Power Electronics and Drive

CHAPTER ONE
Electrical Drive System

By Adane
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 Introduction

• Drive: A combination of prime mover, transmission equipment and

mechanical Working load.

• Electric drive: is electromechanical device for converting electrical

energy to mechanical energy.

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 BASIC ELEMENTS OF ELETCRIC DRIVES

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 Cont..
• Load: usually a machinery to accomplish a given task. Eg-fans, pumps,
washing machine etc.

• Power modulator: modulators (adjust or converter) power flow from the

source to the motor.
• The functions of the power modulator are

• Modulates flow of power from the source to the motor in such a manner that motor is imparted
speed-torque characteristics required by the load.

• It converts electrical energy of the source in the form suitable to the motor.

• Selects the mode of operation of the motor, i.e. Motoring or Braking.

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 Cont..
• Electrical Motors :actual energy converting machine (electrical to mechanical)

Most commonly used electrical motor for speed control applications are the following:

• DC Machines

• Separately-excited and permanent magnet, shunt, series, compound motors and switched reluctance
machines

• AC Machines

• Induction, wound rotor, synchronous, PM synchronous and synchronous reluctance machines.

• Special Machines

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Brush less d.c. motors, stepper motors, switched reluctance motors 5
 Cont..
• Source: energy requirement for the operation the system.

• Control: adjust motor and load characteristics for the optimal mode.
• Sensing Unit
• Speed Sensing (From Motor)

• Torque Sensing

• Position Sensing

• Current sensing and voltage sensing from lines or from motor terminals or from Load

• Temperature Sensing

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 CLASSIFICATION OF ELECTRIC DRIVES
Electric drives may be classified as follows:
1. According to Mode of Operation
• Continuous duty drives
• Short time duty drives
• Intermittent duty drives
2. According to Means of Control
• Manual
• Semi automatic
• Automatic
3. According to Number of machines
• Individual drive
• Group drive
• Multi-motor drive
4. Another main classification of electric drive is
• DC drive
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AC drive 7
 Cont..
According to Number of machines drive systems have three classification
namely

1. Group drive

• One motor is used as a drive for two or more than machines.

• The motor is connected to a long shaft.

• All the other machines are connected to this shaft through belt and pulleys.

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 Cont..
• Advantages:
• Group drive is most economical
• the rating of the motor used may be comparatively less than the aggregate of the individual motors
required to drive each equipment, because all of they may not be working simultaneously.

• Group drive reduces the initial cost of installing a particular industry.

• Cost is less because of investment in one motor which is lesser in HP rating

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 cont..
Disadvantages:

• The use of this kind of drive is restricted due to the following reasons:

 It is not possible to install any machine as per our wish. so, flexibility of lay out is lost.

 The possibility of installation of additional machines in an existing industry is limited.

In case of any fault to the main driving motor, all the other motors will be stopped

immediately

So, all systems will remain idle and is not advisable for any industry.
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 Cont..

Level of noise produced at the site is high.

Because of the restrictions in placing other motors, this kind of drive will result in

untidy appearance, and it is also less safe to operate.

Since all the motors has to be connected through belts and pulleys, large amount of

energy is wasted in transmitting mechanisms. Therefore, power loss is high

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 Cont..
2. Individual drive:

There will be a separate driving motor for each process equipment.

Advantages:
• Machines can be located at convenient places.

• Continuity in the production of the processing industry is ensured to a high level of reliability.

• If there is a fault in one motor, the effect on the production or output of the industry will not be
appreciable.

• Disadvantages:
• Initial cost is very high.
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 Cont..
3. Multimotor drive:

Separate motors are provided for actuating different parts of the driven
mechanism.
Example
• cranes, drives used in paper mills, rolling mills etc.,

• In cranes, separate motors are used for hoisting, long travel motion and cross travel motion.

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 FACTORS INFLUENCING THE CHOICE OF
ELECTRICAL DRIVES
• Nature of electric supply

• Nature of the drive

Whether the particular motor is going to drive individual machine or a group of machines

• Capital and running cost

• Maintenance requirement

• Space ad weight restrictions

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 Cont..

