Specifications

123Motor Control
AC Induction Motor
The AC induction motor (Figure9)
is the workhorse motor for many
industrial applications such as those
for driving pumps, blowers, conveyors,
cranes, etc. It is one of the simplest and
most reliable motor designs and can
range in size from a few watts to many
kilowatts. The induction motor is an
asynchronous motor and is basically an
AC transformer with a rotating shorted
secondary. The primary winding (stator)
is connected to the power source and
the secondary winding (rotor) carries
the induced secondary current creating
a magnetic eld. Torque is produced as
the rotor eld tries to align itself with
the applied rotating stator eld. No
slip rings or commutators are needed
since no source power is physically
connected to the rotor. The most
common designs have three stator
windings and are driven from 3-phase
AC sources. Although direct connection
to AC mains is therefore possible, in
most applications, induction motors
require some form of soft-starter or VFD.
Induction motors “slip” under load. The
amount of slip is directly proportional
to the torque required to drive the load.
Under no-load conditions, no torque
is produced and the rotational speed
is almost exactly the driving frequency
divided by the number of poles in the
stator. These motors are easily speed and
torque controlled by varying the drive
frequency and voltage, respectively.
If constant speed is needed, VFDs
can use position- or speed-detection
feedback to increase the drive frequency
as needed to keep the motor speed
constant under varying loads.
Controllers for AC Induction Motors
AC induction motors operate with
the least torque ripple when the
phase current is sinusoidal. Due to the
inductance of the windings, the phase
can be PWM driven from a xed DC
supply to achieve this current waveform.
The two most common approaches to
induction motor drive include “vector
control” and “direct torque control.”
These techniques are beyond the scope
of this document, but information
is readily available. Suce it to say
that to fully implement these control
techniques, a fairly powerful processor
or DSP is required, but the benets are
many. The result is a VFD (Figure10)
DISPLAY
DRIVER
SWITCH
DEBOUNCER
KEYBOARD
SCANNER
KEYBOARD
SWITCHES
DISPLAY
FRONT PANEL
LOW-SIDE
GATE DRIVERS
HIGH-SIDE
GATE DRIVERS
ANALOG VOLTAGE/
CURRENT SENSORS
IGBT H-BRIDGE
325V DC OR 650V DC
ISOLATIONTRANSCEIVER
DC-DC
CONVERTER
FIELDBUS
RESET
µP
I/0
CURRENT-SENSE AMPS
I/0
TEMPERATURE
SENSOR
AC INDUCTION MOTOR
VR OR
HALL-EFFECT
SENSOR
SENSOR
INTERFACE
VOLTAGE
SUPERVISOR
AC-DC
CONVERTER
AC MAINS (3-PHASE)
230V AC OR 460V AC
VOLTAGE-SENSE AMPS
UART
ADC
ADC
ADC
ADC
ADC
ADC
Figure 10. Variable frequency drive for an AC induction motor.
Figure 9. An AC induction motor.