Electric motor, 3-phase question

Hendrik · Dec 17, 2014

Question for sparkies.
I have a 25 year old 3-phase asynchronous motor of 0.37 kW, driving an overhead door.

The motor is set up for star mode (not triangle).
Between junction point (L1 and L2) I measure 42 Ohm, and the same value between (L1 and L3) and between (L2 and L3).
Between L1 and the star point I measure 21 Ohm, and the same value for L2 and L3.
So all three coils seem to be nicely identical.

I have a new motor with identical specs, although of different make. For this new motor the resistance values are 54 Ohm and 27 Ohm respectively.
The new motor was bought because the old one trips the safety relay in the control box of the overhead door drive at irregular intervals.

Is the old motor bad because the Ohm values are higher? Or is the problem (tripping) somewhere else? I'm really puzzled here.

Note: I'm writing from The Netherlands where we have 230/400 V 3-phase at 50 Hz.

Any and all insight you can provide will be much appreciated. Thanks, Hendrik

JMOR · Dec 17, 2014

'Irregular intervals' sounds like an intermittent problem somewhere, maybe inside motor or outside. Relatively small differences in winding resistance may be accounted for by better/more efficient design of the newer motor or simply different manufacturers. My 2¢ worth.

Chris(WA) · Dec 17, 2014

Have you an amprobe so you can test the actual current draw on each leg? It should be the same on each of the three phases.
If the draw is equal on each phase you probably have a problem other than the motor. Those ohm readings make me think the problem may lie elsewhere.
I would be suspicious of the reversing contactor (not pulling in and making good contact on all three legs every time) or a loose wire connection somewhere.
Could even be the overload is failing and falsely tripping. Seen that before too. An amprobe will tell you what the current is doing.

Janicholson · Dec 17, 2014

With good looking Resistance values, it could be an issue with bearings or, as mentioned, the contactor/relay could be intermittently dropping a phase. Jim

buickanddeere · Dec 17, 2014

The new motor should work just fine. Your old and new resistance readings are made with DC current.
When the motors are powered with 50Hz the XL also known as inductive reactance. The new motor's XL will likely be lower than the old motor's XL. Running current at the same shaft torque and rpm will likely be very close old motor vs. new motor.

Hendrik · Dec 18, 2014

Chris, thanks for your reply.
What is the "reversing contactor"?
Note: I agree with you on the failing overload; have seen that too.
I do not have an amp probe; will borrow one if needed.
Thanks, Hendrik

Texasmark1 · Dec 18, 2014

If you don't have true 3 phase but manufactured 3 ph from single phase in a wye configuration you could have problems also.

We put in a new community well and had 240 available but no 208. So for awhile the power co. did what I said and we had continuous problems with our 3 ph pumps. Once we shelled out about $25k they ran 3 ph. to the pump house and problems disappeared.

Mark

Hendrik · Dec 18, 2014

Thanks Mark.
The quasi 3 phase can indeed cause problems. Our supply is a true 3 phase and has this drive has worked flawlessly for many years.

Hendrik · Dec 18, 2014

Thanks to all who responded.

So the old motor could well be fine, despite the different voltage readings. Since I have the new motor, I will swap them, while preparing myself for further trouble shooting.

This thing even has an rpm control as a safety feature (door closing too fast due to broken spring etc.), which may be acting up.

I'll keep you posted on progress!

Electric motor, 3-phase question

Hendrik

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JMOR

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Chris(WA)

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Janicholson

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buickanddeere

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Hendrik

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Texasmark1

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Hendrik

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Hendrik

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