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 How to check the Windings of a 3-Phase AC motor with an Ohmmeter AuthorGiorgos LazaridisJanuary 4, 2012

PAGE 2 of 2 - Checking the coils of a motor connected in Star or Delta

Checking the coils of a motor in permanent Star connection
Let's first see the leads of the motor in permanent Star connection:

From the schematic diagram you can see that if the first probe of the ohmmeter is connected to one lead (for example 1) and the other probe is connected to another (for example 2 or 3), the ohmmeter must show the total resistance of two coils connected in series. The ohmmeter must show double the resistance of each coil (since all coils have the same resistance).

So, here is what you have to measure: With the first probe connected on lead 1, you connect the other probe to leads 2, 3 and ground. If the motor has no problem, then you must find exactly the same resistance between 1-2 and 1-3, and infinite resistance between 1 and ground.

Then, you repeat the same process, but with the first probe connected on lead 2. Then you connected the second probe on lead 3 and on ground. The resistance between 2 and 3 must be exactly the same like before, and the resistance between 2 and ground must be infinite.

You don't need to make any other measurement to verify that the motor is ok. As you see, with this method, you don't need to have physical access in the motor's electrical connection. You can measure the coils from the protection relay in the electrical cabinet instead. This way, you can verify both the motor AND the power cables:

Of course, it is VERY IMPORTANT to ensure that there is absolutely no power in the circuit!!!

In short
• You must find the same resistance between the three leads that power is applied to the motor.
• There must be absolutely no connection (infinite resistance) between the leads and the ground
• The measured resistance must be double the resistance of the coils.

• Checking the coils of a motor in permanent Delta connection
Let's first see the leads of the motor in permanent Delta connection:

From the schematic diagram it is obvious that there are three points that you can measure with the multimeter without disconnecting the ohmmeter, the three nodes 1,2 and 3. Like before, you must find exactly the same resistance between 1-2, 1-3 and 2-3, as well as between 1-ground and 2-ground.

For educational reasons, let's calculate the resistance that must be measured. We assume that all 3 coils have the same resistance RL. Suppose now that we measure between nodes 1 and 2. It is obvious that the coils between the nodes 2-3 and 3-1 are connected in series. The total resistance of these coils is 2RL. Moreover, the coil between the nodes 1-2 is connected in parallel with the previous 2 coils. So, the total resistance which will be measured with the ohmmeter is the result of the RL parallel to 2RL:

RTOTAL = ( RL x 2RL ) / ( RL + 2RL ) = 2RL2 / 3RL => RTOTAL = 2/3 RL

From the above formula we see that if the coil resistance is 20 Ohms, the resistance that the ohmmeter will measure will be 2/3 x 20, or 0.66 x 20 = 13.3 Ohms

Once again, it is obvious that the coils can be measured either directly into the motor's electrical box, or on the motor protection in the electrical cabinet. Just make sure that there is absolutely no power in the cabinet.

In short
• You must find the same resistance between the three leads that power is applied to the motor.
• There must be absolutely no connection (infinite resistance) between the leads and the ground
• The measured resistance must be 2/3 the resistance of the coils.