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Terminal Winding Resistances for a 3-phase Synchronous Motor

03/26/2022 3:51 PM

Hi guys, hope everyone is well. I got 7.6 ohms reading on terminal to terminal between U-V,U-W, W-V terminal. Is this reading quite normal? Or should I be worried?

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#1

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

03/26/2022 6:08 PM

It depends on the size of the motor what the resistance should be, but that's in the ballpark. The important thing is that all three are equal. Any problem would not affect all equally, so it sounds like you're good.

https://www.electricalengineeringtoolbox.com/2015/12/how-to-test-three-phase-ac-motors.html

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#3
In reply to #1

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

03/27/2022 2:27 PM

Hi Rixter, long time not asking the forum . Thanks for the answer man. Back to work after 2 years almost of quarantine crazy stuff

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#2

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

03/26/2022 11:26 PM

Did you use a DC multimeter to check resistance or an AC resistance meter.

I have an AC checker which can check in circuit resistance at 120Hz and 1KHz.

Even a few shorted turns will not vary the winding resistance very much. You could also measure the winding inductance with an AC LCR meter and look for imbalances.

A more accurate way to measure the resistance is to impress an AC voltage across a winding and measure the current through the coils. You need to use true RMS measuring meters. A close balance between the winding values will show up any problems.

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#4

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

03/28/2022 1:34 AM

We used to meg the windings out, and if too much leakage of current then we would dip them in varnish...

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#5
In reply to #4

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

03/28/2022 3:40 AM

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#9
In reply to #5

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

03/29/2022 8:39 PM

The temperature of the windings plays a role, for operating temperature your readings would be approximately correct, but to get the whole story you need a comparison between cold and warm, you want the warm reading to be about 3 to 4 times the cold reading...then again your reading may not be accurate...

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#10
In reply to #5

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

04/03/2022 1:20 PM

Gee, thanks SolarEagle

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#6

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

03/28/2022 6:51 AM
  • What does the motor manufacturer say?
  • What does the commissioning record say?
  • What do previous maintenance records say?

[Rhetorical questions - NNTR.]

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#11
In reply to #6

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

04/03/2022 1:38 PM

Unluckily they ain't saying a thing. Hi PWSlack

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#7

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

03/28/2022 7:20 AM

<...7.6 ohms...between U-V,U-W, W-V terminal. Is this reading quite normal?...>

Define <...normal...>! For a big motor it's a bit high. For a tiny motor it's a bit low. Size is important and there's nothing in the post to indicate it.

<...worried?...>

One might worry about the lack of confidence in talking directly to the motor manufacturer. That's what its serial number and the telephone number are all about.

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#8

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

03/28/2022 4:25 PM

I'd "worry" about a 3 phase motor with perfectly blanced resistances between terminals in two distinct cases. Resistance infinite or zero. S.M.

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#12

Re: Terminal Winding Resistances for a 3-phase Synchronous Motor

04/06/2022 5:47 PM

Is the motor disassembled?f so,the stator can be tested by applying a low voltage 3 phase to the windings.A ball bearing placed in the center of the stator will roll around on the inside of the stator.If there is a bad spot,it will stop,or drop.

A hack saw blade can also indicate an open winding.sweep it along the inside of the stator with 1/2 voltage applied.An open will feel different.

There is also a tool called a Growler,that can find a bad winding.It is a version of the hack saw method,but can be performed with power off.

A scope set up properly can detect shorted or open windings also.

Use what you have.

http://www.tutorsglobe.com/homework-help/electrical-engineering/growler-test-71596.aspx

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gutmonarch (3); HiTekRedNek (1); PWSlack (2); Rixter (1); SimpleMind (1); SolarEagle (3); Stef (1)

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