Motor Switchgear undersized for motor 1

[IanFletcher]

We have an application where an owner wants to use a 250HP motor to drive a cooling tower fan but he wants to keep his current switchgear that was rated for a 200HP motor.

We can pitch the fan so that it does not require more then 200hp from the motor but is this going to work?

 

[ScottyUK]

Magnetically the motor is still a 250HP motor and will behave like one until it reaches running speed. I think you might well find problems with the the mag trip on the breaker. It will accelerate like a 250HP motor too - could this damage the fan?


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If we learn from our mistakes I'm getting a great education!

 

[IanFletcher]

The fan is rated in excess of 250HP so I am not concerned about the acceleration. Would a slower trip breaker be enough to make it through start up?

 

[ScottyUK]

The problem is that your motor will initially draw between about 6 - 8 times full load current, and this current will be a multiple of full load current on the 250HP motor, regardless of whether you run the motor at a reduced load. If the fan has a long acceleration time the contactor may end up in trouble too because it will not be designed to withstand the starting current of a 250HP motor.


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If we learn from our mistakes I'm getting a great education!

 

[dpc]

If you are in the US, this would be an NEC violation. Motor controller, conductors and circuit protection must be sized based on the motor nameplate rating, not the actual load.

If you don't need a 250 hp motor, put in something smaller. I

 

[IanFletcher]

The fan should be up to full speed in about 2 seconds. Would the addition of a soft starter help?

& thanks for all your comments.

 

[dpc]

Help with what? You would need a soft starter rated for the 250 hp motor.

 

[jraef]

Wiring and installation codes are all about preventing fires. What you are wanting to do is something that, as far as I know, would be technically illegal in any reasonable jurisdiction; for sure in the US and Canada. So any advice given here that would assist you in doing that would be abetting a crime.


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[waross]

I saw a 250 HP contactor applied to a 400 HP motor by an engineering Phd. This was a wound rotor with electronically controlled rotor current. This would be the next best thing to a VFD you could get for a soft start. He had done a series of tests to prove that the contactor was safe to use. It turned out that he had not done a test to interrupt full load current. A stop signal at full load and the contactor arced phase to phase. This resulted in a lot of charcoal in place of a lot of expensive electronics, a bent motor shaft, a bent fan shaft, and parts of a large coupling in every part of the fan room.
But on a ratio of 250 HP to 200 HP it will probably work. Just the same, I don't want to be in either the electrical room or near the motor if the contactor opens under starting current. But that's just me.


Bill
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Jimmy Carter