Electrical drive unit 1 as per IP university_EEE

ELECTRICAL DRIVE
[ETEE-401]
Ms. Amruta Pattnaik
ADGITM
Reference Book: Electrical drive by G.K.Dubey and
Electrical drive by Pillai

Objective:
• The objective of the paper is to facilitate the student with
the basics of Electrical Drives that are required for an
engineering student.

Unit 1:Syllabus
• Dynamics of Electric Drives: Types of loads, quadrant
diagram of speed time characteristics, Basic and modified
• characteristics of dc and ac motors, equalization of load,
steady state stability, calculation of time and energy loss,
• control of electric drives, modes of operation, speed
control and drive classifications, closed loop control of
• drives, selection of motor power rating, class of duty,
thermal considerations.

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

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
• Drives are employed for systems that require motion
control – e.g. transportation system, fans, robots,
pumps, machine tools, etc.
• Prime movers are required in drive systems to provide
the movement or motion.
• Energy that is used to provide the motion can come
from various sources: diesel engines, petrol engines,
hydraulic motors, electric motors etc.
• Drives that use electric motors as the prime movers are
known as electrical drives

1. Electrical drives mainly accomplishes three kinds of work,
i)Starting ii) Speed control and iii) Braking.
2. It can be said that the electrical drives enable us to control the
motor in every aspect.
3. The control of electrical drives is also necessary because all the
functions accomplished by the drives are mainly transient
operations i.e the change in terminal voltage, current, etc are huge
which may damage the motor temporarily or permanently. That’s
why the need of controlling the drives rises and there are various
methods and equipment's to control different parameters of the
drives

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

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

Modern electric drives
• Inter-disciplinary
• Several research area
• Expanding
Machine design
Speed sensorless
Machine Theory
Non-linear control
Real-time control
DSP application
PFC
Speed sensorless
Power electronic converters
Utility interface
Renewable energy

Components in electric drives
e.g. Single drive - sensorless vector control from Hitachi

e.g. Multidrives system from ABB

Motors
• DC motors - permanent magnet – wound field
• AC motors – induction, synchronous (IPMSM, SMPSM),
brushless DC
• Applications, cost, environment
Power sources
• DC – batteries, fuel cell, photovoltaic - unregulated
• AC – Single- three- phase utility, wind generator - unregulated
Power processor
• To provide a regulated power supply
• Combination of power electronic converters
• More efficient
• Flexible
• Compact
• AC-DC DC-DC DC-AC AC-AC

Control unit
• Complexity depends on performance requirement
• analog- noisy, inflexible, ideally has infinite bandwidth.
• digital – immune to noise, configurable, bandwidth is smaller than
the analog controller’s
• DSP/microprocessor – flexible, lower bandwidth - DSPs perform
faster operation than microprocessors (multiplication in single
cycle), can perform complex estimations

Classification of Electric Drives
According to Mode of
Operation
1. Continuous duty drives
2. Short time duty drives
3. Intermittent duty drives
According to Means of
Control
1. Manual
2. Semi automatic
3. Automatic
According to Number of
machines
1. Individual drive
2. Group drive
3. Multi-motor drive
According to Dynamics and
Transients
1. Uncontrolled transient period
2. Controlled transient period
According to Methods of Speed Control
1. Reversible and non-reversible
uncontrolled constant speed.
2. Reversible and non-reversible step
speed control.
3. Variable position control.
4. Reversible and non-reversible
smooth speed control

Group Electric Drive
i. This drive consists of a single motor, which drives one or more line shafts
supported on bearings. The line shaft may be fitted with either pulleys and belts or
gears, by means of which a group of machines or mechanisms may be operated. It
is also some times called as SHAFT DRIVES.
ii. Advantages
A single large motor can be used instead of number of small motors
i. Disadvantages
There is no flexibility. If the single motor used develops fault, the whole process will
be stopped.
Individual Electric Drive
• In this drive each individual machine is driven by a separate motor. This motor also
imparts motion to various parts of the machine.
Multi Motor Electric Drive
• In this drive system, there are several drives, each of which serves to actuate one of
the working parts of the drive mechanisms.
• E.g.: Complicated metal cutting machine tools
• Paper making industries,
• Rolling machines etc.

Advantages of Electrical Drive
 They have flexible control characteristics. The steady state and
dynamic characteristics of electric drives can be shaped to satisfy the
load requirements.
Drives can be provided with automatic fault detection systems.
Programmable logic controller and computers can be employed to
automatically control the drive operations in a desired sequence.
They are available in wide range of torque, speed and power.
They are adaptable to almost any operating conditions such as
explosive and radioactive environments
 It can operate in all the four quadrants of speed-torque plane
They can be started instantly and can immediately be fully loaded
Control gear requirement for speed control, starting and braking is
usually simple and easy to operate.

Selection of Electrical Drives
Choice of an electric drive depends on a number of factors. Some of the
important factors are.
1.Steady State Operating conditions requirements: Nature of speed
torque characteristics, speed regulation, speed range, efficiency, duty
cycle, quadrants of operation, speed fluctuations if any, ratings etc
2. Transient operation requirements: Values of acceleration and
deceleration, starting, braking and reversing performance.
3. Requirements related to the source: Types of source and its capacity,
magnitude of voltage, voltage fluctuations, power factor, harmonics
and their effect on other loads, ability to accept regenerative power
4. Capital and running cost, maintenance needs life.
5. Space and weight restriction if any.
6. Environment and location.
7. Reliability.

Overview of AC and DC drives
DC motors: Regular maintenance, heavy, expensive, speed limit
Easy control, decouple control of torque and flux
AC motors: Less maintenance, light, less expensive, high speed
Coupling between torque and flux – variable
spatial angle between rotor and stator flux

Difference between AC and DC Drive
DC Drive
1. The power circuit and control
circuit is simple and
inexpensive
2. It requires frequent
maintenance
3. The commutator makes the
motor bulky, costly and heavy
4. Fast response and wide
speed range
5. of control, can be achieved
smoothly by conventional and
solid state control
6. Speed and design ratings are
limited due to commutations
AC Drive
1. The power circuit and
control circuit are complex
2. Less Maintenance
3. These problems are not
there in these motors and
are inexpensive, particularly
squirrel cage induction
motors
4. In solid state control the
speed range is wide and
conventional method is
stepped and limited
5. Speed and design ratings
have upper limits

Applications
Paper mills
 Cement Mills
 Textile mills
 Sugar Mills
 Steel Mills
 Electric Traction
 Petrochemical Industries
 Electrical Vehicles

When an electric motor rotates, it is
usually connected to a load which has a
rotational or translational motion. The
speed of the motor may be different
from that of the load. To analyze the
relation among the drives and loads, the
concept of dynamics of electrical
drives is introduced

Torque Equations For translational Systems
The Newton’s Law states that, the net force acting on a body of mass M equals to the rate of change of its
mechanical momentum, which is the product of its mass and its velocity in the direction of the net force. In the
equation form, this is given by
Torque Equations For rotating Systems
For rotational motion (which is the case for rotating electrical machines), the force, the mass and the linear
velocity is equivalent to the torque, the moment of inertia and the angular velocity, respectively.
For rotational motion
T in N-m
J in Kg-m^2
W in rad/sec.

