Electrical, insulation and thermal measurements for motors and drives

March 9th, 2015, Published in Articles: Energize, Articles: Vector


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

Most facilities must obtain maximum life out of their motors because they are expensive to replace, in terms of both money and labour. Used together, thermal imagers can detect potential problems and insulation resistance and electrical tests can determine the cause of the problem.

Handheld thermal imagers such as the Fluke Ti25 can collect heat signatures from a range of motors, from 1000 hp down to 5 hp.

A thermal imager is good for spot checks, to see if motors and associated panels and controls are operating too hot, and for troubleshooting, to track down the specific failed component at fault. They can also check for phase imbalance, bad connections and abnormal heating on the electrical supply.

Insulation multimeters like the Fluke 1587 can perform most of the other tests needed to troubleshoot and maintain motors. When problems occur in a motor, check the supply voltage and then use insulation testing to check the starter and control contacts, to measure the insulation resistance of the line and load circuits to ground, as well as winding resistance phase-to-phase and phase-to-ground.

The motor’s heat signature will tell you a lot about its quality and condition. If the 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.

Should the temperature reading in the middle of a motor housing come 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).

There are three primary causes for abnormal thermal patterns. Typically, most are the result of a high-resistance contact surface, either a connection or a switch contact. These will usually appear warmest at the spot of high-resistance and appear cooler further away from the spot. This thermal image shows a classic pattern in the centre phase connection on the line-side of a breaker. Note how the conductor cools off at the top of the image.

Load imbalances, whether normal or out of specification, appear equally warm throughout the phase or part of the circuit that 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 look cooler than similar ones functioning normally. 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.

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.

Fig. 1 shows a warm bearing (or seal) on the pump. Clearly the access is tight but we can still compare the bearing to the housing around it while Fig. 2 shows another bearing problem with heat
also transferring into the coupling on the right side.

Fig. 1: A warm bearing or seal on the pump. Access is clearly tight but  the bearing can still be compared with the housing around it.

Fig. 1: A warm bearing or seal on the pump. Access is clearly tight but
the bearing can still be compared with the housing around it.

In Fig.3, the motor itself is heating up due to reduced airflow or, more probably, due to misalignment.

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 that enables you to get a motor back into operation the easy way, simply by replacing a cable.

Fig. 2: A bearing problem with heat transferring into the coupling on the right.

Fig. 2: A bearing problem with heat transferring into the coupling on the right.

Insulation testers apply a DC voltage across an insulation system and measure the resulting current. This allows them to calculate and display the resistance of the insulation. The test typically verifies high insulation resistance between the conductor and ground or high insulation resistance between adjacent conductors. Two common examples include testing motor windings for insulation from the motor frame, and checking phase conductors for resistance from bonded conduit and enclosures.

Fig. 3: The motor itself is heating up due to reduced air flow or misalignment.

Fig. 3: The motor itself is heating up due to reduced air flow or misalignment.

Insulation multimeters combine these insulation resistance functions with the other tests 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 and 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 lockout/tagout 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. Good supply is indicative of a motor problem.

Control contacts check

Next, check the control contacts for quality of contact:

  •  Lockout and tagout the disconnect to the starter.
  • Engage the starter manually so that 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’s higher than 0,1 Ω, the 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.


  • Lockout and tagout the disconnect to the starter.
  • Set the insulation tester to the appropriate test voltage (250, 500 or 1000 V).
  • Identify the resistance between these points:
    – Line side of starter to ground.
    – Load side of starter to ground.

To pass these tests, the line and load circuits need to show high resistance. 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 that 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, then 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:

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


  • 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 011 608-8520, vverwer@comtest.co.za

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