INDUCTION MACHINES M ODULE 1 Prepared by: Aswathy Mohandas P

I NTRODUCTION Three-phase induction motors are the most common and frequently encountered machines in industry simple design, rugged, low-price, easy maintenance run essentially as constant speed from no-load to full load called as asynchronous motors described as ―transformer type a.c machine in which electrical energy is converted into mechanical energy. Its speed depends on the frequency of the power source not easy to have variable speed control requires a variable-frequency power-electronic drive for optimal speed control 2

Advantages It has simple and rugged construction. It is relatively cheap. It requires little maintenance. It has high efficiency, good speed regulation and reasonably good power factor. It has self starting torque. Disadvantages It is essentially a constant speed motor and its speed cannot be changed easily. Its starting torque is inferior to d.c motors. 3

P ARTS OF INDUCTION MOTOR 4

C ONSTRUCTION Frame: It is the outer body of the motor. Its function are to support the stator core and winding, to protect the inner parts of the machine and serve as a ventilating housing or means of guiding the coolant into effective channels. Stator: It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel. A number of evenly spaced slots are provided on the inner periphery of the laminations. 5

Rotor: The rotor consists of a laminated core, with slots cut on its outer periphery where windings are placed. The windings may be either of squirrel cage type or wound rotor type. The core is mounted on a steel shaft, provided with bearings on both sides and is supported on end covers attached to the main frame of the motor Shaft and bearings: The rotor shaft is supported by bearings housed in the end shield. 6

T YPES OF ROTOR 7

Squirrel cage rotorWound rotor 8

R OTATING M AGNETIC F IELD D UE TO 3-P HASE C URRENTS 9

At instant 1, ωt = 0°. Therefore, the three fluxes are given by, 10

At instant 2, ωt = 30°. Therefore, the three fluxes are given by, 11

At instant 3, ωt = 60°. Therefore, the three fluxes are given by, 12

At instant 4, ωt = 90°. Therefore, the three fluxes are given by, 13

SLIP 14

ROTOR CURRENT FREQUENCY 15

EQUATION FOR STATOR AND ROTOR EMF S 16

ROTOR CURRENT 17

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ROTOR TORQUE 20

STARTING TORQUE 21

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CONDITION FOR MAXIMUM STARTING TORQUE 23

TORQUE UNDER RUNNING CONDITION 24

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MAXIMUM TORQUE UNDER RUNNING CONDITION 27

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FULL LOAD, S TARTING AND M AXIMUM TORQUES 29

T ORQUE - SLIP CHARACTERISTICS 30

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TORQUE – SPEED CHARACTERISTICS 32

EFFECT OF ROTOR RESISTANCE UPON TORQUE - SLIP OR TORQUE - SPEED CHARACTERISTIC 33

EFFECT OF CHANGE IN SUPPLY VOLTAGE ON TORQUE – SPEED CHARACTERISTIC 34

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POWER STAGES IN AN INDUCTION MOTOR 36

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I NDUCTION M OTOR T ORQUE 39

R OTOR O UTPUT 40

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I NDUCTION M OTOR T ORQUE E QUATION 42

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P HASOR D IAGRAM 46

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C IRCLE D IAGRAM 51

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