Electrical, insulation and thermal measurements for motors and drives

October 29th, 2014, Published in Articles: Vector


Electrical, insulation resistance and thermal measurement are three tests which can troubleshoot motors, drives and associated electrical panels and prolong their operational lifetime.

Used together, thermal imagers can detect potential problems and insulation resistance, while electrical tests can determine the cause.

Handheld thermal imagers can collect heat signatures from a range of motors, from 1000 down to 5 hp. A thermal imager is good for spot checks, to see whether motors and associated panels and controls are operating too hot and, for troubleshooting, to track down the specific failed component at fault. It can also check for phase imbalance, bad connections, and abnormal heating on the electrical supply.

An insulation multimeter can perform most of the other tests needed to troubleshoot and maintain motors. When a motor is having problems, check the supply voltage and then use insulation testing to check the starter and control contacts, measure the insulation resistance of the line and load circuits to ground, and winding resistance phase to phase and phase to ground.

A motor’s heat signature will tell you a lot about its quality and condition. If a motor is overheating, the windings will deteriorate rapidly. In fact, every increase of 10°C on a motor’s windings above its design operating temperature cuts the life of its windings’ insulation by 50%, even if the overheating is only temporary.

If a temperature reading in the middle of a motor housing comes up abnormally high, take a thermal image of the motor and find out more precisely where the high temperature is coming from, i.e. windings, bearings or coupling (a coupling running warm is an indicator of misalignment).

Fig. 1: This thermal image shows a drive cabinet with hot connections on both A and B phases. The exact cause can’t be determined solely from the image, although it may be a load or balance issue.

There are three primary causes for abnormal thermal patterns. Most are typically the result of high-resistance contact surfaces, either connections or switch contacts. These will usually appear warmest at the spot of high resistance. The thermal image shows a classic pattern in the center phase connection on the line side of a breaker. Note how the conductor cools off at the top of the image (see Fig. 1).

Load imbalances, whether normal or out of specification, appear equally warm throughout the phase or part of the circuit which is undersized or overloaded. Harmonic imbalances create a similar pattern. If the entire conductor is warm, it could be undersized or overloaded. Check the rating and the actual load to determine which. Failed components typically seem cooler than similar, normally functioning ones. The most common example is probably a blown fuse. In a motor circuit, this can result in a single phase condition and, possibly, costly damage to the motor.

Fig. 2: A warm bearing (or seal) on the pump. The access is clearly tight but we can still compare the bearing to the housing around it.

Insulation resistance testing

Insulation problems on motors and drives are usually caused by improper installation, environmental contamination, mechanical stress or age. Insulation testing can be combined easily with regular motor maintenance to identify degradation before failure, and during installation procedures to verify system safety and performance. When troubleshooting, insulation resistance testing can be the missing link which enables you to get a motor back into operation the easy way, by simply replacing a cable.

Insulation testers apply DC voltage across an insulation system and measure the resulting current. This allows them to calculate and display the resistance of the insulation. Typically, the test verifies high insulation resistance between a conductor and ground or high insulation resistance between adjacent conductors.

Fig. 3: Another bearing problem with heat also transferring into the coupling on the right side.

Fig. 3: Another bearing problem with heat also transferring into the coupling on the right side.

Two common examples include testing motor windings for insulation from the motor frame and checking phase conductors for resistance from bonded conduit and enclosures. Insulation multimeters combine the insulation resistance functions with the other tests needed to investigate motor, drive, and electrical trouble, from basic supply measurements to contact temperature. The key difference is that insulation resistance tests are performed on de-energised systems while electrical and thermal tests are almost always performed on live operating systems.

Electrical, insulation resistance tests on motors

Visual inspection

  • First, look for a reason not to energise. Remove power from the motor and starter (or drive) following lock out or tag out procedures, and disengage the motor from the load.
  • Conduct a visual, smell and heat inspection, interview the client and check the nameplate. Look for loose connections at the starter and check all fasteners.
  • Use a DMM to check the supply voltage, then the voltage starter contacts. Don’t risk a fire from a possibly shorted motor. If the supply is good, then there’s a motor problem.

Control contacts check

Next, check the control contacts for quality of contact:

  • Lock out and tag out the disconnect to the starter.
  • Engage the starter manually so the contacts close.
  • Set the insulation tester to the low ohms range.
  • Measure the resistance across each set of contacts.
  • The reading should be nearly zero. If it is higher than 0,1 ohm, that set of contacts must be replaced.

Resistance of line and load circuits to ground

Then, measure the insulation resistance of the line and load circuits to ground. However, before doing any insulation resistance testing, you must isolate any electronic controls and other devices from the circuit under test. Then:

  • Lock out and tag out the disconnect to the starter.
  • Set the insulation tester to the appropriate test voltage (250, 500 or 1000 V).
  • Identify the resistance between the line and load side of starter to ground.
Fig. 3: Another bearing problem with heat also transferring into the coupling on the right side.

Fig. 3: Another bearing problem with heat also transferring into the coupling on the right side.

The line and load circuits must show high resistance to pass these tests. As a general rule, AC devices need a minimum 2 MΩ to ground and DC devices need 1 MΩ to ground to ensure safe operation.

± Note: Different companies have different threshold minimums for insulation resistance on used equipment, ranging from 1 to  10 MΩ. Resistance on new equipment should test much higher – from 100 to 200 MΩ or more.

If the load-side resistance values are acceptable, then proceed to the next test. If they aren’t, start tracing the problem: is the insulation breakdown in the load side of the starter, the cables, or the motor?

Winding resistance phase-to-phase and phase-to-ground

Take insulation resistance measurements phase-to-phase and phase-to-ground. Good results include:

  • Balanced comparative low resistance values on all three stator phases.
  • High resistance values on the phase-to-ground insulation test.

Problems include:

  • Gross resistance deficiencies such as a phase-on-phase short.
  • Any winding-to-winding resistance imbalance. If the readings differ by more than a few percent, the motor is probably unsafe to energise.

Contact Val Verwer, Comtest, Tel 608-8520, vverwer@comtest.co.za

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