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Basic Motor Theory - Squirrel Cage Induction Motor



Induction Motor

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Stator Construction

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Squirrel Cage Rotor

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Stator Magnetic Field Rotation

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Rotating Stator Field

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Two Pole Rotating Stator Field

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Squirrel Cage Rotor

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Rotor Slip

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Rotor Current Induction

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Braking, Motoring, Regeneration

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Speed of Magnetic Field

Speed expressed in RPM; N = 2*60*F / P

N = Speed

F = Frequency

P = Poles

Eg.

N = 2*60*60 / 4 = 1800 RPM

N = 2*60*60 / 2 = 3600 RPM

N = 2*60*60 / 6 = 1200 RPM


Rotor Speed

Rotor Speed; N = 120*F / P –Slip

Slip is a function of motor design and load

Motor name plate RPM = synchronous speed -

slip @ full load

Actual slip will increase and decrease as load is

increased and decreased as shown by the

motor manufacturer’s speed torque curve.


Torque

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Motor Torque

Motor Torque = Flux Density * I

Flux Density = K * (V / Hz)

K = Motor Constant

V = Voltage

Hz = Frequency

Motor Torque: = (K * V / Hz) * I

Therefore, Torque is controlled by the V/Hz

ratio


Motor HP Formula

HP = T * N / 5250

T = Torque in foot pounds

N = Speed in rotations per minute


Inertia

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Current Vs. Speed

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Torque Vs. Speed

Motor Speed Control

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1) Changing Motor Pole

2) Alter the Slip

3) Vary the Frequency

Torque Vs. Speed at Multiple

Frequencies

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Current with Varying Load

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Thanks To

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