Single Phase Motor Running Capacitor Calculation

6 min read Jun 26, 2024
Single Phase Motor Running Capacitor Calculation

Single Phase Motor Running Capacitor Calculation: A Comprehensive Guide


Single phase motors are widely used in various applications, including refrigeration, air conditioning, and pumps. To ensure efficient and reliable operation, it is essential to select the correct running capacitor for the motor. In this article, we will discuss the importance of running capacitors in single phase motors and provide a step-by-step guide on how to calculate the required running capacitor value.

What is a Running Capacitor?

A running capacitor, also known as a run capacitor, is an electrical component used in single phase motors to improve their performance and efficiency. It is connected in series with the motor windings and helps to create a rotating magnetic field, which enables the motor to operate efficiently.

Why is a Running Capacitor Necessary?

A running capacitor is necessary for single phase motors because they have a few inherent limitations:

  • Low Power Factor: Single phase motors have a low power factor, which means they require a higher current to produce the same amount of power as a three-phase motor. A running capacitor helps to improve the power factor, reducing the current drawn from the supply.
  • Poor Torque: Single phase motors produce a pulsating torque, which can cause vibration and noise. A running capacitor helps to smooth out the torque, reducing vibration and noise.
  • Inefficient Operation: Without a running capacitor, single phase motors operate inefficiently, leading to overheating and reduced lifespan. A running capacitor ensures efficient operation, reducing heat generation and increasing the motor's lifespan.

Running Capacitor Calculation

To calculate the required running capacitor value, you need to know the following parameters:

  • Motor Rating: The rating of the motor in horsepower (HP) or kilowatts (kW).
  • Voltage: The voltage at which the motor operates.
  • Frequency: The frequency of the power supply.
  • Motor Efficiency: The efficiency of the motor.

The following formula can be used to calculate the running capacitor value:

C (microfarads) = (k x HP x V) / (3.3 x f x E)


  • C is the running capacitor value in microfarads (μF).
  • k is a constant that depends on the motor type and efficiency (typically ranges from 0.5 to 1.5).
  • HP is the motor rating in horsepower.
  • V is the voltage at which the motor operates.
  • f is the frequency of the power supply in hertz.
  • E is the motor efficiency as a decimal value.

Example Calculation

Let's consider an example to illustrate the calculation:

  • Motor rating: 1 HP (0.75 kW)
  • Voltage: 230 V
  • Frequency: 50 Hz
  • Motor efficiency: 80% (0.8 as a decimal value)

Using the formula above, we can calculate the running capacitor value as follows:

C = (0.8 x 1 x 230) / (3.3 x 50 x 0.8) = 24.5 μF

Therefore, the required running capacitor value for this motor is approximately 24.5 μF.


In conclusion, selecting the correct running capacitor value is crucial for efficient and reliable operation of single phase motors. By following the steps outlined in this article, you can calculate the required running capacitor value and ensure optimal performance of your motor. Remember to consult the motor manufacturer's recommendations and specifications for the most accurate results.

Recommended Reading

  • NFPA 70 (National Electric Code)
  • IEEE Standard 519 (Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems)
  • Motor Manufacturer's Specifications and Recommendations


This article provides general information and guidance on running capacitor calculation. It is not intended to replace the manufacturer's recommendations or specifications. Always consult the motor manufacturer's documentation and seek professional advice if you are unsure about the calculation or selection of the running capacitor.

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