Consider the following statements regarding speed control methods of three-phase induction motor

1. We can get speeds greater than the synchronous speed if we inject emf in phase opposition with the existing rotor emf.

2. Two induction motors are said to be differentially cascaded when torque produced by them acting in the opposite direction

3. Rotor resistance method is not economical for a wide range of speed control.

4.  If an emf is injected at an angle α with the existing rotor emf, the power factor of the motor can be improved, and speed changes.

Which of the above statement(s) is/are correct?