Electric Motor Windings Resistance

Insulation resistance failure of an electric motor is one of the first signs that the motor is about to fail.

Electric motor windings resistance.

Values greater than this indicate a potential problem and values significantly greater than this indicate the winding has failed opened. This magnetic field causes a metal bar inside the coil to rotate and power the motor. Often it is required to test the winding of the 3 phase windings with the aid of a multimeter or ohmmeter to determine whether the motor is still good or burnt or shorted. Set the voltage setting of the insulation resistance tester to 500v and check motor windings to earth.

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. Winding dc resistance test follow usace procedure three sets of results total a to b phase b to c phase c to a phase correct values to 40 c using the formula in step 8. During the measurement and immediately afterwards some of the terminals carry dangerous voltages and must not be touched. Record initial and corrected values on motor testing record sheet.

A motor with high resistance will not run or not run with speed control as is the case when a 3 phase motor winding opens while running. 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. Set the voltage setting of the insulation. Each winding leg should have some small resistance reading.

All values obtained from winding dc resistance test should be below 1 megohm and similar in value. Dc voltage of 500 or 1000 v is applied between the windings and the ground of the motor. 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. The meter should not show an open or a short when a winding leg is measured.

As with transformers a motor or generator is broken down into two main components. An electrical induction motor contains windings or wire wrapped around a cylindrical shaft. Studies conducted by ieee and the electric power research institute epri on electric rotating machinery failures show that 48 of motor failures are due to electrical failures. When an electric current flows though the wire a magnetic field is induced inside the coil of wire.

The measurement of insulation resistance is carried out by means of a megohmmeter high resistance range ohmmeter this is how the test works. Consult the motor schematic or a diagram of the motor s windings and use the meter to measure the resistance of the windings. The most common failure mode of a 3 phase ac motor is burnt winding or shorted winding leading to the damage of the motor.