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Application Notes:
Electrical, Insulation and Thermal Measurements for Motors and Drives: Two Testers That Go Great Together. |
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Most facilities need to get maximum life out of their motors, because they are expensive to replace in terms of both money and labor. Electrical, insulation resistance and thermal measurement are three tests that can troubleshoot motors, drives, and associated electrical panels and prolong their operational lifetime. |
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Handheld thermal imagers such as the Fluke Ti30 can collect heat signatures from a range of motors, from 1000 hp down to 5. 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.
About thermal measurementsA motor's heat signature will tell you a lot about its quality and condition. If a motor is overheating, the windings will rapidly deteriorate. 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 percent, 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.
Electrical and insulation resistance tests on motors
1. Visual inspectionFirst, 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.
2. Control contacts checkNext, check the control contacts for quality of contact:
- Lockout and tagout the disconnect to the starter.
- Manually engage the starter, 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.
3. Resistance of line and load circuits to groundThen, 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:
- 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
4. Winding resistance phase to phase and phase to groundTake 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...
Click here to download the full pdf: Electrical, insulation and thermal measurements for motors and drives » | |
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