MRO Magazine

4 Steps For Conducting Electrical And Insulation Resistance Tests On Motors

1. Visual inspection: First, look for any reasons the motor should not be energized. Remove power from the motor and starter (or drive), following lockout/ tagout procedures, and disengage the motor f...


February 1, 2009
By MRO Magazine

1. Visual inspection: First, look for any reasons the motor should not be energized. 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 staff and check the nameplate. Look for loose connections at the starter and check all fasteners.

• Use a digital multimeter (DMM) to first check the supply voltage, followed by the voltage at the starter contacts. Don’t risk a fire from a possible motor fault. If the supply is good, then there is a motor problem.

2. Control contacts check: Next, check the control contacts for quality of contact.

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• 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 close to zero. If it is higher than 0.1 ohms, then that set of contacts needs to be replaced.

3. Measure the insulation resistance of line and load circuits to ground: Before doing any insulation resistance testing, you must isolate any electronic controls and other devices from the circuit under test as the test voltage used could damage them. Then:

• Lockout and tagout the disconnect to the starter.

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

• Identify the resistance between the line side of the starter to ground, and the load side of the starter to ground. To pass these tests, the line and load circuits need to show high resistance. As a general rule and to ensure safe operation, AC devices need a minimum 2 megohms to ground and DC devices need 1 megohm to ground. If the load side resistance values are acceptable, then proceed to the next test. If they aren’t, then determine the cause of 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 per cent, the motor is probably unsafe to energize.


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