460 V motor on 495V

[TDike]

We have a customer running one of our 460V 3-ph 60hz systems on 495V. It's within the +/- 10% tolerance of the motor, but this voltage is sustained and not intermittent. The motor is overheating and tripping the overload. Short of building a custom motor for this application, what can the customer do to reduce his voltage?

 

[edison123]

How is he getting 495 V which is non-standard ?

Muthu

http://www.edison.co.in

 

[TDike]

That is the incoming power to our machine. It measures 495, 493, and 494 on each leg. Can the power company retap the transformer to lower the voltage?

 

[electricpete]

It's within the +/- 10% tolerance of the motor, but this voltage is sustained and not intermittent.

Per NEMA MG1, motors should be suitable for sustained operation within +/- 10%

The motor is overheating and tripping the overload. Short of building a custom motor for this application, what can the customer do to reduce his voltage?

That could be a function of:
1 – the mechanical load
2 – the overload setting
3 – malfunction of overload or high ambient temperature at overload

It is not immediately obvious that high voltage is the problem to address. If anything, high voltage (within reasonable limits) tends to reduce current for motors under load.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[waross]

460V motors run on 480V systems. You are only 3% above the nominal system voltage. Has the motor been checked for overloading? Is the cooling air restricted?
The power company is usually able to drop the voltage in 2_1/2 steps. Failing that if youcan find a couple of 480:12V dry type transformers that are buck boost rated they may be connected in auto-transformer wye configuration to drop the voltage about 12 Volts. I am skeptical that the voltage is causing the heating. How is the no load current?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Seeing sustained 495V is pretty common on 480V systems. Adjust the transformer taps if necessary or if you think that will help. IMHO, 495V should not cause motors to overheat or trip. The actual cause may be elsewhere.



Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

Agree with the others. 495 V is not excessively high for a nominal 480 V system. It could be adjusted via transformer taps, if available, but this will require an short outage.





David Castor

http://www.cvoes.com

 

[edison123]

Here is the no-load current of a 460 V, 60 Hz motor which I tested today @ 50 Hz supply using a variac.

380 V - (Corresponding to 460 V, 60 Hz) - 9.9 Amps

415 V - 12.8 amps

A 9% rise in voltage increased the no-load current by nearly 30%.

I would suggest reducing the voltage at the source.

Muthu

http://www.edison.co.in

 

[electricpete]

At no-load and locked rotor condition, increasing voltage causes increasing current approximately linearly. Yours increases much more than linear... not sure why.... I guess we conclude it is far into saturation or else you applied unbalanced voltage.

Near full load, it should act differently.

Here foe one motor you'll see below 100% voltage, current is decreasing as voltage increases. From 100 – 105% voltage the current is roughly flat. Above 105% it starts increasing.

http://www.motorsanddrives.com/cowern/motorterms12.html

I suppose there is a remote possibility if your way into saturation higher voltage with other factors kicks you over the edge. But in that case I believe the motor is overloaded (or overload set incorrectly) anyway. Properly loaded motor at 10% overvoltage should not trip.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[antigfk]

I agree with everyone saying your 495V is not a likely cause of your motor overload relay tripping. It is quite common for 480V-nominal systems (with 460V motors) to run as high as 500V without causing any motor overload issues. If you have a overload heater relay, you may have problems with high ambient in your control panel housing the overload relay. If solid-state overload relay - ambient temeperature to a point shouldn't affect it. You may just be putting too much load on the motor, or you may have a mechanical issue.

 

[jraef]

To address your initial concerns, the voltage alone is not enough to cause the effect you are seeing.

I think maybe there are other issues at play here. Can you post the make and model of the overload relay, along with heater elements or trip dial settings, motor nameplate data and if possible, measured current? There is more here than meets the eyes, especially of those of us thousands of miles away.




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[edison123]

TDike

How about you post the motor nameplate details and the load currents you're measuring ?

Muthu

http://www.edison.co.in

 

[TDike]

Thanks for the info guys. The motor is a 3/4 HP TEFC motor rated at 208-230/460. Amps are 3.2-3/1.5 with SFA being 4-3.8/1.9 Service factor is 1.5 with a 40C amb continuous rating.

Voltage measured at the site is 495/494/493 with 1.5/1.6/1.7 amps measured. Motor is coupled to a pump moving about 30 GPM and even with the flow control valve turned down the motor still overheats and trips out. The overload is an internal style snap switch similar to those made by Klixon.

 

[waross]

Change out the bad Klixon? Is this pump on a very hot water line?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

The motor is indeed exceeding the rated current, however marginally.

Try the following or a combination there of:

1. Lower tap settings, if possible and see if it helps.( If shut down is a big issue, buy a small transformer just for the motor)
2. Find a way to reduce the load, if possible.
3. Improve cooling.

Failing that up size the motor. Btw, what type of pump is it? Centrifugal pump will see reduced load by restricting the discharge. Positive displacement pump might see an increase.

Rafiq Bulsara

http://www.srengineersct.com

 

[TDike]

Water is 90-100 deg, and pump head is isolated from the motor. This is the second motor, the first one failed. The klixon is non-replaceable on this motor.

 

[edison123]

klixon being an embedded thermister ?

How is no-load (uncoupled/open shaft) current ?

Muthu

http://www.edison.co.in

 

[waross]

Hi edison. Klixon is a trade name for a bimetal disk type overload device.
Is this a good quality motor or a no-name out of China?
The water temperature may be pushing the motor temp up. That takes up a lot of your thermal rise overhead.
If you can come up with a pair of dry type transformers rated 480V:12V you can drop the voltage by about 12 volts. With a motor current of 1.7A the 12 Volt winding must be rated for 1.7A or more.
That works out to 20 VA.
A pair of 50 VA transformers should do the job.
I suspect a mechanical load problem rather than an electrical problem but if you are sure the issue is the voltage,start looking for a pair of transformers.
The transformers should be Buck-Boost rated.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TDike]

The motor is manufactured by Baldor elec in the US, and is a good quality motor that we have been using for many years.

Not getting a lot of help form the technician onsite. They just did a complete remodel of their facility and claim this is the only motor they are having problems with. Of course the other motors may be wound for 480 instead of 460 making them more tolerant of the high voltage.

 

[waross]

I don't believe that Baldor or any other North American manufacturer rates a motor for 480V, 60Hz.
A complete remodel and now having problems with one motor. Stop agonizing and replace it with the next larger size. Tell them that the new motor is rated for 460V or 480V. No other motors having problems is the answer to your voltage problem. There isn't any. The remodeling has apparently lowered the dynamic head and the pump is now overloading.

Bill
--------------------
"Why not the best?"
Jimmy Carter