Ee8353 unit i

EE 8353
Electrical Drives and Controls

Syllabus
• Unit-I Introduction
• Unit-II Drive Motor Characteristics
• Unit-III Starting Methods
• Unit-IV Speed Control of D.C. Drives
• Unit-V Speed Control of A.C. Drives
2

Unit-I Introduction
– Basic Elements, Types of Electric Drives
– Factors influencing the choice of electrical drives
– Heating and cooling curves
– Loading conditions and classes of duty
– Selection of power rating for drive motors with regard to
thermal overloading and Load variation factors
3

Unit-II Drive Motor Characteristics
– Mechanical characteristics
– Speed-torque characteristics of various types of
load and drive motors
– Braking of electrical motors – D.C motors
– Single and three phase induction motors
4

Unit-III Starting Methods
– Types of D.C motor starters
– Typical control circuits for shunt and series motors
– Three phase squirrel cage and slip ring induction
motors
5

Unit-IV Conventional and Solid state Speed
Control of D.C. Drives
– Speed control of DC series and shunt motors
– Armature and field control
– Ward-Leonard control system
– Using controlled rectifiers and DC choppers
– Applications
6

Unit- V Conventional and Solid state
Speed control of A.C. Drives
– Speed control of three phase induction motor
– Voltage control, voltage / frequency control,
– Slip power recovery scheme
– Using inverters and AC voltage regulators
– Applications.
7

What is to be studied now? III Sem.
• EE 8361 Electrical Engg. Laboratory
– 1. Load test on D.C. Shunt & Series motor
– 3. Speed control of DC shunt motor (Armature, Field
control)
– 8. Load test on 3- Squirrel cage induction motor
– 9. Speed control of 3- Slip-ring induction motor
– 10. Load test on 1- Induction motor
– 11. Study of DC & AC Starters
– Six experiments are very important for this subject
8

What is a Drive?
• Verb
– 1. operate and control the direction and speed of
a motor vehicle
• “he got into his car and drove off”
– Synonyms: operate, pilot, steer, handle, manage,
guide, direct, navigate
• “You can’t drive a car without a Speedometer”
• Travel by car, go by car, motor;
• Informal travel on wheels,
• “he drove to the police station”
9

What is a drive (continued)
• Propel or carry along by a force in a specified
direction
– “the wind will drive you onshore”
• Synonyms: power, propel, move, push
• “a two-litre engine drives the front wheels
10

What is a drive (continued)
• Psychology
– An innate, biologically determined urge to attain a
goal or satisfy a need.
• “her emotional drives”
11

Drive
• Alternative name for a medium that is capable
of storing and reading information.
• Examples
– CD-ROM drive
– DVD drive
– Virtual drive
– ZIP drive
– Portable drive
12

Driver
• Noun: a person who drives a vehicle
– “a taxi driver”
– Synonyms: motorist, chauffeur, pilot, operator,
engineer
• a wheel or other part in a mechanism that
receives power directly and transmits motion
to other parts.
13

Speed
• Why to vary Speed?
– FAN
• Summer/Winter
• Day/Night
– Elevator
• Starting/Stopping
14

Torque
• Why to vary Torque?
– Why does Torque of an engine vary with RPM?
– How it is related to Engine Power?
• P = (2  NT)/60
15

Electrical Drives
Drives are systems employed for motion control
Require prime movers
Drives that employ electric motors as
prime movers are known as Electrical Drives
https://www.youtube.com/watch?v=2Gjs7IPOCXs
• A drive consists of various systems combined together for the
purpose of motion control or movement control. This is called
drives
• The drives which employ electric motors for motion control are
called as electrical drives.

• About 50% of electrical energy used for drives
• Can be either used for fixed speed or variable speed
• 75% - constant speed, 25% variable speed (expanding)
Electrical Drives

Conventional electric drives (variable speed)
• Bulky
• Inefficient
• inflexible

Modern electric drives (With power
electronic converters)
• Small
• Efficient
• Flexible

Advantages of Electrical Drives
1. They have flexible control characteristics.
2. The steady state and dynamic characteristics of electrical drives
can be shaped to satisfy load requirements.
3. Speed, torque, power can be controlled in wide limits.
4. They are adaptable to any operating condition.
5. Availability of semiconductor converters employing thyristors, power
transistors, IGBTs, GTOs, linear and digital ICs, and microcomputers
have made the control characteristics even more flexible.

