Winding resistance testing of motors

August 2nd, 2019, Published in Articles: EE Publishers, Articles: Energize, Articles: Vector

Winding resistance measurements detect various faults in motors and transformers: shorted turns, loose connections, broken strands and malfunctioning tap changer mechanisms.

Winding resistance measurements detect problems in motors which other tests may not find. These problems include partial or fully shorted coils, poor crimps or connections, imbalance between phases (improper turns on phases) and incorrect coil (phasing) connections. Studies conducted by IEEE and the Electric Power Research Institute (EPRI) on electric rotating machinery failures show that 48% of motor failures are due to electrical failures.

Winding vs. insulation resistance

As with transformers, a motor or generator is broken down into two main components: insulation and mechanical. The mechanical condition and structure of the rotor or stator affects the winding resistance. Winding resistance testers apply a known DC current through the windings, measure the resulting voltage drop across the winding, and calculate the resistance. One should not apply more than 10% of the winding current rating as this will warm the winding and create a changing resistance value as the copper or aluminum heats up.

For the electrical insulation component, an insulation resistance (IR) instrument is used to validate the condition of the winding relative to ground (outer case of the stator winding). Insulation resistance testers apply high DC voltage which causes a small current through the insulation being tested. The tester then provides a resistance reading. Good insulation should have high resistance and typical values are in the MΩ or GΩ range. When applying DC test voltage, you should never exceed the voltage rating of the motor winding being tested.

Tester requirements

For most common resistance measurements, you can use a regular multimeter set to the ohms (Ω) scale. However, the windings in large motors have low resistance and are very inductive. The tester must therefore safely inject sufficient test current at a more significant test voltage to measure the stator winding safely and timely.

 

Fig. 1: Phase-to-phase resistance measurement.

A higher test voltage will overcome the inductance more quickly (up to 50 times faster than a normal low-resistance meter). A regular multimeter cannot perform winding resistance measurements. Megger’s MTO106 provides up to 6 A of test current and 48 V of open circuit voltage.

The winding resistance tester uses a four-wire measurement with a Kelvin lead set to improve measurement accuracy. This eliminates the resistance of the lead set from the measurement, providing accuracy.

Safety is an important consideration when testing winding resistance. Motor or generator windings can store a large amount of energy when DC current is injected into them during the test (known as inductive charging). This energy must be dissipated safely from the winding after the test current is stopped.

The MTO106 will automatically discharge this energy safely after a test is terminated. The discharge function is passive and allows automatic discharge in the event of inadvertent power loss or if the test leads are accidentally disconnected. The unit also features a visual and audible discharge indicator as a discharge condition occurs.

Why winding resistance tests?

While detecting problems in vital motors or generators is important, finding them before they lead to catastrophic failure is critical. Predictive and preventive maintenance programmes which include regular testing can help detect winding issues early. Winding resistance tests provide information about the condition of the windings.

Test result analysis

Winding resistance test readings can check against factory-stated values. A common diagnostic technique is to compare with previous readings. Since winding resistance is affected by temperature, it is important to use temperature correction factors when applicable.  Winding resistance test results are compared between the three phases (on a 3-phase motor).

A number of standards provide maximum deviation percentages but typical limits are 1 to 3% between the mean average for the three windings. Excessive differences in resistance readings between phases may indicate a possible problem inside the motor. Winding resistance is also used for measuring I2R losses in the winding.

In reality, there is always some resistance, even if it is small. This causes electrical losses which are dissipated as heat. The information in this article relates to testing the stator winding of a motor. Resistance tests on the rotor can typically be done with a low-resistance ohmmeter.

Conclusion

Keeping motors running is critical in a vast number of industries. Knowing the condition of the windings is one important part of ensuring motors’ proper performance.

Contact Corola Argiro, Megger, corolla.argiro@megger.com

 

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