Special Secondary Transformer Voltage Requirements

[yjacket]

We will be installing a specially designed motor that will require a minumum of 460 VAC at the motor terminals. The motor will be controlled by a Variable Frequency Drive. When calculating the volatge drops due to motor leads, supply cable and VFD losses, we need 525 VAC at the line side input of the VFD. Our substation will have a 13.8KV to 480V transformer for the operation of other equipment. Any suggestions on the best way to meet the voltage requirements for the VFD and motor?

 

[pablo02]

What is so unusual about the 460v motor, this is a typical rating for those on a 480v system (designed voltage drop in system)... where is the VFD drive in comparison to your 480v bus? where is the motor in relation to the VFD unit? usually the voltage drop issues are due to cable lengths vs. cable size -- and you calculate the size of cable needed to provide an acceptable voltage drop to the equipment -- the other issue you'll need to address is the voltage rating of the VFD...

 

[busbar]

ANSI C84.1 voltage range is typically 440-504 volts. Pardon my saying, but someone screwed up designing [and consequently buying] equipment that required anything outside of this fairly pervasive standard.

 

[yjacket]

The motor is rated at 350 HP but draws 550Amps and is 20inches in diameter. It is used as a submesible mixing pump. The PF is 76% and efficiency is 79%. Loss through the VFD is 8%.

 

[pablo02]

So, what size cable(s) were you using and what is the cable length? Make sure your VFD is spec'd correctly for this application.. I don't off-hand see a problem with 480 serving this if it's laid out correctly..

 

[peebee]

I agree. Sounds like some wacky calcs going on here. How long are the motor feeders? Over 1000 feet? What is the kVA & %Z rating of the transformer?

Re "Loss through the VFD is 8%": What is your point by that statement? Do you think this will somehow affect voltage drop? The VFD should actually correct voltage drop to some extent, particularly if the VFD is located near the motor. The voltage going into the VFD has very little to do with the voltage out of it.

If you do in fact have a voltage drop problem (and it's not at all clear that you really do), it would not seem that it would take anything more sophisticated than an oversized feeder conductor or adjustment of transformer tap settings to correct.

 

[yjacket]

I may have misled using the term loss, the voltage drop through the VFD is 8% or approximately 38 volts. That gives a maximum voltage at the output of the VFD of 441. Even with zero drop due to cabling, there is insufficient voltage for the motor.

 

[jbartos]

Suggestion: Visit

http://www.actechdrives.com

for MCH Series Installation and Operation Manual,
page 9, MH Series Rating
indicates:
Input Voltage (400/480 VAC, 50-60Hz)
Output Voltage (0-400/460 VAC)
(at nominal current Amps and power kVA)
Therefore, the drive located at the motor will provide 460V output.
The drive input is supposed to have 480V. If the voltage on the drive input is too low, an autotransformer at the drive input may be needed.

 

[peebee]

One of us is a bit confused with what's going on with the VFD.

Typically, a solid-state VFD rectifies to DC and then reinverts to AC. The AC output is generated at the VFD and therefore the term "voltage drop through the VFD" is rather meaningless. In fact, many VFD's can and will output a higher voltage than they are receiving (particularly in overspeed applications). They are actually very similar to UPS systems without the batteries. There should be no 38-volt drop across the VFD unless it was programmed to do so.

Do you have this kind of VFD or something else?