Introduction to Electric Drives

INTRODUCTION
TO
ELECTRIC DRIVES
Prof. Rozina R. Surani
Asst. Prof.
Electrical Engineering Department
VGEC-Chandkheda
1Prof. Rozina R. Surani 22-Dec-17

 Introduction of Electric Drives.
 Comparison of AC and DC drives.
 Operating Principle of Electric drives.
 Advantages and Disadvantages of AC and DC drives.
 Application of AC and DC drives.

 Motion control is required in large number of industrial and
domestic application like transportation systems, rolling mills,
paper molls, textile mills, machine tools, fans, pumps ,robots,
washing machine etc.
 Systems employed for motion control are called as Drives.
 It may employ any of the prime movers (Diesel or Petrol engines,
Gas or steam turbines , Hydraulic motors and electric motors)
for supplying mechanical energy for motion control.
 Drives employing electric motors are known as Electrical
Drives.
In other words,
 A Drive is a combination of various systems combined together
for the purpose of motion control.

 Do not pollute the environment. Electric energy can be generated and
transported to the desired point economically and efficiently.
 Unlike other prime movers there is no need to refuel or warm-up the motor.
 They are available with wide range of torque, speed and power.
 Electric motors have high efficiency, low no load losses, short time overloading
capability. Compared to other prime movers they have longer life, lower noise,
lower maintenance and cleaner operation.
 Electric braking can be employed. Therefore they are having flexible control
characteristics.
 Can operate in all four quadrants of sped –torque plane. Electric braking gives
smooth deceleration and increase life of equipment compared to other forms
of braking. Considerable saving of energy when regenerative braking is
utilized.
 In the past, Induction and Synchronous motors were employed mainly in
constant speed drives and for variable speed, only DC Motor s were used but
now a days AC motors also used in variable speed drives due to development of
semiconductor converters.The reason is due to the presence of commutator
and brushes, DC motors have number of disadvantages.

 Domestic uses.
 Machine tool drives and Belt Conveyors.
 Cranes and Hoist work.
 Lifts and Elevators.
 Lathes, Milling and Grinding Machines.
 Punches, Presses and Shears.
 Shapers and Slotters.
 Blowers ,Fans and Pumps.
 Drilling machines.
 Air Compressors.
 Electric Traction-Electric trains (In India, 25kV, 50Hz single phase AC
supply ), Electric buses, Trams(tramways) & trolleys.
 Sugar Mills, Flour Mills, Printing Machinery, Wood industry, Cement
Mills, Mining Work, Textile Mills, Paper industry, Ship Propulsion,
Rolling Mills………………………….and Many More ……

The electric drive system has five main functional
blocks:
 A power Source (Utility Supply)
 Power Modulator (Power Electronic Interface)
 A Motor (speed-torque chara. Must be compatible to the
load requirements)
 Mechanical Load (ex. Fans, pumps, robots,
trains,washing m/c)
 Control unit and Sensors

 The power source:
 Utility Power Supply: 1-Phase /3-Phase, 50 Hz in India.
 Low and medium power motors are generally fed from 400
V supply.
 Higher rating motors may be rated 3.3 KV, 6.6 KV, 11 KV
and higher.
 In case of Aircraft and space application 400 Hz AC supply
is generally used.
 In main line traction a high voltage supply is preferred-in
India 25 KV, 50 Hz supply is employed.
 Underground traction system employ low voltage 500-750V
dc .
 In western India (Bombay to Igatpuri) 1500 V dc is used.

The Power Modulator (Power Electronic
Interface):
 Interfaces the motor with the power source and provides the motor
with adjustable voltage, current and frequency. The function of
converters is to convert the electric waveform of the power source to
a waveform that the motor can use. For example the available
power source is AC and the motor is DC series motor, then the
converter converts ac into dc.
 During transient operations such as starting, braking and speed
reversal, it restricts source and motor currents within permissible
value.
 Converts electrical energy of the source in the form suitable to the
motor e.g. if the source is dc and an induction motor is to be used
then power modulator is required to convert dc into variable
frequency ac.

AC to DC converters
/Rectifiers
DC to AC
Converters/Inverters
AC to AC
converters/AC
controller
AC to AC converters/
Cyclo converters
DC to DC
converters/Chopper
10Prof. Rozina R. Surani
Types of Converters
22-Dec-17

 1. AC to DC Converters/ Controlled Rectifiers:
 Input is fixed AC voltage and output voltage may be fix
DC or Variable DC.
 Diode based rectifiers / half controlled thyristor based
/fully controlled rectifier.
 Single phase/ three phase.
2. DC to AC Converters / Inverters:
 Input voltage is fixed DC and output voltage is variable
AC /fixed AC/variable frequency.
 PWM inverters.
 Shape of output waveform is square wave or stepped
wave.

 3. AC to AC Converters/AC controller:
 Input is fixed AC voltage and magnitude of output
voltage will be Variable AC.
 Frequency of input voltage and output voltage is 50 Hz.
4. AC to AC Converters/ Cyclo converters:
 Input is fixed AC voltage and output voltage will be also
fix AC.
 Frequency of input voltage and output voltage will be
different.

5. DC to DC Converters / Choppers/ DC
transformers:
 Input voltage will be fixed DC and output voltage will
be variable DC.
 Output voltage can be varied steplessly by controlling
the duty ratio i.e. turn ON and turn OFF time of a
switch such as IGBTs, GTOs, Power MOSFETs and
Thyristors.

