Starting an Induction Motor with Rotor Already Rotating from External Means


Thread Starter


I would like to know what could happen to the starting current of a Squirrel cage Induction Motor (11 kV, 4MW, 350% Full Load Amps as Starting current) if I rotate the rotor by means of another motor or Turbine up to half of the rated speed or more and then close the breaker of motor. Will the motor take high current it usually would? Could there be any significant decrease in the starting current? Please clear the funda behind high starting current.

Thanks in advance..!!
Stormrider... using a smaller motor to accelerate a larger motor is called the "Pony Motor" method!

1) Sometimes an error is made when the smaller motor's unable to provide the necessary breakaway-torque.

2) Sometimes,especially if the driven-machine is a pump, fan, or compressor, the smaller motor is unable to maintain the necessary acceleration-torque during runup.

3) Sometimes you will have to include a clutch to separate the smaller motor, after reaching full-speed because personnel Safety requirements.

If you're sure that about Starting-current is only 3.5 time FLA, there are other, more economical, methods.

Phil Corso
There are two main aspects to "high starting current"; first comes the true "Inrush Current", which is the current drawn for a fraction of a second to establish the magnetic fields in the stator windings, then comes the Accelerating Current, aka "Starting Current".

Using a pony motor will NOT affect the Inrush Current at all, you still have to magnetize the windings. That current draw is the result of the fact that for the first instant, there is no back EMF in the motor, even though it is already spinning, because there are no magnetic fields yet. So for about 1 to 1-1/2 cycles, there is virtually no impedance in the circuit other than the resistance of the wire so current will spike to a level commensurate with the available fault current in the system at the motor terminals. As soon as the magnetic fields are established and the rotating rotor bars cut the lines of force, you have impedance and it drops to the Accelerating Current level.

Accelerating Current is high because the relative frequency of the stator and rotor are far apart, so the power factor is very low, as in .1 or .2PF. that means your source must supply all of the reactive current as well as the active current, and it will amount to around 600% of the FLC of the motor. That current level will remain that way until the speed increases to about 80% of slip speed, then drops off rapidly to the Running Current. So using a pony motor does affect the ACCELERATING current <i>profile</i> in that if you are already at 50% speed, you only have to suffer the high current draw for the remaining time it takes to get to 80% speed. The actual MAGNITUDE of the Accelerating Current will still be about the same, it's just the duration that is lowered. But that usually results in having less of an effect on the supply system and therefore causing less of a voltage drop, which is the desired effect in most cases.