Electric Motor Starting Capacitor Selection

6 min read Jun 26, 2024
Electric Motor Starting Capacitor Selection

Electric Motor Starting Capacitor Selection: A Comprehensive Guide


Electric motors are widely used in various industries and applications, and their efficient operation relies on proper starting and running conditions. One crucial component in ensuring the smooth operation of an electric motor is the starting capacitor. In this article, we will delve into the world of electric motor starting capacitor selection, discussing the importance of capacitors, types of starting capacitors, and key factors to consider when selecting the right capacitor for your motor.

Why are Starting Capacitors Important?

Starting capacitors play a vital role in the operation of electric motors, particularly in single-phase induction motors. These motors require a high starting torque to overcome the inertia of the load and accelerate quickly to the desired speed. The starting capacitor provides the necessary boost in torque to achieve this.

Starting capacitors perform two primary functions:

  • Torque enhancement: Capacitors provide a phase shift between the voltage and current, increasing the starting torque of the motor.
  • Energy storage: Capacitors store energy during the starting phase and release it quickly to help the motor accelerate.

Types of Starting Capacitors

There are two primary types of starting capacitors used in electric motors:

Film Capacitors

Film capacitors are the most common type of starting capacitor used in electric motors. They consist of a thin film of insulating material, such as polyester or polypropylene, sandwiched between two metal electrodes. Film capacitors are known for their high reliability, long lifespan, and low dissipation factor.

Ceramic Capacitors

Ceramic capacitors are another type of starting capacitor used in electric motors. They consist of a ceramic material with metal electrodes deposited on either side. Ceramic capacitors have a higher capacitance per unit volume than film capacitors, making them suitable for more compact designs. However, they have a lower reliability and shorter lifespan compared to film capacitors.

Key Factors to Consider when Selecting a Starting Capacitor

When selecting a starting capacitor for your electric motor, consider the following key factors:

Capacitance Value

The capacitance value of the starting capacitor should be selected based on the motor's specifications and the load it will drive. A higher capacitance value provides more starting torque, but increases the cost and physical size of the capacitor.

Voltage Rating

The voltage rating of the starting capacitor should match or exceed the motor's voltage rating. Failure to do so can result in premature capacitor failure.

Current Rating

The current rating of the starting capacitor should be sufficient to handle the motor's starting current. Insufficient current rating can lead to overheating and capacitor failure.

Physical Size and Mounting

The physical size and mounting style of the starting capacitor should be suitable for the motor's design and application.

Operating Temperature

The operating temperature of the starting capacitor should be compatible with the motor's operating temperature range.

Reliability and Lifespan

The reliability and lifespan of the starting capacitor should be considered, as a failed capacitor can lead to motor failure and downtime.


Selecting the right starting capacitor for your electric motor is crucial for efficient and reliable operation. By understanding the importance of starting capacitors, types of capacitors available, and key factors to consider, you can ensure that your motor operates smoothly and efficiently. Remember to consult with a qualified electrician or engineer if you are unsure about the selection process.