit to see what they have to say about modifications and their approvals would give you a clear picture of what your options are.
I would be interested to see a circuit layout of your hoist in its original state so then the suggestions that have been made could be adapted to the circuit with minimum change to original and this just might be ok with them.
Another option is to ring WorkCover and speak to an inspector, one who knows about Vehicle Hoists and see what they say about modifications.

Keith.

eskimo
22nd Feb 2017, 09:50 AM
Another option is to ring WorkCover and speak to an inspector, one who knows about Vehicle Hoists and see what they say about modifications.

Keith..

Based on my personal experience, I very much doubt Workcover, the Safework mobs or any other govt department will tell one what to do, what is allowed, whats not etc etc...all they will do is wait for the Sh*& to hit the fan and then find someone to pin on.
My experience also tells me that that they do not care about near misses either!!

eskimo
22nd Feb 2017, 09:56 AM
366732[/ATTACH]

I hope your going to tidy all that cabling up one day..even a few cable ties wont go astray:2tsup:

BobL
22nd Feb 2017, 11:53 AM
I hope your going to tidy all that cabling up one day..even a few cable ties wont go astray:2tsup:

Yeah its an old photo from when I first set it up. It's all tied down now. I've even covered the mains wires with the missing outer sheath!

RE: Worksafe.
Asking for advice will result in being told vague things like you have to comply with the relevant OHS regs.
If you are not a place of employment they may not even want to know about it,

Richard_C
5th Jun 2017, 07:03 PM
AS / NZS 1418.9:1996
2.11 ELECTRICA L EQUIPMENT AND CONTROLS
Electrical equipment and controls incorporated in vehicle hoists shall comply with the requirements for electrical equipment and controls given in AS 1418.1.

The operative part here is:

"Electrical equipment and controls incorporated in vehicle hoists"

The VFD is not incorporated into the vehicle hoist, it is part of the workshop supply.

Cheers
Richard

Bohdan
11th Aug 2017, 01:57 PM
Comparing a 3ph induction motor with a single phase motor, when it is loaded up to above the rated power, both motors will start to "slip" meaning that they will slow down.

The 3ph motor will inherently try to come back up to speed so it will keep going overloaded but the single phase motor requires a starter winding to be switched on to accelerate it up to speed so it will stall.

3ph motors can often be run for a short time above their rated power but a single phase motor will stall.

In the case of a motor that is lifting a car hoist the duty cycle is very short and the manufacturer can overload it just to get a better figure for advertising purposes.

So if you are replacing a 3ph motor that is run at full power or sometimes slightly above full power then you will require a larger single phase motor just to guarantee that it will keep going.

To work out how much larger you need to measure the current at max load and from that work out what the true power being used is.

Another problem is that the motor manufacturer may be going cheap on the amount or quality of steel in the motor so at full power it is already slipping severely and it is border line at rating that they give it.

BobL
11th Aug 2017, 07:03 PM
3ph motors can often be run for a short time above their rated power but a single phase motor will stall.

I haven't noticed this with my motor testing where all motors are run to the stall point.
When an SP motor stalls the centrifugal switch kicks back in anyway.

Single phase motors can usually easily exceed their rated power and keep working for for just as long as 3P motors.
Both types of motors will quickly heat up and if a 3P motor has proper over current protection on it this will kick in and turn the motor off.
Single phase motors usually have thermal cut out on them that will eventually cut their power.

Here is a HP/RPM curve for a 1490 RPM SP motor rated at 1.3HP.
The 1.3 HP rating is reached @ about 1425 RPM and by applying more back torque it generated > 1.4HP and would have sat there working away until the thermal cut out had kicked in.
Applying even greater back torque the RPM dropped to ~1350 PM the power dropped to just over 1.3 RPM and would also have kept working until the the thermal cut out had kicked in
369156

sossity
16th Sep 2017, 05:36 PM
Excuse me for the long story here. I wanted to come back and finish this in case anyone else is having problems with a converted hoist. There was not much on the web about converting hoists form 3phase to single phase, but hopefully this might help anyone who tries this afterwards. What I did find on the web certainly helped me so here's something back.

I always suspected the motor as being faulty, even though it was brand new and would run and take a fair bit of load. I couldn't get past the fact that power is power, and single phase 2.2kw should lift the same as 3phase 2.2kw, once it is running, so in my mind, power is power, no starting involved. (The hoist takes no load as it starts until the car is lifted off the suspension). Anyway, the reason I was suspicious of the motor was because I had converted my 2.2kw 3Ph star wound mill motor to delta, and when I did that, I could see that one of the poles had never been working. This was because when they welded the copper wires together, one of the wires had burnt back about ¼” and was clearly not joined, not by a mile. I’ve practiced a lot with light gauge welding, and although welding copper wires isn’t hard when you know how, its not something everyone can do. At the time I saw the motor in my 30 year old mill and it had been running all its life at lower power, I though thought there would be quite a lot of motors running out there that are not capable of full power because of bad welding.

