# calculation of start capacitor

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### #1 martinez

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Posted 27 April 2005 - 07:37 AM

I have a motor with two capacitors:
a run capacitor and a start capacitor.

The capacity of the start capacitor is 100µF at 50Hz.
What would be the capacity at 60Hz ?

Is there a way to calculate this or should it be tested ?;q;e;

### #2 marke

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Posted 27 April 2005 - 10:10 AM

Hello Martinez

Aim to keep the capacitor current the same. That means you need to reduce the value of the capacitance as the frequency is increased, and you need to reduce the capacitance as the voltage is increased.

The current = v x 2 x pi x f x C

At the same voltage but 50Hz rather than 60Hz, I would select 100 x 5 / 6 = 83 uF

Best regards,

### #3 martinez

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Posted 27 April 2005 - 11:14 AM

I think there must be a formula to calculate the phase shift of the current as a function of the frequency.
If the angle of the winding is 90° the current should also be shifted over 90°...

Am I mistaking ?

### #4 marke

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Posted 27 April 2005 - 06:47 PM

Hello Martinez

The current through a capacitor always leads the voltage by 90 degrees.
There will be a variation in the effective phase shift due to the inductance/resistance of the start winding, but I do not believe that this will have a significant effect.

Best regards,

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