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INDUCTION__MOTOR - LECTURE NOTES ON INDUCTION MOTOR Facts...

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LECTURE NOTES ON INDUCTION MOTOR

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Facts about IM About 65% of the electric energy in the United States is consumed by electric motors. In the industrial sector alone, about 75% is consumed by motors and over 90% of them are induction machines.
Why IMs are popular they are rugged reliable easy to maintain relatively inexpensive. Their power-to-density ratios (output power to weight) are higher than some other motors.

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a b c i i i
Rotating Field φ b φ a φ c Time t 1 t 3 t 2

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i a i c i b φ b φ c φ a φ a φ c φ b
max 2 3 φ max 2 3 φ - max 2 3 φ max 2 3 φ - max 2 3 φ - max 2 3 φ max 2 3 φ max 2 3 φ max 2 3 φ At t 1 At t 2 At t 3 i a i c i b φ b φ c φ a

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Rotating Field min / rev f 60 sec / rev f n s = = The speed of the airgap flux is one revolution per one ac cycle. The time of one ac cycle The speed of the airgap flux f 1 = τ

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Number of Poles i a i c i b i a i b i c
Number of Poles If each phase has one coil, it is a two-pole machine The rotor moves 360 degrees for one complete ac cycle. If each phase has two coils, it becomes 4- pole machine The rotor moves 180 0 mechanical for every one complete ac cycle.

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Number of Poles min / rev p f 120 pp f 60 n s = =
Why IM rotates? i l B F v l B e = = ) n , ( f e φ = ) i , ( f T φ =

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Apply 3-phase to Stator Apply 3-phase to Stator Field rotates in airgap Voltage induced in rotor circuit e = B l n Torque is developed in rotor wdgs f = B l i Motor rotates z e i = e i
Apply 3-phase to Stator n = 0 Voltage induced in rotor circuit e = B l n = 0 Torque is developed in rotor wdgs f = B l i = 0 Motor slows down What if motor speed is n s ? 0 z e i = =

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Apply 3-phase to Stator Load demands certain Torque Motor develops equivalent force f = B l i What is the motor speed? Force demands equivalent current Current demands equivalent induced voltage z i e = Induced voltage demands equivalent speed e = B l n
What is the motor speed?

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