Dynamics of motor-load combination
• Fundamental Torque Equations
The dynamic relations applicable to all types of motors and loads.
J = Polar moment of inertia of motor load
Wm = Instantaneous angular velocity
Tm = Instantaneous value of developed motor torque
TL = Instantaneous value of load torque referred to motor shaft
Any motor-load system can be described by the following fundamental torque equation during
dynamic condition:
Tm-TL = ± d/dt(J*Wm)…….(1)
Tm=TL ± [J d/dt(Wm)+ Wm d/dt(J)]……….(2)
This equation is applicable for variable inertia drives such as mines winder, industrial robots etc
If dJ/dt =0 i.e. constant inertia
Then Tm=TL ± J d/dt(Wm)
+ Sign is for acceleration
- Sign is for deceleration m

Acceleration or deceleration depends on whether Tm is greater
or less than TL where TL must be a passive load .
In order to accelerate in forward direction, Tm –TL must be
positive; which means that the applied electrical torque must be
larger than the load torque.
In order to decelerate, the net torque must be negative; the
electrical torque must be made smaller than the load torque and
the motor operates in braking mode –more on this later.
Note that the speed is always continuous. A discontinuity in
speed (i.e. step change in speed) theoretically will require an
infinite torque.
If Tm=TL, the motor will continue at the same speed if it were
running or continuing to rest if it were not.
J d/dt(Wm) is called a dynamic torque .it is present due to
transient condition i.e. when the speed of the drive varies.

Types of loads
Active Load
• Load torques which has the
potential to drive the motor
under equilibrium conditions
are called active load torques.
• Such load torques usually
retain their sign when the drive
rotation is changed (reversed)
Example
• Torque due to force of gravity
• Torque due tension
• Torque due to compression and
torsion etc
Passive load
• Load torques which always
oppose the motion[motor torque]
and change their sign on the
reversal of motion are called
passive load torques
Example:
• Torque due to friction, cutting etc.

Components of Load Torques:
The load torque can be further divided in to following components
(i) Friction Torque (TF)
Friction will be present at the motor shaft and also in various parts of
the load. TF is the equivalent value of various friction torques referred to
the motor shaft.
(ii) Windage Torque (TW)
When motor runs, wind generates a torque opposing the motion. This is
known as windage torque.
(iii) Torque required to do useful mechanical work.
Nature of this torque depends upon particular application. It may be
constant and independent of speed. It may be some function of speed,
it may be time invariant or time variant, its nature may also change with
the load’s mode of operation.

Friction torque
 Generally friction and windage torques are grouped together and can be
expressed as Dw where D is friction constant.
 The magnitude of friction torque depends on the speed.
 Figure below shows variation in friction torque during rotation in the positive
direction and negative direction.
 This friction at standstill is called static friction. When the motor is to be started the
torque developed by the motor must overcome the friction torque. Otherwise,
motor will not run

ELECTRICAL DRIVE
[ETEE-401]
Ms. Amruta Pattnaik
ADGITM

Objective:
• The objective of the topic is to know about the different
types of mechanical load.

Torque Equations For translational Systems
The Newton’s Law states that, the net force acting on a body of mass M equals to the rate of change of its
mechanical momentum, which is the product of its mass and its velocity in the direction of the net force. In the
equation form, this is given by
Torque Equations For rotating Systems
For rotational motion (which is the case for rotating electrical machines), the force, the mass and the linear
velocity is equivalent to the torque, the moment of inertia and the angular velocity, respectively.
For rotational motion
T in N-m
J in Kg-m^2
W in rad/sec.
If dM/dt =0
Fp-Fv=m dv/dt

In steady state ,angular speed
(w) is constant
So Te=TL
In transient state
Te=T L+J *dw/dt

Components of Load Torque, TL
1. In general, the load torque TL can be classified into two
types: the passive load torque (frictional torque) and the
active load torque.
2. Frictional toque exists only when there is motion and it
always opposes the driving torque.
3. Active load torque[useful load torque] on the other hand, is
independent of the direction of motion.

Frictional torque
Moving parts of the motor and load constitute the frictional
torque.
There are several types of frictional torque
Static friction –standstill position of motor where speed is
equal to zero
Coulomb friction – exists in bearings, gears, coupling and
brakes. It is almost independent of speed.
Viscous friction – exist in lubricated bearings due to the
laminar flow of the lubricant. It is directly proportional to
the speed.
Windage friction – occurs due the turbulent flow of air or
liquid. It is directly proportional to the square of speed

Speed (rad/sec)
Friction Torque(N-m)
Static Torque
coulomb Torque
Viscous Torque
Windage Torque
Friction Torque(N-m)
Speed (rad/sec)

Speed-Torque convention/Multi-quadrant operation
1. A four-quadrant or multiple-quadrant operation is required in industrial as well as
commercial applications. These applications require both driving and braking, i.e., motoring
and generating capability.
2. Some of these applications include electric traction systems, cranes and lifts, cable laying
winders, and engine test loading systems.
3. In multi-quadrant operation or four quadrant operation, motor accelerates or decelerates
depending on whether motor torque is lesser or greater than load torque.
4. During motor acceleration, it should supply not only the load torque, but an additional
component of load current to overcome the inertia.
5. Motor positive torque produces the acceleration in forward direction. In this, the motor
speed is positive when the motor is rotating in forward direction.
6. 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.
7. A motor can be controlled in such a way that it operates in two cases; motor action and
braking action.
8. 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.
9. Similarly, these two actions are performed in case motor operating in reverse direction, i.e.,
(reverse motoring and reverse braking actions).

