## 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

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