 A Motor:
 Motors commonly used in electrical drives are:
 DC motors:
 shunt, series, compound
 Universal motors
 permanent magnet motors
 AC motors:
 Induction motors-squirrel cage, wound rotor and linear
 synchronous motors-wound field
 Special motors:
 Brushless DC motors
 Stepper motors for position control
 Switched reluctance motors.
 DC servo motor

 Mechanical Load:
Fans, pumps, traction,milling machine, rolling,
textile………………etc.
 Control Unit:
 Controls for power modulator are provided in the
control unit.
 It consists of firing circuits which employ linear and
digital integrated circuits and transistors and a
microprocessor when sophisticated control is required.

 Costly
 Control circuit are complex
 Need trained engineers

Based on
supply
• AC drives
• DC drives
Based on
running
speed
• Constant speed drives
• Variable speed drives
Based on
number
of motors
• Single motor drive
• Multi motor drive
Based on
control
parameter
• Constant torque drives
• Constant power drives

 Advantages:
 AC motors are Cheaper as compared to DC motors.
 AC motors requires less maintenance than DC motors.
 Disadvantages:
 Power converters for AC drives are more complex.
 Power converters for AC drives generates more harmonics in
supply system as well as load systems.
22-Dec-17

Constant Speed Drives and
Variable Speed Drives:

Group
Drive
Individual
Drive
Multi
motor
Drive
Electrical
Drives

 Horsepower = RPM x Torque (ft-lb) /5250
 1 Horsepower (HP) = 746 Watts = 0.746 kWatts
 This torque formula implies that the torque is
proportional to the horsepower rating and inversely
proportional to the speed.
We can categorize drive applications by their
operational torque requirements:
 Constant Torque Loads
 Constant Horsepower Loads

What is an AC drive.mp4
22-Dec-17

The process of selecting an adjustable AC or DC
drive is one where load is primary consideration.
When considering load characteristics, the
following should be evaluated:
 What type of load is associated with the
application ?
 What is the size of the load?
Load Characteristics

Does the load involve heavy inertia ?
What are the motor considerations ?
Over what speed range are heavy loads
encountered ?
Load Characteristics

Constant Torque Load
In this group, the torque demanded by the load is
constant throughout the speed change
25Prof. Rozina R. Surani22-Dec-17

The load requires the same amount of torque at low
speeds as at high speeds. Loads of this type are
essentially friction loads
Examples: Conveyors, Extruders, and Surface
Winders

Constant horsepower Load
The horsepower demanded by the load is constant
within the speed range. The load requires high
torque at low speeds.
Examples: Center-driven winders and Machine tool
spindles

Variable torque load

Constant horsepower, torque varies inversely with
speed.
 Applications: Metal cutting tools operating over
wide speed range, mixer, extruder and special
machines where operation at low speed may be
continuous
Constant torque, horsepower varies as the speed
 Applications: General machinery hoists,
conveyors, printing press
 .

Horsepower varies as square of the speed, torque varies
with speed
 Applications: Positive displacement pumps, some
mixers, some extruders
Horsepower varies as cube of the speed, torque varies
as square of speed
 Applications: All centrifugal pumps and some fans
(Note that fan power may vary as the power of speed)

High inertial loads
 Applications: Are typically associated with
machines using flywheel to supply most of the
operating energy, punch press

 A DC motor may operate
in one or more modes (or
quadrant) in variable
speed applications. The
major advantage of using
DC motor is that the ease
of its control.
 The speed of the DC motor
is controlled by applying a
variable DC input for
below rated speed control.
For above rated speeds, the
motor is controlled by
applying variable current
through its field winding.
 For reversing the direction
of rotation, either polarity
of the supply voltage
(which is applied to
armature terminals) or the
direction of field current
has to be changed.
 By using DC motors, it is
possible to obtain smooth
speed control over a wide
range in clockwise as well
as anti-clockwise
directions.
Prof. Rozina R. Surani 3222-Dec-17

 The torque of a DC motor is
proportional to the armature current
which in turn depends on the difference
between back emf and applied voltage.
 Therefore, it is possible to make the
motor to develop positive or negative
torque simply by controlling the applied
voltage to a greater or lesser than the
back emf.
 Thus an armature controlled DC
machine is inherently capable of
operating different modes or quadrants,
generally it is known as four-quadrant
operation of a motor.
 In multi-quadrant operation or four
quadrant operation, motor accelerates or
decelerates depending on whether motor
torque is lesser or greater than load
torque.
 During motor acceleration, it should
supply not only the load torque, but an
additional component of load current to
overcome the inertia.
 Motor positive torque produces the
acceleration in forward direction. In this, the
motor speed is positive when the motor is
rotating in forward direction.
 During motor deceleration, the resultant or
dynamic torque has a negative sign. This
torque assists with motor developed torque
and maintains the motion by extracting the
energy from stored energy.
 Hence the motor torque is considered as
negative if it produces deceleration. A motor
can be controlled in such a way that it operates
in two cases; motor action and braking action.
 Motor action converts the electric energy into
mechanical energy and it produces forward
motion, hence it called as motoring action,
whereas braking action converts mechanical
energy to electrical energy which gives forward
braking motion, it is termed as generator.
Prof. Rozina R. Surani 3422-Dec-17

The benefits of Electric Drives are to be
quantified and Energy Saving has been the
prime reason for employing electric drives.
Improved process control and increase life
expectancy of Electric Motors are another
added advantages.
22-Dec-17

Introduction to Electric Drives

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Introduction to Electric Drives