3 phase motors

Hi to all,
Most our motors can be wired for 3 phase high voltage or low voltage. Most our motors are ran at 480v but some are ran at around 230v. I have noticed that motors say 208, 230 for low voltage and 480 for high. I never relay thought about it much, but does the 208 3 phase wire the same as the 230 v low voltage? I would guess if a machine was set up for 230v 3 phase and you tried to run 208 v 3 phase it could burn up wires because of the larger wires needed for the 208v. I think most places do not use 208 3 phase very much and most use 480 3 phase. Thank you for any input on this. I am just making sure I understand the differences.

Sincerely:
Maintenance Man

it'll be fine its fairly normal to run 208 with 230 motors

You have to read ALL the information on the nameplate.
If there are several voltages, then there are also several currents, and possible several power ratings. And some of the data on the nameplate may be for 50Hz, and other for 60Hz.

Some demanding applications may require a boost in voltage when a 230v motor is ran on 208. Most motors seem to get along just fine. Equipment such as CNC etc may have more demanding requirements and mfg instructions should be followed.

Typical Tolerance is +/- 10% of data plate voltage. After that the motor may be damaged to some degree and have a shortened life span.

Strictly speaking, a 230V motor running on 208V is being starved for voltage to a small degree. You would expect the maximum output capacity of a motor running that way to be reduced slightly also.

All of that is true but, if the motor has a little extra thermal capacity built into it, you can do it and all will be well.

And, yes, you do have to follow the nameplate data for the actual voltage you are using.

http://www3.sea.siemens.com/step/default.html

You can study basic ac motors theory here and a lot more!

Aother useful manual is here

http://literature.rockwellautomation.com/idc/groups/literature/documents/at/drive-at001_-en-p.pdf

DickDV said:

Strictly speaking, a 230V motor running on 208V is being starved for voltage to a small degree. You would expect the maximum output capacity of a motor running that way to be reduced slightly also.

All of that is true but, if the motor has a little extra thermal capacity built into it, you can do it and all will be well.

And, yes, you do have to follow the nameplate data for the actual voltage you are using.

Click to expand...

Now Dick.... NORMALLY I revere your answers as pure gospel... but THIS one is causing me consternation. It was MY understanding that as you LOWER the input voltage, the AMPS go up (because of the way a squirrel cage induction motors function) to COMPENSATE. The result is that the power, and the horsepower, and therefore the CAPACITY of the motor remains the same. There is no "starving" for voltage, or reduction of capacity.

I see you did touch on the "extra thermal capacity". This would be to COMPENSATE for the increased amps drawn by the motor to maintain it's rated horsepower when the voltage is lowered to 208vac.

The nameplate will ALSO reflect the FLA rating for 208vac as higher than the FLA for 230vac. If the manufacturer puts this higher amp rating on the nameplate, specifically for 208vac, it only stands to reason that the motor has the wire size and heat dissipating capacity to tolerate these extra amps and lower volts.....

Stationmaster

Stationmaster, I agree with everything you posted so I believe our understandings match.

When I said that the capacity of the motor might go down a bit when "starved" for voltage, exactly what you described happens.

The reduction in capacity is just as you mention---in the thermal capacity. For example, take a motor that is designed for 230V with a service factor of 1.0 (this is often the case with IEC motors), and run it on 208V. If you load the motor to the full nameplate hp or kw, the amps will go up to compensate and the heat generated in the motor will also go up due to I^2R losses. The motor will also slip a little more. Since this motor has no excess thermal capacity at 230V, it will run into overtemp problems being fully loaded at 208V. So, you have to back off a little on the load at 208V to keep the motor within its design temp.

That's why I said the output capacity would be reduced slightly. If the motor had a service factor over 1.0, then it has excess thermal capacity and the motor could be fully loaded and only the available service factor margin would be reduced a bit.

I think we are thinking the same on this.

As to "revering" my opinions on here, I would urge caution! My intentions are good but, even at 64 years old and 21 years in this field, I still am learning from all of you and have to "adjust" my understanding of things more often than I care to admit. We have the best possible arrangement on here exchanging information and adding to each others' experiences. Let's keep it that way and mix a little fun with it from time to time too!

Thanks for the nice compliment.