Chapter 1 (a)

Introduction to Electrical Drives

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 1.1 Definition

• Drives
 Systems employed for motion control.
 may employ any prime movers such as diesel or petrol
engines, gas or steam turbines, hydraulic motors for
supplying mechanical energy for motion control
• Electrical Drives
 Drives employing electric motors
• Motion control is required in large number of industrial and
domestic applications like transportation system, rolling mills,
paper mills, machine tools etc.

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 Definition…cont

Electric drives for motor is used to draw

electrical energy from the mains and
supply the electrical energy to the
motor at whatever voltage, current and
frequency necessary to achieved the
desired mechanical output

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 1.2 Introduction

 Study of electric drive system involves controlling

electric motor in steady state and dynamic operation,
taking into account the characteristic of mechanical
loads and behaviours of power electronic converter.

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 1.3 Requirements of an Adjustable
Speed Drive
1) Stable operation :
The torque speed characteristic of the motor should be
such that stable operation of the drive is assured in all the
four quadrants over a wide range of speeds. The drive
must be controllable with regenerative braking to
maintain constant speed if the load overhauls the motor.

2) Have a good transient response :

Drive should return to its original operating condition very
quickly in case of disturbance. If there is a step change in
torque or speed the drive must attain its new operating
point quickly without any overshoots.

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 1.4 Basic Components of An Electric
Drive System
 5 main functional blocks:
1) Power source
2) Electronic converter
3) Motor
4) Mechanical load
5) Controller

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 Basic Components of An Electric
Drive System…cont
1) Power source
 Provides the energy that the drives system needs.
 2 major types is AC and DC
 Sources can be either single phase or three phase systems
 3- phase 50 Hz supply is use for normal industrial and commercial
electrical drive applications
2) Converters
 Main function is to transform the waveform of a power source to
that required by an electric motor in order to achieve the desired
performance.
 Provide adjustable voltage, current and/or frequency to control
the speed, torque or power of the motor.
 4 basic types of converters:
1. dc to ac
2. dc to dc
3. ac to dc
4. ac to ac

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 Basic Components of An Electric
Drive System…cont
2) Converters

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 Basic Components of An Electric
Drive System…cont
3) Controllers
 Basic function is to monitor system variables,
compare them with some desired values and then
readjust the converter output until the system
achieves the desired performance.
4) Motors
 Electric motors should be selected to match the
intended performance of the load.
 Types of motors:
1) DC – separately excited, shunt, series, compound.
2) AC – Induction and synchronous
 Fed from DC and AC supply.
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 Basic Components of An Electric
Drive System…cont
5) Mechanical Load
 Exhibit wide variations of speed- torque characteristics. Load torques are
generally speed dependent.
 Load characteristics can be grouped into one or more of the following types:
1) Torque independent of speed
 The power that the load consumes is linearly dependent on speed.
 Hoist or the pumping of water or gas against constant pressure
2) Torque linearly dependent on speed
 Mechanical power is proportional to the square of speed.
 Motor driving a dc generator connected to a fixed- resistance load, and the field of the
generator is constant
3) Torque proportional to the square of speed
 Load power requirement is proportional to the cube of the speed and may excessive at high
speed.
 Fans, centrifugal pumps and propellers.
4) Torque inversely proportional to the speed
 Usually requires a large torque at starting and at low speed.
 Milling and boring machine.
 This is why the motor of an electric saw does not always get damaged when the saw disk is
blocked

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 Centrifugal pump

Warman centrifugal pump in a Single Stage Radial Flow

Coal Handling and Preparation Centrifugal Pump
Plant (CHPP) application
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 Hoist

Builder's hoist, with A hoist on the Trump

small petrol engine International Hotel &
Tower-Chicago
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 1.5 Advantages using electrical drive

1) Flexible control characteristics

2) Available in wide range of torque, speed and power
3) Do not pollute the environment
4) Can operate in all the four quadrants of speed- torque
characteristics.
5) Adaptable to almost any operating conditions
6) High efficiency, low no load losses and considerable short
time overloading capability

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 1.6 Disadvantages using electrical
drive
1) The power failure completely disabled the whole of the
system.
2) The application of the drive is limited because it cannot use
in a place where the power supply is not available.
3) It can cause noise pollution.
4) The initial cost of the system is high.

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1.7 Factor affecting drive selection

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1.8 DC Vs AC Drives

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Thank You For Your Attention

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 Steady state
 A condition of a physical system or device that does not change over time, or in
which any one change is continually balanced by another, such as the stable
condition of a system in equilibrium.
 A stable condition that does not change over time or in which change in one direction
is continually balanced by change in another.
 A non equilibrium state of a system through which matter is flowing and in which all
components remain at a constant concentration.

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Dynamic or transient condition

 Occur in electric drive systems when the operating

point changes from one steady state condition to
another, following a change introduced in system
variables.
 These variables may be mechanical, such as speed,
torque or electrical such as voltage, current.

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