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INTRODUCTION
AC
induction motors are the most common motors used in industrial motion
control systems, as well as in main powered home appliances. Simple and
rugged design, low-cost, low maintenance and direct connection to an AC
power source are the main advantages of AC induction motors.
Various types of AC induction motors are available in the market.
Different motors are suitable for different applications. Although AC
induction motors are easier to design than DC motors, the speed and the
torque control in various types of AC induction motors require a
greater understanding of the design and the characteristics of these
motors.
This application note discusses the basics of an AC induction motor;
the different types, their characteristics, the selection criteria for
different applications and basic control techniques.
BASIC CONSTRUCTION AND OPERATING PRINCIPLE
Like
most motors, an AC induction motor has a fixed outer portion, called
the stator and a rotor that spins inside with a carefully engineered
air gap between the two.
Virtually all electrical motors use magnetic field rotation to spin
their rotors. A three-phase AC induction motor is the only type where
the rotating magnetic field is created naturally in the stator because
of the nature of the supply. DC motors depend either on mechanical or
electronic commutation to create rotating magnetic fields. A
single-phase AC induction motor depends on extra electrical components
to produce this rotating magnetic field.
Two sets of electromagnets are formed inside any motor.
In an AC induction motor, one set of electromagnets is formed in the
stator because of the AC supply connected to the stator windings. The
alternating nature of the supply voltage induces an Electromagnetic
Force (EMF) in the rotor (just like the voltage is induced in the
transformer secondary) as per Lenz’s law, thus generating another set
of electromagnets; hence the name – induction
motor. Interaction between the magnetic field of these electromagnets
generates twisting force, or torque. As a result, the motor rotates in
the direction of the resultant torque.
STATOR
The
stator is made up of several thin laminations of aluminum or cast iron.
They are punched and clamped together to form a hollow cylinder (stator
core) with slots as shown in Figure 1. Coils of insulated wires are
inserted into these slots. Each grouping of coils,
together with the core it surrounds, forms an electromagnet (a pair of
poles) on the application of AC supply. The number of poles of an AC
induction motor depends on the internal connection of the stator
windings.
The stator windings are connected directly to the power source.
Internally they are connected in such a way, that on applying AC
supply, a rotating magnetic field is created.
FIGURE 1: A
TYPICAL STATOR
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