How to Wire a Motor Capacitor

August 28, 2023

A capacitor failure is usually the first sign of a bigger problem. Maybe an issue with your start switch, a low voltage problem, or the load is more than the motor can bear. So, if your capacitor has failed, make sure you do some investigating before replacing it. In this article, we'll cover:

  1. Why do capacitor fail (avoid the same problem reoccuring)
  2. Steps to confirm if your capacitor failed
  3. How to wire a motor capacitor
  4. What types of motors need capacitors
  5. What is a capacitor
  6. How AC single-phase induction motors work
  7. Bonus: a video of a capacitor exploding

Why Did Your Capacitor Fail?

Your capacitor might be failing if your motor is slow to start, fails to start, or when you hear a constant buzzing during operation. Typically, wesee capacitors fail due to overloading, which can lead to a short circuit, open circuit, or deterioration.

Start capacitors, in particular, aren’t designed for continuous operation. They just can’t dissipate heat fast enough. If they stay in the circuit for too long, they’ll simply overheat and fail...which usually isn’t an issue unless something’s up!This is a sign that your switch may be malfunctioning, or you have a low voltage problem, or the motors being overloaded.

Short cyclingalso leads to issues with start capacitors. If your motor is starting and stopping a lot in a short period, the capacitor doesn’t have enough cool down time before it needs to perform work again.

We see this a lot with our agriculture customers - high vibration, on an auger motor for example, leads to a poor connection at the capacitor’s terminals, which creates a higher resistance and excess heat. To avoid this failure, just make sure the capacitor’s connected properly.

Excess heat in the atmosphere and sub-zero temperatures can also affect the ability of the capacitor to perform its job, leading to failure.

Lastly. Capacitors are a “wearable” component. They’ll deteriorate over time and are meant to be replaced regularly. The rate of wear depends on how often it starts and stops and the environment it’s in. In a perfectworld, you could see a capacitor last ten-plus years. But we do see scenarios where customers are replacing capacitors every few years.

How to Test a Motor Capacitor

  1. Step One - First, perform a simple visual inspection of the capacitors.Check for any leakage, cracks, or bulges, and see if the membrane on the top of the capacitor is still in place.
  2. Step Two - If there are no visual indicators of failure, you’ll need to test the capacitance of the capacitor with a multimeter. To do this, you need to ensure that power has been cut from the circuit and then discharge the capacitor.Remove the capacitor from the circuit and connect your multimeter to the capacitor terminals.If your multimeter can test for capacitance, select that mode, wait a few seconds, and note the reading - which will be a microfarad rating. The rating should be within a range that you can find on the label of the capacitor.

If you confirmed your capacitor failed, you’ll want to determine what actually causedthe failure before replacing the capacitor.This is probably an issue with the switch, a voltage problem, or a load problem.

How to Wire a Motor Capacitor

Due to the nature of their design and intended use, capacitors can hold a dangerous and potentially deadly charge for some time after being removed from power. You must discharge the capacitor before handling. This can be done simply by placing ascrewdriveracross the two terminals.

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.

Once you’re ready to replace the failed capacitor,locate the ratings on the side of the old capacitor. You’re looking for the microfarad or M-F-D rating and the voltage.Also, note the shape and dimensions of the old capacitor to ensure that the new one can easily be mounted in the same place.Then, head over to to find your capacitor.Once on thesite, use the menu on the left-hand side, search for and select the correct voltage and MFD ratings. To lower the search resultsfurther, you can also select start or run capacitor and the capacitor shape. Make your selection from the search results. If youre unsure of your selection or if you’d like any assistance, contact our team using the link at the bottom of this article.

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

  1. Step 1 – Cut power from the circuit.
  2. Step 2 – Locate and discharge the capacitor safely. You can discharge a capacitor by placing an insulated screwdriver across the terminals.
  3. Step 3 - Double check that the new capacitor’s ratings match the old one.
  4. Step 4 - Remove the old capacitor and install the new one.I prefer to do one at a time so that you don’t need to label the wires.
  5. Step 5 – Resume power to the circuit and test the motor.

What Types of Motors Need Capacitors?

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

What Does a Capacitor Do in a Motor?

Capacitors are electrical components that are wired into a circuit to mitigate power issues. They can build and hold an electrical charge to be used later when needed by the system.

So, why do single-phase AC induction motors need capacitors?On its own, single-phase power can’t create the rotating field needed to start an electric motor or produce enough torque to move the load.But with a start capacitor wired into the circuit, the motor starts.And with a run capacitor, torque is more constant.

Capacitors play a big role in the operation of your single-phase AC induction motor. Without them, your motor won’t be able to perform its basic functions. It’s important that you try to avoid common causes of failure and replace your capacitors as soon as a failure is detected.

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.

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.

Figure1sourced 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.

Bonus: Video of a Capacitor Exploding

For training purposes, we wanted to show some of our newer team members what a failed capacitor looked like. So naturally, we blew up a capacitor.


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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