Motor Voltage

[djrupprecht]

Hi,

I am trying to determine what would be the better route. A 700HP motor at 480VAC or a 700HP motor at 4160VAC. What types of things are affected by going from one to the other? I think gather from talking to other people that a higher voltage would require less current to be supplied to the motor? Is this correct? Therefore all the cables etc could be smaller? I am not 100 percent on how motor voltage will affect everything. I know the basic motors equations and can see from those why, but I am looking for a physical answer. Something that makes sense. Any help would be great.

Thanks

 

[LarryFine]

DJ, welcome to the forum!

How do you define "better"? What's more important here, engineering or cost?

As a generality, the higher voltage can be "more efficient," electrically speaking, but, while smaller wire can be used, higher-voltage equipment can cost more.

I believe the best bet would be to price it both ways.

 

[skeshesh]

Another consideration, specially with practices/regulations for worker safety increasing, is cost & saftey of maintanence/future upgrades as the higher voltage terminations/equips/etc. are generally more costly maintain. A bit more information on the system/purpose would be nice.

 

[Jraef]

It's a judgment call. I have seen plants where they implement a standard that any motor 250HP and up must be MV. I have also seen plants where the limit is 1000HP.

I have hooked up a couple of 900HP 460V motors, it was no picnic. I have also hooked up a 250HP 4160V motor, that was no picnic either. 4160V is "Medium Voltage", so all of the gear, cable etc., besides being a lot more expensive, is much more difficult to work with. The wire, although "smaller" in terms of the copper diameter, has very thick and stiff multi-layered insulation and (usually) a metal shield; terminations must have accommodations for that shielding and boots to prevent flashover; bending radii are much larger etc. etc. You must also have special training and certification (in many parts of the country) to work on MV systems, so long term service issues must be taken into consideration. On one project I did 10 years ago, the Consulting Engineer failed to consider any of these issues and the Water District had no electricians certified to work with MV systems. So the District must now pay to have a service contractor on retainer 24/7.

But of course, if you are the contractor angling for that contract, that's a different story...

 

[djrupprecht]

This all helps thanks.

The application is refridge compressor, VFD driven. The project lead was wondering if you could get a 700HP VFD at 480V. He was thinking this would be medium voltage, but I guess you can do it at 480V also. Another Engineer I had talked to said that the cables and the motor would have to be larger, and seemed to think that 4160V would be the way to go. Although, I see there are more things to consider here.

 

[masterinbama]

I would stick with 4160 if you have it available on site. FLC on a 700HP 480 volt motor is in the neighborhood of 840 amps. 4160 will be roughly 100 amps.

 

[Jraef]

VFD makes it a horse of a different color, you should mention things like that up front.

A 4160V VFD for 700HP will be the same as for 1500-2000HP because they can only make them so small, so the cost will be very high compared to a 480V VFD. A 480V VFD that size would likely cost about $75K, a 4160V VFD will be more like $200K, plus the associated switchgear.

 

[Besoeker]

VFD makes it a horse of a different color, you should mention things like that up front.

A 4160V VFD for 700HP will be the same as for 1500-2000HP because they can only make them so small, so the cost will be very high compared to a 480V VFD. A 480V VFD that size would likely cost about $75K, a 4160V VFD will be more like $200K, plus the associated switchgear.

I agree.
For a 550 kW VFD I'd go LV rather than MV.
More practical and cost effective. For all new.
For a retrofit with an existing MV motor and MV supply the MV drive might be the better option.

 

[djrupprecht]

Thanks all. Very informative

 

[weressl]

VFD makes it a horse of a different color, you should mention things like that up front.

A 4160V VFD for 700HP will be the same as for 1500-2000HP because they can only make them so small, so the cost will be very high compared to a 480V VFD. A 480V VFD that size would likely cost about $75K, a 4160V VFD will be more like $200K, plus the associated switchgear.

Power availability at the different voltages, distance from source to drive, from drive to motor will all impact the economics. At the end it is all about the bottom line. Gather all info, cost each item, create a balance sheet and it will give you the answer.

700HP 480V motor is larger than NEMA MG-1 scope, so it is a non-standard moptor.