# Re: Re: OT: help with motor run capacitor sizing

```Original poster: <a1accounting@xxxxxxxxxxxxx>

Hi David,

```
I stand corrected. There is a relatively simple formula assuming you need a phase shift of 90deg between the currents of the windings. C=(R^2)/L where C is the capacitance to be put in series in Farads, R is the series resistance of the coil in ohms and L is the series inductance of the coil in Henries.
```
I did double check it, very quickly, because no 2*pie*f  is odd, it looked ok.
I don't know if it produces practical values of C.

```
Note the current will be different in the winding with C compared to the winding without the C (assuming identical windings) so you may have to change the power supply voltage to compensate. Assuming the impedance of L > R then the voltage must be reduce to avoid over heating.
```
Bob
>
> From: "Tesla list" <tesla@xxxxxxxxxx>
> Date: 2006/06/20 Tue PM 01:46:45 EDT
> To: tesla@xxxxxxxxxx
> Subject: Re: OT:  help with motor run capacitor sizing
>
> Original poster: <a1accounting@xxxxxxxxxxxxx>
>
> Hi David,
>
> I don't  think anyone replied to you so.
>
> There is probably no one formula is it would be dependent on the L R
>
> You could try putting a diode in series with each winding. The idea
> being that each winding is powered by alternative half cycles. You
> may have to do this at a lower supply voltage because the L of the
> windings will not limit the current.
>
> Bob
>
>
>
>  >
>  > From: "Tesla list" <tesla@xxxxxxxxxx>
>  > Date: 2006/06/18 Sun AM 12:41:02 EDT
>  > To: tesla@xxxxxxxxxx
>  > Subject: OT:  help with motor run capacitor sizing
>  >
>  > Original poster: David Speck <dave@xxxxxxxxxxxxxxxx>
>  >
>  > Terry,
>  >
>  > May I ask the EE types on the list for a little help with a not
> quite TC item.
>  >
>  > I have a number of two coil motors, some of them steppers, that I
>  > would like to use as simple AC motors by wiring them as  capacitor
>  > run motors.  One winding would run directly across the power supply,
>  > and the other winding would run in series with  an AC cap to provide
>  > a phase shift.
>  > Is there a formula that would suggest the optimum run capacitor value
>  > based on the inductance of the windings, or the current draw through
>  > one winding at their rated voltage?
>  >
```