How to Test an Electric Motor: A Complete Guide on it

Using motors is a part of your work if you belong to automotive, steel, or any other heavy machinery. Indeed, these devices have made hectic tasks a matter of no time by converting electrical or hydro energy into mechanical power. Useful as it sounds, it comprises complex parts for its function. If anyone of them malfunctions, it can cost you a lot of time and money. Hence, you must know how to test an electric motor with time.

What to Test on A Motor?

For electric motor testing, you need to know what parts or factors to check periodically to minimize the potential delay.

Bearings and Shaft

Bearings of the motor are the component that has most of the load on it. As time passes, it is worn out and may cause damage to other machinery if not changed with time. Therefore, you have to keep an eye on it.

Testing bearing and the shaft is easy. For that, all you need to do is to move it and check whether it is rotating smoothly. If not, lubrication might help, or you need to replace it completely with the new one.

Motor Windings 

In a motor, windings provide a path to current flow, which as a result, creates a magnetic field that rotates the gears. Besides, they play a significant role in motor function; you need to inspect them regularly.

For that, you need a multimeter to see the ohm readings of the wires and terminals. You will test the windings for each of the wires and check whether all the components of windings have a regular flow of current. Then, you will test them with each other to check the shortage in phases.


As a motor runs on electricity, you need to check its power supply. For that, again, you can use a multimeter. Excellent power ratings vary from motor to motor, so you must consider the type before choosing the threshold value.

Check the user manual before conducting a power test for complete guidance. If done otherwise, you could complicate the process wasting your time and maybe damaging the motor in the end. SO hire a professional if you don’t have any experience.

Motor Fan

While testing the components, people mostly forget it is the motor fan. Not knowing how much it is crucial, you may not maintain it. A fan can keep your motor cooler, increasing its life span. Thus, you have to make sure it is clean and free of debris.

Meanwhile, you are cleaning your motor, make it a habit to rotate your fan and check whether it is moving smoothly or not. If not, lubricate it or change it to the new one to protect your motor from overheating.

Tools for Motor Testing

For conducting electric motor testing successfully, it is crucial to have a good tool kit on hand.

Digital multimeter (DMM) 

A multifunctional tool, a digital meter is a power pack of voltmeter, ammeter, and ohmmeter. You can use a multimeter for checking power loss, excessive current levels, and fluctuating resistance readings in a circuit. A screen to see the value on a DMM, a dial to choose the value to measure, and input jacks to connect the test leads. 

Clamp-on ammeter 

Used to measure both AC and DC currents, a clamp-on ammeter measures the strength of the magnetic field produced around the conductor. A pair of jaws present on this ammeter makes it easy for you to clamp it on the wire or other testing object. By using a clamp meter, you can check the current without disconnecting the whole system. Clamp-on ammeters are convenient and hence the first choice of technicians. 


With a megger or meg-ohmmeter, you can test the insulation level of a wire. It does this by giving a high volt, low DC charge and assesses the current leakage, if any. After that, it displays the value on the screen, making it handy for everyday insulation maintenance. 

Non-contact thermometer

Ideal for determining the temperature from a distance, a non-contact thermometer measures thermal radiations emitting from moving or hard-to-reach parts of the machine using temperature sensors. Likewise, it is shaped like a radar gun, so you can use it to check the temperature of an apparatus from a safe point.

Power quality analyzer

Another multifunctional tool for testing motors, a power quality analyzer, is expensive than the above tools. But the functions it provides, such as

  • Calculating power in neutral as well as all three phases
  • Measuring Inrush currents and swells
  • Integrating with compatible software makes it worth every single penny 

Methods of Motor Testing

For a good maintenance program, you need to plan out the technique you choose from several methods. Typically, there are two classes of motor testing;

  • Online dynamic testing provides the data for power quality and specifies under which conditions you should operate a motor. With this type of testing, you should collect the readings on power condition, voltage level and imbalance, load and torque from a moving motor. You can then perform an analysis on these values and assess the motor performance effectively.
  • Whereas, offline static analysis determines the functioning of each part in an electrical motor and given current and voltage. You can use it to check for broken rotor bars, end rings, and misalignment in the components. For maintaining your motor, you have to perform it regularly when the machine is not running to figure out issues on the spot.

