4160V vs. 600V motor

[RegDunlop]

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?

 

[busbar]

 
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.

 

[aolalde]

Some extra benefits are; keep the original 600 V system capacity and prevent voltage drops during the 500 HP starting.

 

[jraef]

Another possible benefit (especially if future 4160V motors are planned) is that many utilities will offer "bulk rate" power contracts to users who buy it at 13.8kV and provide their own transformer / switchgear equipment. That may not necessarilly be offered or worth it for 1 500HP motor however.

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"

 

[RegDunlop]

Thanks for the responses!

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.

 

[aolalde]

Your calculated currents are fair close to actual motors current. For 71.9 amperes you will need at least 6AWG size wire 5kV-90°C cover. And 750 kCM 600 V-90 °C cover for 498 Amperes. If the distance from the motor to the control is large, the voltage drop shall be considered.

 

[RegDunlop]

Thanks aolalde.

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?

 

[jsummerfield]

The client is speaking of INRUSH current. For across the line starting, assume the inrush current about six times the full-load Amps from the time you push start until the motor and load are up to speed. With a 600 Volt system and 500 hp motor the inrush will be a very big problem unless you are quite near a very stiff power source.

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

 

[aolalde]

As John explained the "inrush" current is the momentary overcurrent that motors demand during the starting process. The RMS value of this current could be 5 to 6.5 times the full load current.

 

[rbulsara]

My two cents..

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..

 

[bigbillnky]

Does the NEC not require motor branch circuit conductors to be calculated at 125% of motor current, and those conductors must be rated to carry that current at 75* Celsius, not at 90* Celsius?