What are the directions of the rotor conductor, stator magnetic field, and rotor magnetic field? DOo they all have the same direction of rotation? How are they related to each other? Is the relative speed between the rotor conductor and stator magnetic field the same as the relative speed between the rotor conductor and rotor magnetic field?
In an induction motor, the stator and rotor magnetic fields rotate in the same direction at exactly the same speeds. The rotor (and thus also its conductors) rotate at a speed and direction that is determined by the operating torque. The rotor turns at a speed that is less than the speed of the magnetic fields Unless the rotor driven by an external machine or the magnetic fields are reversed while the motor is running.
Obviously, when the motor is energized with the rotor at rest, the magnetic field almost instantaneously achieves a speed that is determined by the applied frequency. The torque produced accelerates the rotor and connected load to a speed that is not more than a few percent below the speed of the magnetic fields. That speed varies with load, but rotor speed dropping more than a few percent would mean that the motor is overloaded.
The diagram below shows motor and load torque vs. speed for a passive load. The difference between the rotor speed and the magnetic field speed is called slip. The speed of the magnetic fields is the synchronous speed shown by a vertical dashed line. At the steady-state operating speed, the slip shown on the diagram appears to be practically zero, but it is actually about 2 percent.