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Thread: Another VFD question
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25th Nov 2016, 01:26 PM #1
Another VFD question
Can you use a VFD to control the speed of a Single Phase 240 volt motor.
Shane
Got the square peg in the round hole, now can't get it out !!
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25th Nov 2016, 02:23 PM #2Most Valued Member
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No
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25th Nov 2016, 05:58 PM #3Most Valued Member
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Short answer, see above
Long answer, read on
Would you want to? I doubt performance much below 50Hz is going to be much good, one of the reasons some single phase motors are replaced with 3 phase motors is that they run smoother. I assume as single phase motor is slowed it will get even rougher. I don't think small 3 phase motors cost all that much and would give a better result unless changing the motor has its own issues. You might be able to go over 50Hz ok.
Possible? I wonder, and as always I might be missing something. It may depend on the VSD, maybe it would seen it as a fault maybe it won't? Maybe it would let the smoke out, maybe it won't. I know you can turn on the output of at least one VSD with no motor connected with no faults and no immediate release of smoke. What it would think of a load across two phases or operation for more then about 20 seconds I have no idea.
Depending on the motor even starting it could be an issue.
As I don't have a spare single phase motor laying about that I can think of I'm tempted to try it with a 10" ceiling fan. But I'm not sure I want to spend $150 finding out and would a 30W load tell the full story anyway? We'll see.
Stuart
Could you run 3 single phase motors in delta?
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25th Nov 2016, 06:28 PM #4
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25th Nov 2016, 06:51 PM #5Most Valued Member
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Had to find out.
For whats is worth it seems to work just fine on a Huanyang. Granted a 2.2kW VSD likely didn't even notice the fan was there, but it worked.
Not sure what would happen with a cap start motor. My guess is it would be fine, but that's a WAG.
Would it still start in the right direction? I think so
Would the start winding draw to many Amps or over heat because it was running for longer? I suspect not but don't really have the foggiest.
Would it blow the caps up? I don't think so, but again really don't know.
So I might hold off on that test for a little while and see what others think, as the motor I'm thinking of using is my scraper grinder.
**that's another thing, with a cap start motor I think you'd want to stay above its starter winding cutout speed.
***maybe with a cap start cap run motor you could do away with the starter windy/cap altogether?
Stuart
I see your 454 and raise you 1256.....twice
https://en.wikipedia.org/wiki/Allison_V-3420Last edited by Stustoys; 25th Nov 2016 at 07:10 PM. Reason: **, ***
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25th Nov 2016, 07:57 PM #6
Well it's not normal practice and I wouldn't bother to do it, but would it work? Don't see why it wouldn't slow the motor down, the voltage reduces somewhat linearly with frequency, keeping above the centrifugal switch cut out would be important, but the bottom line is variable speed controls for single phase motors cost less than three phase vfd's. So why would you bother? Also as Stuart noted, cogging and rough running might be an issue at lower Hz.
This one is top of the line in single phase speed control https://www.jaycar.com.au/240v-10a-d...r-kit/p/KC5478
Down market you can get something like this... New AC 220V High Power Control Speed Electric Motor Replacement Module TH285 | eBay
or even cheaper still. AC 220V 4000W SCR Variable Voltage Regulator Motor Speed Control Controller Fan | eBay
A VFD even the cheapest is still 10 times the cost of the cheapest SCR chopper speed control.
Ray
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25th Nov 2016, 08:51 PM #7Senior Member
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Not quite..
The kicker is
"Note: This project is designed for brush type motors and is not suitable for induction motors or shaded pole motors."
Now, they do have a kit for induction motors :
https://www.jaycar.com.au/speed-cont...otors/p/KC5509
The kicker here is
"
Project does not work for motors with centrifugal switch
"
No doubt for the reasons discussed above.
-russ
Edit: a quick search suggests vfds are available for single phase, eg VFD for single phase induction motor - EMA Inc.
-but not for cap start, and only for 'centrifugal loads' eg fans and pumps. I suspect starting torque is the issue.
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25th Nov 2016, 09:04 PM #8Diamond Member
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This is another approach for a kit project vfd which works for both single and 3 phase motors to 1.5Kw, but not motors with any kind of centrifugal switch. The design is from Silicon Chip magazine of April and May of 2012, with revisions in 2013. Jaycar have marked down the price of kits ATM, but it is a fairly complex build and you definitely need to know your way around high voltage circuitry. Might be just as economic to get a s/h 3phase motor and low cost vfd, most of the hard work is already done then.
https://www.jaycar.com.au/speed-cont...otors/p/KC5509 Good luck
Rob. Ahh,
Edit: Toooo sloooow as usual, Russ beat me to it.
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25th Nov 2016, 09:52 PM #9Most Valued Member
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The VSD does give you the option to go above 50Hz though, maybe he wants more speed!
This would say to me that they only operate below 50Hz. With a fan you would likely overload the motor above 50Hz anyway*, not an issue for what most of us use them for.
Would the carrier frequency effect the caps?
Stuart
p.s. I still think that without a compelling reason Eskimo still has the correct answer. But maybe it will come in handy one day.
*If I remember BobL's dyno tests well enough it likely wouldn't go much faster anyway as above 50Hz power is pretty constant so you would quickly reach a point that there wouldn't be enough power. Not sure what would happen then, my guess is the motor would slow stop.Last edited by Stustoys; 25th Nov 2016 at 09:54 PM. Reason: p.s.
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25th Nov 2016, 11:20 PM #10Senior Member
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That's the question isn't it, why? I recall from the silicon chip article, and a few other references, some power saving can be obtained for things like pool pumps by throttling back a bit, the same would apply to fans in maybe a ventilation system.
Where the power increases non-linearly with speed. But not for machine tool type loads.
-russ
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