Speed (rad/sec)
Torque(N-m)
Forward motoring
Speed +
Torque +
Power ( +)>0
It takes power from source
and act as a motor
Forward braking
Speed +
Torque -
Power ( -)<0
It draws power from load to source
and act as a generator
Reverse motoring
Speed -
Torque -
Power ( +)>0
It takes power from source to load
in reverse direction
and act as a motor
Reverse braking
Speed -
Torque +
Power ( -) <0
It draws power from load to source in
reverse direction
and act as a generator
Torque(N-m)
Speed (rad/sec)
First QuadrantSecond Quadrant
Third Quadrant
Fourth Quadrant

Four quadrant operation of a motor driving
a hoist load
1. This hoist consists of a cage with or without any load. A rope,
generally made up of a steel wire is wounded on a drum to raise
the cage and a balance weight.
2. This balance weight or counterweight magnitude is greater than
that of empty cage, but less than the loaded cage.
3. For each quadrant of operation, direction of rotation, w, load
torque TL, and motor torque Tm are shown in figure
4. Consider that the load torque is constant and independent of
motor speed

Anti clock wise direction clock wise direction

1st Quadrant Operation
1. The hoist in which the loaded cage is moving upwards.
2. The direction of rotation of motor, w will be in anticlockwise
direction i.e., positive speed.
3. The load torque acts in opposite direction to the direction of motor
rotation.
4. To raise the hoist to upwards, the motor torque, Tm must act in the
same direction of motor speed, w.
5. So both motor speed and motor torque will be positive. To make
these as positive, the power taken from the supply should be
positive. This is called forward motoring.

2nd Quadrant Operation
1. The hoist in which unloaded cage is moving upwards.
2. The counterweight is heavier than the unloaded cage and hence
hoist can move upwards at a dangerous speed.
3. To prevent this, motor must produce a torque in the opposite
direction of motor speed, w in order to produce brake to the
motor.
4. Therefore, the motor torque, Tm will be negative and motor speed,
w will be positive.
5. This quadrant operation is called forward braking.

3rd Quadrant Operation
• The empty cage is hoisting down.
• The downward journey of empty cage is prevented by the torque
exerted by the counterweight.
• So the direction of motor torque, Tm should be in the same direction of
motor rotation-w.
• Due to the downward movement of the cage, the direction of rotation is
reversed, i.e., w is negative and hence Tm is also negative.
• Since the machine acting as motor in reverse direction, it receives the
power from the supply and hence power is positive.
• This quadrant operation is called reverse motoring

4th Quadrant Operation
1. Loaded cage is moving downwards.
2. The loaded cage is moving downward (of which weight is more
than counterweight), the motion takes place without use of any
motor.
3. There will be a chance to go downward at a dangerous speed
because of loaded cage.
4. To limit the speed of the cage within a safe range, the electrical
machine must act as a brake.
5. In this the direction of the motor, w is negative and hence the
motor torque Tm is positive to decrease the speed of the motor.
6. Thus, the power is negative that means the electrical machine
delivering power to the supply.
7. This phenomenon is called as regenerative action. This quadrant
operation is called reverse braking.

Objective:
• The objective of the topic is to know about the equivalent
drive parameters.

Equivalent Values of Drive Parameters
1. Different parts of the load may be coupled through different mechanisms, such as
V- belts, crankshaft, gears etc.
2. These parts may have different speed and different types of motions such as
Rotational
Translational

Loads with Rotational Motion
Let’s consider a motor driving two loads, one coupled directly to the
shaft and other through gear with n & n1 teeth
J0 =moment of inertia of motor & load directly coupled to its shaft (kg - m2)
ω m=motor speed (rad/sec)
T l0 = load torque of directly coupled load (N - m)
J1 =moment of inertia of load coupled through a gear (Kg – m^2 )
ωml =speed of load coupled through a gear (rad/sec)
Tl1 =torque of load coupled through a gear (N - m) Wm increase, if
teeth is less
and vice versa

𝑛
𝑛1
= a1 ∴ a1=gear tooth ratio
⇨
𝜔 𝑚1
𝜔 𝑚
=
𝑛
𝑛1
= 𝑎1 …………………(1)
If the losses in the transmission are neglected then kinetic energy due to
equivalent inertia must be same as kinetic energy of various moving parts
1
2
𝑗 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝜔 𝑚2 =
1
2
𝑗0 𝜔 𝑚2 +
1
2
𝑗1 𝜔 𝑚12 …………………………………….(2)
From equation 1 and equation 2 ,we get
𝑗 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 = 1/𝜔 𝑚2( 𝑗0 𝜔 𝑚2 +𝑗1 𝜔 𝑚12 )
= 𝑗0 +
𝑗1 𝑎12……………………………………………(3)
Power at the motor & load must be same, if transmission efficiency of the
gears is 𝜂1,then
𝑇𝑙 𝜔 𝑚 = 𝑇𝑙0 𝜔 𝑚 +
𝑇 𝑙1 𝜔 𝑚1
𝜂1
…………………………..(4)
As the power is seen by the motor is higher than the power is at rotational
system .
So total power at rotational load =output power/efficiency=
𝜂1
From equations 1 and 4,we get 𝑇𝑙 = 𝑇𝑙0 +
𝜂1 𝜔 𝑚
⇨ 𝑇𝑙 = 𝑇𝑙0 +
𝑇 𝑙1 𝑎1
𝜂1
…..(5)

If there are m other loads with moment of inertias J1,J2,….,Jm &
gear teeth ratios of a1,a2,….,am then
𝑗𝑒𝑞.= 𝑗0 + 𝑗1 𝑎12 + 𝑗2 𝑎22 + 𝑗3 𝑎32 + ⋯
If there are m other loads with moment of inertias J1,J2,….,Jm &
gear teeth ratios of a1,a2,….,am & transmission efficiencies η1, η
2,….., η m then
𝑇𝑙 = 𝑇𝑙0 +
𝑇 𝑙1 𝑎1
𝜂1
+
𝑇 𝑙2 𝑎2
𝜂2
+……..

Loads with Translation Motion
Let’s consider motor driving two loads, one coupled directly to its shaft & other
through a transmission system
J 0 =moment of inertia of motor & load directly coupled to its shaft (Kg - m2)
ωm=motor speed (rad/sec)
Tl0 =torque of directly coupled load (N - m)
M1 =mass of load coupled through a transmission system (Kg)
v1 =velocity of load coupled through a transmission system (m/sec)
F1 =force of load coupled through a transmission system (N)

If the losses in the transmission are neglected then kinetic energy due to
equivalent inertia J must be same as kinetic energy of various moving parts
Similarly, power at the load & motor should be same, thus if efficiency of
transmission be 𝜂1 then,
If there are m other loads with translational motion with velocities
v1,v2,….,vm & masses M1,M2,….,Mm then,

Unit 1:Syllabus
• Dynamics of Electric Drives: Types of loads, quadrant
diagram of speed time characteristics, Basic and modified
characteristics of dc and ac motors, equalization of load,
steady state stability, calculation of time and energy loss,
control of electric drives, modes of operation, speed
control and drive classifications, closed loop control of
drives, selection of motor power rating, class of duty,
thermal considerations.

