How To Tell If Electric Motor Capacitor Is Bad

4 min read Jun 26, 2024
How To Tell If Electric Motor Capacitor Is Bad

How to Tell if an Electric Motor Capacitor is Bad

An electric motor capacitor is a crucial component that helps to regulate the flow of electrical energy in a motor. If the capacitor goes bad, it can cause the motor to malfunction or even fail altogether. In this article, we'll discuss the common signs and symptoms of a bad electric motor capacitor and how to diagnose the problem.

Common Symptoms of a Bad Electric Motor Capacitor

Motor Won't Start

If the motor won't start at all, it could be a sign that the capacitor is bad. A faulty capacitor can prevent the motor from generating the necessary torque to start rotating.

Motor Runs Slowly or Erratically

If the motor is running slowly or erratically, it may be due to a weak or dying capacitor. As the capacitor degrades, it can cause the motor to struggle to maintain a consistent speed.

Motor Overheats

A bad capacitor can cause the motor to overheat, leading to premature wear and tear on the motor's internal components.

Motor Vibrates Excessively

Excessive vibration can be a sign of a bad capacitor, as it can cause the motor to wobble or shake violently.

How to Diagnose a Bad Electric Motor Capacitor

Visual Inspection

Perform a visual inspection of the capacitor and look for signs of physical damage, such as cracks, burn marks, or rust. Check the capacitor's terminals for corrosion or wear.

Capacitance Measurement

Use a multimeter to measure the capacitor's capacitance. A faulty capacitor will typically have a lower capacitance reading than the rated value.

ESR (Equivalent Series Resistance) Measurement

Measure the capacitor's ESR using an ESR meter. A high ESR reading indicates a bad capacitor.

Voltage Test

Apply the rated voltage to the capacitor and measure the current flow. A bad capacitor will typically draw more current than expected.

What to Do if You Find a Bad Electric Motor Capacitor

If you've diagnosed a bad capacitor, it's essential to replace it with a new one of the same rating and type. Failure to do so can cause further damage to the motor or other components.


A bad electric motor capacitor can cause a range of problems, from motor failure to overheating and excessive vibration. By recognizing the common symptoms and performing a few simple tests, you can diagnose a bad capacitor and replace it with a new one. Remember to always follow safety precautions when working with electrical systems, and consult a professional if you're unsure about any aspect of the diagnosis or replacement process.

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