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Thread: VSD power tests
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3rd Jul 2014, 12:22 AM #121Cba
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example:
If the 50 Hz rating is 240V, that is a V/Hz ratio of 4.8:1
If the 60 Hz rating is 288V, that is the same V/Hz ratio of 4.8:1
The end result is the same torque. But the rpm is 20% faster.
Since power=torque*rpm, 20% more rpm is 20% more power.
So far I cannot see a mistake.
Now, can the VFD deliver these 288V? No problem I think. The input to the VFD is 240V AC into a bridge rectifier. The rectifier charges the bus capacitor to 240V*1.41= 338V. These are fed to the motor via a pulse modulated H bridge.
PS: For those interested, this is a good description on how a VFD creates the variable voltage output. This is based on a 3-phase input VFD, but single phase is very similar just with lower voltages: What is a Variable Frequency Drive?
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3rd Jul 2014, 12:31 AM #122Most Valued Member
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What is the peak V of 240VAC? i.e not RMS
Stuart
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3rd Jul 2014, 12:36 AM #123
I should let Ray do this....
338 is the peak voltage obtained, not the average. The voltage also drops to 0v at one point. Luckily there is more time spent at the crest of the wave than at the 0v point, so the average is not 169v, but more like 215. The maths is a bit tricky but it basically amounts to AC voltage in/1.1.
Being that the voltage change is happening at 50hz, like all PWM the VFD controller see's it as a fixed voltage and not a changing one.
Bob, have you got a volt meter on the output of the VFD?
cheers,
Ew1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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3rd Jul 2014, 01:04 AM #124Member: Blue and white apron brigade
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Not yet , I'm hopefully borrowing a true RMS voltmeter tomorrow.
While I am interested in what goes into or out of the VFD , my main interest is quantifying the Torque/HP output of motors connected to VFDs so I'm not going to spend a lot of time measuring electrical parameters in these experiments.
I currently only have one drum and that suits a 19 mm shaft size, I need to make 3 more 16, 25 and 28 mm so I can test other motors.
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3rd Jul 2014, 03:18 AM #125
You don't get more than maximum voltage out of the vfd, whatever that voltage is. Let me explain.
1. There is some maximum RMS voltage that the VFD can deliver.. depends on input voltage, output waveform and switching efficiency etc...
Whatever, the details don't matter, what matters is that that voltage is the maximum the vfd can supply.
2. Now let's choose a base frequency... it's usual practice to choose the motor nameplate rated frequency, could be 50Hz, could be 60Hz, could be 100Hz... for the sake of this explanation it doesn't matter, so long as the base frequency is high enough that you are not too far above the motor rated V/Hz.. we just pick a reasonable number... say it's 50Hz. Obviously it doesn't matter what the mains input frequency is either...
Ok we now have a maximum voltage and a base frequency, the way a V/Hz vfd works is that it increases the voltage and frequency in a somewhat linear fashion from zero to the base frequency. There is usually a bit of extra voltage applied down at lower frequencies, sometimes called "torque boost", but it's essentially a straight line from zero up to the base frequency. This region is called the constant torque region. The torque is essentially constant for a given current, and as a result the horsepower output increases pretty much linearly with frequency, up to the point that the voltage output reaches it's maximum, and that happens at (you guessed it ) the base frequency..
Now what happens as we increase the frequency beyond the base frequency, there is no more voltage available ( remember that's what defines the maximum) and the current drawn is still the same, (we are still loading to nameplate current ), so with the same power output and increased rpm, the torque drops. That operating region is called the constant power region.
It's easy to grasp if you just remember the formula Horsepower = torque * rpm
Now let's look at that graph again..
I hope it's starting to make sense..
On a side issue, what's the maximum RMS 3 phase voltage you can get from a 240V single phase VFD.. ok, let's see... the DC bus voltage is easy to calculate, that's 240*1.414 = 339V, but we had to rectify it to get DC, so we only have half the peak to peak voltage, so the highest voltage AC waveform we can generate directly is only 120VAC, so what does that mean for the 3 phase output?
Well, we can only get 120*1.732 = 208V AC between phases... so in going from single phase to three phase we have gone backwards. We can get back some of the drop in voltage by mucking about with the waveform using the PWM to produce non-sinusoidal waveforms that have higher RMS values, and that's exactly what some VFD's actually do with fancy signal processing to maintain the sinusoidal interphase waveform.
As to Chris's suggestion of getting 288V 3 phase out of a 240V single phase input...unlikely in practice.. . it's a long way from starting point of 208V to 288V... you'd probably end up with a square wave, and I don't know of any VFD's that work that way. ( or regulations that would allow the EMI ) Also when you get to the point of loading the DC bus to that extent the losses due to ripple on the DC bus will start to become significant, depending on how big the caps are I guess... Not to mention that the waveform at that extreme would be essentially pulsed DC, and the stator windings generally don't take too kindly to DC.. ( remember the v/hz limits ) saturating and overheating would be the most likely result.
But, as I've explained above, it's immaterial, the vfd maximum output voltage occurs at the base frequency. ( I just took the long way round to say it. )
Ray
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3rd Jul 2014, 08:05 AM #126Philomath in training
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Bob, you may already have the numbers for this, but with my thinking triggered by Ray's description of how a VFD works, are you able to calculate an efficiency for the VFD (at the various speeds)? During a previous discussion on these, the statement was made that VFDs are more a efficient way to drive a motor. From my reading of VFD specs I would have said that there are losses incurred in the frequency conversion process and so the answer is that direct supply is more efficient - although due to equipment speed or other requirements that may not be possible.
Michael
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3rd Jul 2014, 10:06 AM #127Most Valued Member
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I hadn't thought of that, frankly. On the face of it, not a bad idea except for a few issues.
