Adhering to utility in rush requirements starting a large HP motor

[BSBoulder]

We purchased a vintage 1957 800 HP motor with a 10x14 ball mill. Unfortunately the local utility will not let us start the motor across the line but have limited us to 162 amps (4160 service) for start up.

We have been told that there are several ways to accomplish this, capacitor banks being the most economical.

We cannot use an MCC or VFD. Each presents problems for us.

final issue is that there is no data available on the motor from GE (mfg). They cannot find any files no the motor and the data plate is missing.

 

[dpc]

The magical 162 A. Ask them how they came up with that number.

A rough calculation shows this motor would likely draw between 650 and 700 A locked rotor amps if started across the line. You would need a huge amount of capacitance to reduce the locked rotor current to 162 A, and it might still exceed this value as the motor accelerates and the power factor improves.

If you really want to run this mill, my suggestion is to find a local EE who can work with the local utility to come up with a more reasonable approach. But if you wanting to use a 50-year old motor, something tells me you probably will be reluctant to hire a consultant.

We cannot use an MCC or VFD

Not sure what you mean by "MCC" - reduced voltage starting? Maybe you could buy a 50 year generator to run the motor.

 

[LionelHutz]

Is this mill clutched?

GE motor on a ball mill - Is this a synchronous motor?

If no clutch, using a soft-starter or other form of reduced voltage starter will likely not produce enough torque to get the mill turning.

The cap bank might work but honestly I don't see it capable of reducing the current to 150%. Even if it would work, I expect you'll need something like 4MVAR of capacitors which is a fairly large bank and not as cheap as you may think. You'll also produce some very decent transients switching a capacitor bank that large.

A soft-starter might work if the mill has a clutch.

A soft-starter with a smaller capacitor bank might work if the mill has a clutch.

A VFD would be the safest bet, especially if the mill does not have a clutch.

 

[waross]

I have read an article on the possibility of using capacitors to reduce the starting current of a motor, and the calculations for capacitor sizing.
I have never seen the technique used, or heard of it being used, or read about it actually being used.
The reduction in starting current is based on supplying the reactive component of the starting current with capacitors, locally, or correcting the power factor of the starting current. The capacitors must be switched out in steps as the motor accelerates.
Starting current is highly reactive and it takes a lot more capacity to correct the power factor of a motor at or near locked rotor than it does to correct the power factor of a running motor.

A rough calculation shows this motor would likely draw between 650 and 700 A locked rotor amps if started across the line. You would need a huge amount of capacitance to reduce the locked rotor current to 162 A

I agree dpc.

Bill
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