Replacing Start and Run Capacitors on AC Electric Motors

August 12, 2022

Over the years, electric motors have changed drastically. You can find hundreds of designs on the market these days, but that doesn’t mean a motor alone will get the job done. Many applications require control periphery or accessories to be added to the circuit so that the electric motor system can handle the application.

Among the list of accessories needed to run your electric motor safely and efficiently, are start and run capacitors. We’ll explore how to connect a capacitor to an AC Motor by explaining what start and run capacitors are and how to replace them below.

What is a Capacitor?

A capacitor is an electrical device added to an electric circuit to help mitigate power issues. Capacitors hold an electrical charge that can be used by the device powered by the circuit as needed.

In this article, we’ll cover the two main types of capacitors used with electric motors

Start Capacitor

A start capacitor holds a charge that it uses to help the motor at start up, providing additional torque so that motor can turn the load from a stand still. Start capacitors are wired into the auxiliary winding circuit of the motor and are disconnected from the main winding circuit by the centrifugal switch once the motor has reached a predetermined speed (usually 75% of the rated speed).

For more information, refer to the wiring diagram further on in this article.

Run Capacitor

A run capacitor is wired into the main coil circuit and is never disconnected from the circuit. The run capacitor holds a charge to help mitigate power issues while the motor is running. They help to smooth out the flow of power and improve the performance and efficiency of the motor.

What Types of Motors Need Capacitors?

Electric motor start and run capacitors are used with AC single-phase induction motors. You’ll most often find these motors in domestic appliances:

How AC Single-Phase Induction Motors Work

AC single-phase electric motors have two winding circuits, the main winding and the auxiliary/starting winding. The two windings are connected in series with a centrifugal switch that disconnects the auxiliary winding from the main winding after start-up. See below circuit diagram.

Figure 1 sourced from

At start-up, the start capacitor sends a charge through the auxiliary winding; this charge is out of phase from the main winding, creating a rotating magnetic field to torque the rotor. The start capacitor provides enough torque to start the motor under load and quickly bring it up to speed. Once the motor has reached a predetermined speed, the centrifugal switch disconnects the auxiliary winding from the main winding. The motor continues to receive power from the main winding circuit.

Replacing Your Start and Run Capacitors

Before we begin, we’d like to note that all electrical work should be completed by a certified electrician. Hiring a certified electrician can help save you and your businesses from damages to your equipment or worse, physical harm to a person. eMotors Direct is not responsible for any damage or injury that may result from these directions.

Here is a step-by-step on replacing your capacitors. This explanation will work for both start and run capacitors.

  1. First, start by turning off the power to the system.
  2. Locate and discharge capacitor.
    1. You can discharge a capacitor by placing an insulated screwdriver across the terminals.
  3. Now you need to ensure that the new capacitor ratings match the one you are replacing.
  4. Label wires, or better yet, take a photo to ensure that you properly wire the new capacitor in place.
  5. Install the new capacitor with the same setup and wiring as the old capacitor.
  6. Finally, resume power to the system and test the motor.


Many electric motor systems require peripheral accessories to ensure a safe and efficient operation. In the case of AC single-phase induction motors, this accessory is the capacitor. Start and run capacitors hold an electric charge to provide additional torque at start up and to smooth the current while running so that the motor operates efficiently and without damage.

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