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

PAGE 1 of 2 - How to check the Windings of a 3-Phase AC motor

The rotor of a squirrel-cage motor
There are several different AC motor types, each one with different operating and mechanical characteristics. The most common type though is the -so called- squirrel cage rotor. It is called squirrel-cage, because it's rotor looks like the exercising wheel found in squirrel or hamster cages.

A typical 3-phase squirrel-cage motor has six connection leads in the electrical connection box for the three coils. If someone works with AC 3-phase motors, then it is important to know how to connect these motors in Star and Delta connection, and how to detect an electrical problem. There are basically 4 problems that the motor windings can suffer from:

• Broken coil (infinite coil resistance)
• Short-circuited coil (less than normal or zero coil resistance)
• Leaking coil to ground (current leaking from one coil to ground/neutral)
• Two or more coils short-circuited with each other (current leaking from one coil to another coil)

• All of the above problems can be detected with a simple Ohm-meter. First of all, you need to understand how the coils are connected with the six motor leads that exist in the electrical connection box. Here is a photo from a typical electrical connection box:

As you can see, there are indeed six leads arranged in two rows. Since each coil has 2 endings, it is easy to understand that these six leads can be separated in three pairs, and each pair is connected to one coil. It sounds logical to separate these 3 pairs in a vertical pattern, but that's not the way it goes. Instead, the pairs are in cross-pattern like this:

You may now wonder, why the coils are connected in cross-pattern and not vertically... The answer is simple: To easily bridge a permanent connection. A permanent connection is when the motor is connected in either Star or Delta, and this connection is not supposed to changed during the operation of the motor. This is usually done if the motor is small (smaller than 3.5 KWatts) or if the motor is driven by an inverter or some kind of electronic driver. A permanent connection is accomplished with 2 or 3 metallic bridges.

Suppose for example that the motor is connected in Star. In a star connection, each one of the 3 phases (R-S-T) is connected at one end of each coil. The other ends of the coils are connected together in a common point. A star connection can be easily accomplished simply by bridging one of the two horizontal rows in the connection box of the motor. The phases are then connected on the leads of the other horizontal row:

 This is why we call this type of connection "Star" (or Y) A star connection is accomplished simply by bridging one horizontal row in the connection box This is a photo from the connection box of a motor connected in Star

Suppose now that the motor is connected in Delta. In Delta connection, the end of each coil is connected with the start of another coil. The 3 coils are then connected in a circle, creating thus 3 nodes. The 3 phases are then applied on these nodes. A Delta connection is easily accomplished by bridging the 3 columns in the connection box vertically:

 This is why we call this type of connection "Delta" (from the Greek letter Δέλτα) A Delta connection is easily accomplished by bridging the 3 columns in the connection box vertically This is a photo from the connection box of a motor connected in Delta

There is one last thing to know: All 3 coils must have the same resistance. That is of-course easy to understand why, so there is no need to explain any further.

Checking the coils of an unconnected motor
Being a motor unconnected, means that there are no Star or Delta bridges on its leads. This is the most straight-forward case to understand. All you have to do is try to find the coil pairs in the electrical connection box. Let's give numbers to the 6 leads:

Suppose that you start from lead 1 and you want to find it's pair. You connect the first probe of the ohm-meter to lead 1, and then you connect the other probe of the ohm-meter to leads A, B, C, 2, 3 and ground (motor chassis).If the motor has no problem, then you must find infinite resistance between all leads and ground, except from one lead. This one cannot be lead A though, because -as we said- coils are connected in cross pattern.

Finally, you repeat the same process with the first probe on lead 3, and the second probe on leads A, B, C and ground. Now, you know exactly which of these 3 leads pairs with lead 3. If for example you found that lead B pairs with 1 and lead C pairs with 2, then obviously lead A pairs with 3.

Additionally, you can compare the resistances between the pairs. If they are all equal, then you have a very well manufactured motor. A tolerance of 5% (and some times 10%) is generally accepted. If the difference is bigger, this could mean that some wires of the coil are short-circuited and the overall coil length is shorter. This is the most difficult problem to identify in such a motor. Usually, if the motor has short-circuited windings, it won't take long until this specific coil it is totally destroyed.

In short
• You must find the same resistance between 3 pairs of leads ONLY
• These pairs must be in cross-pattern as explained before
• There must be absolutely no connection (infinite resistance) between all other combinations
• There must be absolutely no connection (infinite resistance) between the leads and the ground