How To Test and Check Single phase Electric Motors
Thursday - 19/03/2020 14:09
There are several types of single phase motors. What is however common to them all is that they have a Start Winding, a Run Winding, and a Common connection between them as shown below:
Testing of single phase motors is pretty easy if certain basic steps are followed. The objective of any AC motor test is to determine the health status of the motor. The basic steps in ascertaining the health of any motor are given below:
(a) General Inspections
(b) Earth Continuity and Resistance Test
(c) Power Supply Test
(d) AC Motor Winding Resistance Test
(e) Insulation Resistance Test
(f) Running Amps Test
For the single phase motor, do the following:
(1) Check the appearance of the motor. Check for burnt, damage to body or cooling fan or shaft.
(2) Manually rotate motor shaft to examine bearing condition. Look out for smooth and free shaft rotation. If shaft rotation is free and smooth, bearing is possibly in good condition, otherwise consider replacing.
(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. Examine the name plate thoroughly.
Earth Continuity and Resistance Test
With a multimeter, measure the resistance between motor frame (body) and earth. A good motor should read less than 0.5 ohms. Any value greater 0.5 ohms indicate trouble with the motor.
Power Supply Test
For single phase motors, the expected voltage is about 230V or 208V depending whether you are using the UK or America voltage system. Check that the correct voltage is applied to the motor.
AC Motor Winding Resistance Test
Check the motor winding resistance or ohms reading with a multimeter. Since there are three terminals – S, C, R –in a single phase motor, measure winding resistance:
C to S, C to R and S to R. Measured Value S to R should be = C to S + C to R
As a rule to single phase motors, the following applies:
(1) Ohms reading between S and R should give the maximum reading of resistance
(2) Ohms reading between C and R should give the lowest reading of resistance
(3) Ohms reading between C and S should give some intermediate value between that for S to R and C to R
Any deviation signifies a possibly bad electric motor or a motor that requires repairs.
Insulation Resistance Test
Insulation resistance failure of an electric motor is one of the first signs that the motor is about to fail. Insulation resistance is usually measured between motor windings and earth using an insulation tester or megometer. Set the voltage setting of the insulation resistance tester to 500V and check motor windings to earth. Check C to E, S to E, R to E. Minimum test value for a good electric motor is at least 1MΩ
Running Amps Test
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. Deviations from rated FLA could signify problems with the motor under test.