Ain Shams University
Faculty of Engineering
CHEP
Energy Senior
Student Name: Khaled Gamal Hafez Abulazm
Student ID: 12p5129
Course Instructor: Dr. Hany M. Hasanien
Course Name: Electric Drives
Course Code: ERGY 553
Subject: Report on ac drives control
strategies
Due Date: 21-May-2017
Introduction
Ac motors exhibit highly coupled, nonlinear, and
multivariable structures as opposed to much
simpler decoupled structures of separately
excited dc motors.
The control of ac drives generally requires
complex control algorithms that can be performed
by microprocessors or microcomputers along with
fast-switching power converters. They require
control of frequency, voltage, and current for
variable-speed applications. The power
converters, inverters, and ac voltage
controllers can control the frequency, voltage,
or current to meet the drive requirements. These
power controllers, which are relatively complex
and more expensive, require advanced feed-back
control techniques such as model reference,
adaptive control, sliding mode control, and
field-oriented control.
The ac motors have a number of advantages; they
are lightweight (20 to 40% lighter than
equivalent dc motors), are inexpensive, and have
low maintenance compared with dc motors. Ac
drives are replacing dc drives and are used in
many industrial and domestic applications.
Types of ac drives
1. Induction motor drives
2. Synchronous motor drives
Induction motor drives
The speed and torque of induction motors can be
controlled by
1. Stator voltage control
2. Frequency control
3. Stator voltage and frequency control
4. Rotor voltage control
5. Controlling number of poles
To meet the torque-speed duty cycle of a drive,
the voltage, current, and frequency control are
normally used.
1. Stator voltage control
In this type of control the frequency is
constant and stator voltage varied to vary the
speed of induction motor. The stator voltage
can be varied by three-phase
Ac voltage controllers,
Voltage-fed variable dc-link inverters
Pulse-width modulation (PWM) inverters.
The torque produced by running three phase
induction motor is given by low slip region
(sX)2 is very very small as compared to R2. So,
it can be neglected. So torque becomes
Since rotor resistance, R2 is constant so the
equation of torque further reduces to
We know that rotor induced emf E2 V. So, T
sV2.
However, due to limited speed range
requirements, the ac voltage controllers are
normally used to provide the voltage control.
The ac voltage controllers are very simple. AC
voltage controller is an electronic module
based on either thyristors, TRIACs, SCRs or
IGBTs, which converts a fixed voltage, fixed
frequency alternating current (AC) electrical
input supply to obtain variable voltage in
output delivered to a resistive load.
Modes of operation
On-and-off control
Phase angle control
Frequency control
Actual speed N is given:-
The actual speed can be changed by changing
synchronous speed. But synchronous speed Ns can
be changed by the changing the stator frequency
F so theoretically speed can be control by only
F .
If the voltage is maintained fixed at its rated
value while the frequency is reduced below its
rated value, the flux increases.
This would cause saturation of the air-gap flux,
and the motor parameters would not be valid in
determining the torque-speed characteristics.
At low frequency, the reactance decreases and
the motor current may be too high. This type of
frequency control is not normally used.
Stator voltage and frequency control
In this technique torque developed by the
Induction motor is directly proportional to the
V/F ratio. The region below the base or rated
frequency should be accompanied by the
proportional reduction of stator voltage so as
to maintain the air gap flux constant. If the
ratio of voltage to frequency is kept constant,
the flux remains constant. By varying the
voltage and frequency the torque and speed can
be varied. The torque is normally maintained
constant while the speed is varied.
Rotor voltage control
In a wound-rotor motor, an external three-phase
resistor may be connected to its slip rings. The
developed torque may be varied by varying the
resistance Rx.
The typical torque-speed characteristics for
variations in rotor resistance.
This method increases the starting torque while
limiting the starting current. However, this is
an inefficient method and there would be
imbalances in voltages and currents if the
resistances in the rotor circuit are not equal.
A wound-rotor induction motor is designed to
have a low-rotor resistance so that the running
efficiency is high and the full-load slip is
low. The increase in the rotor resistance does
not affect the value of maximum torque but
increases the slip at maximum torque. The wound-
rotor motors are widely used in applications
requiring frequent starting and braking with
large motor torques (e.g., crane hoists).
Because of the availability of rotor windings
for changing the rotor resistance, the wound
rotor offers greater flexibility for control.
However, it increases the cost and needs
maintenance due to slip rings and brushes.
The wound-rotor motor is less widely used as
compared with the squirrel-case motor.
The three-phase resistor may be replaced by a
three-phase diode rectifier and a dc converter,
as shown in Figure 16.6a, where the gate-turn-
off thyristor (GTO) or an insulated-gate bipolar
transistor (IGBT) operates as a dc converter
switch.
The inductor Ld acts as a current source Id and
the dc converter varies the effective resistance
Re = R (l - k)
where k is the duty cycle of the dc converter
and the motor speed can be controlled by varying
the duty cycle.
The portion of the air-gap power, which is not
converted into mechanical power, is called slip
power. The slip power is dissipated in R.
Controlling number of poles
The synchronous speed is given by,
So it is possible to change the synchronous
speed by changing the number of poles.
The no of poles can be changed by changing the
connection of stator winding with the help of
simple switching.
The no of the poles in the ratio of 2:1 .so two
different speeds are obtained.
