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## Basic Motor Theory - Squirrel Cage Induction Motor

Induction Motor Stator Construction Squirrel Cage Rotor Stator Magnetic Field Rotation Rotating Stator Field Two Pole Rotating Stator Field Squirrel Cage Rotor Rotor Slip Rotor Current Induction Braking, Motoring, Regeneration 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 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 Current Vs. Speed  Torque Vs. Speed

Motor Speed Control 1) Changing Motor Pole

2) Alter the Slip

3) Vary the Frequency

Torque Vs. Speed at Multiple

Frequencies Current with Varying Load Thanks To Go to Top of Page