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I've read on many forums, publications and specification sheets that it's not possible to run a capacitor start motor using a VFD. The start winding of such a motor is momentarily powered until the motor achieves full speed. Leaving it powered, will cause the start winding to overheat. Consequently, a VFD can't power such a motor because of the capacitor has been selected for a given frequency of 60 Hz.

Is it possible to add another circuit between the motor and VFD to power such a motor? I can see removing the start capacitor and connecting it directly to another three phase leg and then use a circuit to detect the back EMF to disengage the start winding. Is 120 degree phase difference sufficient to start it?

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  • \$\begingroup\$ Many important details are omitted from your question, but consider the case of STARTING the motor at the designed frequency, and then using VFD after it is running. However, there is also the possibility (probability) that a simple capacitor-start motor won't perform all that well from a VFD anyway. \$\endgroup\$ May 19 '16 at 15:19
  • \$\begingroup\$ Makes no sense using VFD with single phase motor. Change the motor with 3 phase motor. \$\endgroup\$ May 19 '16 at 15:42
  • \$\begingroup\$ @MarkoBuršič Makes a lot of sense if you want to control the speed of a motor. \$\endgroup\$
    – user148298
    May 19 '16 at 15:46
  • \$\begingroup\$ @RichardCrowley As long as you start the motor at or below the design frequency with a minimum starting speed, it should be fine. Correct? \$\endgroup\$
    – user148298
    May 19 '16 at 15:48
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    \$\begingroup\$ @user148298 then use a proper motor that can handle variable frequency control. Single phase motors are simply not designed for VF control. Period. That is why most low HP drives can take a single phase input, and convert it to three phase output. \$\endgroup\$
    – R Drast
    May 19 '16 at 16:40
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Here are two diagrams from Fitzgerald, Kingsley, Umans, “Electric Machinery” 4th ed. They show the basic configuration and performance for capacitor start and permanent-split capacitor motors.

enter image description here

Capacitor-start motors are used for loads such as compressors that require high starting torque. With reduced frequency, the impedance of the start winding will be reduced and the impedance of the capacitor will be increased. The result will be reduced phase shift and reduced current in the start winding. As a result the starting current will be reduced. If the motor is applied to the load for which was originally selected, the motor will probably not be capable of starting the load. If the motor is repurposed to drive a fan or centrifugal pump, it would not need to be capable of producing very much starting or low-speed running torque.

The start winding is normally disconnected by a centrifugal switch at about 75% of synchronous speed. That would prevent the motor from being operated below that speed because the start winding and capacitor would quickly overheat if not disconnected. An alternative method of disconnecting the capacitor would prevent that problem.

It might be possible to permanently connect a much lower value capacitor to the start winding. The capacitor would need to be selected so that the continuous current in the start winding would not overheat it. It would be difficult to determine the safe current and the required capacitor value. The resulting starting and low-speed running torque would be extremely low.

The phase difference between the main-winding current and the start-winding current should be close to 90 degrees for 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 with limiting the current in the start winding to a safe value.

It may not be impossible to run a capacitor-start motor with a VFD, but it certainly doesn’t seem to be a very satisfactory alternative.

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    \$\begingroup\$ It would make a lot more sense to eliminate the capacitor and have the VFD drive the start winding with an appropriately phased output. Of course, then you might as well make things more balanced... which leads right back to a three phase motor. \$\endgroup\$ Aug 14 '16 at 1:31
  • \$\begingroup\$ Cool! You could start the motor as usual at 60 Hz and as the start windin \$\endgroup\$
    – user148298
    Sep 2 '16 at 22:11

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