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FredRosse (Mechanical) (OP)
20 Sep 06 13:03
For an Induction Motor with across the line starting.

Some indicate that the initial instantaneous Inrush Current is much larger than Locked Rotor Amps (LRA ), while others state that they are the same thing.

Which is correct?

Please help this ME

Thanks in advance.
Helpful Member!(2)  CJCPE (Electrical)
20 Sep 06 16:54
The initial motor current is a combination of magnetizing current that lasts only a couple of cycles or so plus locked rotor current that lasts until the motor has accelerated to a speed at which the current has significantly reduced. When people talk about inrush current being the same as locked rotor current, they are really not talking about the instantaneous inrush but the more sustained initial part of the acceleration current.
Helpful Member!(2)  electricpete (Electrical)
20 Sep 06 19:52
The instantaneous peak during the first quarter cycle after motor energization can be up to 2.82 times the rms locked rotor current.  The actual peak depends upon the phase of the voltage at time of circuit closing (will vary between closure attempts) and on the circuit decay constant.

This is due to the presence of an exponentially decaying dc component.  Model an a series R/L circuit with suddenly applied voltage of magnitude. The response has a steady state sinusoidal component and an exponentially decaying dc component.  The initial value of the dc component in the worst case phase closure has the value of the peak of the sinusoidal component.  This can just about double the peak.  Doubling the normal peak of sqrt(2) times rms LRC gives 2*sqrt(2) times LRC max instantaneous peakl

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jraef (Electrical)
20 Sep 06 20:37
Excellent responses.
This is a pet peave of mine. Industry professionals misue the term "inrush current" when they mean "starting current" all the time.
Eng-Tips: Help for your job, not for your homework  Read FAQ731-376 pirate

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