How to check the Windings of a 3-Phase AC motor with an Ohmmeter
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)
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:
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| A star connection is accomplished simply by bridging one horizontal row in the connection box |
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| This is a photo from the connection box of a motor connected in Star |
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| This is why we call this type of connection "Delta" (from the Greek letter ΔÎλτα) |
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| A Delta connection is easily accomplished by bridging the 3 columns in the connection box vertically |
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| This is a photo from the connection box of a motor connected in Delta |
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.
Then, you repeat the same process but this time you connect the first lead of the ohm-meter to lead 2, and the second lead to leads A, B, C, 3 and ground. Notice that you do not need again between 2 and 1 since you've measured already before. Again, you must find infinite resistance between all leads and ground, except from one lead. This lead cannot be the one opposite to 2 (which is the lead B) and of course cannot be the lead that pairs with lead 1 (found from the previous measurement).
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
Then, you repeat the same process but this time you connect the first lead of the ohm-meter to lead 2, and the second lead to leads A, B, C, 3 and ground. Notice that you do not need again between 2 and 1 since you've measured already before. Again, you must find infinite resistance between all leads and ground, except from one lead. This lead cannot be the one opposite to 2 (which is the lead B) and of course cannot be the lead that pairs with lead 1 (found from the previous measurement).
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
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:
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
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
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.
How to Identify Three Phase Windings
A procedure has been adopted by the IEC for identifying three phase winding connections. Letters and numbers are used as follows. The high voltage (HV) terminals have upper-case letters e.g. A-B-C, R-Y-B, U-V-W, L1-L2-L3 and the low voltage (LV) terminals have lower-case letters e.g. a-b-c, r-y-b, u-v-w, l1-l2-l3. Each winding has a start numbered 1 and a finish numbered 2. The choice of letters and numbers tends to be a national preference, see the table below for a rule of thumb guide:
