Testing 575V Motor On 480V System 1

[Teflon]

Can anyone point me in the direction of a formula or table calling out derating factors for running a 575V motor on a 480V system (both 60Hz)? I need to test run a machine before it ships, and I'd like some idea of the torque loss due to the lesser voltage.

Thanks in advance,

Tef

 

[busbar]

Applying a pair of 480—120-volt 1ø transformers in an open-delta autotransformer configuration can provide 600V 3ø 3-wire for the equipment under test. Each 1ø transformer should be sized at 44% of the 600V-load kVA.

 

[jbartos]

Suggestion: A resistor in each line in series with motor could be found less expensive solution and accomplish about the same results as the transformers. The torque T ~ |E|**2.

 

[cbarn24050]

hi, you should not run a loaded motor under volt, it will burn out very quickly if it cannot reach correct speed.

 

[DanDel]

jbartos, are you saying that series resistors will increase the voltage to the motor?

 

[electricpete]

The current for given load will go by approx 575/480 ~ 1.19 (assuming motor is near full load where contribution of magnetizing current is negligible).

The heating from core losses will decrease, but let's neglect that (conservative) for simplicity.

To maintain thermal performance, you would need to derate to 480/575 ~ 83% of full load to keep current <= full load current.

As was mentioned by cbarn torque is also a factor. It will decrease ~ V^2 ~ 70%. That raises a question of whether the motor will stall during starting, and whether excessive heating will occur during starting.

I think that for a general purpose NEMA design B motor (breaqkdown torque 175%-250% of rated) driving a pump/fan load, there will be no stall during starting. However, the heating that occurs during the start will be increased and needs to be evaluated considering the load inertia, torque-speed characteristics, and starting method.

There has been a lot of discussion on starting heating on this forum. Here is something that I wrote on the subject.

http://www.geocities.com/pschimpf/RotorHeatingSymbols.htm

 

[electricpete]

The punchline of that document is that the total rotor heating during starting of a motor is the final kinetic energy times the &quot;weighted average&quot; of
(Telec/(Telec-Tmech)) with 2s being the weighting factor, and s=1..s_operating being the averaging interval.

Total stator heating is roughly proportional to total rotor heating. Proportionality constant is R1/R2 (all quantities referred to stator side).

Sorry for going off on a tangent.

 

[jbartos]

Clarification: It appears that the original posting intent is to test the 575V motor on available 480V power supply. If this is not the case, then a transformer will be needed to obtain the 575V motor rated terminal voltage for some tests, at least.

 

[Teflon]

Thanks for the information, all. Mechanical tells me we have a 2.0 SF on the motor application.

To be safe, I'll have the motor mechanically decoupled. I just need to spin them up to check operation.

Once again, this is simply a quick test. Commissioning will be on the specified 575V system.