Induction Machines Fundamentals

• Induction machines are the most widely used of all electric motors. They offer the following attractive features:

–Generally easy to build and cheaper than corresponding dc or synchronous motors

–Rugged and require little maintenance –Offer reasonable asynchronous performance –A manageable torque-speed curve –Stable operation under load –Generally satisfactory efficiency –Range in size from few Watts to several MW

Induction Machines Fundamentals

• Motors account for about 70% of industrial energy consumption

• Some disadvantages of induction motors are: –Speeds not as easily controlled as dc motors* –Draw large starting currents, typically 6-8x their full load

values* –Operate with a poor lagging power factor when lightly

loaded

Induction Machines Construction

Squirrel-Cage Wound Rotor

Two types of motor:

Construction

• An induction motor has two main parts – A stationary stator

• consisting of a steel frame that supports a hollow, cylindrical core

• core, constructed from stacked laminations (why?), having a number of evenly spaced slots, providing the space for the stator winding

• A revolving rotor – composed of punched

laminations, stacked to create a series of rotor slots, providing space for the rotor winding

– one of two types of rotor windings

– conventional 3-phase windings made of insulated wire (wound-rotor) » similar to the winding on the stator

– aluminum bus bars shorted together at the ends by two aluminum rings, forming a squirrel-cage shaped circuit (squirrel-cage)

Construction

Construction

Cutaway in a typical wound-rotor IM. Notice the brushes and the slip rings

Brushes

Slip rings

sf120p

ns =

How does an induction motor work? 1. Apply AC three-phase current to stator winding to produce rotating

magnetic field.

2. Rotating magnetic field induces voltages in rotor windings resulting with rotor currents.

3. Then, rotor currents will create rotor magnetic field.

4. Constant speed stator magnetic field will drag rotor magnetic field.

ns

n

ns: Synchronous speed (the speed of stator rotating field in rpm).

n : Rotor speed (rpm).

SLIP: It is defined as the difference between synchronous speed and the rotor speed divided by synchronous speed.

s

s

nnns −

=