Soft start for rotary phase converter 5

[nelsonhoover]

Is there any way to soft start a 60 HP rotary phase converter on 1ø? This is a more rural area and 3ø isn't available. The electric company here didn't really like the idea of starting a 60 HP motor without a soft start. There wouldn't be any load on the motor at startup, of course.

 

[Marke]

Hello Waross

The classic motor starting theory does not apply in this situation as you never actually "start" the motor, you run it from zero Hz. By that I mean that it never operates under high slip conditions so we can ignore all that theory.

Reducing the V/Hz will reduce the voltage at any given frequency. This will reduce the magnetizing current and the flux in the iron. The net result is that the maximum torque output from the motor shaft will be reduced. This could be a problem in getting the motor rotating in the first place.

You would need to use a VFD in an open loop V/Hz mode. If you ran it in sensorless vector mode, it would probably get totally confused as the measured characteristics would be well outside of the acceptable values for that size of drive.

Provided that less than 20% torque was sufficient to accelerate the motor and load, you should be able to use a drive rated at less than 20% of the motor rating.

Best regards,

Mark Empson

http://www.lmphotonics.com