Leads of a three phase induction wye-wye motor

[ePOWEReng]

Hello. Does anybody know how to find the correct 'pairs' of leads for a induction motor that is connected in a double wye configuration with the two wyes completely electrically disconnected. The two 'wye points' are not accessible, only the six phase leads. We are able to use an ohm meter to separate out the two wyes, but now the A phase, B-phase and C-phase needs to be "paired up" between to two wyes to form the right connections, does anybody know of a way to do this????? Thanks for any help you may have.

 

[waross]

vehazle
First the phase rotation meter. The ones that I have used are typically quite small and rugged. There are three coloured leads, and two lights. One old model has two small (1/2" Dia.) windows with a movable target thaat apears in one or the other window. You connect the three leads to the circuit under test and one or the other light illuminates to indicate A-B-C or C-B-A.
Inverted Y, This does not seem compatible with connecting A1-A2, B1-B2, C1-C2, I suspect a misunderstanding or a typo.
Maybe others will have some ideas.
There is another test meter that is often found in company with the phase rotation meter. The two are often used together when starting motors for the first time, when the direction of rotation is critical, or a large number of motors are involved.
This is the motor rotation meter, or indicator. It has three leads that are connected to the motor leads. The motor under test is turned by hand and the indicator shows the correct connections, A-B-C or B-C-A to achieve that rotation.
Voltages. If you can arrange three phase 25 volt testing in the shop, you MAY get better results.
Single phase testing with 25 volts. Imagine a Tee that is labled A1, B1 across the top, and C1 on the bottom. That is the electrical connection you have with a single phase voltage applied across A1-B1.
As you can see, C1 is tied to the center point. Electrically, the C phase winding is 90 deg out of phase with the A-B connection, however, with a three phase rotating machine connected as a single phase transformer, I honestly don't know what to expect.
However your readings seem to indicate that the physical position is such that virtually no voltage is being induced in the "C" winding, and it is reading the voltage at the center point of the A-B pair. However, I don't quite trust the readings.
I would have more confidence in a test with three phase 25 volts in the shop. If you can apply enough voltage in the shop that the machine rotates, I would trust the tests even more. If you can arrange a pony motor to spin the generator in the shop you can probably get a good test.
Let us know how the testing progresses.
yours

 

[petronila]

Hello Vehazle,

I think if you can´t find the problem with all above explanations, and with a surge test, the better way is dissambling the machine and testing the stator winding in a shop, i think the winding could be damage.

Regards

Petronila.

 

[mc5w]

In Electricpete's method, one possible test connection would produce a phase diagram consisting of 2/3 of a hexagon with 2 tails attached to it. The phase to phase voltages would either be 100% or 173% (sqrt3) of the applied 3-phase voltage. Essentially, you measure the 5 phase to phase voltages and use whatever phase diagram makes the most sense.

 

[waross]

After sleeping on it, i realized what was causing the odd voltages on the unconnected winding. You are applying a single phase voltage in the test. All the induced voltages are in phase, and some voltage is being induced in the unconnected winding. Your test voltage is inducing flux in the stator and the rotor. Whatever winding the flux returns through will have a voltage induced in it.
The voltage induced in the unconnected winding is either adding or subtracting from the voltages that would be seen on the unconnected winding if no voltage was induced in it.
Still, the unconnected winding is the one that is identified at each step in the test.

 

[vehazle]

Sorry for letting this drop for so long due to internet availability and presssure to figure out the problem. We finally figured out the reasons for the strange induced voltage readings and were able to identify the correct lead phasing/marking.

As it turns out the test using a single phase AC voltage on wye1 and measuring the induced voltages on wye2 worked as it was intended. The problem was that the spare generator that we found the strange readings was not exactly the same design as the generator in question.

The generator we were having problems with was manufactured within the last few years by another manufacturer than the spare generator which was built about 8 years ago. The newer, problematic generator was connected in a weaved or laced (not sure of the correct terminology) pattern whereas the older, spare generator was built in a lapped (again not sure of the correct terminology).

I will attempt to clarify in this text format.

For the problem generator in the weaved/laced configuration, imagine as I described above as an upright Y and and inverted Y. From left to right at the top you have a2,c1,b2 and along the bottom b1,c2,a1.

For the older, spare generator in the lapped configuration, imagine something like "3E" where from top to bottom on the "3" you have a1,b1,c1 and on the "E" you have c2,b2,a2.

In either case the corresponding phases on each wye are 180deg apart but the directions are opposite from eachother when looking down the axis of the generator, ie a1 is away from you and a2 is towards you. This gives you a North pole and a south pole on opposite sides.

If you care to take the time to draw these two scenarios out and sketch the wye phasing check procedure you will see that the laced/weave configuration is quite symmetrical whereas the lapped configuration is only symmetrical for one combination. This explains why we obtained the strange readings on the spare generator.

The wye phasing check procedure was developed with the manufacturer of the newer generator with the laced/weaved pattern and we did not know at the time that there could be another method for the coil connections so we were suprised by the results from the spare generator in the lapped configuration. The key to figuring out that there were two different constructions was that we have yet another generator at this site that was originally built about 8 years ago by the first manufacturer. We performed the wye phasing check on this generator and obtain the symmetric results that were predicted. This initially really confused us until we found out that this third generator was sent back to the second generator manufacturer for rewinding. During this rewind the second manufacturer changed the coil connections from the lapped to the laced/weave configuration. After some discussions with the engineers at both manufacturers we determined the source of the discrepancy between predicted results and the those obtained on the spare generator.

Ultimately we performed the check on the problem generator and determined that the leads were indeed marked incorrectly and hence were connected incorrectly which were the source of the problem.

I wanted to post a follow-up in case it could help someone else in the future and to thank everyone here for their suggestions and help. I also want to thank Hold6448 and his team for helping to figure this problem out.