Some of the common issues motors face are rotor and winding failure. To lessen these problems, you can test the motor from a variety of proven methods. Common approaches are

Electric Motor Rotating Test

To avoid damage to the machinery, you have to check the direction in which the motor is rotating before connecting it to a load. For that, you will need a phase rotation meter with six leads, three on the motor side and three on the supply side, while checking a three-phase motor.

Motor Impulse Test

The impulse test tells you about how your system will withstand sudden voltage conditions and power surges. Usually, the motor gets overvoltage due to weather conditions, voltage variations from AC to DC, and voltage equipment changes. Therefore, you have to test the motor after these situations occur.

Insulation Resistance Test

Resistance of insulation decreases with the increase in its temperature. This test makes sure the motor has adequate resistance so that the flow of current does not stops. According to IEEE 43 standard, this test needs a temperature rectification of 40C.

Wound Rotor Test

With a wound rotor test, you can reach the main cause of the problem as it isolates the statorrotor, and resistance bank. Like the working of a transformer, any changes in the rotor (secondary) circuit having a resistance bank will cause a change into the stator (primary) circuit and vice versa.

Megger Testing

Megger tests check for the insulation resistance of a motor. As a good approach, Combining it with another type of test is best to detect all the faults in winding completely.

Winding Resistance Testing

By a digital multimeter, you can check for winding resistance to ensure you have wired all the circuits properly and have no loose connections. In addition, this resistance also allows enough flow of charges through the coil for its effective working.

HiPot Test

High Potential Test reviews the weakness in the insulation of cable or wire. To conduct this test, you will apply current between the circuit and the motor frame. Notice the leakage current and the corresponding resistance of that point. Note the values as lower readings of meg-ohm show damaged insulation.

Surge Test

Note that you have to perform a Surge test under IEEE 522 standard. The surge test helps in the detection of potential motor burnout and failure. It indicates the voltage level according to the type of motor and its winding. You can identify failing insulation and dead shorts with this test.

Core Loss Test

Core Loss tests look into the difference between the input and output power and check it against the standard value. It is a strong indicator that the motor requires replacement to maintain the system’s highest performance.

Polarization Index Testing

PI test checks for the fitness of motor. Correspondingly, it gives you the stats on how much moisture, dirt is built up inside the motor and its effects on insulation. For that, you take the values of insulation 10 minutes apart. Their ratio is your Polarization index value.

DC step voltage testing

Step Voltage also looks for the insulation integrity in a circuit. You should perform it after finding the value of PI and increase the voltage after a given time. Outcoming values will plot a graph for you, which is if non-linear, your motor has insulation problems. You have to conduct this test under IEEE 95 Standard.

Voltage Drop Test

The voltage drop test provides information about the resistance in high ampere circuits. Similarly, it is one of the quickest tests used for evaluating the quality of a circuit operation. You can take the voltage drop values as a sign of maintenance and cleaning the motor circuitry.

Vibration Analysis

Vibration analysis is an online test that gives the data on a motor according to a schedule. Using a MEM sensor, you will collect the data in the shape of a waveform. With this test, you can check for bearing defects, resonance conditions, rotor and stator faults, and broken welds or loose bolts.

Rotor Bar Test

To examine any damaged rotor bars, you can apply the growler testsingle-phase rotor test, and high current rotor test. With these tests, you can look into the discontinued current flow and current fluctuations. In a high current rotor test, you can also perform a visual inspection on the surface of the rotor to reveal damaged spots using an infrared camera.

Automated Test

With the advancement of technology, modern motor testing uses the benefits of automatic fault testing methods. Therefore you can easily find an automated diagnostic tool to perform all electrical tests in one device. By using automated testing, the chances of having operators error minimize to 0, and you get more accurate results.

