Motor LRC - Disconnect requirements

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xformer

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Master Electrician
I am curious as to motor design. can someone cite real world examples of motors that have such a LRC that they require a disconnect of a greater horsepower rating than the HP rating of the motor?
 
The "HP rating" of a disconnect takes into account the possibility of it having to break the LRC of the motor, that's why you need the HP rating as opposed to just the current rating that is equal to the motor FLA. So no.

Although the HP rating is based on a basic 4 pole motor (as the NEC FLC tables are), motors with high pole counts can have significantly higher FLC ratings than 4 pole motors, but the LRC will be roughly the same because it's a function of the kVA/kW and that involves Power Factor, which also changes with pole count in the opposite direction. So bottom line, the HP rating of the disconnect has LRC covered regardless of the motor design.
 
Jraef said:
The "HP rating" of a disconnect takes into account the possibility of it having to break the LRC of the motor, that's why you need the HP rating as opposed to just the current rating that is equal to the motor FLA. So no.

Although the HP rating is based on a basic 4 pole motor (as the NEC FLC tables are), motors with high pole counts can have significantly higher FLC ratings than 4 pole motors, but the LRC will be roughly the same because it's a function of the kVA/kW and that involves Power Factor, which also changes with pole count in the opposite direction. So bottom line, the HP rating of the disconnect has LRC covered regardless of the motor design.
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I understand the purpose of the HP rating. Do manufactures make a 10 HP motor with locked rotor code letter of S?
 
xformer said:
I understand the purpose of the HP rating. Do manufactures make a 10 HP motor with locked rotor code letter of S?
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I've never seen one, but that doesn't mean it doesn't exist. I have seen Code R on a 900HP 575V motor being used on cryogenic submersible pumps for a natural gas tanker, it was a one-off special purpose motor design and we had to use VFDs as soft starters because of that LRC code and the fact that all the power came from generators on board.
 
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