Can you operate a VFD over 60 Hz
Can you use a vfd to run a capacitor start motor?
Here are two diagrams from Fitzgerald, Kingsley, Umans, "Electric Machinery" 4th Edition. They show the basic configuration and performance for capacitor start and permanently split capacitor motors.
Capacitor starting motors are used for loads such as compressors that require high starting torque. With a reduced frequency, the impedance of the starting winding is reduced and the impedance of the capacitor is increased. The result is reduced phase shift and reduced current in the starting winding. This reduces the starting current. If the motor is applied to the load originally selected for, the motor is likely to be unable to start the load. If the motor is converted to drive a fan or a centrifugal pump, it does not have to be able to generate a lot of starting or slow running torque.
The starting winding is normally separated by a centrifugal switch at around 75% of the synchronous speed. This would prevent the motor from operating below this speed as the starting winding and capacitor would quickly overheat if they were not separated. An alternate method of disconnecting the capacitor would prevent this problem.
It may be possible to permanently connect a much lower value capacitor to the starting winding. The capacitor would have to be chosen so that the continuous current in the starting winding does not overheat it. It would be difficult to determine the safe current and the required capacitor value. The resulting starting and low speed torque would be extremely low.
The phase difference between the main winding current and the starting winding current should be close to 90 degrees for a maximum starting torque. It is possible that a 120 degree difference between the winding voltages could provide an acceptable phase difference for the currents. There would still be a problem limiting the current in the starting winding to a safe value.
Running a capacitor starter motor with a VFD may not be impossible, but it certainly doesn't seem like a very satisfactory alternative.
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