Nature and classification of load torques
• Load torques can be classified into two categories: active load torque &
passive load torque.
• Active Load Torque
• It has potential to drive motor under equilibrium condition.
• Such load torque usually retains their sign when the direction of the
drive rotation is changed.
• Torque due to gravitational force, tension, compression & torsion
come under this category.
• Passive Load Torque
• Torque which always opposes the motion is called passive torque.
• Their sign change on the reversal of motion.
• Torque due to friction, windage, cutting etc. fall under this category.

Nature of load torque depends on
application

A low speed hoist
Torque is constant & independent of
speed.
 At low speed, windage torque is
negligible.
 Net torque is mainly due to gravity that is
constant & independent.
Torque is Constant (Independent of Speed)

Following requirements for
paper mill drive must be
fulfilled
1) To manufacture different
types of paper we should be
able to vary the speed of
entire series of rolls.
2) Relative speeds of rolls
should be constant otherwise
tearing of paper may result.
3) We should be able to
adjust the speed of any group
of rolls relative to others in
order to allow for the draw of
the paper.
4) All above requirements are
fulfilled by the use of Schrage
motor
Paper Mill Process
Paper mill drive: Constant torque i.e. coulomb friction dominates over
other torque components.

Torque is Function of Speed
• Torque is proportional to square of speed (T α ωm ^2): It has low
starting torque. Examples of such loads are; axial & centrifugal
pumps, centrifugal compressors, fans, ship propellers etc.
• Torque is linearly proportional to speed (T α ωm): Examples of
such loads are mixers and stirrers.
• Torque is inversely proportional to speed (T α 1⁄ωm): Here
developed power is nearly constant. It is approximately hyperbolic
in nature. Examples of such loads are lifts, lathes, wire drawer,
winders, reciprocating rolling mills etc.

• Torque is inversely proportional to speed
(T α 1⁄ωm):
Here developed power is nearly constant. It is
approximately hyperbolic in nature. Examples of such loads
are lifts, lathes, wire drawer, winders, reciprocating rolling
mills etc

Rolling MillsDepending upon type of mill selection of motor has to satisfy following
conditions.
1. Motor should have robust construction to withstand severest duty
2. It should be capable of developing short time torque to the extent of
2 or 3 times of rated torque.
3. It should be capable of having wide speed variation.
4. It should be in a position to maintain preselected speed within
tolerances from no load to full load. Therefore it should have quick
response to change.

TEXTILE MILLS
1. Loom motors operate in atmosphere which is laden wool-lint and moisture.
2. Further the motor should high starting torque of about 2to3 times full load
torque.
3. Loom motors have to start and stop many times in a day.
4. this increases the motor heating during starting.
5. Motors of low temperature rise under full load conditions are therefore
selected do that under working conditions of frequent starting and stopping
permissible temperature rise is not exceeded
6. Keeping above requirements in view totally enclosed, fan cooled, high
torque squirrel cage motors are selected
7. Where only wool lint is present and no moist atmosphere ,screen protected
motors are used.
8. Splash proof motors are used in wet locations such as dye house.
9. Brush shaft adjustable speed motor is used on printing machine

Electrical drive unit 1 as per IP university_EEE

Punches Presses & Shears
• Since heavy sudden loads are applied , D.C. compound
motor or slip ring motor with flywheel is used. Here torque
is inversely proportional to speed

LATHE, GRINDING & MILLING
MACHINE

Lifts
• High smooth accelerating torque of 200 to 250% full load
torque at starting, high over load capacity and pull out
torque an maximum degree of silence are essential
requirements.
• Motor are normally one hour rated for duty cycle of 150 to
180 starts per hour.
• D.C. compound motors or slip ring motors suit the above
requirements

Belt conveyors
• Belt conveyors are used for moving sand, coal and others
raw materials.
• Heavy loads have to accelerated.
• High torque squirrel cage or preferably slip ring
motors should be made as per requirement.
• Due to outside working conditions and presence
of grit and dust in atmosphere ,totally enclosed
surface cooled motors are used.

Mines
• Motor used inside mines should have flame proof
enclosure.
• For mine winders, it is essential to have both speed
control as well as breaking capacity
• Ward Leonard system is most effective drive.
• Slip Induction motor with D.C. dynamic breaking is also
used.
• We may also used D.C. motor supplied from controlled
rectifier

• Torque is proportional to square of speed (T α ωm ^2): It has low
starting torque. Examples of such loads are; axial & centrifugal
pumps, centrifugal compressors, fans, ship propellers etc.

Centrifugal Pump
1. Where pump speed does not come within the range of the fixed motor speed
,V belt drive is employed.
2. starting torque necessary for centrifugal pumps is about 40 to 50% of full
load torque.
3. It is advisable to use slip ring motors for reciprocating pumps and squirrel
cage motors for centrifugal pumps.
4. For reciprocating pumps starting torque may be 100 to 200 % of full load
torque
5. It is advisable to use slip ring motors for reciprocating pumps and squirrel
cage motors for centrifugal pumps

Marine DriveElectric Drive propeller of ship is universal .
Propeller: A propeller is type
of fan that transmits power by
conversion rotational motion
into thrust. A pressure
difference is produced
between the forward and rear
surfaces of the airfoil-shaped
blade, and a fluid (such as air
or water) is accelerated
behind the blade.
It has following Advantages.
1) Position of prime mover Speed control and reversing in case of electric
propulsion are easy and can be carried out from any convenient position instead
of signaling to engine room
2) Position is not fixed by the propeller shaft as such there is greater flexibility in
the layout.
3) In Case of very big ships, electric drive will be by three phase induction or
synchronous motors
4) Small ships can be driven by D.C. motors, which are controlled by means of
ward Leonard method.

