The magnetic field of a three-phase motor rotor is caused by?

The magnetic field of a three-phase motor rotor is primarily caused by current flow in the rotor. In a three-phase induction motor, the rotor is typically designed as a squirrel cage, which induces a current as the rotating magnetic field produced by the stator interacts with the rotor bars. This induced current creates its own magnetic field, which adds to the overall magnetic field of the rotor.

It's important to understand that this interaction between the stator's rotating magnetic field and the induced current in the rotor is what leads to torque production and the functioning of the motor. The rotor does not initiate its own field when stationary; it requires the influence of the stator's magnetic field to produce the current that generates the rotor's magnetic field.

While resistance in the squirrel cage, impedance of the stator, and voltage in the end bells are relevant factors in the motor’s overall performance, they do not directly result in the creation of the magnetic field in the rotor itself. The fundamental source of the rotor's magnetic field is thus the current flow induced in it due to the action of the stator magnetic field.