Test Single-phase Electric Motors

For testing a single-phase AC motor, follow these simple steps.

  • General Inspections; See the basic wiring of the motor. Rotate the shaft and fan with your hand to see if they are running smoothly or not. This way, you will inspect that the basic components are good for the run.
  • Capacitor test; With a multimeter, test the power supply voltage and the capacitor terminal voltage. Capacitor voltage should be 1.7 times more than the supply Voltage.
  • Extension Cable Test; For this test, power off the system first. Then test the points near the motor and power supply. If the circuit is intact, the tester will beep.
  • Winding Resistance Test; Test the resistance readings removing all the extra components. Measure the resistance in each of the winding. If they conform to designated values, your motor is ready to roll.

In case your motor doesn’t pass the capacitor test, check/replace the capacitor or wiring. Also, if it fails the extension test, check the cable or connector, and if it fails the winding resistance test, replace the motor.

Testing Three-phase AC Motors

For testing a three-phase AC motor, follow these simple steps.

  • General Inspections; See the appearance of the motor and look for any burnt or damaged area. Rotate the shaft and fan with your hand to see if they are running smoothly or not. After that, test it with amps test and access the value compared to the values proved on the motor manual.
  • Earth Continuity and Resistance Test; With a multimeter, measure the resistance between the motor frame and earth. It should be less than 0.5 OHMS, or else it requires repairing
  • Supply Test; If you have a 230/400V three-phase motor, the normal voltage value will be 230V between phase to neutral point, and 400V between supply lines. Check it using a multimeter and ensure the good condition of the motor.
  • Winding Continuity Test; Test the continuity from phase to phase. If any phase fails the test, the motor is burnt; otherwise, the motor is good to go.
  • Winding Resistance Test; Test the resistance readings from phase to phase. Each winding will give you nearly equal value to each other. Otherwise, there is insulation breakage.
  • Insulation Resistance Test; Inspect the insulation readings from phase to phase and phase to the motor. Note that failed insulation resistance indicates failed motor circuit and that it need change.
  • Running Amps Test; Using a clamp-on meter, check the full load amps or FLA and compare it with the value on the nameplate or manual. If values deviate from each other, you need to change the motor under test.

Prepare your Test Program

You can prepare your testing program by taking help from the following steps.

  1. Start personal preparation by taking deeper knowledge of the system and circuits
  2. Analyze the machine you will test based on its operation ratings, safety, logic, and environmental criticality. Discuss these factors before the test with your team for deeper insight.
  3. Check your weapons. See the tool’s batteries, power cords, voltage clips, and updates on software if you are using one.
  4. Check the operational status of your tester before going into the field.
  5. Test as much circuit with maximum load as possible since you are devoting time and energy to it.
  6. Confirm identified anomalies, as it exists if you are getting a compromised reading during the test. If not, you can have a bad reputation in the market or be accused of using bad tools or both.
  7. Try using different technologies in the same test environment to minimize erroneous results.
  8. In the end, Generate readable reports so that your clients can see the results and benefit from them.


Wrapping up the discussion, testing a motor at the right time can save you from damages in the future. You can conduct different types of tests and check their windings, frames, and insulation for this purpose. However, it is advised to conduct the test periodically to minimize the chances of burnt electric motor.

Testing motors often require repeated wiring and is tough when the machine is complex. Here at Cloom, we offer custom wiring assembly with attention to each detail so that your devices are easy to maintain. 

Hey, I am John, General manager of Cloom and OurPCB.

I am a responsible, intelligent and experienced business professional with an extensive background in the electronics industry.

Reach me at to get a quote for your projects.

Lets Cloom Suport Your Projects

Get a free wire harness or cable assembly sample?

Cookies help us deliver our services. By using our services, you agree to our use of cookies.


Fill in your details and we’ll get back to you in 24 Hours.


Fill in your details and we’ll get back to you in 24 Hours.