Permanent Capacitor for a Single Phase Motor Calculation

Selection of capacitance on motor capacitors

Selection of a permanent capacitor for a single phase motor implies the consideration of technical and economical aspects.

As the winding of a single phase motor can be done in very different ways (division of the winding space between the main winding and the auxiliary winding, selection of the number of winding turns and sections of the winding, and so on), it is not possible to give universal rules to determine the capacitance and the working voltage of the capacitor for a certain power of the motor.

It is then always necessary to apply the criteria established by the motor manufacturer.

However, following it is exposed a calculation procedure with the only aim of being useful for a first evaluation and give an approximate idea of the values of the permanent capacitor:

It is considered that in general, for each CV of power, a motor capacitor requires

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HOW TO CHOOSE THE RIGHT CAPACITOR

CRITERION FOR THE SELECTION OF THE RIGHT CAPACITOR:
A capacitor motor does not appear to be highly affected by the capacitance reactive power, therefore, it is not necessary to use an accurate capacitance value. It will be possible to choose a capacitance reactive power equalising the inductive-reactive power

TYPICAL CHOICE PARAMETERS:

TURNS RATIO n:
Although it is possible to choose this ratio on the basis of a large number of combinations,usually the main / auxiliary winding turns ratio is chosen in order to generate a voltage on the capacitor closest to its rated values.

VOLTAGE ON CAPACITOR VC :
The following is a formula able to approximately calculate the voltage on the capacitor. If the voltage measured at both ends of the auxiliary winding is equal to n*Vp (where Vp is the voltage measured at both ends of the main winding and n is the turns ratio), the voltage at both ends of the capacitors can be estimated as follows

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