REFRIGERATION & AIR CONDITIONING
In vapor compression system of refrigeration motor is used
to drive the compressor.
 This motor is controlled by thermostat .
When motor is restarted , it has to drive compressor
against high speed pressure.
Motor should therefore be required developing starting
torque of 200 to 250% full load torque.
 For small unit air conditioners, capacitor type single phase
230 V motor with D.O.L. starter is most commonly selected.
High torque squirrel cage induction motor or slip ring
induction motors are used for large installations
For very large plant size, synchronous motor, driving turbo
compressor may be found suitable specially when P.F.
correction is also required

• Torque is linearly proportional to speed (T α ωm):
Examples of such loads are mixers and stirrers

Universal series motor
• Universal series motor is used for house hold
appliances such as Mixer, Washing Machine,
Sewing Machine, Vacuum Cleaner

(d)
Nature of load torque (a) T = constant (b) T a ωm2
(c) T a ωm (d) T a 1⁄ωm

Load torque that vary with angle of
displacement of the shaft
• In machines having crank shafts such as reciprocating pumps and
compressors, frame saws, etc., the load torque is a function of the
position of the crank, i.e. the angular displacement of the shaft or
rotor of the motor.
• Load torque (TL) can be resolved in to two parts; a constant torque Tc
and a variable torque TL’, which changes periodically in magnitude
depending on the angular position of the shaft

Load torques that vary with time
Load variation with time can be periodic and repetitive in certain applications.
One cycle of the load variation is called a duty cycle.
The variation of load torque with time has a greater importance in the selection of a suitable
motor.
Classification of loads that vary with time:
(a) Continuous, constant loads: Centrifugal pumps or fans operating for a long time under the
same conditions, paper making machines etc.
(b) Continuous, variable loads: Metal cutting lathes, hoisting winches, conveyors etc.
(c) Pulsating loads: Reciprocating pumps and compressors, frame saws, textile looms and
generally all machines having crank shaft.
(d) Impact loads: Apparent, regular and repetitive load peaks or pulses which occurs in rolling
mills, presses, shearing machines, forging hammers etc. Drives for such machines will have heavy
fly wheels.
(e) Short time intermittent loads: Almost all forms of cranes and hoisting mechanisms, excavators,
roll trains etc.
(f) Short time loads: Motor generator sets for charging batteries, servo motors used for remote
control of clamping rods of drilling machines.

• Loads of the machines like stone crushers and ball mills are
characterized by frequent impact of small peaks so they are
classified as continuous variable loads rather than the impact
loads
• One and the same machine can be represented by a load torque
which either varies with the speed or with the time.
• For example, a fan load whose load torque is proportional to
the square of the speed, is also a continuous, constant load.
• Load torque of a crane is independent of the speed and also
short time intermittent nature.
• Rocking pumps for petroleum have a load which vary with
angular position of the shaft, but also be classified as a pulsating
load

Hyperbolic speed-torque characteristics,
where load torque is inversely proportional
to speed or load power is constant.

ELECTRICAL DRIVE
[ETEE-401]
By
Amruta Pattnaik

Steady State and Transient Stability
It is quite important to investigate the stability of the electric drive when
its equilibrium state is disturbed.
Equilibrium speed: Speed at which motor torque becomes same as load
torque is known as equilibrium speed.
Stability: The stability of motor-load combination is defined as the
capacity of the system which enables it to develop forces of such a
nature as to restore equilibrium after any small departure therefrom.

There are two types of disturbances:
Changes from state of equilibrium take place slowly which is
related to steady state stability.
Sudden & fast change from the equilibrium state which is
related to field of transient stability

Let us examine the steady state stability by referring the
speed – torque characteristic of a certain load when there
is change in the speed caused by disturbances

1. For this system, let the
disturbance causes a reduction
of Δwm in speed.
2. At new speed motor torque (T)
is greater than the load torque
(TL ) therefore, motor will
accelerate and operation will be
restored to A,
3. Similarly, an increase of Δwm
in speed caused by disturbance
will make load torque greater
than the motor torque, resulting
into deceleration and
restoration of operation to point
A. Hence, the drive is steady
state stable at point A
System 1

• Let us now examine for this system.
• A decrease in the speed causes the load
torque to become greater than the motor
torque, drive decelerates and operating
point moves away from B.
• Similarly, when speed increases,
the motor torque becomes
greater than the load torque that
will move the operating point
away from B. Thus, B is an
unstable point of equilibrium.
System 2

Stable or unstable
Stable Un stable Stable Un stable

Criteria for Steady-State Stability
After a small displacement from the equilibrium, the torque equation is
T-Tl = d/dt (J*Wm)
⇨(T+∆T)-(Tl+∆Tl)=J*d/dt(Wm+∆Wm)
So the equation becomes
Where
J=moment of inertia (Kg - m2)
ωm=motor speed (rad/sec)
Tl =torque developed by Load (N -m)
T=torque developed by motor (N - m)
Δωm=a small displacement in motor speed
from the equilibrium (rad/sec)
ΔTl =a small displacement in load torque
from the equilibrium (N - m)
ΔT=a small displacement in motor torque
from the equilibrium (N - m)
Hence
………………….(1)
………………….(2)
………………….(3)
………………….(4)
………………….(5)
Put equation 4 and 5 in equation 3
………………….(6)
………………….(7)

Where, (Δ wm )0= Initial value of deviation in speed
Based on the value of exponent there are three cases
 Exponent > 0: The speed deviation will increase with time and the system
will move away from the equilibrium, results in unstable system
• Exponent < 0: The speed deviation will decrease with time and the system
will move towards the equilibrium, results in stable system
• Exponent = 0: The equitation is insufficient to discuss about stability
The exponent will always be negative if
………………….(7)
………………….(8)

Measurement of Moment of Inertia
Moment of inertia can be calculated by two ways
Theoretically: if dimensions & weight of various parts of load and motor are known
Practically: Retardation Test
In retardation test the drive is run at the speed slightly greater than the rated
speed and then supply to it cut off
Drive continues to run due to stored Kinetic Energy and decelerate due to
rotational mechanical losses
Variation of speed with time is recorded. At any speed wm, Power P consumed
in supplying the rotational losses give by
From retardation test (dωm)⁄dt at rated speed is obtained. Now drive is
reconnect to the supply and run at rated speed and rotational mechanical power
input to the drive is measured.
This is approximately equal to P. Now, J can be calculated from above
equation.

Modes of Operation:
An electrical drive operates in three modes:
• Steady state
• Acceleration including Starting
• Deceleration including Stopping

Steady state
• Steady state operation takes place when motor torque
equals the load torque. The steady state operation for a
given speed is realized by adjustment of steady state
motor speed torque curve such that the motor and load
torques are equal at this speed.
• Change in speed is achieved by varying the steady state
motor speed torque curve so that motor torque equals the
load torque at the new desired speed

Acceleration and Deceleration modes
• Acceleration and Deceleration modes are transient
modes. Drive operates in acceleration mode.
• Whenever an increase in its speed is required. For this
motor speed torque curve must be changed so that motor
torque exceeds the load torque.
• Time taken for a given change in speed depends on
inertia of motor load system and the amount by which
motor torque exceeds the load torque.

