Delta connected motor failing surge test ????

[GaPwr]

I read the information Below from DougMSOE color #EF2929]25 Jan 04 0:58 thread[/color] from a previous thread and it looks like what i am seeing on a motor that I have spent 100's of hours testing. This motor is Delta connected, has 180 coils,60 groups, 20 in each phase, so i agree it is not testing the whole circuit i have tested each indvidual coil, each group, and each phase,i have reconfigured to a WYE and tested that way too. now what i am seeing is when I surge(call 1-2,2-3,3-1)phase 1-2 on 2-3 and ground 3-1 it shows good when I surge 1-2 and 3-1 ground 2-3 it shows bad. and when i surge 2-3 and 3-1 ground 1-2 it shows bad. now wht i am seeing is a wiggle with one surge tester Baker ST115 and basicly the same thing with my Baker D12000 but just spread out more. is this a normal attribute I will find in a delta connected motor or is there something not connected correctly also i have tested two other DELTA motors got the same surge results. but my thoughts were were that we wound those motors so we could have done it wrong can any one explain the relationship between the surge test and a delta connected motor.
also i have ran one of these motors and the RPM is below slip and synchronous. could this cause it ???????????????????????????????????

25 Jan 04 0:58
DougMSOE
Usually, a surge tester comparies one coil to the other, at least the Baker and PJ electronics equipment does. The surge testers that I have used have a common lead that is the "neutral" for both coils. The other 2 leads go to the other ends of their respective coils. BUT, with a delta winding you need to disconnect all of the coils. Otherwise you will get a cross feed from the coil "not under test". This will destroy the test results.
One more point, if you are doing this test to a machine that has already been wound you are wasting your time as the nature of the iron core will allow the surge to go about 2 turns around the coil before the 'surge' has been attenuated to an extent that the volts /turn is less than the actual line voltage. Take a look at any book on surge attenuation.

 

[electricpete]

I've never gotten into the nitty gritty details of surge testing. I don't know if there are special connection requirements for delta motors.

I do know that there are more likely to be seen ghost anomalies when testing with rotor in place due to remnant flux effects. Your comment about high slip (you didn't say what load) might imply rotor anomaly. Is the rotor in place during the test?

I guess you are judging based on phase comparison. Have you tried comparing one phase against itself with varying voltage?


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(2B)+(2B)' ?

 

[rhatcher]

GaPwr,

I'm having trouble understanding your post. Are you connecting the black ground lead to one of the phases while testing the other phase(s)? If so, this may be your problem. When testing a three phase winding the three red leads connect to the motor phase leads and the single black lead connects to the stator frame. This is different that the test of single coil where two red leads are connected to one side of the coil and the third red lead and the ground lead are connected to the other side of the coil.

 

[GaPwr]

First to Electricpete the surge test was not done with rotor in place and yes I have checked one phase from one side and then from the otherside and i have done this on all the groups and all three phase's and I still am getting same results two test bad one good.

 

[GaPwr]

To Ratcher: no I am connected the black lead to ground and the three red to each phase taking a reading go to the next phase. i am pretty positive @ this point that a delta connected motor just simply has this trait with a aurge tester when connected as such what i was hoping for is some verification i guess from people in our industry that can put my mind at ease about what i am seeing with this delta connected motor i have tested several delta connected motors sense i did the one in question and all hae that trait or wiggle in two legs just want to know if others have seen this?

 

[rhatcher]

Ga Pwr,

There are no special connections required for a delta connected motor. If you are connecting the surge tester correctly, the same for wye or delta, the results that you see are correct. A bad reading indicates a shorted or grounded phase.

Having said that, if you are getting the same bad result with multiple motors and if the same phase is indicated as bad each time then I would suspect a problem with the surge tester leads or the surge tester itself.

Another possibility is if you are testing the motor while it is resting on a steel table or on a concrete floor with steel reinforcement bars (rebar). In this case, the table, or floor, will affect the surge test by changing the apparent inductance of the windings closest to the table, or floor.

 

[GaPwr]

yes we thought the same thing so we lifted the motor and set it on top of wooden blocks to eliminate those influences still had the same result i agree with you that there is no special connections. we also have tested several 1 circuit and 2 circuit WYE's all good clean surge's. and what is the likelihood of two surge tester's being bad? and also the likelihood of three different motors having shorted coils or turns that have just been rewound?

 

[biggeorgetechnical]

electricpete seems to be suggesting testing at lower voltages, gradually increasing the voltage - to see if the separation "comes on" as the voltage is increased. This is a suitable method to use.

This is a different technique than comparing at the full "Surge comparison" test voltage. Baker called this stepping/ramping up technique "pulse to pulse", or - in the old days "nested acquisition" Schleich MTC2 calls the latest modernized variation of this "peak to peak" analysis, and I don't know what PJ or the various other vendors say.

Delta winding can be tricky to understand results, the "bad" winding is usually the pattern shifted to the right, and separated from the other two curves.

In a wye winding, the "bad" or shorted winding is usually the winding shifted to the left.


I can tell you that D12000 and ST115 used the same SCR type surge generator, but ST115 uses analog 2 channel CRT comparison (2 Red, 2 Black) D12000 has a digital storage oscilloscope so the waves can be stored into the memory bank and compared by the above technique. Also it used an "open delta" version of phase selector switch(1 Red hot, 2nd Red return, 3rd Red open) that can contribute to interference patterns as DougMSOE may have been alluding to. Not likely in your case, but to be sure you can use an extra jumper wire to ground the floating leg manually.

If you have a photo of the surge curves you could share that would be real handy.

Finally in answer to your comments about the wiggle on the delta windings. As it sounds like this is steady response (i.e not jittering/jumping about. If jittering/jumping about it could be possibly be SCR commutation effects) based on your description I agree this is most likely not any winding flaw. Subtle differences in the tie ups in the surge ring, iron, even and where the motor leads connect can be seen as separation/interference pattern. Perhaps your Delta config is done a bit different physically than your wyes.

Also, I have seen cases where lifting the "G" lead can cancel some of the wave reflections by open circuiting the capacitive path to ground. Try it and let us know.

Thanks,
George


"When you go looking for problems, you will find them... But they may not be the problems you went looking for!"

 

[rhatcher]

what is the likelihood of two surge testers being bad and also the likelihood of three different motors having shorted turns or coils that have just been rewound?

The likelihood of two different surge testers being bad is slim. The likelihood of three different motors that have just been rewound having shorted turns, shorted coils, or a wrong connection is in the realm of possibility. This is especially true if they were wound by the same person and if they are the same winding design.

One test that may help you to determine if there is a problem in the winding or not is the open stator impedance test. Energize the stator (no rotor) with a low voltage source while being certain that the currrent does not exceed full load amps. Measure the phase currents. They should be balanced. If they are not, rotate the power supply leads and see if the problem follows the power supply or if it stays with a particular motor phase. This eliminates unbalanced voltage as the cause of the unbalanced current. For rotation of the leads, I mean that if you start with power supply phase A connected to motor phase T1, B-T2, C-T3, rotate them to C-T1. A-T2, B-T3. If the unbalanced current stays on motor lead T1, for example, the problem is the winding. If it follows power supply phase A regardless of which motor lead it is connected to, the the current unbalance is casued by an unbalanced power supply voltage.

If the open stator impedance test indicates an unbalance in the winding this is an indication of a winding defect. Of course, the same cautions about being on a steel table or near a reinforced concrete floor still apply.