First, I've never come across a multi-speed 7.5HP motor. Let's leave the cost aside......
Second, all the smaller 4 pole/8 pole motors I know of have significantly less HP at the 8 pole setting than the 4 pole, so the torque tradeoff wouldn't be good.
Third, you either need 2 VFD's and a good interlock system or 1 VFD and take mains power off of it before switching the motor over so the VFD can't be tuned to the motor.
Fourth, PeteF looked into this for his F3 and came to the conclusion that the HP at the lower frequency on the 4 pole setting was the same as the HP produced 'natively' on the 8 pole setting, so the complexity in point 3 wasn't justified.
Fifth, the loss of torque at slow speeds would still be there whereas a mechanical reduction is giving torque multiplication.
I have had quite a lot of bad things to say about Kopp variators but they do give close to a 10:1 speed range with adequate torque transmission in a reasonably compact package. Combine this with the inbuilt 10:1 back gears (which would be unaffected by a motor/variator swap) and you have a very flexible speed setup allowing speeds from 30 to 300 rpm then 300 to 3000 rpm in high range. I rarely go over 1500 rpm but I'd hate to lose the ability to do so. OTOH having to occasionally swap another belt on a layshaft to a different ratio wouldn't really bother me.
My totally uninformed opinion has always been that the best you were likely to do was say 20 Hz to 100 Hz giving a 5:1 range with 'reasonable' torque. Bob's single example here is showing 3:1 yet others are claiming much better is possible. My thinking is, without dyno testing, just how much torque is available for use at the speed extremes? Generally at low speeds on my lathe it's because I've either got something big in the chuck, or I'm running a big drill bit, boring bar etc into heavy metal. I can't afford not to have a lot of torque available. With my old Premo, I often used to bring the spindle to a stop when turning something in the 200 to 250mm diameter range, especially if it was an interrupted cut on a casting, as you can't 'play' with it, you have to get under the skin.
All of which is why I'm very interested in what Bob's doing.....
PDW
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3rd Jul 2014, 10:15 AM #128Member: Blue and white apron brigade
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I have input V/I to the VFD but only output current. I will put V meter on the output.
I have some dental work being done this morning so it could take some time to report back.
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3rd Jul 2014, 10:22 AM #129
On the Antrac the motor is a 4/6 pole, and whilst the power rating are different they are something like 3hp/2.25hp. Now i actually thought that the machine had more power than this, because it can really move some metal even at the lower speed.
The other option for 2 speeds, if you don't want to have to change belts or you need more than one belt to transfer the power, is to have 2 sets of pulleys always spinning. You have the driven pulleys on bearings on the output shaft and put a sliding dog clutch on the shaft in between the 2 pulleys. A simple shifter and you have the convenience of an easy shift and no belts to mess about with. It may just be me but as soon as you have belts to change you don't do it as often as you should or would if it was just a lever.
Ew1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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3rd Jul 2014, 03:21 PM #130Most Valued Member
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Well I was hoping you wouldnt need such a large motor. But I did some maths and it doesnt look so clever. If you have the gearbox split the motor speeds then you can great control(with less than +/-15hz you cover the gaps), but not the range you want (just over 3:1). If you have the gearbox give you a second range below the two motor speeds you then have the speed range you want(over 10:1) but you're back to needing +/-25 Hz to cover the gaps.
Even if one of the above works you end up with a 4/8speed lathe anyway.
No, switching poles is fine with mains on...as long as output is off. As for interlocks, easy enough if you choose to, but I'm not so sure they are needed. But given the above and other issues doesn't really look like a starter unless your forced into it.
Stuart
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3rd Jul 2014, 05:14 PM #131
Out of curiousity I had a look at the Kopp variator design, and it looks like the ideal solution..
If you were to go away from that concept, I'd be thinking a 10kw BLDC servo ( CNC spindle style ) would be a better solution than an induction motor with VFD.
Or if you wanted to keep costs down, rather than looking at CNC spindle type motors, it might be worth while to look at the vast array of BLDC motors and controllers that are made for electric vehicles.. plenty of 10KW hub motors at reasonable prices
Ray
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3rd Jul 2014, 08:08 PM #132Cba
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You were right Ray. Thanks for taking the time to put me right. I was completely stuck in my thinking error. With 240VAC input, there is indeed no easy way to recreate a near sinusoidal shape output of more than 240Vrms. Not with a simple straight VFD.
There are 110V single phase input VFD's, that can directly output 220V, but these cost more as they employ a voltage doubler input stage. There are also two stage inverter VFD's that can create any output voltage, but those are even more costly. It would have been too nice... Chris
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3rd Jul 2014, 10:31 PM #133Most Valued Member
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4th Jul 2014, 12:33 AM #134Member: Blue and white apron brigade
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4th Jul 2014, 10:00 AM #135Most Valued Member
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I do all the time, not sure about the 'supposed to' part. Mine is the older 3 ball model but in pretty good condition really, however it does suffer from a bit of 'stiction' at times so I find varying the rpm by maybe 50 or so every now & then is good and every so often I wind it through its entire speed range - usually in low range as I don't really want my spindle bearings running at 3000 rpm unnecessarily. Also you can try to do constant surfacing speed when facing off which is handy. Usually I'm too lazy and just hit the power feed.....
The Kopp variator is an interesting design, more flexible than say a CVA or Holbrook with fixed gear speeds at the expense of some fragility but not close to the sophistication of the Monarch 10EE drive system. OTOH it's also a lot simpler than a 10EE.
The 10EE crowd have the same issues in swapping to a VFD setup and usually they have to go to at least a 7.5HP motor plus keep the back gearing to get satisfactory performance. Some have gone to sophisticated DC drive systems with success and it's something to keep in mind. A long way out of my personal comfort zone WRT electrical stuff, though.
PDW
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