If you get lower resistance readings each time, your motor should be taken out of service as soon as possible. Remember heat breaks down motor insulation, so if you are not sure about a motor you have just tested, try testing the motor again while it is still warm from use. We hope this video will clear up how and why to use these meters. When a megohmmeter is used properly you will be able to easily identify insulation damage.
Triple Phase Training made the video above as a free tool for all to use. Otherwise your test may not be reliable or accurate. Depending on what class of insulation you are testing will have an effect on the amount of ohms the meter should read. To test high class insulation you should use a megohmmeter capable of testing into tera ohms. Time and heat break the motor’s insulation down. Using a megohmmeter properly will test any motor insulation. This is an insulation test of motor windings, commonly referred to as megging, to megger, or to meg a motor. We are testing the motor’s insulation, more specifically between the motor windings and the motor casing. If a fraction of current flows, it can be measured by these highly sensitive meters. Motor insulation is rated and built to withstand varying temperatures. Depending on the insulation class of the motor, a good motor could measure well into Tera Ohms or 1,000,000,000,000 ohms! It specifically tests the motor windings insulation. Why do we need to measure that high of resistance? Rather than thinking in terms of measuring resistance, think of this as a testing device. One Mega Ohm is equivalent to 1,000,000 Ohms. Have you ever wondered what a megger test does? What does a megohmmeter do? It is a specific meter designed to measure mega ohms….
Deviations from rated FLA could signify problems with the motor under test.An instructional video on how to megger a motor and the proper use of a megohmmeter.
With the motor running, check the full load amps (FLA) with a suitable meter or preferably a clamp on meter and compare with the motor name plate FLA. Minimum test value for a good electric motor is at least 1MΩ Set the voltage setting of the insulation resistance tester to 500V and check motor windings to earth. Insulation resistance is usually measured between motor windings and earth using an insulation tester or megometer. Insulation resistance failure of an electric motor is one of the first signs that the motor is about to fail. (3) Ohms reading between C and S should give some intermediate value between that for S to R and C to RĪny deviation signifies a possibly bad electric motor or a motor that requires repairs. (2) Ohms reading between C and R should give the lowest reading of resistance
(1) Ohms reading between S and R should give the maximum reading of resistance Measured Value S to R should be = C to S + C to RĪs a rule to single phase motors, the following applies: Since there are three terminals – S, C, R –in a single phase motor, measure winding resistance:Ĭ to S, C to R and S to R.
Check that the correct voltage is applied to the motor.Ĭheck the motor winding resistance or ohms reading with a multimeter. Any value greater 0.5 ohms indicate trouble with the motor.įor single phase motors, the expected voltage is about 230V or 208V depending whether you are using the UK or America voltage system. A good motor should read less than 0.5 ohms. With a multimeter, measure the resistance between motor frame (body) and earth. (3) As with all testing and inspections, the motor name plate provides valuable information that will help to ascertain the true health of the motor. If shaft rotation is free and smooth, bearing is possibly in good condition, otherwise consider replacing. Look out for smooth and free shaft rotation.
(2) Manually rotate motor shaft to examine bearing condition. Check for burnt, damage to body or cooling fan or shaft. For the single phase motor, do the following: