Capacitor Calculation For Single Phase Motor

This document has some general information:

http://www.kemet.com/Lists/ProductCatalog/Attachments/158/F9000_GenInfo_MotorCaps.pdf

If you have a nameplate on the motor, the safest thing would be to do a search on that or contact the manufacturer.

Is there any information on the motor name plate?

Is the capacitor a start, run, or permanent split type? You need to know this before going any further.

How to decide:-

1. If there is a centrifugal switch, then it won't be a permanent split cap.

2. If there is a centrifugal switch and there is only one cap then it is most likely a start cap, and whilst you can use a run cap in lieu of a start cap, the size required is much larger and may not be a realistic option.

3. If there are two caps, then one will be a start cap and one will be a run cap, the larger one will be the start cap. You can verify this by metering the wiring...The start cap will be disconnected from the circuit when you open the centrifugal switch.

Assuming it's a start cap, then a good guess for that size motor would be around 100 microfarad.

The formula is:- C = (I*10⁶)/(2π*f*V)

Where C =capacitor size in microfarads, I = full load current in amps, π = pi = 3.14, f= supply frequency, V = supply voltage.

This should give you a figure around 100 - 120µf. Note that start caps do not have a specific size as do run caps, they have a range. Get one within that range and you will be OK.

The cap should have a voltage rating of at least 360 volts.