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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Capacitor-start single phase induction motor

Single Phase AC Induction Motors
AC single phase induction motors are classified by their start and run characteristics. An auxiliary starter winding is placed at right angles to the main stator winding in order to create a magnetic field. The current moving through each winding is out of phase by 90 degrees. This is called phase differential. After the motor has reached approximately 75% of operating speed, the auxiliary winding is disconnected from the circuit by a centrifugal switch.
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Capacitor Start / Induction Run Motors
Capacitor start / induction run motors are similar in construction to split phase motors. The major difference is the use of a capacitor connected in series to start windings to maximize starting torque.

The capacitor is mounted either at the top or side of the motor. A normally closed centrifugal switch is located between the capacitor and the start winding. This switch opens when the motor has reached about 75 percent of its operating speed.

Capacitors in induction run motors enable them to handle heavier start loads by strengthening the magnetic field of the start windings. These loads might include refrigerators, compressors, elevators, and augers. The size of capacitors used in these types of applications ranges from 1/6 to 10 horsepower. High starting torque designs also require high starting currents and high breakdown torque.

Capacitor start / induction run motors typically deliver 250 to 350 percent of full load torque when starting. Motors of this design are used in compressors and other types of industrial, commercial, and farm equipment.

Capacitor start induction run motors of moderate torque values are used on applications that require less than 175 percent of the full load. These are used with lighter loads like fans, blowers, and small pumps.
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Capacitor-start single phase induction motor vedio