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#3150 08/07/01 10:48 PM
Joined: Jan 2001
Posts: 13
Bgame Offline OP
It's been a while since I last posted and I hope all is well for everyone. I must admit that I feel rather foolish even asking a question like this but I could find nothing in the code or any refererance material that I came across pertaining to my problem so here it goes. The corperation that I work for recently closed down a plant in Canada and sent some of the equipment to the plant that I work in - all motor voltages are 575 Volts and they are running at 480 volts. I have voiced my concern but no one seems to want to be accountable for having them on the floor. Now the plant Manager has told me that I have to look for the "grey area" in the code, but this seems more like a grey area in the laws of physics! I am sure that any arguements that I make will fall on deaf ears, but I would like to know what to expect as noone that I know has ever heard of such a thing. Thanks in advance.

#3151 08/07/01 11:45 PM
I don't believe it challenges the laws of physics.

AC motors should operate well ±10% of nameplate voltage.
517.5 V should be okay. At -18%, you are stretching the lower limit quite a bit.
You can't expected the rated torque, speed, or horsepower output. But you could put the motors on 120 V and I would still expect them to spin.

#3152 08/07/01 11:49 PM
Joined: Jan 2001
Posts: 13
Bgame Offline OP
Oh, they spin fine but I'm kinda worried about the heat generated 'cause doesn't the amperage go up proportionally to the voltage going down?

#3153 08/08/01 12:14 AM
>doesn't the amperage go up proportionally to the voltage going down?
To output the same horsepower, yes. But odds are that the motors are not being asked to perform full rated load.

In other words, a 50 HP motor at 575 V is a 10 HP motor at 120 V.

Simply derate the horsepower by 20-25% and the amps probably won't be a problem. The lower voltage probably is not as efficient.

#3154 08/08/01 03:53 AM
Joined: Oct 2000
Posts: 5,392
while that angle could be pursued , 110-4 would still be violated.

#3155 08/08/01 06:52 AM
Joined: Dec 2000
Posts: 4,294
You might be able to use boost transformers to up the voltage at the motors if they're causing you trouble.

#3156 08/08/01 07:37 AM
Joined: Oct 2000
Posts: 2,749
Please see Table 430-150. Full-Load Current Three-Phase Alternating-Current Motors
 The following values of full-load currents are typical for motors running at speeds usual for belted motors and motors with normal torque characteristics.
Motors built for low speeds (1200 rpm or less) or high torques may require more running current, and multispeed motors will have full-load current varying with speed. In these cases, the nameplate current rating shall be used.
The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges of 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.

Also, always use this table for FLC and refer to 430-6 in the 1999 NEC.

This has been the accepted practice for many years.

Joe Tedesco, NEC Consultant
#3157 08/10/01 08:32 AM
Joined: Aug 2001
Posts: 31
I have a couple of concerns, more so than code answers. I think that all the code concerns were addressed already.

I’ve had this question a few times in technical support (I went from steel mills to flying a desk....I still haven’t got used to it.)

I would verify that the motors do not have 2 sets of connections in the pecker head. A lot of 575 v motors are in reality 460/575. If it does, just change the connections to your existing bus voltage.

If you are across the line starting these (I just conferred with one of my peers about this) your motors have to be way over-sized to be running like you say. The de-rating factor is 36% because of the change in current. Remember that current is not linear to torque when voltage and frequency are fixed. It concerns me also about the temperature rise times involved with this and the insulation ratings in general.

The drive way of looking at this (which is what I do all day) is as follows. The volts/Hz ratio is 9.58 based on a 575/60 relationship. With that said, take 460/575 which yields .80. 80% of 60Hz is 48Hz. This is the real key; the motor has no idea or cares what is going on until 48Hz with a VFD. After 48Hz the torque starts tapering off just like it would if you had a basic 460/60 trying to run the motor beyond 60Hz. If torque is a concern at higher speeds, realize that it's not going to perform.


PS. I really enjoy reading this forum...thanks all.

Carl Lee Tolbert
Technical Support
AC Drives
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