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 correspond can be separated in three pairs, and each pair is connected to one coil. It seems logical to separate these 3 pairs in a vertical pattern, but that is not correct. 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 this: To easily bridge a permanent connection. A permanent connection is when the motor is connected in Star or Delta, and this connection cannot be changed. This is usually done if the motor is small (smaller than 3.5 KWatts) or the motor is driven by an inverter or some kind of electronic driver. A permanent connection is usually 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.
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
@Ian Ellis there are 6 connectors on a 3-phase squirrel cage motor. You can test them with multimeter, ohm-meter or battery by following the instruction if the first page of this theory, at the bottom (Checking the coils of an unconnected motor)
How would you go about testing the start & end of each motor winding prior to powering up the motor, either by the use of a multimeter or a battery and a volt meter. I understand that one could power up the motor and check the current being pulled by each phase, if there is a serios unbalance on a single winding, then turn that winding around, BUT I would like clarity on how to sort out the windings start and end before power up.
@George excellent! I am working on a simple coil checker tool (that is why i uploaded this theory after all). I will check this link and see if i can add it as a function. ;) thanks.
NEVER WORK ON AC system by yourself, ALWAYS have another person there as the SAFETY person. To switch off the power, and/or save your butt/ life if anything goes seriously wrong
@Paul - there are two methods for checking phase of windings mentioned on cr4.globalspec.com/thread/71498
One uses a push button DC voltage and a galvanometer .
The other uses live AC voltage on 1 winding against the other 2 windings connected in series to see if there is a net 0V AC or double the test winding Voltage, BUT beware YOU ARE WORKING WITH LIVE AC POWER. So make sure that you have tested for earth faults. [A multimeter is not really the greatest here a megger would be better] AND DO NOT TOUCH ANYTHING WHEN LIVE - use INSULATED PROBES
@Paul :D :D i'll tell you a little secret. The motor that is the photos, is a repaired motor that had a burned coil. The (not so good) technician that repaired the coils connected one of them the opposite direction. I tried hard to find a way to measure that problem, but had no luck with simple instruments. You connect it in the supply and if it makes noise and does not rotate, or rotate with no torque, then change connection...
At 5 January 2012, 11:03:36 user Paul wrote: [reply @ Paul]
Just wonderin...
Some time ago I also had to figure out how to connect a 1.5KW three phase motor but on this motor there was not such a nice board for connecting the wires. It basically had 6 loose wires dangling from it.
A big problem would be if one of the phases is connected backwards relative to the other 2.
How do you check for that?
In the end I just (sort of) guessed, but I took some extra precautions to prevent destroying a perfectly good motor.
I guessed right but didn't feel happy about it.
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