Basic elements of an electric drive/General
electric drive system/Block diagram of an
electrical drive system.

Sources
• In India 1- phase and 3- phase 50 Hz ac supplies are readily available.
• Low power drives are fed from 1- phase supply.
• Rest of the drives are powered by 3 phase source.
• In case of traction drives, even at high power levels, 1- phase supply is
used.
• Most drives are powered from ac source either directly or through a
converter link.

Power Modulators
Functions :
1. Modulates the flow of power from the source to the motor
2. During transient operations, such as starting, braking and speed
reversal, it restricts source and motor currents within permissible
value
3. 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 employed ,
then the power modulator is required to convert dc into a variable
frequency ac.
4. Selects the mode of operation of the motor, i.e. motoring or braking.

Classification of power modulators
Power modulators are classified as follows :
Need for a converter arises when the nature of the electric power is
different than what is required for the motor.
1) Converters ( AC to DC)
2) Inverters (DC to AC)
3) AC voltage controllers (AC to AC)
4) DC choppers (DC to DC)
5) Cyclo converters (frequency conversion)

Converters
AC to DC Converter :
The above given converter converts fixed ac voltage in to fixed dc
voltage
Such converters are called as Uncontrolled rectifiers.

Inverters(DC to AC):
Inverters are employed to get a variable frequency ac supply from a
dc supply.

AC Voltage Controllers (AC to AC):
Ac voltage controllers are employed to get variable ac voltage of the
same frequency.

DC Choppers (dc – dc) :
• They are used to get variable voltage dc from a fixed voltage dc
and are designed using semiconductor devices such as power
transistors,
• IGBTs, GTOs, power MOSFETs and thyristors. Output voltage can
be varied steplessly by controlling the duty ratio of the device by
low power signals from a control circuit.

Cycloconverters (Frequency conversion):
• It converts fixed voltage and fixed frequency ac to variable voltage
and variable frequency ac.
• They are built using thyristors and are controlled by firing signals
derived from a low power control circuit.
• Output frequency is restricted to 40% of supply frequency in order to
keep harmonics in the output voltage and source current within
acceptable limits.

Motors
Most commonly used electrical drives are
• DC motors : Shunt, Series, compound and permanent
magnet
• Induction motors: Squirrel cage, wound rotor and linear
• Synchronous motors: wound filed and permanent magnet.
• Other motors: Brushless DC motors, stepper motor and
reluctance motors.

Sensing Unit
(i) Speed sensing:
• It is required for implementation of closed loop speed
control schemes.
• Speed is usually sensed by using tachometers.
• When very high speed accuracies are required, as in
computer peripherals and paper mills etc., digital
tachometers are used.
(ii) Current sensing:
• Use of current sensor employing hall effect.
• Involves the use of non-inductive resistance shunt in
conjunction with an isolation amplifier, which provides
amplification and isolation between power and control
circuits.

Control Unit
• Controls for power modulator are provided in the control
unit.
• Nature of the control unit for a particular drive depends on
the power modulator that is used.
• When semiconductor converter are used, the control unit
will consists of firing circuits, which employ linear and
digital integrated circuits and transistors and a
microprocessor when sophisticated control is required.

Types of Electric drives
Generally classified into 3 categories:
Group Drive
Individual Drive
Multimotor Drive
Group Drive :
• If several group of mechanisms or machines are organized on one
shaft and driven or actuated by one motor, the system is called a
group drive or shaft drive.
• The various mechanisms connected may have different speeds.
• Hence the shaft is equipped with multistepped pulley and belts
for connection to individual loads.
• Application: Floor/rice mill use a single motor drive to do more tasks
parallely.

