Motor locked rotor current

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The overload relay protects based on a time-current curve that is deliberately scaled to be lower than the thermal damage curve of the motor, meaning that the trip will take place BEFORE the motor is damaged and the higher the current the faster it trips. At locked rotor current it would trip in the number of seconds pursuant to the Overload Class; Class 10 trips in 10 seconds, Class 20 trips in 20 seconds, etc. NEMA motors are generally designed as Class 20, IEC motors are all Class 10. So if you have a NEMA design motor, the thermal damage curve can withstand more than 20 seconds at Locked Rotor.

So if you are not using an overload relay, do so. If you are, make sure it is properly sized /adjusted per the manufacturer’s instruction. The concerns from your engineer should be taken care of by understanding this.
 
Jraef said:
The overload relay protects based on a time-current curve that is deliberately scaled to be lower than the thermal damage curve of the motor, meaning that the trip will take place BEFIRE the motor is damaged and the higher the current the faster it trips. At locked rotor current it would trip in the number of seconds pursuant to the Overload Class; Class 10 trips in 10 seconds, Class 20 trips in 20 seconds, etc. NEMA motors are generally designed as Class 20, IEC motors are all Class 10. So if you have a NEMA design motor, the thermal damage curve can withstand more than 20 seconds at Locked Rotor.

So if you are not using an overload relay, do so. If you are, make sure it is properly sized /adjusted per the manufacturer’s instruction. The concerns from your engineer should be taken care of by understanding this.
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However the cabinet, starters and control circuits were all built by 2 other coworker who have some knowledge of electrical but are by no means licensed journeymen (doubt they figured proper feeder size, feeder taps, ground, over current heaters etc). One motor starter heaters already melted for a compressor

Also the cabinet and apparatus mention were fitted for a pilot plant with different motors. In this situation I was asked by owner to tie in this single 5 hp motor into a breakout box powered by a starter already in this cabinet mentioned so the overload is likely NOT sized proper for this 5 hp motor being added but not parallel with any other operating motors at this time

The disconnect I added to be within 50 ft of motor from company junk pile has 3 inline fuses rated 30 amperes. However, I calculated if non time delay fuses were to be used, then fuses were to be rated at 25 amperes or 300% of nameplate FLA with a service factor rating and temp rise mentioned in specs
 
Design B code letter has an approximate mid-range kVA/HP of 3.3 kVA/HP (3.15 to 3.55). That means your locked-rotor amps at 460V is 35.87A (430.251(B), for selection of disconnects and controllers) lists 5HP LRA at 460V as 46A). Why the hassle?
 
No
topgone said:
Design B code letter has an approximate mid-range kVA/HP of 3.3 kVA/HP (3.15 to 3.55). That means your locked-rotor amps at 460V is 35.87A (430.251(B), for selection of disconnects and controllers) lists 5HP LRA at 460V as 46A). Why the hassle?
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I also want to learn as much and know as many formulas before taking my journeyman’s license test
 
topgone said:
Design B code letter has an approximate mid-range kVA/HP of 3.3 kVA/HP (3.15 to 3.55). That means your locked-rotor amps at 460V is 35.87A (430.251(B), for selection of disconnects and controllers) lists 5HP LRA at 460V as 46A). Why the hassle?
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The design code letter and the locked rotor code letter are not the same thing.
 
You are fighting a battle you can't win with the weapons you were given. Replacing the overloads with the proper ones is a start. VD is likely to influence the current flow, especially at LR. I like CC fuses but not sure how you will get them into an old disconnect recovered from the junk pile.
 
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