4160V vs. 600V motor
4160V vs. 600V motor
(OP)
I have a client who has a 600V supply and wants run a 500HP motor. They also want to know if it would be worth while to take their 13.8 kV service, step it down to 4160V and run the motor with that. I realize the cable will be smaller due to less current but is there any other benefits? Enough to offset the cost of a transformer?





RE: 4160V vs. 600V motor
With 13.8kV service, voltage profile will be better. Two new transformers will be needed plus 15kV switchgear.
Depends a lot on the capacity of existing 600V service, and acceptability of new-motor starting-voltage drop. If LV>MV transformer is used, suggest 600? to 4160Y—high-resistance grounded arrangement. Another consideration is whether any other 4160V motors are anticipated.
References are Beeman's 1955 Industrial Power Systems Handbook and IEEE Std 141 (red book) chapter 3.
RE: 4160V vs. 600V motor
RE: 4160V vs. 600V motor
In general you would have slightly lower losses in the transformer than you would experience adding 500HP to the 600V transformer, but as busbar mentioned it all has to do with the existing 600V service capacity. For instance if it is so oversized now that adding 500HP to it is possible without other ill effects, then adding that load may actually improve the overall efficiency of the system compared to what it is now.
"Venditori de oleum-vipera non vigere excordis populi"
RE: 4160V vs. 600V motor
I've been trying to figure what current each will draw and subsequently what size of cable will be required.
I calculated (assumed efficiency of 80% and pf of .9):
600V:
I = (500*746)/(1.732*600*.8*.9) = 498 A
4160:
I = (500*746)/(1.732*4160*.8*.9) = 71.9 A
Does that look correct? What size of cable will that require? What I'm getting doesn't make sense.
RE: 4160V vs. 600V motor
RE: 4160V vs. 600V motor
I have another question. The client asked if end rush current would be an issue for the 600 volt motor. I usually work in controls so I don't even know what that means - can someone explain it to me and will it be an issue?
RE: 4160V vs. 600V motor
I would prefer the 4160 V solution. I used 12,470 V dropped to 2300 V on a remote site for a motor in that approximate size range. A similar site was evaluated for 480 V. The compressor station was about a marathon away from the nearest substation. The calculated voltage drop was way more than acceptable.
John
RE: 4160V vs. 600V motor
RE: 4160V vs. 600V motor
For multiple motors.. it makes sense to go with medium voltage. There is no real point in stepping it down to 4160V for a single motor. Either use 13.8kV or 600V.
Also if any speed control/adjustable speed is required such as VFD, 600V is a better bet.
Most of the benefit of MV motor controls vs LV are (or were) in the maintenance cost. LV electromechanical starters becomes a maintenance nightmare because of large currents associcated with LV. But a solid state RV starter or a VFD overcomes all that and can provide adjustable speed. So now-a-days for one or 2 motors I would say LV is ok. You may have to get a liquid cooled VFD or if a solid state RV starter you can cut off electronics once the speed is reached..
RE: 4160V vs. 600V motor