ELECTRICAL DRIVE
[ETEE-401]
BY
Amruta Pattnaik

Speed control and Drive application

• Speed Control and Drive Classification are the Drivers
where the driving motor runs at a nearly fixed speed are
known as Constant Speed or Single Speed Drives.
• Multi-speed drives are those which operate at discrete
speed settings.
• Drives needing step less change in speed and multispeed
drives are called Variable Speed Drives.
• When a number of motors are fed from a common
converter, or when a load is driven by more than one
motor, the drive is termed as multi-motor drive.

• Constant Torque refers to maximum torque capability of the drive
and not to the actual output torque, which may vary from no load to
full load torque.
• The Constant Power Drive and Constant Power Mode (or region)
are defined in the same way.Ideally it is desired that for a given speed
setting, the motor speed should remain constant as load torque is
changed from no load to full load.
• In practice, speed drops with an increase in the load torque. Quality
of a speed control system is measured in terms of speed-regulation
which is defined as
• If open-loop control fails to provide the desired speed regulation,
drive is operated as a closed-loop speed control system

Closed loop control of Drives
1. In a control system, there are two types of systems, one is
open loop and the other is closed loop control system.
2. In open loop control system the output has no effect on the
input, i.e the controlling phenomenon is independent of the
output, on the other hand
3. closed loop control system is much more advanced and
scientific, here the output is fed back to the input terminal
which determines the amount of input to the system, for
example if the output is more than predetermined value the
input is reduced and vice-versa.
4. In electrical drives feedback loops or closed loop control
satisfy the following requirements.
Protection
Enhancement of speed of response
To improve steady-state accuracy

Types of closed loop control of drives

1. Current Limit Control of Drives
1. Current Limit Control of Drives scheme of Fig is employed to limit the converter
and motor current below a safe limit during transient operations.
2. It has a current feedback loop with a threshold logic circuit.
3. As long as the current is within a set maximum value, feedback loop does not
affect operation of the drive.
4. During a transient operation in Current Limit Control of Drives, if current
exceeds the set maximum value, feedback loop becomes active and current is
forced below the set maximum value, which causes the feedback loop to become
inactive again.
5. If the current exceeds set maximum value again, it is again brought below it by
the action of feedback loop.
6. Thus the current fluctuates around a set maximum limit during the transient
operation until the drive condition is such that the current does not have a
tendency to cross the set maximum value, e.g. during starting, current will
fluctuate around the set maximum value.

2.Closed loop torque control
• This type of torque controller is seen mainly in battery
operated vehicles like cars, trains etc. the accelerator
present in the vehicles is pressed by the driver to set
the reference torque [T]. The actual torque [T] follows
the 𝑇∗
which is controlled by the driver via accelerator.

3. Closed Loop Speed Control
1. There are two control loops, which is an inner loop and outer loop.
2. The inner current control loop limits the converter and motor current or motor torque
below the safe limit.
3. Suppose the reference speed Wm
* increases and there is a positive error ΔWm, which
indicates that the speed is needed to be increased.
4. Now the inner loop increases the current keeping it under maximum allowable current.
5. And then the driver accelerates, when the speed reaches the desired speed then the motor
torque is equal to the load torque and there is a decrease in the reference speed Wm
which indicates that there is no need of any more acceleration but there must be
deceleration, and braking is done by the speed controller at maximum allowable current.
So, it can say that during speed controlling the function transfers from motoring to
braking and from braking to motoring continuously for the smooth operation and
running of the motor.

4. Closed Loop Speed Control of Multi Motor
Drives:
• When mechanical part of the load is of large physical dimension it
becomes desirable to share the load between several motors.
• Such multi-motor drives are also employed in electric and diesel
electric locomotives, rapid transit vehicles and some paper machines.
• In a locomotive because of different amount of wear and tear, all
wheels do not have the same diameter. Therefore, for a given speed
of train they would revolve at different speeds.
• Consequently, the driving motor speeds will also be different. In spite
of different speeds it is essential that torques are shared equally
between different motors; otherwise when one motor is fully loaded
others will be under loaded, and thus, the rated locomotive torque will
be less than the sum of individual motor torque ratings.
• Here also a single outer speed loop, with speed feedback derived
from a suitably located speed sensor, is enough.
• Because each motor has its own torque control loop, in spite of their
different rotational speeds, they are made to share the torque equally

Speed Sensing
Sensing of speed is required for implementation of closed-
loop speed control schemes.
Speed is usually sensed using tachometers coupled to the
motor shaft.
Speed sensors are(i) Tachometer and (ii)digital tachometer
and (iii) Dc drive by back emf sensing method

Tachometer
 A tachometer is an ac or dc generator with a high order of linearity between
its speed and output voltage.
A dc tachometer is built with a permanent magnetic field and sometimes with
silver brushes to reduce contact drop between the brush and commutator.
The tachometer output voltage consists of a ripple whose frequency
depends on its speed.
At low speeds, adequate filtering can only be done by a filter with a
large enough time constant to affect the dynamics of the drive.
Special large diameter tachometers with a large number of
commutator segments are sometimes built to overcome this problem.
Tachometers are available to measure speed with an accuracy of ±
0.1%. Tachometer should be coupled to the motor with a torsionally
stiff coupling so that natural frequency of the system consisting of
rotor of the motor and tachometer lies well beyond the bandwidth of
the speed control loop.

Digital Tachometer
1. When very high speed accuracies are required, as in computer
peripherals and paper mills etc., digital tachometers are used.
2. A digital tachometer employs a shaft encoder which gives a
frequency proportional to the motor speed.
3. Encoder consists of a transparent plastic or aluminium disc
mechanically coupled to the motor shaft.
4. Transparent plastic disk is alternately painted black on its peripheri to
provide alternately transparent and non-transparent parts.
5. In an aluminium disc, a number of holes or slots are uniformly made
around its peripheri.
6. An opto-coupler unit, consisting of a light source and a light sensor,
is so mounted that the disc will run in between light source and the
sensor.
7. Sensor senses the light source whenever a transparent part/slot/hole
crosses opto-coupler and a voltage pulse is produced.
8. Frequency of the pulse train is proportional to the speed of shaft.
9. Pulses are counted over a specific period to obtain a number
proportional to speed

Dc drive by back emf sensing method
• In dc drives, speed can be sensed without a tachometer when
field current is held constant.
• Use is made of the fact that the beak emf is directly proportional
to speed when flux is held constant.
• The back emf is measured by deducting from motor terminal
voltage a signal equal to its armature resistance drop.
• Accuracy of measurement is affected by difficulty in sensing
armature current accurately due to the presence of ripple,
variation of flux due to field supply disturbance and variation of
temperatures of field and armature windings.
• Method is inexpensive and provides speed measurement with an
accuracy of ± 2% of base speed.

