Overload or motor failing?

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cmaki

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sheboygan, WI USA
I've got a motor on one of our machines that I think might be failing but I'm not sure. Its a goofy german built thing that is running at 380VAC. Sometimes when it has a load on it the motor will stall and stop. sometimes it wont. Once and a while it will trip the motor overloads but normally it doesn't. One time while it was acting up I was able to get an amp draw of one leg. right before it stalled I got 0 amps. The very next run it shot to 16 amps and tripped the overload but the sound was the same. Is it possible that the overloads are failing without tripping? Or one of the phases in the overload is going bad and stopping the power on that one phase causing the motor to stall?
The machine is a panel saw for cutting long boards. The motor in question is the first small cut and the larger motor cuts the boards fully. The motors are moving so its hard to get a reading from the motor itself.

Or am I totally wrong and the motor is going? That was my first thought until I saw the amp readings.
 
Overloads do not interrupt the flow of current, they just watch it and control some other device that controls it, ie a motor contactor. So it's highly unlikely that the overlad relay is the problem here, in fact it sounds like it is doing its job. If you truly read 0 amps on one phase and the motor makes a funny sound, that sounds as though your contactor is failing, as in a contact is broken inside. Sometimes it makes, sometimes it doesn't. Replace the entire motor starter. Even though the OL relay is apparently still working, the added stress it has been seeing from repeated tripping on single phasing might make it less accurate now, plusnif it is old, it may no longer attach to a new contactor even if it is the same brand. It's easier to just replace them both, especially on something this small.

Do NOT try to just replace the contacts. If this was made in Germany, it likely has Siemens or Moeller controls and even though they might sell replacement contact kits, those IEC contactors are NOT designed to have them replaced. The Germans are laughing all the way to the bank on that issue, because they know Americans will buy the replacement contacts, then the contactor will almost immediately fail mechanically and they get to sell you another one too. In Germany, they NEVER would consider replacing contacts, so the mechanical design basis is for all the parts to wear out at about the same time. That, by the way, is one reason IEC contactors are cheaper than NEMA contactors.
 
cmaki said:
There is a 480 transformer with 380Y coming off one side. I can post a picture of the transformer if you'd like.
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Really common mistake by the way. Those motor will be running very hot and close to overloading all the time. Whomever installed it would have been better off connecting them to 480V without the transformer, it's closer to the correct ratio of voltage and frequency. 380V 50Hz is 7.6 V/Hz, the same thing a 460V 60Hz motor is designed for. It would spin 20% faster, but that's going to happen anyway. By putting in the 380V transformer, the V/Hz ratio is cut to 6.33V/Hz, lower than an acceptable 10% deviation. So the motors put out less torque, which means to do the same work, they will draw more current and over heat. That's likely what caused your contactor to fail, IEC contactors have little to no fudge factor to speak of.

This is assuming, by the way, that these are AC induction motors and there is no VFD involved.
 
cmaki said:
Correct assumption however it is a 380volt motor.
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Ok, so again, 380V 50Hz motor, you are now giving it 380V 60Hz, that's what is wrong. You either have to give it the correct frequency at that voltage, ie with a VFD, or give it the correct V/Hz ratio by getting rid of the transformer, and living with the higher speed.

Unless by some long shot miracle, the motor came from Brazil, where they use 380V 60 Hz.
 
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The 50/60 Hz rating on the transformer does not mean that it will convert 60 Hz to 50 Hz.
The motor nameplate clearly shows 50 Hz, and a voltage range of 380-415 volts. So in addition to using too high a V/F ratio the 380 volts is at the low end of the allowed range, making the current overload even worse.
Did the transformer come with the unit or was it obtained seperately?
 
Look more carefully at that motor nameplate, it clearly says, on the far right side, "440V 60Hz", which is close enough for government work to 460V. So again, the initial mistake was the transformer. Totally unnecessary and in fact detrimental. Lose it.

You likely still have a broken contactor however, assuming your reading of 0 amps on one phase was real. The bad decision on the transformer may have lead to that failure in the long run by running it hot, but when you replace it, toss the transformer too.
 
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