Advantages
• In this type of drive a single machine whose rating is smaller than
the sum total of all connected loads may be used, because all the
loads may not appear at the same time.
• This makes the drive economical, even though the cost of the shaft
with stepped pulleys may seem to be high.
• Less installation cost.
• Any Fault that occurs in the driving motor renders all the driving
equipment idle.
• Efficiency low because of losses occurring in the energy transmitting
mechanisms (Power loss)
• Not safe to operate and Noise level at the working spot is high.
• The location of the mechanical equipment being driven depends on
the shaft and there is little flexibility in its arrangement.
• Single motor drives a no of machines through belt from common
shaft.
Disadvantages

Individual drive
• If a single motor is used to drive or actuate a given mechanism and
it does all the jobs connected with this load , the drive is called
individual drive.
• Application is lathe machine.
• All the operations connected with operating a lathe may be
performed by a single motor.
• In lathe machine, a single motor drives a shaft, moves the feed,
drives a cooling and lubricating pumps.
• If some part require different speeds, the gearing arrangement is
used.
• In some cases it is possible to have the drive motor and driven load
in one unit.

Disadvantages
The efficiency may become poor over several
operations , due to power loss.
Advantages
• No dependency on other drives.
• More efficient and easier to install.
• Complete operation control is possible .
• Cleaner and safer.

Multimotor drive
• Each operation of the mechanism is taken care of by a separate drive
motor.
• The system contains several individual drives each of which is used
to operate its own mechanism.
• Separate motors are provided for actuating different parts of the
driven mechanism.
• Applications:
Metal cutting machine tools,
Paper making machine, travelling cranes, rolling mills, etc.
• Such a system facilitates automatic control for each operation,
increasing the overall productivity and ensures minimum running
conditions of the various mechanism.

Advantages
• Each machine is driven by a separate motor it can run and stopped
as desired.
• Machines not required can be shut down and also replaced with a
minimum of dislocation.
• There is a flexibility in the installation of different machine’s.
• In the case of motor fault, only its connected machine will stop
where as others will continue working undisturbed.
• Absence of belts and line shafts greatly reduces the risk of accidents
to the operating personnel.
Disadvantage
• Initial high cost.

ABB Rolling mill drive system
Medium Voltage AC Drive Series
ACS 6000

Factors affecting the Selection of Electric Drive
Following are the factors which influence the selection of motor to drive
the load :
Limits of speed range :
• The range over which the speed control is necessary, for the load,
similarly how hard is it to control the speed and the speed regulation
also affects the choice of the motor.
Efficiency :
• The motor efficiency varies as load varies so the efficiency
consideration under variable speed operation affects the choice of the
motor

Braking :
• The braking requirements from the load point of view.
• Easy and effective braking are the requirements of a good drive.
Starting requirements :
• The starting torque necessary for the load, the corresponding starting
current drawn by the motor also affects the selection of drive.
Power factor :
• The running of motors with low power factor value is not economical.
• While the power factors varies with the load conditions in some
motors.
• Hence the type of the load and the running power factor of the motor
are the essential considerations while selecting a drive.

Load factor :
• There are variety of types of load conditions like continuous,
intermittent, impact etc.
• Such load variation factor and duty cycle of the motor influences the
selection of drive.
Availability of supply:
• The motors are available are a.c. or d.c. drive.
• But the availability of supply decides the type of the motor to be
selected for the drive.
Effects of supply variations :
• There is a possibility of frequency supply variations.
• The motor must be able to withstand such supply variations.

Economical aspects :
• The size and rating of motor decides its initial cost while various
losses, temperature rise decides its running cost.
• The economical aspects must be considered while selecting a
drive.
Reliability of operation :
• It is important o study the conditions of a stable operation of reliability
of operation of an electric drive.
Environmental effects :
• It is possible that the atmosphere where an electric drive is to be used
may contain some chemical gases, fumes, humidity, etc.
• Such a contaminated atmosphere or a humid atmosphere also affects
the choice of motor for a drive.

Power losses and Heating of Motors
An electric motor consists of following losses
1. Copper losses Due to armature and field winding
2. Core losses Due to hysteresis in the magnetic material.
3. Mechanical losses Due to friction and windage.
Without cooling system, the motor cannot dissipate heat to the external
medium. Therefore its temperature increases to very high value. Hence
it must be provided with cooling unit to limit the temperature rise to a
permissible value.

