How to Test a Single-Phase Motor with a Multimeter

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When a single-phase motor fails to start, becomes hot, trips its overcurrent device or runs intermittently, there could be various causes leading to these symptoms. At times, the main problem could be lying within the power supply, such as the motor controller or the branch circuit conductor. The other possibility could be that a driven load has jammed.

When your single phase motor becomes faulty, it could be as a result of burnt wire or winding failure such as deteriorating bearing or insulation deterioration. To figure out the exact cause of these symptoms, you might consider using various diagnostic tools; however, a multimeter is deemed to be the best. Therefore, in this article, we shall be discussing how to test a single-phase motor with a multimeter.

The process of testing a single-phase motor with a multimeter

Assessing the condition of your single phase motor is relatively easy once you follow the necessary steps. The primary goal of testing an AC motor is to determine its health status. Here are the essential steps that you should consider following when ascertaining the health status of your motor;

  • Testing electrical motor impulse
  • General inspection
  • Resistance and earth continuity test
  • Testing for power supply
  • Testing winding resistance in the single-phase motor
  • Testing insulation reactance
  • Testing running amps

Testing electrical motor impulse

Testing electrical motor impulse is one of the integral parts of maintaining an electrical motor. Here are some essential things you should be concerned about when testing your motor;

  • Check if the insulation has deteriorated or the impulse testing is faulty
  • Can you diagnose weak insulation in your motor using a DC resistance, Megger, Inductance or a HiPot?
  • Check whether your motor operates well after failing an impulse test
  • Assess if your motor is capable of operating accordingly despite having a turn-turn short.

General inspection

Do the following for a single-phase motor:

  • Assess motor appearance. You should be looking for burns, any damage to the body, the shaft or cooling fan.
  • You should then rotate the motor shaft manually in order to examine its bearing condition. Check out for the rotation of a smooth and free shaft. When the shaft turns smoothly and freely, then the bearing is probably in a good state; otherwise, you should consider replacing it with a new one.
  • Once that is done, you should thoroughly examine the name plate. Here you will have valuable information that will help you figure out the actual health status of your motor.

Testing earth continuity and resistance

Using your multimeter, test the resistance between the motor frame and the earth. The readings of a good motor should not be more than 0.5 Ohms. Otherwise, any reading more than 0.5Ohms shows that your motor is faulty.

Testing power supply

230V or 208V is the expected voltage levels of a single-phase motor. This level depends on whether you are using the UK or an American voltage system. You should always assess if the correct supply of voltage is supplied to the motor.

Testing winding resistance in the single motor phase

Use your multimeter to test for winding resistance or checking on the Ohms reading from your motor. This is how you should measure winding resistance in a single-phase motor since they feature three terminals; S, C and R. Measure winding resistance C to S, C to R and S to R. S to R measured values should be equal to C to S + C to R. The following rules apply to single-phase motors;

  • The maximum reading of resistance of the motor should be the Ohms reading that is between S and R.
  • Whereas the lowest reading of resistance of the motor should be the Ohms reading that is between C and R
  • C and S Ohms reading offer some intermediate values that are between that for S to R as well as C to R.

Presence of any deviation indicates a faulty electrical motor or your motor requires some repairs.

Testing insulation resistance

The failure of insulation resistance in a motor is one of the significant signs indicating that your motor is faulty. Usually, insulation resistance is measured between motor winding and earth while using a megohmmeter or an insulation tester. Set your insulation resistance tester voltage settings to 500V and then assess the motor winding to earth. Assess C to E, S to E and R to E. Usually, 1MΩ is the minimal test value for an electric motor that is in a good state.

Testing the running amperage

With your motor running, check on the maximum load amps using a suitable meter and then compare it to the motor name plate FLA. Presence of any deviation from the plate name FLA signifies that the motor being tested is faulty.

Why is it essential to test and electrical motor?

Electrical faults are often the primary causes of motor failure; therefore trying to figure out ways of rectifying this problem is essential. Testing your single phase motor using various electrical diagnostic tools, including a multimeter, will help you locate the chief cause of a problem to your motor.

This makes it easy to solve problems that cause electrical motor failure. When you discover any fault within your electrical motor, it will help you save money and your time since you will be able to prevent sudden shutdowns.


Whenever a single-phase electric motor fails, it is difficult to figure out the cause of the problem by visualizing. You should note that a motor that is placed in a store might or might not work accordingly irrespective of its physical appearance. Conducting a quick check-out using a multimeter or an Ohm meter might help you figure out the cause of the problem.

However, there is plenty of information you will have to collect and weigh before outing it into perspective. When testing your single phase motor, at no point will the power be required. Therefore, ensure you disconnect the motor if it is connected to the power source before commencing on your test. As we come to a conclusion we hope that this article will help you solve all faulty symptoms experienced by your single-phase electric motor.

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