A couple of weekends ago, I finally made the effort to take the motor down off the hoist. It was easier than I thought, although putting it back was just as hard as I expected, because its at the top of one of the hoist posts.
When I checked out the motor, the run and start capacitors both read the correct uf on a dvm. I came up with the idea of putting the caps on a megger, and the start cap had high resistance after it was charged but the run cap had only 20k and wouldn’t charge over 50V, on the 250V setting. I got a new capacitor. I noticed the motor was hard to turn over by fingertips. If I opened the motor end caps just 0.5mm, it turned over fine. I ended up taking 0.5mm off the inside of the end cap. I also replaced the bearings which were cheap chinese (same as the motor) . The rotor run out was oK at 0.03mm although one bearing also had radial play of 0.03 also (separate to the run out on the rotor), and felt very loose. I suspect its because they were soft metal and had been run with a lot of preload pushing sideways so had worn in just minutes of use.
The centrifugal switch had welded on one contact and burnt as permanent open on the other. The effect was I got high volts on the start cap even the motor was disconnected which gave me a nice surprise.

The motor coils checked out ok on a megger, and with u1 to u2 the same resistance as z1 to z2. It shouldn’t have been but I haven’t looked at motor design at all for 30 years and didn’t know better. The wire gauge was the same for the U coil as the Z coil so it seemed reasonable.
I put it back together and it ran with a horrible shriek. I should go back and say when I first tried the motor to lift a 1.5 ton van, it lifted about 1.2tons by my guess, then gave up. The motor was relatively quiet. As I tested things, after about 20 tests/lifts it began to shriek. This was still there after I fixed the bearings the run capacitor and the centrifugal switch. Even with no load on the bench.
I thought the shriek was due to electrical imbalance because it went away immediately I switched off even though the motor was still running down.
So I was back to my original suspicion, one of the windings was not welded correctly which would cause the imbalance. You can see here the motor looks ok when I opened it up. If you think the coils look discoloured or burnt, they definitely don’t in real life. Just like new really. Well, it is a new motor.

370004

I had to cut all the lacing away and of course it was the last junction I took off that had the problem. This is normal for me. If there are two outcomes, I always get the wrong one first. Lucky people say you make your own luck. People like me say they are tossers. You can see here that one of the junctions covered with fiberglass tube is burnt.

370005

It turns out I was right, and the welding was pretty bad. I‘d guess they hadn’t cleaned the enamel off the copper properly at all, and in any case, you need a certain level of skill to weld copper wire this thin. Something like this could not be done by a robot so I think its likely to be done badly in cheap motors. In my opinion, there will be many motors out there that are not running to their full potential due to bad welding, but how many are run at full load to know?
In the next picture, you can see on the left the colour of the original welds. This is actually after I cleaned it up with a fibreglass pen.

370006

One wire must not have been connected or maybe originally connected through a very thin section which was heating up and going high resistance and losing power. This was the split phase coil and I suspect would cause an unbalanced ripple current in the capacitor and probably that’s why the capacitor was gubbed. ‘Scuse the photos. I have a cheap old phone as a camera just now and I just couldn’t get it to focus. Later on I put a bit of paper behind it so it would focus a bit better. On the right, you can see my new weld after I cleaned everything. There is no comparison in quality, but you might need to take my word due to the bad photos. After welding the junctions properly, I got 2.4ohms on the split phase coil and 1.6 on the main coil. Initially, I thought that burnt section had burnt the insulation off the side of one of the poles where I couldn’t see it. In the end I used the megger again, with the earth on a metal rod down the side of the pole where the burn would have been, and the positive on the coil. Testing showed no problem with insulation, so no damage elsewhere from that burning. I was not sure about the different resistance values though and thought maybe there was more damage somewhere. I had asked for advice from a member on here and he was sure the coils would not be the same, only on 3 phase would they be the same, and he convinced me to go to the effort of lacing up the coils again and give it a go. I was getting a bit pessimistic after all this I must admit.

I did lace it all up again. Gives you all sorts of little cuts due to the sharp aluminium and you need to pull the string very tight. Took me about 2 hours to do this and get it all rigid and glued.

370007

Did it work? Do fish drink water!

370008

It can lift my 1 ton car no problems at all. The motor doesn’t even slow down as it takes the load. It can even start it when it is already half up. I think with the 1.5ton van, I might be a struggle to start it once its half up, but everyone else I read about that’s converted from 3 phase motor to single phase motor live with that and just lower the vehicle and take another run at it to get it higher.

I know what you are thinking, if I had a mate we could just lift that wee car onto a couple of boxes, no need for a hoist. What can I say - billy no mates here

So, I think I can conclude this means power is power and a single phase 2.2kw motor will lift the same thing as a 2.2kw 3 phase motor. It won’t start under as high a load as the 3phase motor though. The moral of the story is if you buy new cheap chinese thing for an unknown system (and not just a direct replacement in a known good system where a fault on new is obvious), you could be causing all sorts of problems making it hard to know what is at fault. I had completely rebuild the hoist and made new nuts for the screws and everything. There was a lot that could be causing the fault and I’d never seen the hoist run. i did a lot of checking of the hoist before taking the motor down because it was so heavy. Turns out it was the brand new motor.

BobL
16th Sep 2017, 11:33 PM
I had asked for advice from a member on here and he was sure the coils would not be the same, only on 3 phase would they be the same, and he convinced me to go to the effort of lacing up the coils again and give it a go. I was getting a bit pessimistic after all this I must admit.

I dont recall saying that I, "was sure they would not be the same", more like "usually not the same".
Anyway that's splitting hairs a bit and am really pleased you got it working.
Well Done.

Have you had a chance to asses the temperature rise of the motor at all?

Keith_W
17th Sep 2017, 05:37 AM
Hi sossity,
Glad that you have the hoist working as you want it.
Interesting exercise on fault finding, as the motor is new did you think of returning it or was it out of warranty. Did you inform the place where you bought the motor of the manufacturing faults in their motor and seek some form of compensation for the work and investigations