3 phase motor interesting ohm readings

[Mayfran]

Customer returned a burnt up motor. 460VAC 1HP. Nothing special about the motor or application other than the load does shift in a cyclical pattern. We are driving an offset cam.

Initial inspection looked good. Wires 1,4,7 are red 2,5,8 are black and 3,6,9 are white. It is a dual voltage motor and was wired in WYE configuration for the high voltage. 4 to 7, 5 to 8, 6 to 9. with 1,2,3 connected to the line

Ohm readings:
7 to 8 = 6.8 ohm
7 to 9 = 6.8 ohm
8 to 9 = 6.8 ohm
1 to 4 = 1Meg
2 to 5 = 3.4 ohm
3 to 6 = 1Meg
1 to 2 = 1Meg
2 to 3 = 1Meg
3 to 1 = 1Meg

I checked insulation with a megger and on 1,2,3 all readings instantly went to >1G ohm. 7,8, and 9 readings seemed to 'charge up' to about 22M ohm.

The connections at 4,7 and 5,8 and 6,9 were all good connections with ring stakons and screws. Connections were taped very well.

Any ideas on why 1 to 4 and 3 to 6 would read so high? I would think that if those went, the 9 to 7 connection would be susppect also.

As an after thought I measured 1 to 9 and 1 to 8 the readings were at about 225K ohms. I reversed the leads and the readings went above 1Meg. Again I assume I am charging up the field and by reversing the leads discharging and recharging. I've never had to take some of these reading before because I've never seen these conditions where the two windings would open.

Oh, by the way. This is the second time this customer has returned same usage motor. I was not available to look at the previous failure to compare.

Additional note I fuess the customer's power runs at 70hz, somewhere in Mexico so I can't tell you how good the power is.

thanks for any feedback or ideas.

Jim

 

[Mayfran]

Additional information, after discussion I meggered 4,5,and 6.
4 = 343M ohm to Gnd
5 = >1G ohm
6 = 343M ohm

Interesting so I measured and 4 t0 6 are shorted. So evidently I had a phase to phase short which ble open to the line side causing these readings.

by why an internal phase to phase? I measured from 1 to4,5,6 and measured 2 to 4,5,6 and 3 to 4,5,6 and other than the open circuits between 1-5 and 3-6..

But what if wires were incorrect and I had 1 to 6 and 3 to 5. Would that allow the motor to run for a month or so before generating enough heat/damage to short phase to phase? I would almost expect the motor to act as if it were single phasing. ??

 

[waross]

Your readings confirm that the motor is shot. some leads have shorted together and many have burned clear.
If the initial failure is to ground near the wye point, a lot of current may be developed by auto-transformer action and a lot of damage may be done before enough current flows in the lines to trip the protection.
Why an internal phase to phase? Why not. A lot of strange things can and do happen when motor windings start arcing internally. Probably one phase supply failed and the two leads were welded instead of burning free.


Bill
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"Why not the best?"
Jimmy Carter