Electrical, Insulation and Thermal Measurements for Motors and Drives

Pumps & Systems, June 2008

Due to the expense and labor required, most facilities need to maximize the life of their motors. Electrical, insulation resistance and thermal measurement are three tests that can troubleshoot motors, drives and associ­ated electrical panels and prolong their operational lifetime. Ther­mal imagers can detect potential problems and insulation resistance and electrical tests can deter­mine the cause.

Handheld thermal imag­ers can collect heat signatures from a range of motors, from 1,000-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 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.

About Thermal Measurements

A motor's heat signature reveals a lot about its qual­ity and condition. If a motor is overheating, the windings will rapidly deteriorate. In fact, every increase of 10-deg C on a motor's windings above its design oper­ating temperature cuts the life of its windings' insulation by 50 percent, even if the overheating is temporary.

If a temperature reading in the middle of a motor housing is 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. (If a coupling is running warm, it may be an indicator of mis­alignment.)

There are three primary causes for abnormal thermal patterns; typically, most result from a high-resistance contact sur­face, either a connection or a switch contact. These will usu­ally appear warmest at the spot of high-resistance, cooling off the further away from the spot.

Load imbalances, whether normal or out of specification, appear equally warm throughout the phase or part of the circuit that is undersized/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, 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 possible costly damage to the motor.

About Insulation Resistance Testing

Insulation problems on motors and drives are usually caused by improper installation, envi­ronmental contamination, mechanical stress or age. Insu­lation testing can easily be combined 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 to putting a motor back into operation the easy way, by simply replac­ing a cable.

Insulation testers apply a DC voltage across an insulation system and measure the result­ing current 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. 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 above 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-energized systems, while electrical tests (and thermal) are almost always performed on live, operating systems.

Electrical and Insulation Resistance Tests on Motors

1. Visual Inspection

First, look for a reason NOT to energize. 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.

Do not risk a fire from a pos­sibly shorted motor. If the supply is good, then there is a motor problem.

2. Control Contacts Check

Check the control con­tacts for quality of contact:

1. Lockout and tagout the disconnect to the starter.

2. Manually engage the starter so the contacts close.

3. 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 ohms, the set of contacts needs to be replaced.

3. Resistance of Line and Load Circuits to Ground

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:

1. Lockout and tagout the disconnect to the starter.

2. Set the insulation tester to the appropriate test voltage (250, 500 or 1000 V).

3. 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 megohms to ground and DC devices need 1 megohm to ground to ensure safe operation.

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

If the load side resistance values are acceptable then proceed to the next test. If they are not, then start tracing the problem. Is the insulation breakdown in the load side of the starter, the cables or the motor?

4. 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.

Problems:

• 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 energize.

 

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