Inrush Current

Dear SIr,

Could you please guide me why A.C motor draw high current at start?

and is there any difference between inrush current and lock rotor current?

http://en.wikipedia.org/wiki/Inrush_current#Motors

When the shaft is not moving, enough current has to flow in the start coils to overcome inertia. Once the motor is running, the run coils can keep it in motion for much less current. Locked rotor amps is the current pulled when the shaft is prevented from turning. That is a much higher current draw.

It has to do with the conversion of electrical power to the work of turning the shaft of the motor and whatever physical load is connected to the shaft. If the shaft isn't turning, then there is no power conversion and thus all the power goes directly through the windings and back to the source -- the only thing limiting the current is the resistance of the motor windings. Once the shaft starts to turn, more and more of the power is converted into mechanical work and so less of it is left over to return to the source.

My Most respected Sir Steve,

Please correct me about inrush current. I just studied that at start motor rotor is at stand still condition (just like secondary of the transformer)

which is shorted at ends because in cage type induction motor rotor construct with the help of Al bars shorted from both end with cu ring...

As i study that (Rotor induced voltages=Slip x Stater voltages) means at start our (s= slips is at 1) so voltage induced on the rotor which is short circuit and cause of inrush current but as motor start moving slips goes down means induced voltages on rotor reduce and current goes down.

Man, I'm sorry, but I can't make out what you are trying to ask. It's partially because I'm not well versed on the mechanics of a motor, so I probably wouldn't be able to answer even if I could understand you.

Anyone else?