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Anonymous Poster #1

Short Circuit

04/09/2013 9:19 AM

dear friends

when short circuit accur at bus, why motor supply current to the bus?

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#1

Re: Short Circuit

04/09/2013 9:41 AM

Because although supply voltage has dipped (due to short circuit), motor is still spinning.

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rogerggbr
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#2

Re: Short Circuit

04/09/2013 10:39 AM

When short circuit occurs, the motors are not powered by the system voltage, but keep spinning for short period of time due to inertia, and work as induction generators (for induction motors) or synchronous generators (for synchronous motors), and hence contribute short circuit current during the initial cycles of the fault. Generally small motors have minimum contribution to the short circuit current and they are neglected for the short circuit calculation.

This link will also help:

http://www.geindustrial.com/publibrary/checkout/SC-Motor?TNR=White%20Papers|SC-Motor|generic

- MS

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#3

Re: Short Circuit

04/10/2013 9:42 AM

Because at the instant of short circuit, the terminal voltage at the motor collapses. But, there would still be residual magnetism in the motor's core. Also, the rotor of the motor would still be in rotation, due to the dynamic inertia of the motor's rotor as well as that of the connected load. With magnetism & rotation, the motor would behave as a generator and thus would feed into the fault.

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#4

Re: Short Circuit

04/10/2013 12:21 PM
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