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  1. #1
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    Default Single phase Vs 3 phase motor ratings.

    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
    To grow old is mandatory, growing up is optional.

  2. #2
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    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.
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  3. #3
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    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.

  4. #4
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    Quote Originally Posted by KBs PensNmore View Post
    ... 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

  5. #5
    BobL is online now Member: Blue and white apron brigade
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    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.

  6. #6
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    Thanks everyone for their comments. 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
    To grow old is mandatory, growing up is optional.

  7. #7
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    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.
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  8. #8
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    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

  9. #9
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    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
    ScreenShot105.jpg

    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.

  10. #10
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    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

  11. #11
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    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

  12. #12
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    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

  13. #13
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    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
    Equipment er.... Projects I own

    Lathes - Sherline 4410 CNC
    Mills - Deckel FP2LB, Hardinge TM-UM, Sherline 2000 CNC.

  14. #14
    BobL is online now Member: Blue and white apron brigade
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    Quote Originally Posted by sossity View Post

    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?

  15. #15
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    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.
    20170212_161753.jpg
    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.
    20170212_161611.jpg
    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.

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