How to Test Motors

What methods are available for testing motors? This page offers an easy-to-understand explanation of the basics.


If you’re like most people, the phrase “motor system” probably summons an image of a car. Today, motor systems play an essential role not only in cars, but also in a variety of other fields. What test methods are available when you need to check the quality or safety of a motor, or pinpoint an issue when troubleshooting?

This page provides a detailed introduction of basic test methods for motor systems.

Motor drive systems (inverters): An essential component in a variety of industrial fields

Motor drive systems are used in a variety of industrial fields to convert a fixed voltage to a variable voltage for use in providing drive power. Because they excel in terms of efficiency and controllability compared to simple, directly connected motors, motor drive systems are able to deliver cost savings along with increased performance and longer service life.

Generally speaking, motors used in electric vehicles are probably the most familiar example of this class of device. Such motors are also used to power compressors in many household appliances such as air conditioners and refrigerators. It’s also typical for them to be used at construction sites, chemical plants, and manufacturing machinery, although consumers rarely encounter such installations directly.

Therefore, it’s no exaggeration to say that today’s convenient lifestyles would not be possible without the benefit of motor systems. For machines, motor systems are an extremely important type of electronic device. As a result, it’s necessary to ensure safety, verify quality, and, in the event of a problem, to pinpoint and deal with the underlying issue in an appropriate manner.

Example of an inverter efficiency measurement system using Hioki’s power meters

Testing resistance between motor phases

Phase resistance testing makes it possible to investigate whether each phase in a motor exhibits the appropriate resistance value. For example, for a motor with a, b, and c terminals, the resistance of the following terminal combinations is measured: a-b, a-c, and b-c. If the measurements yield resistance values ranging from several ohms to dozens of ohms, it’s safe to conclude that there are no issues between the motor’s phases.

However, if there is any combination of phases for which the measured value is zero, you can conclude that a short exists between those phases. A short occurs when wires have come into contact with each other, for example as the result of torn insulation. Ordinarily, electricity can be converted into various forms of energy by resistance.

However, shorts between wires cause large currents to flow without being converted into another form of energy. By contrast, if the resistance between two phases is infinite, then there is likely a break in one of the wires. With phase resistance, it’s necessary to measure all combinations (not just the resistance of individual phases), to check whether there’s a problem at one point or at multiple points, and to take action accordingly.

Testing a motor’s insulation resistance

Insulation resistance testing is used to check for degradation in wire insulation. Such testing is performed not only after problems are found, but also on a regular basis to prevent such issues. Continuing our example for phase resistance above, a motor with a, b, and c terminals would be tested by measuring the wiring between terminal a and the motor body, between terminal b and the motor body, and between terminal c and the motor body.

Insulation Tester

A special tester known as an insulation resistance meter is used to measure insulation resistance. The higher the insulation resistance value, the greater the insulating effect. If the measurement indicates infinite resistance, you can conclude that the insulation resistance is normal. However, a measurement of 0 Ω suggests the possibility that the wire in question is leaking current.

Current leaks are extremely dangerous. Exercise caution as current leaks can cause machinery malfunctions as well as electric shock, fire, and even death.

Performing impulse winding tests on motors after rotor installation

Measurement by Hioki’s impulse winding tester

Impulse winding testing is a technique for investigating the condition of a motor’s coil. The condition of the coil can be investigated by applying a high voltage with low energy, known as a pulse voltage, to both ends of the coil. This testing technique is unique in that it can be carried out after the rotors have been installed since it quantifies and displays a high-precision response waveform.

In addition to wire breaks in the coil, it can also detect the following issues:

- Defects such as multi-turn shorts that are difficult to detect using traditional testing methods
- Minuscule changes in inductance
- Insulation defects between motor windings that are prone to be obscured by noise

This simple method detects problems by comparing the test waveform for the motor under test to the waveform response generated when the impulse voltage was applied to a known-good motor. It can also be used to check for difficult-to-identify problems like multi-turn shorts by quantifying the measured waveforms.

Identifying issues through motor testing

Motor systems are revolutionary systems that play an essential role in our daily lives in a variety of fields, including in automobiles, household appliances, and plant equipment. Consequently, it’s critical that testing to verify quality and safety and to pinpoint issues generate results that are as accurate as possible. This page has introduced some of the basic methods used to test motors.
Remember to ensure that you have a sufficient understanding of the sort of test results you require before carrying out testing.


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