Why Single Phase Motors Need Capacitors

Single-phase alternating current (AC) motors are designed to carry a given load however these motors need an extra boost to get and sometimes keep the load moving. A motor capacitor is an electrical storage unit that stores and releases energy to increase the current to one or more copper windings of a single-phase motor to create this extra boost and increase the motor torque.

The figure below displays 25 µF, 115V AC motor starting capacitor electrolytic capacitor:

Single-phase motors need capacitors primarily to improve starting and running performance. Unlike three-phase motors, single-phase motors do not inherently produce a rotating magnetic field, which is crucial for smooth and continuous operation. Let’s delve into why capacitors are essential in single-phase motors:

  1. Starting Torque: Single-phase motors inherently lack starting torque. To overcome this, capacitors are used to create a phase shift between currents in multiple motor windings. Essentially, a capacitor connected in series with a “start” winding shifts the phase of the current in that winding relative to the current in the “main” or “run” winding. This phase shift creates a rotating magnetic field, which is required to start the motor. Once the motor reaches a certain speed, a centrifugal switch or electronic relay disconnects the start winding (and the capacitor) to allow the motor to continue running on the main winding alone.
  2. Running Performance: In some types of single-phase motors, like permanent split capacitor (PSC) motors, capacitors are not just used for starting but also remain connected during operation. The continuous presence of the capacitor improves the motor’s efficiency, power factor, and running torque. This configuration supports a smoother and more efficient operation, making PSC motors popular for applications requiring consistent and reliable operation, such as in fans and air conditioners.
  3. Power Factor Correction: Capacitors help in correcting the power factor of the motor. A poor power factor, typically a lagging power factor caused by the inductive load of the motor, means that more apparent power (measured in VA, or volt-amperes) is needed to produce actual work (measured in watts). By introducing a capacitor, which provides a leading power factor, the overall power factor of the motor is improved. This correction reduces the phase difference between voltage and current, thereby improving the efficiency of power delivery from the power source to the motor.
  4. Reducing Electrical Noise: Capacitors can also help filter out electrical noise in the power supply, which can improve the longevity and reliability of the motor. Noise reduction is particularly important in sensitive electronic environments or in residential settings.

Overall, capacitors are crucial for the efficient and effective operation of single-phase motors by facilitating the start-up and improving running characteristics. Their use enables these motors to be used in a wide range of household and industrial applications where three-phase power is not available.

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