Heating Curves
• Assumptions
– Machine is considered to be a homogeneous body having
a uniform temperature gradient, i.e., it has the same
temperature throughout the body. (All the points at which
the heat is generated have the same temperature; all the
points at which the heat is dissipated to the cooling
medium are also at the same temperature)
– Heat dissipation taking place is proportional to the
difference of temperatures of the body and surrounding
medium. No heat is radiated.
– Rate of dissipation of heat is constant at all temperatures
59

Basics
• Integral of dx/x = log x
• Integral of dx/(x-a) = log (x-a)
• Integral of dx/(a-x) = log(a-x)
62

Heating Curves (contd..)
68
0
m
Time (t)
Figure shows the heating curve of the machine

Cooling curve
• If the machine is switched off from main supply or when load
on machine is reduced, the machine cools.
• It cools to the ambient temperature when it is switched off.
• It cools to the temperature attained by power losses at reduced
load.
• When the machine is switched off, there is no heat generation
and all heat stored in the machine is dissipated to
surroundings.
• Cooling takes place when heat generation is less than heat
dissipated.
69

• X-axis – Time (t)
• Y-axis Temperature
76

Loading Conditions
• An electric motor, under steady state operation, develops
electromagnetic torque of such a magnitude which can
counterbalance the actual load torque TL of the connected
equipment and an opposing torque Tmech corresponding to the
losses that take place in gear and transmission mechanisms.
• Under transient conditions, the motor torque has to
overcome the inertia torque Tdyn also.
• Hence, in general, the torque developed by the motor should
be expressed as T = TL + Tmech + Tdyn
77

Loading Conditions (Continued)
• The combined load torque (TL + Tmech) is determined from the
torque-time plot of the connected load. A typical example is
shown in Fig. a).
• In order to determine the variation of inertia torque with
respect to time, the speed-time curve, an example of which is
shown in Fig. (b) and the moment of inertia of the rotating
masses J must be known.
• Now with the help of the equation, T = TL + Tmech + Tdyn it will
be possible to obtain the torque-time curve of the driving
motor (Fig c) which is called duty cycle of the motor.
78

Loading Conditions (Continued)
• Since, initially J remains unknown, the torque-time graph can
be plotted by taking into account only TL & Tmech.
• After determining the power rating of the machine using the
above torque-time graph, it can be corrected for the presence
of inertia torque by increasing the rating obtained by 15-20
percent.
• Knowing J of the chosen motor, the exact torque-time curve
can be plotted and a more correct estimation of the rating of
the driving motor can be made.
79

Classes of motor duty
Duty cycle of a motor :
• Relationship between the active (operating) time and the inactive
(resting time) of an equipment or machine.
• In other words, it is expressed as the ratio of active time (operating)
to the total time period.
Electric motors, for example, are rated on the basis of continuous
duty (non-stop operation lasting an hour or more) or intermittent
duty (alternate period of rest and operation lasting, 5, 30, or
60 minutes).
Duty cycle = Active (operating) time period (or) ON time
Total time period (ON time + OFF time)

Types of motor duty
The duty cycles of the motor can be classified into Eight categories as
follows:
i. Continuous duty
ii. Short time duty
iii. Intermittent periodic duty
iv. Intermittent periodic duty with starting
v. Intermittent periodic duty with starting and braking
vi. Continuous duty with intermittent periodic loading
vii. Continuous duty with starting and braking
viii. Continuous duty with periodic speed changes

Continuous Duty
 It denotes the motor operation at constant load torque for long
duration of time.
 As a result the temperature of the motor reaches steady state value.
 This duty is characterized by a constant motor loss.
Here N indicates the duration of the operation.
θmax indicates maximum temperature rise.
Duty Cycle = N = 1
N

Examples - Continuous Duty
Motors used for :
1. Compressors.
2. Fans.
3. Centrifugal pumps.
4. Paper mill drives.
5. Conveyors.
Oilfield centrifugal pump Vacuum pump in paper mills

Short time duty
 This denotes the operation at constant load during a given time,
followed by a rest of sufficient duration.
 In this, time of drive operation is considerably less than the heating
time constant and machine is allowed to cool off to ambient
temperature before the motor is required to operate again.
Duty Cycle = N
N+R
N indicates the duration of operation.
R indicates the period of rest.
R

Examples - Short time duty
Motors used for :
1.Domestic appliances like mixer.
2. Battery charging units.
3. Lock gates.
4. Bridges.

