How can we test the three phase motor by multimeter on continuity and resistance test. What should be the value of the resistance in all three winding.
Should not be more than 0.5 ohms, greater than that results in insulation failure and would cause electric shock.
The value would be the sum of the resistance of individual windings if tested while connected.
But the best practice is to break the delta and star connections and read as between start and finish end of each winding. You don’t expect a standard value but certainly not zero. The values are supposed to be uniform for the 3 phases. The values depend on the size of the motor.
As for the insulation resistance test, a value of at least 1Mohm between phases and above 50Mohm between a phase and earth is acceptable. The value of Resistance of the windings depends on the size and the material used. The resistance of the windings should be balance. L1 and L2 = L2 and L3 = L3 and L1. The megohm readings should be at least 5meg ohms…L1 to ground L2 to ground L3 to ground. The shaft rotor should turn very smoothly at no load. But should be balance and equal in All three phases
There are always certain measurements that could be made when testing a 3-phase motor using a multimeter which include:
- Continuity tests,
- Resistance measurements and
- Checking for short circuits
as explained below:
Here is the step-by-step procedure to Test a 3-Phase Motor:
Steps to Test a 3-Phase Motor
Step-1: Preparation
Step-2: Continuity Test
Step-3: Resistance Test
Step-4: Short Circuit Test
Step-5: Ground Test
Step-1: Preparation
First, make sure the operation of the motor is switched off, and that there is no connection in any way to a power supply to avoid a chance of electric shock to oneself or damage to the multimeter.
It is important for you to read up on the terminal leads of a motor among which are U1, U2, V1, V2 W1 and W2.
Step-2: Continuity Test
Set your multimeter to the continuity setting.
Continuity check between pairs of terminals should be done:
- Test between U1 & U2
- Test between V1 & V2
- Test between W1 & W2
If the sound produced is a continuous beep or the low resistance reading then the coils are fine. If there is no continuity, the motor winding may be spoilt.
Step-3: Resistance Test
Change the multimeter setting to measure resistance. Touch the two probes together on the multimeter and turn the selector dial to the ‘resistance’ setting.
Take a measurement of the resistance between each of the terminals as shown below:
- U1 to U2
- V1 to V2
- W1 to W2
The readings on the resistance should not be very much off but the values should be almost the same in all coils. It is considered that significant differences can refer to imbalance or to the presence of a fault in the winding.
Step-4: Short Circuit Test
Again with the multimeter still set to Ohms, check shorts between the different coils:
Test between
- U1 & V1,
- U1 & W1,
- V1 & W1.
These pairs should in fact be completely discrete; there should be absolutely no overlap. If continuity exists, it is possible there is a Short between the coils and may be an indication the motor has a problem.
Step-5: Ground Test
Connect one probe of the multimeter to a terminal (for example, U1) and the other one should be connected to the motor casing (earth). Do the same for V1 and W1.
Absence of any grounding issues can be inferred from a reading of infinite resistance. A low resistance reading means that a winding can be shorted to ground usually a very serious fault.
With the outlined step-by-step procedures, it is possible to test a 3-phase motor for various problems through the use of a multimeter.
If any test is abnormal, then some further evaluation may be needed or the help of a professional.