How to: Drive a 4160V motor, with 480V source???

[MilesD]

This is a puzzle for me, I have some Ideas, but I could use some more ideas.

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Problem:
Test a 4160 Volt NEMA G motor/pump combination on a skid, given a 480 Volt source.

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Things I have:
I have a 480 Volt source, with a 400 Amp circuit breaker.
I also have a 500kVA transformer that I can use to step up the voltage to 4160 Volts. (4160/Y480)
I have a 4160 Volt, 150 HP motor connected to a pump.
I have the pump connected to water that the pump will move to test the motor/pump skid

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This is what I think I know:
1. The motor will take 10 to 15 seconds to spin up to 1760 rpm.
2. The draw for the motor during startup will be up to 120 Amps (19.1 full load Amps * NEMA G ~6.2 times), this is about 170% of the transformer FLA, and will require over 1000 Amps at the 480 Volt source.
3. Because of 1. and 2. above, and the inrush on the transformer, I am sure the 400 Amp circuit breaker cannot be used to close into a circuit containing only the step-up transformer and the motor.
4. The basic circuit will go something like this:
-480 Volt, 400 Amp circuit breaker
-Cable
-4160/480V 500kVA transformer
-Cable
-4160 Volt 150HP motor

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My solutions:
Solution 1.
Put a current limiting drive on the 4160 Volt portion of the circuit between the transformer and the motor. This would limit current on the motor to 2 or 3 times the FLA of the motor until it is up to speed. The FLA of the motor is 19.1 Amps at 4160 Volts. I calculate that would require 166 Amps on the 480 Volts side of the transformer. If the drive can limit current to 38.2 Amps, then the 480 Volt side would only need 332 Amps.

My problem with this solution is that the drive is expensive, hard to find, and this setup is a onetime test, we will never do it again. What will I do with the drive when I am done.

Solution 2.
Put a current limiting drive on the 480 Volt side of the transformer. If I can limit current to under 400 Amps on the 480 Volt side of the transformer, maybe I can start the motor and the transformer at the same time. A 480 Volt drive is easy to find, and we can sell it when I am done with it.

My problem with this solution is that I don't trust myself. I can model inrush and motor start up current, but I am not a drive expert. I have no idea what kind of drive will do this. I am thinking that a VFD will not do the job. This is where I think I need help, but I could be totally wrong, and in need of help with the whole solution.

Solution 3.
I can add a 800 Amp circuit breaker to the 480 Volt panel. 800 Amps is the limit of the panel. This may make one of the solutions above more feasible.

Solution 4.
Some smart engineer on the internet tells me the right answer. At first this looked like an easy problem to me. I sketched the circuit, did some quick calculations, and said we will do this. Then I looked closer and said we need more parts. And now, I am asking for help.

What say you???

 

[davidbeach]

Given all that, I'd start off with your solution 2, bringing the motor and transformer up together. This will avoid transformer inrush.

I have not checked any of your numbers so this response is somewhat theoretical.

 

[CJCPE]

I think solution 2 is the right answer. The VFD will start the motor at a low frequency and ramp the frequency up to accelerate the motor without ever operating at high slip. That means that the motor can produce 100 to 150% of rated torque without drawing more than 100 to 150% of rated current. The VFD needs to be set up in the V/Hz mode not sensorless vector. The V/Hz must be constant with no low frequency boost otherwise the transformer could saturate.

 

[waross]

If you have an 800 amp thermal magnetic breaker, you may want to try it. The impedances of the transformer and the feeders and the thermal delay of the breaker may get you started. Set the magnetic trips on high. If the breaker trips before the motor is up to speed, then look for a drive.
If money is no object then use a VFD.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[NAZ55]

Is this a three phase motor?

 

[jraef]

15 seconds seems like an awful long acceleration time, leading me to think you are accelerating with flow in thew pump. Can you start the motor with no flow on the pump, i.e. with a valve that could be kept closed and then opened slowly only AFTER the motor is at full speed? If you can keep the acceleration to a second or two, that transformer is likely capable of starting that motor, especially if you follow waross' advice on the breaker.


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[LionelHutz]

I would first try to start the pump full-voltage if you either have or can get the 800A circuit cheap.

Otherwise, I would buy a 250amp soft-starter and use it. Most soft-starters will limit the current. This would likely be cheaper than buying a 800A retail priced breaker or buying a VFD.

If you do use a VFD then the VFD is more of a frequency controller than a current limiter. You ramp the frequency which will keep the motor current low. You basically operate the motor at rated slip all the time meaning you don't see the starting inrush current. So, your idea of starting current levels required does not really apply when using a VFD.

In any case, you need to add the magnetizing current of the transformer to the motor current. This is why I recommend a 250A soft-starter or 250A VFD.

 

[NAZ55]

Is 400A breaker instantaneous trip or inverse time?