• Environment and location

• Nature of load

• Whether the load requires light or heavy starting torque

• Whether load torque increases with speed remain constant

• Whether the load has heavy inertia which may require longer straight time

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 Cont..
• Electrical characteristics of motor
• Starting characteristics,

• running characteristics,

• speed control and

• Braking characteristics

• Size, rating and duty cycle of motors

Whether the motor is going to the operator for a short time or whether it has to run
continuously intermittently or on a variable load cycle

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 Cont..
• Mechanical considerations
• Type of enclosures, type of bearings, transmission of drive and Noise level.
• Due to practical difficulties, it may not possible to satisfy all the above considerations.
• In such circumstances, it is the experience and knowledge background which
• plays a vital role in the selection of the suitable drive.

• The following points must be given utmost important for the selection of motor.
• Nature of the mechanical load driven
• Matching of the speed torque characteristics of the motor with that of the load
• Starting
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conditions of the load. 17
 Load Conditions In Motor
• The load requirements are in either of
 Speed control
 Torque control

• Depending upon the load requirements the motor has to be chosen

• Classification of loads
• Constant Torque
• Torque linearly dependent on speed
(Ex- motor driving a DC generator connected to a fixed resistance load [generator field value is kept constant])

• Torque proportional to square of speed

(Ex- fans, sentrifugal pumps, propellers)

• Torque inversely proportional to speed

(Ex-milling and boring, machines)
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 Cont..

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 Different type of industrial loads
There are three types of industrial loads under which electric motors are required to work. they are

1. Continuous load

• Load is continuous in nature

Ex- Pumps or fans require a constant power input to keep them operating.

2. Intermittent load
 Motor loaded for short time and then shunt off for sufficiently longer duration. Eg: kitchen mixie.

 Moor loaded for short time and shunt off for short time .

3. Variable or fluctuating load

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 Drive Motor Characteristics
Types of DC machines

1. DC Generator

2. DC Motor

Types of AC machines

1. Transformers
(a) Single phase (b) three phase

2. Alternators

3. Synchronous motor

4. Induction motor
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Single phase (b) three phase 21
 Types of DC motors

Fig Various types of DC motors : (a) Separately excited ,(b) shunt excited ,( c) Series motor , (d)
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compound motor.
 Characteristics of Dc Motors
The performance of DC motor can be judged from its characteristics curves.

1. Electrical characteristics

Torque / Armature current characteristics

Speed / Armature current characteristics

2. Mechanical characteristics

Speed / Torque characteristics

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 DC Separately Excited Motor

Fig. Your text hereequivalent circuit of a DC separately excited motor

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 The Principal equations of DC machine

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 Cont..

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 Speed and Torque separately excited Dc motor

Fig. Speed-torque characteristics of separately excited motor

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 Characteristics of Shunt DC Motor
• Shunt dc motor is a motor whose field circuit gets its power directly across
the armature terminals of the motor

• When the supply voltage to a motor is assumed constant,

• There is no practical difference in behaviour between separately excited and

shunt DC motor.

• Unless otherwise specified, whenever the behaviour of a shunt motor is

described, the separately excited motor is included too.

• The resulting torque-speed characteristic of a shunt dc motor that is just a

straight
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line with a negative slope 28
 The equivalent circuit of a shunt dc motor

Fig. The equivalent circuit of a shunt dc motor

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 Series D.C. Motor
• A series d.c. motor is a dc motor whose field windings consist of relatively few
turns connected in series with the armature circuit.

• A distinct feature of a series motor is that the field current Is is equal to the
armature current IA and the current drawn from the supply IL

(1)

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Fig.1 Series motor circuit
 Induced torque in a series D.C motor
• A series dc motor is due to the fact that the flux is directly proportional to the armature
current, at least until saturation is reached.

• As the load on the motor increases, its flux increases too.