Current Sensing of Electrical Drives
• Current Sensing of Electrical Drives is required for the
implementation of current limit control, inner current control
loop of closed-loop speed control, closed-loop torque control of
a dc drive, for sensing fault conditions, and for sensing speed in
dc drives by back emf sensing method.
• In order to avoid interaction between control circuit, carrying
low voltage and current, and power circuit involving high
voltage and current and sometimes harmonics and voltage
spikes, isolation must be provided between the two circuits.

Current sensing method
1. one method is employing Hall-effect. It has the ability to
sense current direction and is commercially available for a
wide range of currents (few amperes to several hundred
amperes) with a typical accuracy of 1% up to 400 Hz:
2. Other method that it involves the use of a non-inductive
resistance shunt in conjunction with an isolation amplifier
which has an arrangement for amplification and isolation
between power and control circuits

3. In current control loop of a variable speed drive, accurate
sensing of current is not necessary, and therefore, the drop across
a suitable winding, e.g. inter-pole winding in a dc machine, is
often used for Current Sensing of Electrical Drives.
4. Isolation amplifier may consist of any one of the following
circuits: Voltage drop across the shunt is filtered, amplified,
modulated and then applied to primary of an isolation
transformer. Output of the transformer is demodulated by a phase
sensitive demodulator, filtered, buffered and applied to output
terminals. This method allows the sensing of current direction.

5. Current transformers
1. Current in three-phase ac circuits can be sensed using the circuit as shown in Fig.
2. Current transformers (CT) are used to provide isolation.
3. The current transformer output is rectified, applied across resistor R and then filtered.
4. Voltage drop V0 is proportional to the current in ac lines.
5. When used in variable frequency inverters care should be taken to avoid saturation at
low frequencies.
6. Major limitation of this method is that it cannot sense the phase of currents.
7. In case of fully-controlled rectifiers, dc link current is proportional to ac line currents.
8. Therefore, in dc and ac drives fed from fully-controlled rectifier, dc link current can be
sensed indirectly by sensing ac line currents of rectifier by the method .

Phased lock loop control
• A PI controller ideally should provide perfect speed
regulation. However, due to imperfections in sensing and
control circuits, the closed-loop schemes described earlier
can at best achieve a speed regulation of 0.2%.
• The Phase Locked Loop Control (PLL) can achieve a
speed regulation as low as 0.002% which can be useful in
conveyers for material handling, paper and textile mills,
and computer peripherals.

ELECTRICAL DRIVE
BY
Amruta Pattnaik

Phased locked loop control (PLL)
• A PI controller ideally should provide perfect speed
regulation.
• However, due to imperfections in sensing and control
circuits, the closed-loop schemes described earlier can
at best achieve a speed regulation of 0.2%.
• The Phase Locked Loop Control (PLL) can achieve a
speed regulation as low as 0.002% which can be useful
in conveyers for material handling, paper and textile
mills, and computer peripherals.

1. Two pulse trains reference pulse train of frequency f * and the feedback pulse train of
frequency f are compared in a phase detector.
2. Output of the phase detector produces a pulse-width modulated output Vc.
3. Pulse-width of Vc depends on the phase difference between the two input pulse trains and
polarity depends on the sign of phase difference (i.e. lag or load) between them.
4. The output of the phase detector is filtered by the loop filter to obtain a dc signal and applied
as control voltage to a voltage controlled oscillator (VCO); the output of which is the
feedback signal f.
5. Because of the closed-loop, VCO output frequency changes in a direction that reduces the
phase difference.
6. When steady state is reached, f becomes exactly equal to f* and the loop is said to have
locked.
7. Control voltage required by VCO to produce f equal to f* comes from the phase difference
between the two input signals.
8. If now f* is altered, f will follow the change and control voltage required by VCO will be
obtained by the adjustment of phase difference between the two input signals

Closed loop speed control by using PLL
1. An electrical drive employing Phase Locked Loop Control is shown in Fig.
2. The VCO is replaced by converter, motor and speed encoder.
3. Output of the loop-filter forms the control signal for the converter.
4. It alters the converter operation such that the motor speed adjusts to make the
frequency of speed encoder output signal f equal to the frequency of reference
signal f*.
5. By changing f* the motor speed can be changed.
6. Excellent speed regulation is the main feature of this drive. However, it has two
important
7. Disadvantages:
 transient response is slow and
 it has a low speed limit below which it becomes unstable.

Closed Loop Position Control:
1. A Closed Loop Position Control scheme is shown in Fig.
2. It consists of a closed-loop speed control system with an inner current
control loop inside an outermost position loop.
3. Current and speed-loop restrict the current and speed within safe limits,
enhance the speed of response, reduce the effects of nonlinearities in the
converter, motor and load (such as nonlinear transfer characteristic of
converter, coulomb friction, variation of parameters due to temperature
and friction) on the transient and steady state performance of the position
control system.
4. Position controls are required in a number of drive applications, e.g. feed
drive in machine tools, schrew down mechanism in rolling mills.

Thermal Loading of the machine
• Motor temperature is occurred when it operates because
of losses, then it reaches a steady state value.
• Steady state temperature depends on power loss, which
in turn depends on the output power of the machine.
• Since temperature rise has a direct relationship with the
output power, it is termed Thermal Loading on the
machine,

Selection of Motor Power Rating

Three objectives for Selection of Motor
Power Rating
1. To obtain a suitable thermal model for the machine
which can be utilized in calculation of motor ratings for
various Classes of Motor Duty.
2. Categorization of load variation with time into certain
standard categories which are termed as Classes of
Duty of motor.
3. To present methods for calculating motor ratings for
various classes of duty.