Intermittent duty
 It consists of periodic duty cycles, each consisting of a period of
running at a constant load and a rest period.
 Neither the duration of running period is sufficient to raise the
temperature to a steady-state value, nor the rest period is long enough
for the machine to cool off to ambient temperature.
 In this duty, heating of machine during starting and braking operations
is negligible.
Duty Cycle = N
N+R
N indicates duration of the operation.

Examples - Intermittent duty
Motors used for :
1. Hoist.
2. Lift.
3. Traction motors.
4. Trolley buses.
Gearless machine for elevators (lifts)

Components of gearless machine Geared machine for elevators (lift)

Intermittent periodic duty with starting
 This is intermittent periodic duty where heat losses during starting
cannot be ignored.
 Thus, it consists of a period of starting, a period of operation at a constant
load and a rest period.
 The operating and rest periods being too short for the respective steady
state temperature to be attained.
 In this duty, heating of machine during braking is considered to be
negligible, because mechanical brakes are used for stopping or motor is
allowed to stop due to its own friction.
Duty Cycle = S + N
S+ N+R
S indicates starting period.

Examples :
Motors used for
1. Machine tools
2. Metal cutting lathes.

Intermittent periodic duty with starting and braking
 This is the intermittent periodic duty where heat losses during starting
and braking cannot be ignored.
 Thus, it consists of a period of starting, a period of operation with a
constant load, a braking period with electrical braking and a rest period;
with operating and rest periods being too short for the respective steady
state temperatures to be attained.
Duty Cycle = S + N + B
S+ N+ B+R
B indicates the period of braking.

Examples :
Motors used for
1. Suburban electric trains.
2. Billet mill drive.

DC Series motor
Synchronous motor

Continuous duty with intermittent periodic loading
 It consists of periodic duty cycles, each consisting of a period of
running at a constant load and a period of running at no load, with
normal voltage across the excitation winding.
 Again the load period and no load period being too short for the
respective temperatures to be attained.
 This duty is distinguished from the intermittent periodic duty by the
fact that a period of running at a constant load is followed by a period
of running at no load instead of rest.
N indicates duration of the operation.
V indicates operation on no load
condition.
Duty Cycle = N
N + V

Examples - Continuous duty with intermittent
periodic loading
Motors used for :
1. Pressing.
2. Cutting.
3. Shearing and
4. Drilling machine drives.

Continuous duty with starting and braking
Consists of periodic duty cycle, each having
 a period of starting,
 a period of running at a constant load and
 a period of electrical braking
 there is no period of rest.
The duty cycle for this class is 1.
Duty Cycle = S + N + B
S + N + B
B indicates the period of braking.

Examples - Continuous duty with starting
and braking
Blooming mill
Motor room which houses the motor that
droves the heavy mill bloom rolls

Continuous duty with periodic changes in speed
 This class indicates a sequence identical duty cycle, each having a
period of running at one load and speed, and another period of
running at different speed and load.
 Again both operating periods are too short for respective steady-state
temperatures to be attained.
 Further there is no period of rest.

N1, N2 and N3 indicates operation at three different motor speeds.
B1, B2 is the duration of electric braking.
S is the duration of starting.
Duty Cycle = S + N1 = B1 + N2 = B2 + N3
X X X
where X = S + N1 + B1 + N2 + B2 + N3

Selection of Power rating for Drive motors with
regard to load variation factors
or
Determination of Motor Ratings
108

a) Continuous duty and constant load
109

b) Continuous duty and variable load
113
• The calculation of rating for this type of load is on the
approximation that the actual variable motor current
according to the load can be replaced by an equivalent Ieq
which produces same losses in the motor as actual current
• This equivalent current Ieq is determined as:
• Motor loss P1 = Constant loss PC ; independent of Load
(iron loss & friction loss) +
Copper loss PCU; load dependent
(variable loss)

