Soft starting a Single Phase Induction motor.

[gawnieonengineering]

The voltage flicker is significant when I start my single phase capacitor split motor. I want to use a soft starter on the motor, yet I fear that the auxiliary components or starting components will not be able to handle the applied voltage for a prolonged period of time and overheat. Does anyone have experience with soft starting single phase motors?

 

[jraef]

Yes, don't do it. Capacitors in the cap start motors get into an "argument" with the SCRs in the soft starter and it becomes a race to see which one fails first. The cap changing current looks like a short circuit to the SCRs and they can self commutate (turn themselves on when you don't want them to) causing damage to the SCRs, and the high harmonics from the SCR firing can over heat the caps and cause them to swell and fail over time. It will last for a while and may outlast the warranty on the soft starter, but not much more than that.

If you are having severe voltage drop on startup of a small single p\hase motor, it's much more likely that you have an improperly sized circuit somewhere in the mix. Find and fix that problem before you chase it with hardware.

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[LionelHutz]

It will work, but generally speaking the life of the capacitors is reduced. It's not a huge deal because the starter only stresses the caps during the start. There is no negative effects once running. We have customers who've been using soft-starter for years without any issues and others that have to replace the start capacitor a little more often. The permanent split phase motors seem to work better than the motors with the switched start winding. You can basically just limit the current a bit on startup and you will also get the benefit of ramping the current up instead of the sudden surge the full-voltage starter causes. Regular capacitor checks will ensure the caps survive and don't cause larger failures.

If you really have power limitations then you might be better served by using a VFD as a phase converter and installing a 3-phase motor.

 

[gawnieonengineering]

The idea is to run the motor as a starter motor and then as a generator. A single phase output is needed. Reliability is an important issue, therefore diminished capacitor life is not desirable. I realise it is case dependant, but how severe is this capacitor wear?
Are there other ways to soft start a single phase motor?

 

[Strong]

"Are there other ways to soft start a single phase motor?" You can keep your motor and install a soft-start magnetic coupling. Take a look at this product:

http://www.fluxdrive.com

Walt

 

[LionelHutz]

The capacitor and winding are in series. I would ensure the capacitor can handle the di/dt caused by the sudden switching of peak voltage across the inductor/capacitor series circuit.