Heating and Cooling Curves of
Electrical Drives:

Let the machine, which is assumed to be a homogeneous
body, and the cooling medium has following parameters at
time t:
P1 = Heat developed, joules/sec or watts.
P2 = Heat dissipated to the cooling medium, joules/sec
or watts.
W = Weight of the active parts of machine, kg.
h = Specific heat, Joules per kg per °C.
A = Cooling surface, m2.
d = Coefficient of heat transfer or specific heat
dissipation, joules/sec/m2/°C.
θ = Mean temperature rise, °C

• During a time increment dt, let the machine temperature
rise be dθ. Since,
C is the thermal capacity of the machine, watts/°C, and D the heat
dissipation constant, watts/°C. Heat dissipation mainly occurs
through convection. Typical values of d are in the range of 40 of
600 W/m2/°C.
………(1)
………(2)
………(3)
Simplify the equation (1)
………(4)Where
………(5)

The first order differential equation (3) has a solution
Constant of integration K is obtained by substituting the temperature rise at t = 0
in Eq. (4.6). When the initial temperature rise is θ1, Eq. (4.6) has a solution
τ, which has the dimension of time, is known as the heating (or thermal) time
constant of the machine.
In Eq. (9) as t = ∞, θ = θss. Thus θss is the steady state temperature of the machine
when it is continuously heated by power P1. At this temperature, all the heat
produced in machine is dissipated to the surrounding medium.
………(6)
………(7)
………(8)
………(9)

Let the load on machine be thrown off after its temperature rise reaches a value θ2. Heat loss
will reduce to a small value P′1 and cooling operation of the motor will–begin. Let the new value
of heat dissipation constant be D′. If time is measured from the instant the load is thrown off,
then
Solving this first order differential equation subjected to the initial condition, θ = θ2 at
t = 0, gives
θ′ss is again steady state temperature rise for new conditions of operation and τ′ is
known as the Cooling (or thermal) Time Constant of the machine.
If motor were disconnected from the supply during Heating and Cooling Curves of
Electrical Drives then P′1 = θ′ss = 0, suggesting that the final temperature attained
by the motor will be ambient temperature.
Substituting in Eq. (4.11) gives
………(10)
………(11)
………(13)
………(12)
Where
………(14)

Eqs. (9) and (14) suggest that both heating time constant τ and
cooling time constant τ′ depend on the respective heat dissipation
constants D and D′, which in turn depend on the velocity of
cooling air.
In self cooled motors, where cooling fan is mounted on motor
shaft, the velocity of cooling air varies with motor speed, thus
varying cooling time constant τ′. Cooling time constant at
standstill is much larger than when running. Therefore, in high
performance, and medium and high power variable speed drives,
motor is always provided with separate forced cooling, so that
motor cooling be independent of speed.

The Figure shows the variation of motor temperature rise with time during
Heating and Cooling Curves of Electrical Drives.
Thermal time constants of a motor are far larger than electrical and mechanical
time constants.
While electrical and mechanical time constants have a typical ranges of 1 to
100 ms and 10 ms to 10 s, the thermal time constants may vary from 10 min to
couple of hours.

Classes of Motor Duty in Electrical Drives:

IS: 4722-1968 categorizes various load time variations
encountered in practice into eight standard Classes of
Motor Duty in Electrical Drives:
1. Continuous duty.
2. Short time duty.
3. Intermittent periodic duty.
4. Intermittent periodic duty with starting.
5. Intermittent periodic duty with starting and
braking.
6. Continuous duty with intermittent periodic
loading.
7. Continuous duty with starting and braking.
8. Continuous duty with periodic speed changes.

1. Continuous Duty (Fig. (a)):
This duty is characterized by a constant motor loss.
Paper mill drives, compressors, conveyers, centrifugal pumps
and fans are some examples of Classes of Motor Duty in
Electrical Drives.
2. Short Time Duty (Fig. (b)):
In this operation, the machine can be overloaded until
temperature at the end of loading time reaches the permissible
limit.
 Some examples are: crane drives, drives for household
appliances, turning bridges, sluice-gate drives, valve drives, and
many machine tool drives for position control.

3 Intermittent Periodic Duty (Fig. (c)):
It consists of periodic duty cycles, each consisting of a period of
running at a constant load and a rest period.
In this Classes of Motor Duty in Electrical Drives, heating of machine
during starting and braking operations is negligible.
Some examples are pressing, cutting and drilling machine drives.
4. Intermittent Period Duty with Starting (Fig. (d)):
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; with operating and rest periods, being
too short for the respective steady-state temperatures 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.
Few examples are metal cutting and drilling tool drives, drives for fork
lift trucks, mine hoist etc.

5. Intermittent Periodic duty with Starting and Braking (Fig. (e)):
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.
Billet mill drive, manipulator drive, ingot buggy drive, schrew-down
mechanism of blooming mill, several machine tool drives, drives for electric
suburban trains and mine hoist are some examples of this duty.
6. 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.
This Classes of Motor Duty in Electrical Drives 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.
Pressing, cutting, shearing and drilling machine drives are the examples.

7. 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 main drive of a blooming mill is an example.
8. Continuous Duty with Periodic Speed Changes:
Consists of periodic 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.

Motor Rating Various Duty Cycles:
Various Motor Rating Various Duty Cycles can be broadly
classified as:
Continuous duty.
Fluctuating loads.
Short-time and intermittent duty.

Load Equalization in Electrical Drives:

Motor speed torque at
fluctuating loads
1. Load equalisation is the process of smoothing the fluctuating load.
2. The fluctuate load draws heavy current from the supply during the peak
interval and also cause a large voltage drop in the system due to which the
equipment may get damage.
3. In load equalisation, the energy is stored at light load, and this energy is
utilised when the peak load occurs. Thus, the electrical power from the
supply remains constant.
4. The load fluctuation mostly occurs in some of the drives.
5. For example, in a pressing machine, a large torque is required for a short
duration. Otherwise, the torque is zero.
6. Some of the other examples are a rolling mill, reciprocating pump,
planning machines, electrical hammer, etc.

7. In electrical drives, the load fluctuation occurs in the wide
range. For supplying the peak torque demand to electrical drives
the motor should have high ratings, and also the motor will draw
pulse current from the supply.
8. The amplitude of pulse current gives rise to a line voltage
fluctuation which affected the other load connected to the line.

Method of Load Equalization
1. The problem of load fluctuation can be overcome by using the fly-
wheel.
2. The flying wheel is mounted on a motor shaft in non-reversible
drives.
3. In variable speed and reversible drive, a flywheel cannot be
mounted on the motor shaft as it will increase the transient time of
the drive.
4. If the motor is fed from the motor generator set, then flywheel
mounted on the motor generator shaft and hence equalizes the
load on the source but not load on the motor.
5. When the load is light, the flywheel accelerated and stored the
excess energy drawn from the supply.
6. During the peak load, the flying wheel decelerates and supply the
stored energy to the load along with the supply energy. Hence the
power remains constant, and the load demand is reduced.
7. Moment of inertia of the flying wheel required for load equalisation
is calculated as follows

Electrical drive unit 1 as per IP university_EEE

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Electrical drive unit 1 as per IP university_EEE