• As seen earlier, an increase in flux in the motor causes a decrease in its speed.

• The result is that a series dc motor has a sharply drooping torque-speed characteristic.
• The developed torque is

(2)

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 Cont..
• The flux in this machine is directly proportional to its armature current (at least until metal
saturates).

• Therefore, the flux in the machine can be given by

(3)

• where Kf is a constant. Thus,

• (4)
Fig. 2 Magnetization Curve of series motor
• Series d.c. motors are therefore used in applications requiring very high torques.

• Example: starter motors in cars, elevator motors, tractor motors etc.

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 Derivation of the torque-speed characteristic

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 Cont.

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 Cont..
• The speed – torque curve of series motor will vary, according to Eq.(9) as
shown in Fig.3.

Fig.3 the torque-speed characteristic of a series dc motor

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 Cont..
• It can be easily, from Eq. (9), being noted that the speed – torque characteristic of
series motor is a hyperbola with asymptote at the speed axis.
• To plot this hyperbola, substituting Te =0 in Eq. (9) yields,

• i.e., the no load speed at the ideal no load running of the series motor is infinite
(excessive speed).
• Therefore, at very small load torques are likely to cause rapid increase in the motor
speed n which is dangerous to the motor mechanical construction (armature
winding, bearings, and commutator structure).
• This is the case when we say that the motor “races”.
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 Cont..
• Series motors can usually protected against the danger of excessive speeds by a positive
connection to their load before starting.

• However, for small motors, below 200W, the mechanical losses in the motor may be
sufficient to prevent excessive no load speed.

• In high power motors, even a threefold increase, n = 3nn , where nn is the normal (rated)
speed may result in mechanical over stresses dangerous to the armature .

• Therefore, these machines should not be run at no load condition.

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 Cont..
• On the other hand, the starting torque Tst can be calculated by setting n = 0 in Eq. (9), thus

• Since the values of RT and Rs are usually very small, hence the starting torque of the series
motor is considerably large compared with that of separately excited and shunt motors.

• This feature makes it preferable to be used to start heavy loads such as electric vehicles,
electric trains and elevators.

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 Compound d.c. Motor
• The mechanical characteristics of the compound motor occupy intermediate position between
the characteristics of the shunt and the series motors due to the fact that the compound motors
contains both the shunt and the series windings.

Fig.4 Compound motor connection diagram.

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 Cont..
• Fig.5 depicts the speed-torque characteristics of the three types of d.c. motors namely, shunt
,series and compound. As is clear from these characteristics, the compound motor at ideal idle
running has an ultimate no load speed no .

• Beside, when compared with shunt motor, the compound machine develop stronger starting
electromagnetic torque , and when compared with series motor , it exhibits more “ rigid ”

mechanical characteristics.

Fig.5 Compound motor

mechanical characteristics as
compared with shunt and
series motors.

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 Applications Of DC Motors

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 Induction Motor
• The induction machine is the most rugged and the most widely used machine in industry.

• The induction machine has a stator and a rotor mounted on bearings and separated from the stator by an air
gap.

• However, in the induction machine both stator winding and rotor winding carry alternating current.

• The alternating current (ac) is supplied to the stator winding machine.

• The induction machine can operate both as a motor and as a generator.

• However, it is seldom used as a generator supplying electrical power to a load.

• The performance characteristics as a generator are not satisfactory for most applications.

• The induction machine is extensively used as a motor in many applications.

• Of all the ac motors the poly-phase induction motor is the one which is extensively used for various kinds of
industrial drives.
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 Induction Motor

Two types of rotor construction is normally

used for three phase induction motor.

•Squirrel-cage rotor: Motors employing this

type of rotor are known as squirrel-cage

induction motors.

•Phase-wound or wound rotor: Motors

employing this type of rotor are variously

known as phase-wound motors or wound

motors or slip-ring motors.

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 Torque–speed characteristics
the torque speed characteristics of
IM can be derived.

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 Cot..

Torque-speed characteristics
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