# Variable Speed Drive For Single Phase Capacitor Start/run Motor

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### #1 Zaaf

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Posted 25 December 2013 - 02:44 PM

I am building a VFD for single phase Capacitor Start/Run Motor. There are two windings the main and start/run winding. Main winding having low resistance means high power and start/run winding has twice the resistance of main winding means low power. I have read everywhere that for reversing the direction of motor we have to switch the leads. My one question is that if I connect capacitor to the main winding instead of start/run winding will the motor get burn? As a matter of fact I have done this thing on pedestal fan motor and it runs fine and I can change the direction of the motor this way. I have asked about this thing from a motor winder and he said that this way motor can get burn. My second question is that if the motor will get burn what could be the reason because I think the load will always be on the main winding and until unless this way if both windings in this configuration are not able to produce the required magnetic field then there is possible chance that it will get burn.

I have read that capacitor creates around 80° to 90° leading phase shift of current in the start/run winding which creates the rotating magnetic field and motor runs in forward direction. If we change the leads for the start/run winding the current will lag 80° to 90° in the start/run winding and motor will run in reverse direction (am I correct here because I have little doubt). According to my thinking same should happen when I will connect the capacitor to the main winding instead of start/run winding and motor will run in reverse.

Now, I come to my third and final question for which I have almost written an essay above that if I build a two phase inverter with 90° phase shift between them will I be able to run the motor? My main concern was that will I burn the start/winding with this technique or should I change voltage level in the second phase which will I give to the star/run winding so that the current is limited in this winding?

Regards.

### #2 yuri

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Posted 27 December 2013 - 06:17 PM

For a multiple phase supply wiith the phase shift of say 90 degrees the motors have to be designed appropriately  for this shift to work efficiently. Otherwise a usual motor will not run smoothly, but in a sort of jerking way with much energy losses. You can swap the run and the start windings how you want  and the motor may run sometimes, but usually would not for too long.

### #3 Zaaf

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Posted 28 December 2013 - 05:23 AM

For a multiple phase supply wiith the phase shift of say 90 degrees the motors have to be designed appropriately  for this shift to work efficiently. Otherwise a usual motor will not run smoothly, but in a sort of jerking way with much energy losses. You can swap the run and the start windings how you want  and the motor may run sometimes, but usually would not for too long.

I was also thinking the same thing what you have said after I posted this topic. I also thought that the capacitor is selected according to the start/run winding to produce the required phase between the two currents and connecting the same capacitor to the main winding will not produce the required phase shift. Am I correct on this one?

There are some motors which have equal main and start/run winding. Will swapping the capacitor work on these types of motors?

I want to ask one more thing that if I know the phase difference between the two currents and I make a 2 phase VFD according to it, will it work?

Regards.

### #4 yuri

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Posted 28 December 2013 - 05:04 PM

With a capacitor there will always be the angle shift 90 degrees no matter to which winding you connect it to. Only the impendance of the start one is such that it is designed to be connected to mains through a capacitor of a certain value, while the run one directly to the mains.

One can make a 2 phase inverter with 90 degrees phase shift only, I think they are being produced and it would be easier to buy one ready made.

Regards.

### #5 Zaaf

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Posted 29 December 2013 - 02:48 PM

With a capacitor there will always be the angle shift 90 degrees no matter to which winding you connect it to. Only the impendance of the start one is such that it is designed to be connected to mains through a capacitor of a certain value, while the run one directly to the mains.

One can make a 2 phase inverter with 90 degrees phase shift only, I think they are being produced and it would be easier to buy one ready made.

Regards.

yuri, thanks for guiding me.

I have programmed a microcontroller creating 2-phase SPWM with 90° phase shift between them. I have partially tested it on the motor because I had lack of hardware at that time and now I will test it completely.

There are some commercially available single phase inverter and they can only work at centrifugal loads and they pretty expensive than three phase inverters. I have seen two phase inverter in Air Jet Fabric Weaving Machine but it's type of control card that can only work with that machine.

I want to ask that can a VFD work on single phase compressors which have capacitor start motor and it's type of hard to start load. Single phase VFD certainly cannot work with these type of motors but I think two phase VFD can work. What considerations should I take when building VFD for capacitor start single phase motor?

Regards.

### #6 Zaaf

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Posted 29 December 2013 - 03:41 PM

I want to add one more thing that I have read in a article that auxiliary (start/run) winding in permanent capacitor motor has higher voltage than main winding and we have to use different voltage levels for separate windings. Below is the link of the article:

http://ivobarbi.com/..._Telles_Ivo.pdf

Regards.

### #7 yuri

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Posted 01 January 2014 - 11:33 AM

Happy New Year !

Then, let's use 2 different sources for the both windings.

Regards

### #8 Zaaf

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Posted 01 January 2014 - 03:01 PM

Happy New Year !

Then, let's use 2 different sources for the both windings.

Regards

Happy New Year!

I am really thankful to you for helping me.

Regards.

### #9 VeichiElectric

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Posted 11 May 2016 - 05:53 AM

Here is an article about VFD for single phase Capacitor Start/Run Motor. I think it might help with your problems.

http://goo.gl/kImG6O

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