How to Test an Electric Motor

March 9, 2023

While we have talked about preventive and predictive maintenance in previous articles (read this article for definitions and differences), here we’ll be taking a closer look at the specific tests and tasks associated with the maintenance of your basic 3-phase electric motor.

Variety of Electric Motor Tests Used

Electrical Tests

-Insulation Resistance (IR)

-Polarization Index (PI)

-Measurement of current and voltages of each phase

-Measurement of motor resistance, inductance, and capacitance

-Motor Current Signature Analysis (MCSA)

Physical Checks

-Lubrication and lubricant level checks

-Visual inspections

Motor Adjustments


-Belt tension

-Bolt tightness

Other Electric Motor Tests

-Thermal Scanning

-Ultrasonic Testing

-Vibration Analysis


While preventive maintenance is performed on a single set of observations and test results, predictive maintenance will take the results of many tests over time and use the trends (like slow increases in vibrations or electrical resistance) to predict the best time to complete a maintenance task before a total breakdown.

Taking a Closer Look – Electrical Tests

Insulation Resistance – IR

A megohmmeter is applied between the winding and the ground for one minute at a pre-determined voltage level, testing for insulation resistance.

Polarization Index – PI

The PI is an extension of the IR test, running for a total of ten minutes to find the ratio between the one-minute and ten-minute test. This ratio is termed the polarization index. (only used on 2300V and larger machines)

Winding Resistance

A digital low-resistance ohmmeter (DLRO) measures the resistance within the windings, testing for high resistance connections and misconnections within the windings.

Motor Current Signature Analysis – MCSA

This test detects rotor currents that have been introduced to the windings.

Taking a Closer Look – Physical Tests

Many bearings require regular relubrication intervals. Ensure that you are relubricating according to manufacturer processes, using manufacturer-recommended lubricant. The purging of old lubricant may be required to avoid any contamination. Additionally, oil lubricated bearings should have the oil tested using either a laboratory or oil analysis equipment.

Here is a tutorial to relubricate your electric motor.

Taking a Closer Look – Adjustments

Visual Inspection

The motor should be inspected for any loose, damaged or missing components, any leaking lubricant, any evidence of contamination, and any blockages to airflow.


Laser alignment equipment is used to check the alignment of the motor and the equipment it is driving. This allows the user to perfect the alignment of their assets to prolong bearing life. This can also be used to check belt alignment.

Taking a Closer Look – Other Tests

Thermal Scanning

An infrared thermometer or camera is used to detect overheating and/or slight changes in temperature.

Ultrasonic Testing

Ultrasound technology is used to detect the condition of the bearings and even the level of lubricant in the motor.

Vibration Analysis

An FFT spectrum analyzer is used to digitize vibration signals. Data is collected over time and used to compare changes in the motor’s condition.


While we have visited preventive and predictive maintenance before, this deep dive will give you a better understanding of the tests used to monitor your electric motor and related components. Using the findings of these tests you’ll be able to put a maintenance plan in place that will keep your motor running in tip-top shape, saving you time and money.

Read more in this linked EASA article.

This article is posted with permission by EASA.

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