Testing 575V Motor On 480V System
Testing 575V Motor On 480V System
(OP)
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
Thanks in advance,
Tef





RE: Testing 575V Motor On 480V System
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.
RE: Testing 575V Motor On 480V System
RE: Testing 575V Motor On 480V System
RE: Testing 575V Motor On 480V System
RE: Testing 575V Motor On 480V System
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
RE: Testing 575V Motor On 480V System
(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.
RE: Testing 575V Motor On 480V System
RE: Testing 575V Motor On 480V System
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.