Motors

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Gac66610

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Kansas
Question my son and I have, (he's preparing for a test, and I can't talk him into joining this forum :blink:)

What size dual element fuse is required for the branch circuit protection for a three-phase 50hp 480v motor? Auto-transformer starting with code letter A.
 
Little Bill

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Gac66610 said:
Question my son and I have, (he's preparing for a test, and I can't talk him into joining this forum :blink:)

What size dual element fuse is required for the branch circuit protection for a three-phase 50hp 480v motor? Auto-transformer starting with code letter A.
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Any more info than that?
 
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Gac66610

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Kansas
Yeah I know, this was a practice exam question.
He came up with (c) 150a .... 65a X 115% = 74.75, 74.75 X 175% = 130.8 , next higher 150a
I came up with (b) 125a .... 65a X 175% = 113.75 , next higher 125a
the tests answer sheet had (a) 100a .... 65a X 150% = 97.5 ,next size higher 100a


Now looking at all this, I believe I used the wrong % for dual element, I used motors other than wound-rotor
assuming this is a wound-rotor motor, question didn't state either way

sure has been awhile
 
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Gac66610

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Location
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well, i can tell that is what they used for the answer they give.

i don't deal much with motors

figured this would be the best place to find out which is correct, The question is in OP its all the info i have on it
 
Little Bill

Little Bill

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Gac66610 said:
well, i can tell that is what they used for the answer they give.

i don't deal much with motors

figured this would be the best place to find out which is correct, The question is in OP its all the info i have on it
Click to expand...

That is why I ask if you had more info. Seemed there were some key parts missing!
 
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Gac66610

Senior Member
Location
Kansas
Little Bill said:
That is why I ask if you had more info. Seemed there were some key parts missing!
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Figured that was why, I just didn't have anything else. I don't even know where this practice exam came from ..

Dennis Alwon said:
I would go with 125. 65 * 1.75 (430.52) and 430.250
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Kind of what I originally thought, I have been known to talk myself out of the answers, even correct ones :ashamed1:

Thanks to all for replies
 
kwired

kwired

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Dennis Alwon said:
I would go with 125. 65 * 1.75 (430.52) and 430.250
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Likely the best answer, question is poorly written. That answer based on 430.52 is maximum overcurrent device the question asked for what overcurrent device is required. The correct answer for that is anything that will allow the motor to start but does not exceed values in T430.52 or the exceptions if it still will not start, but that was not one of the answers to choose from.
 
Besoeker

Besoeker

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Gac66610 said:
well, i can tell that is what they used for the answer they give.
Click to expand...
I think it unlikely for a few reasons.
The question mentions an autotransformer starter. That would most likely be used with a cage motor.
A wound rotor motor would likely have a stepped resistance starter. That gives better torque and lower supply current than an ATX starter would during starting.
Wound rotor motors are not all that common at such low powers. 50HP is 37kW. The smallest I've seen was 210kW but most have been upwards of 500kW.
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