Can anybody explain in simple language the meaning of magnetizing current? or why there is a difference between magnetizing current and real current in an electric motor being powered by a generator? and why is the magnetizing current returned to the generator and apparently doing no work? Surely the real power KW drives the motor.

 

In an electric motor, the torque is developed by the interaction between the stator current and a magnetic field.

With an induction motor, there is no permanent field: instead, the magnetic field is developed by current flowing in the stator winding. This therefore has inductance.

In an inductance, the current produces a flux and the changing flux sets up a back emf that resists current flow. The developed emf is proportional to the rate of change of current. When the winding is connected to an alternating supply, the back emf must match the applied emf. So, if you connect a motor stator to an ac supply, you will find that the current flow lags the applied voltage by 90 degrees. (if the voltage is a sine curve, the current will be a negative cosine curve.)

To find the instantaneous power, multiply the value of voltage at any time by the current at the same time. The resulting product will be a sine function at twice the frequency ( sine A x cos A = 1/2 sin 2A). Over a cycle, this has an average value of 0.

Effectively, the energy put into the magnetic field on one half-cycle is returned to the system on the other, so does not appear on your power bill. However, since the motor will not function without the magnetic field, and the magnetic field needs the current to exist,you cannot operate an induction motor without the magnetising current, and the related effect of a lagging power factor.

Hope that helps.

 

Thank you for your time and help. I will study this further and get in touch with you later.