Fluctuating Load pattern
114
I
I2
I1 I4
I3 In

c) Short time duty
• In short time duty, time of motor operation is considerably
less than the heating time constant (1) and motor is allowed
to cool down to the ambient temperature before it is required
to operate again.
• If a motor with a continuous duty power rating of Pr is
subjected to a short time duty load of magnitude Pr, then the
motor temperature rise will be far below.
• The maximum permissible value θper (is never attained) and
the motor will be highly underutilised.
• Therefore motor can be overloaded by a factor K(K >1) such
that the maximum temp. rise just reaches the permissible
value θper.
117

d) Intermittent periodic duty
122

Types of Enclosures
• Enclosed within a cover to protect them from dust – as well as
to protect the operating personnel from coming in contact
with them.
• Motors used under different environments will require
different types of enclosures.
• Type must be specified along with the type of motor when
ordering for a particular application
123

Screen protected type
• Enclosure of this motor has large openings, which are covered
by wire mesh screens. This makes ventilation possible, but
doesn’t protect the motor from dirt or dust. Provides
protection from , coming in contact with the motor, i.e., rats,
squirrels and also human beings
124

Drip proof type
• If the motor is to be used in a location where the atmosphere
is damp or if it is likely to be submerged or of it is to be used
outdoor, a drip proof enclosure must be specified.
• These motors have water tight connection at the conduit
entrance and the body is made of rust resistant material.
125

Totally enclosed type
• If the motor is to be used in a very dusty atmosphere such as
in coal handling plants or saw mills etc. a non ventilated type
of enclosure is used.
• Since the motor is enclosed, the heat dissipation to the
atmosphere is designed to be effected by providing a fan for
cooling.
• In spite of this, the heat dissipation cannot be as good as in
ventilated motors.
• Therefore, the size of the motors which are totally enclosed is
generally less than 50 HP.
126

• Sometimes large size motors with totally enclosed covers are
fitted with a duct (or) pipe through which clean air is forced
into the motor from outside the building (or) area.
• These enclosures are called pipe ventilated enclosures.
127

Flame proof type
• If the motor is to be used in a potential explosive atmosphere,
it must be enclosed in special type of strong covers.
• These covers must be able to withstand any explosion of gas
inside the motor without transmitting the flame to the
outside atmosphere.
• Motors with such enclosures are used in coal mines etc.
128

Load equalisation
• When the fluctuation of load is too wide occurring within a
small interval of time as is the case in reciprocating pumps,
electric hammers, rolling mills etc. the equalization of load
becomes a necessity.
• Because of the fluctuating load, the motor draw a very heavy
current during high load condition which may cause a large
voltage drop of the line.
• This may affect other consumers who will experience voltage
fluctuations.
• Also the motor experiences a shock during each cycle of load
variation.
• Therefore, the equalization of load is achieved by means of a
flywheel connected to the load shaft 129

Variation of Load Torque, Motor Torque
and Speed with Time
130
Load Torque TLh
TLl
Motor Torque
Speed
Time -

• In order that the flywheel may operate effectively, the motor
should have dropping characteristic.
• When heavy load is applied, the motor speed decreases and
flywheel will supply kinetic energy to the motor.
• During light load condition, the motor speed increases and
the flywheel stores the energy.
• Thus the load on the motor is equalized.
• Suppose that the motor speed decreases linearity with the
increase in torque, then the variations of speed, load torque
and motor torque with time is shown in the graph/figure.
131

• For linear variation, if ω0 is the no load speed and ‘T ’ the no
load torque and if ω and T are the speed and torque at any
instant of time and ωr and Tr the rated speed and torque of
the motor, the following relations are true:
– ω0 no load speed
– T no load torque
– ω speed
– T torque
– ωr rated speed
– Tr rated torque
133

1. The enclosure of a 20 kW motor is equivalent to
a cylinder of 70 cm diameter and 100 cm
length. The motor weighs 500 kg assuming that
the specific heat is 700 J/kg/◦C and that the
peripheral surface of the enclosure of the motor
alone is capable of heat dissipation of 12.5
W/m2/◦C. Calculate the heating time constant of
the motor and its final temperature rise. Assume
the efficiency of the motor as 80 percent.

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