vfd to control single phase motor from 3 phase input?

[Michael Weber]

Due to several previous questions in this forum about using vfd's to convert single to 3 phase I feel this is an appropriate question. Can a single phase motor's speed be controlled by using 2 legs of a vfd output. The vfd input is 3 phase in this case. Will it damage the motor? Would have ask downstrairs somewhere but need a fast response. thanks

[Rod Sheridan]

Most single phase induction motors cannot be operated below rated speed.

So the simple answer in most cases is no.

Regards, Rod.

[Mike Henderson]

You can run a single phase motor on a VFD but there's some issues.

A three phase motor has the windings wired so that they produce a rotating magnetic field. A single phase motor can be viewed as having two rotating magnetic fields (or magnetic vectors), one going clockwise and one counterclockwise. Starting a three phase motor doesn't require any special starting circuitry because the magnetic field is rotating in only one direction (set by the way the field coils are wired).

But starting a single phase motor does require some special starting circuitry. Basically, what the starting circuit does is favor one of the rotating magnetic vectors. Once the motor gets turning, the starting circuit is usually disconnected and the motor follows the favored vector.

So your problem is to get the motor started, which you can probably do as long as you don't reduce the frequency too far. But the other problem is to keep the starting circuit disconnected once you get the motor running.

Many single phase motors have a centrifugal switch which disconnects the starting circuit once the motor is up to some percentage of full speed, maybe 75% of full speed. So if you slow the motor down below 75% of full speed, the centrifugal switch will kick in and reconnect the starting circuit.

This may or may not cause problems. Some starting circuits were not designed for continuous operation and will overheat the motor. Others will not cause any problems.

But that brings up another problem with both three phase and single phase motors. Regular motors (not designed for variable speed operation) have fans connected to the shaft (may be inside the motor). They are designed to run at a certain speed in order to keep the motor cool. When you run the motor at low speeds, the motor is likely to overheat. You may have put a separate fan blowing on the motor to keep it cool, especially if you load the motor fairly high.

So my recommendation to you is to try it if you have all the components. Start it the first time at 60Hz. Then try starting it at 50Hz, etc. and see what happens. Just monitor the temperature of the motor and don't let it get too hot.

Mike

[One other comment - most motors will not perform very well at twice the rated Hz or beyond. I'd plan not to run a 60Hz motor beyond about 90-100Hz.]

[Tom Veatch]

...Many single phase motors have a centrifugal switch which disconnects the starting circuit once the motor is up to some percentage of full speed... So if you slow the motor down ... the centrifugal switch will kick in and reconnect the starting circuit.

This may or may not cause problems...

Almost guaranteed to cause problems. Can't say it's true in every single case, but every start capacitor I've dealt with was a electrolytic that was not designed for continuous operation. If the centrifugal switch doesn't cut the start capacitor out of the circuit, it will be toast inside of a minute of operation and you'll have leaking electrolyte to clean up and a need for a new capacitor.

That's not the case with run capacitors. They are designed for continuous operation.

[Michael Weber]

Rod, Mike and Tom, Thanks for the fast replys. I didn't think it was a particularly good idea and wasn't sure about the capacitor. I'll have to talk to the person in charge of the project and let him decide what he wants to do after I update him with your info and recommendations. Thanks again.

[Mike Henderson]

Almost guaranteed to cause problems. Can't say it's true in every single case, but every start capacitor I've dealt with was a electrolytic that was not designed for continuous operation. If the centrifugal switch doesn't cut the start capacitor out of the circuit, it will be toast inside of a minute of operation and you'll have leaking electrolyte to clean up and a need for a new capacitor.

That's not the case with run capacitors. They are designed for continuous operation.
Electrolytic capacitors can be used continuously - they were used in power supplies for electronic equipment for many years. I can't say if the start cap on a motor would fail under continuous use or not, but I would suspect you could keep power on it and it would be okay. You could test it by wiring a spare one across the power line and see what happens.

My reason for doubting that it would fail quickly is that occasionally a motor will not start due to the load and the operator does not notice it until the thermal switch kicks in and turns it off. During that time (which could be several minutes), the start circuit is engaged and I've never heard of a start cap failing in that situation. It's hard for me to imagine designing a system that would fail under those conditions.

Mike

[Chip Lindley]

Could you tell us to what application you are thinking of applying this configuration? What h.p./rpm single phase motor is involved? I'm SO curious now!

[Michael Weber]

Sure Chip. Actually an issue at work non woodworking related. We're designing a heat pump operated swimming pool heater. Person in charge ordered a bunch of stuff without really knowing what was needed. He got the vfd to change the motor speed as a way of water flow regulation but the 2HP pump has a single phase motor. I would have used a modulating valve but I wasn't consulted. My connection to the project is providing the acquisition and control software to be written in Labview.