**Control of three phase induction motors using slip power recovery scheme**

The phasor diagram shows the stator and rotor currents in an induction motor, when an emf of slip frequency is injected into its secondary circuit. The power input to the induction motor is

Resolving currents on E1, we have

Multiplying by E1, we get

= (Stator copper loss)+ (iron loss)+(power transferred to the secondary circuit)

Resolving emf , we get,

Referring to stator side

Adding (1-s)E1I2’, we get

Power transferred to the secondary circuit

P2 = rotor copper loss + slip power + mechanical power output.

- The air gap power P2 is nearly constant

- If losses are neglected, the mechanical power output and the slip can be varied, by regulating the slip power or injected voltage Ej at slip frequency sf.

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