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 Post subject: Motor contribution
PostPosted: Wed Feb 22, 2012 6:38 am 
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Location: Charlotte, NC
Someone told me yesterday that ETAP has an option to limit motor contribution current to 2 cycles, can anyone explain this? He thought it had something to do with the motor shorting itself out?


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PostPosted: Wed Feb 22, 2012 4:01 pm 
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Motor contribution is normally for five cycle it means that in an event of the short circuit motor will be acting as a generator because of its inertia and it will produce asymmetrical current (Sin+DC Component+ exponentiation decay). This asymmetrical current from the motor (which in the event of short circuit acting as generator) will last for five cycles.

The protective devices have to interrupt the contribution from (motor + utility + other sources) in order for the safe operation of the systems.

ETAP has an option in the settings where you can limit the motor contribution for particular number of cycles. Normally five cycles are consider enough for motor contribution.


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PostPosted: Fri Feb 24, 2012 8:53 am 
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you can also read this beautiful article about the details.


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PostPosted: Mon Feb 27, 2012 11:28 am 
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You would wan to run the numbers with and without motor contribution. Five cycles of motor contribution, which would add about 20 to 25 percent to fault current, could cause a breaker to trip sooner and result in lower IE values. So you want to take worse case. The other thing is that motor current during a fault is specific to a point on the circuit. You could have say 20KA flow through the breaker to a fault, and 4K from the motor flowing to the fault. But if the fault is between the breaker and the motor, the breaker only sees the 20K, not the 24K? So you have to pay attention to what is flowing through the breaker.


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PostPosted: Mon Feb 27, 2012 2:54 pm 
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Thanks for the comments, I don't do studies anymore and this was something I have not heard of before. Interesting concepts.


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