An induction electric motor initially operates in a vacuum, very close to the synchronous speed.
In this machine, the resulting rotary field is produced by the magnetizing current. In this state the sliding and relative movement between the rotor and the magnetic fields are small, even so, the rotor frequency is also reduced.
The relative motion is small, and consequently, the induced voltage in the rotor rod and the flow of the current in the rotor are also small. As the rotor frequency is much smaller, its reactance is approximately zero and the rotor current is almost in phase with the rotor voltage.
The rotor current then produces a small magnetic field and an angle a little greater than 90 ° behind the resulting magnetic field. It should be noted that the stator current must be very large even when empty, as it will supply most of the resulting rotating field.
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For this reason electric induction motors have large unladen currents compared to other types of motors. The induced torque keeps the motor running.
As the magnetic field of the rotor is small, the induced torque is also small, but large enough to oppose the rotational losses of the motor.