VFD for a single phase motor

[tjagiello]

Hi guys,

I've got a 250W single phase motor with a start capacitor and I'd like to hook up a VFD (variable frequency drive) to it. Is something like this at all possible? If not what else can I use to control the speed of such a motor?

Regards,
Tom

 

[waross]

It is not worth the trouble. Get a three phase motor and run that off a VFD rated for single phase supply. This issue has been discussed at length on this site.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

There are VFDs for single phase motors, but they cannot be used for capacitor start versions, So if you must change the motor, use a 3 phase motor and a VFD that can be fed with 1 phase power as Bill suggested.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[tjagiello]

Hi again,

thought you might suggest this. The main problem is however that the motor is attached to a pump and the whole unit looks like this:

http://barismo.com/uploaded_images/pump_02-788132.jpg

so if I get a new motor I'll have to figure out a way to connect the pump and the motor.

Thanks for the replies none-the-less.

Regards,
Tom

 

[jraef]

Actually that looks like a fairly basic motor with some sort of adapter to attach the pump. Shouldn't be to hard to find a 3 phase replacement. Take the whole thing down to a good motor supplier.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[Skogsgurra]

What you want to do has been made in an HVAC type of installation where 44 (or was it 42?) single-phase motors were connected to one VFD. The VFD was set to output 230 V and the motors were connected phase-phase distributed between all three phases. Seems to work for that particular installation where speed ratio wasn't more than about 1:3.

The problem with the capacitor is that it becomes less and less optimal when leaving design frequency. But, on the other hand, it doesn't need to be optimal when speed and torque goes down, which is valid for a fan. Not sure about your pump. But, if torque is speed squared, it will probably work.

But, again, as Jeff says, do not expect to start at zero speed. You will probably need to start at, or near, nominal frequency. If that is a no-no, then you are probably stuck with a three-phase motor and a mechanical job.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[tjagiello]

Hi guys,

thanks for all the tips.

I'd like to start at whatever speed the user wants, so it might be a bit of a problem.

Do you think it's possible to use a DC motor instead and control it via PWM, without a VFD?

Regards,
Tom

 

[jraef]

Anything is possible. But if you use a simple DC motor and drive, you will have brush maintenance to deal with. If you use a BLDC motor and drive, it will likely cost as much as a 3 phase AC motor and drive.

Gunnar,
The other major issue on a Cap start 1 phase motor and a VFD is that there will be a centrifugal switch that shorts the caps out at 90% speed (or thereabouts) and as you slow the motor down the switch disengages, placing the caps back in the circuit when they are no loner needed. Worse yet is if the user selects a speed where the switch cycles on and of repeatedly. They are not built to have much hysteresis. You end up destroying the caps, the transistors or both.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Hi jraef; Not all motors are created equal. I have seen one design where the centrifugal switch has a lot of hysteresis. The start winding is cut out, as you said, at 85% r 90% of full speed. However at shut down, the motor coasts down to a couple of hundred RPM before the centrifugal switch resets.
I imagine this type of motor would work as Gunnar said. The initial start would have to be at or near rated speed to activate the centrifugal switch. After that the speed could be varied as long as it was kept above the reset speed of the centrifugal switch. Given the characteristics of centrifugal fans, it could work well for air handling units. Protection may be a challenge. I imagine the system on slow speed and a failing bearing slows the motor so that the centrifugal switch cuts in. At low voltage and frequency the start winding and capacitors will not be very effective. but on the other hand small motors are often cheaper to replace than to repair.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MantelServices]

Not all capacitive start motors are the same. You can successfully use a single phase VFD on a capacitor start motor if it is a permanent capacitor start motor or a shaded pole single phase motor. If your motor is a switched capacitor start motor then you are out of luck. (With some motor rewiring it is possible but I have not done this myself). I have successfully used the following

http://www.controlres.com/pdf/stratus.pdf

to control a single phase permanent capacitor motor in a fan application and it works very well. This controller can be switched from 3 phase to Single phase VFD control with a DIP switch setting.
Good luck

 

[jraef]

Yes, that is why I said "There are VFDs for single phase motors, but they cannot be used for capacitor start versions", which is what he said he had.

Shaded Pole are not capacitor start, and PSC are considered separate because the caps are in the circuit all the time, not switched in and out during starting.


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