Motor Insulation Resistance Test Procedure

Figure 4 typical record of insulation resistance of a mill motor.

Motor insulation resistance test procedure.

For a three phase motor insulation resistance is usually measured between each motor winding or phase and between each motor phase and motor frame earth using an insulation tester or megger. During the measurement and immediately afterwards some of the terminals carry dangerous voltages and must not be touched. Curve b shows same values corrected to 20 c see page 22 giving a definite downward trend toward an unsafe condition. Regular checks of motor insulation resistance the key to prolonging the life of any electrical device is periodic checks maintenance the insulation resistance of stored and active motors should be checked regularly.

At a the insulation resistance decreases due to aging and dust accumulation. In an industrial facility the recommended procedure is to perform periodic tests and record the results so that damaging trends can be detected and corrected to prevent an outage. The insulation resistance tester generally known by its trade name megger is capable of providing critical information regarding the condition of motor insulation. Insulation resistance test insulation resistance failure of an electric motor is one of the first signs that the motor is about to fail.

Hands down without a doubt one of the most universally searched topics on ec m s website from a historical perspective is insulation resistance testing as a result some of our most popular evergreen articles like understanding insulation resistance testing continue to top the charts when it comes to most popular content. Since 80 of electrical maintenance and testing. A if the insulation resistance of a new cleaned or repaired motor that has been stored for some time is less then 10 mω the reason might be that the windings are humid and. Developed early in the 20th century the insulation resistance ir test is the oldest and most widely used test for assessing the quality of insulation.

Curve a shows test values as measured. Figure 3 typical scale on the megger insulation tester. Z 5 date of test r eading in megohms the graph below shows an example of the insulation resistance readings on an electric motor. The measurement of insulation resistance is carried out by means of a megohmmeter high resistance range ohmmeter this is how the test works.

The insulation resistance test is the second test required by the electrical safety testing standards. The international electrical testing association neta specification maintenance testing specifications for electrical power distribution equipment and. For a three phase motor the isolation resistance can usually be measured between each motor winding or phase and between each motor stage and motor frame earth by an insulation tester or megger 4. Loss of insulation resistance of an electric motor is one of the first signs of motor failure.

Historically many field electricians use the somewhat arbitrary standard of 1 megohm per kv. Set the voltage. Ground insulation test of a motor. Also acceptable values of insulation resistance depend upon the equipment you re testing.