1 6.1 INTRODUCTION TO POLYPHASE INDUCTION MACHINES Two types of motor:Squirrel-CageWound Rotor
2 6.1 INTRODUCTION TO POLYPHASE INDUCTION MACHINES How does an induction motor work?Apply AC three-phase current to stator winding to produce rotating magnetic field.Rotating magnetic field induces voltages in rotor windings resulting with rotor currents.Then, rotor currents will create rotor magnetic field.Constant speed stator magnetic field will drag rotor magnetic field.ns: Synchronous speed (the speed of stator rotating field in rpm).n : Rotor speed (rpm).nsnSLIP: It is defined as the difference between synchronous speed and the rotor speed divided by synchronous speed.
3 6.1 INTRODUCTION TO POLYPHASE INDUCTION MACHINES The speed of rotor magnetic field with respect to rotor isThe relative motion of stator flux and the rotor conductors induces voltages of frequency (fr is called slip frequency)The rotor speedMechanical angular velocity
5 6.2 CURRENTS AND FLUXES IN POLYPHASE INDUCTION MACHINE Developed rotor winding of an induction motor with its flux-density and mmf waves in their relative positions for (a) zero and (b) nonzero rotor leakage reactance.
6 6.2 CURRENTS AND FLUXES IN POLYPHASE INDUCTION MACHINE Reactions of a squirrel-cage rotor in a two-pole field.Figure 6.6
7 6.3 INDUCTION MOTOR EQUIVALENT CIRCUIT Stator equivalent circuit for a polyphase induction motor.Counter emf generated by the resultant air-gap flux
8 6.3 INDUCTION MOTOR EQUIVALENT CIRCUIT Rotor equivalent circuit for a polyphase induction motor at slip frequency.
9 6.3 INDUCTION MOTOR EQUIVALENT CIRCUIT Single-phase equivalent circuit for a polyphase induction motor.Models the combined effect of rotor resistance and shaft load
10 6.3 INDUCTION MOTOR EQUIVALENT CIRCUIT Alternative form of equivalent circuit.Electromechanical power is equal to the power delivered to this resistance
11 6.4 ANALYSIS OF THE EQUIVALENT CIRCUIT Pmech is not the net power but it includes the losses such as friction, windage.Output power and torque from the shaft is
12 6.5 TORQUE AND POWER BY USE OF THEVENIN’S THEOREM (a) General linear network and (b) its equivalent at terminals ab by Thevenin’s theorem.
13 6.5 TORQUE AND POWER BY USE OF THEVENIN’S THEOREM Equivalent circuits with the core-loss resistance Rc neglected.
14 6.5 TORQUE AND POWER BY USE OF THEVENIN’S THEOREM Induction-motor equivalent circuits simplified by Thevenin’s theorem.