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AC induction motor fundamentals
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By
Rakesh Parekh, Microchip Technology
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Industrial Control Designline
(05/07/2009 9:46 AM EDT)
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Introduction
AC induction motors are among 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.
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.
Rotor
The rotor is made up of several thin steel laminations
with evenly spaced bars, which are made up of
aluminum or copper, along the periphery. In the most
popular type of rotor (squirrel cage rotor), these bars
are connected at ends mechanically and electrically by
the use of rings. Almost 90% of induction motors have
squirrel cage rotors. This is because the squirrel cage
rotor has a simple and rugged construction. The rotor
consists of a cylindrical laminated core with axially
placed parallel slots for carrying the conductors. Each
slot carries a copper, aluminum, or alloy bar. These
rotor bars are permanently short-circuited at both ends
by means of the end rings, as shown in Figure 2. This
total assembly resembles the look of a squirrel cage,
which gives the rotor its name. The rotor slots are not
exactly parallel to the shaft. Instead, they are given a
skew for two main reasons.
The first reason is to make the motor run quietly by
reducing magnetic hum and to decrease slot
harmonics.
The second reason is to help reduce the locking tendency
of the rotor. The rotor teeth tend to remain locked
under the stator teeth due to direct magnetic attraction
between the two. This happens when the number of
stator teeth are equal to the number of rotor teeth.
Click here to read the rest of the paper.
Included is the basics of an AC induction motor;
the
different types, their characteristics, the selection criteria for
different applications and basic control techniques.
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