Motor Winding Resistance Unbalance Test

They were on two different subjects.
(1) Regarding unbalance in winding resistance - If the resistance that if the unbalance in phase resistance is more than 2% the motor has to be rewound.
However, this answer was given assuming the motor is wire wound. However, if it is a motor with formed coils, there exists a possibility that the motor could be partially repaired by detecting the defective coil and repairing/replacing the defective coil.

(2) The other part of the discussion was about the impedance of the winding. To calculate the actual impedance of the winding of an operating is very complicated and is not necessary in this case where we just trying to establish balance/unbalance in the motor winding.
In a small repair shop or in the maintenance department of a factory you do not have any elaborate equipment a laboratory may have for these measurements. Therefore, this is a simple arrangement to check the imbalance in the winding wherein you inject sufficiently high voltage to detect inter-turn short which may not be detected by the micro-ohm-meter as these measurements are done at a voltage as low as 5 to 10 V. Since this test rig requires simple instruments which are generally all repair shops and factory maintenance departments it easily possible to conduct this test by them.

To assess the healthiness of a wire wound motor winding a simple test can be carried out in a repair shop where lack of proper testing instruments is normal in small shops. What you need is a three phase variac 440V 50 Hz or 460V 60 Hz for this test. Connect the variac to the stator with rotor out. Increase the voltage gradually till the winding draws current equal to the rated current. Record current in each phase, and record voltage across each phase as well. To take care of unbalance in voltages, if there is any one may calculate average voltage. The static impedance is given by the simple formula Vph/Iph. However, at measuring stage only you will know if there is any defect if there is considerable difference in currents drawn.

For this test you need a 30-50 Amp variac. A 50 amp variac covers up to 50 HP motors and a tong tester which can measure both current and voltage. In addition if you have micro ohmmeter you can assess most of the motor faults. I have not mentioned megger because it is almost a part of the body of every industrial electrician. Elaborate dynamic impedance / inductance are not required in practice.

Leave your comment (Registered user only)


10/3/2019 6:34 PM
Excellent article.
Regarding allowable tolerance of 2% just check CSA C392 standard: "Testing of three-phase squirrel cage induction motors during refurbishment", Update No. 1 (July 2011), Update No. 2 (2012)
3/21/2019 5:16 AM
What electrical standard does it mentioned 2% is allowable tolerance for imbalanced phase to phase readings of induction motor?
12/8/2017 10:39 PM
If motor unbalance resistance its cause motor winding is damaging
4/6/2017 7:43 PM
If motor given reading test.  Unbalance resistance phase to phase. And phase to ground. +2000 ,meg ohm. Can i start. Motor
1/15/2017 8:06 AM