Department : Electrical & Electronic Engineering

Name : Dr. Surendra Kumar Tripathi

Designation : Associate Professor
Semester : VIth
Subject : Special Electrical Machines
Subject Code : KEE-061
Topic : Concept of Constant Torque and Constant Power
Controls.
 Concept of Constant Torque and
Constant Power Controls
Constant Torque Loads
Constant Power Loads
 Constant Torque Loads
Constant torque loads require the same amount of torque at low speeds
as at high speeds.
Torque remains constant throughout the speed range, and the
horsepower increases and decreases in direct proportion to the speed.
Constant torque loads include most positive displacement and
reciprocating pumps and compressors as well as traction drives and
conveyors.
With constant torque loads, the torque is not a function of speed.
As speed is changed, the load torque will remain
fairly constant and the horsepower will change
linearly with the speed.
Constant Torque Loads

For example, if the speed increases by 50%, then the

power required to drive the operation will increase
50% while the torque remains constant.
 Constant Power Loads
Constant horsepower loads require high torque at low speeds and low
torque at high speeds, which means constant horsepower at any speed.
Constant horsepower loads include grinders, winding machines and
lathes.
For constant horsepower loads, the torque loading is a function of
speed up to 100% operating speed.
As the speed of the operation is decreased, the torque increases so that
the horsepower required remains essentially
constant.
Constant Power Loads
 Variable Torque Loads
Variable torque loads require much lower torque at low speeds than at
high speeds.
The torque required varies as the square of the speed and the
horsepower required varies as the cube of the speed.
Variable torque loads include most centrifugal and axial pumps, fans
and blowers and many mixers and agitators.
 Variable Torque Loads
As the speed is decreased, the torque will decrease by the square of the
speed decrease and the horsepower required decreases by the cube of
the speed decrease.
Constant Torque and Constant
Power Operation
(Base Speed Torque/Power Characteristics)
Constant Torque and Constant
Power Operation
(Rotor Speed -Torque Characteristics)
 Applications of Constant Torque
Operation
• Traction drives
• Compressors
• Conveyors
• Positive displacement pumps
• Mixers and Hoists etc
 Applications of Constant Power
Operation
• Metal Cutting Machine
• Milling Machine
• Paper Machine
• Film Production Line Coiler, Uncoiler, etc
Thank
You
Department : Electrical & Electronic Engineering
Name : Dr. Surendra Kumar Tripathi
Designation : Associate Professor
Semester : VIth
Subject : Special Electrical Machines
Subject Code : KEE-061
Topic : SEIG Operating Principle, Equivalent Circuit,
Characteristics, Applications
 Induction Generator Working
Theory
Just like a DC Machine, a same induction machine can be used as an
induction motor as well as an induction generator, without any
internal modifications. Induction generators are also called as
asynchronous generators.
Before starting to explain how an induction (asynchronous)
generator works, I assume that you know the working principle of an
induction motor.
 Induction Generator Working
Theory
In an induction motor, the rotor rotates because of slip (i.e. relative
velocity between the rotating magnetic field and the rotor). Rotor tries
to catch up the synchronously rotating field of the stator but never
succeeds.
If rotor catches up the synchronous speed, the relative velocity will be
zero, and hence rotor will experience no torque.
But what if the rotor is rotating at a speed more than synchronous
speed?
 Working of Induction Generators
Consider, an AC supply is connected to the stator terminals of an
induction machine. Rotating magnetic field produced in the stator
pulls the rotor to run behind it (the machine is acting as a motor).
Now, if the rotor is accelerated to the synchronous speed by means of
a prime mover, the slip will be zero and hence the net torque will be
zero. The rotor current will become zero when the rotor is running at
synchronous speed.
If the rotor is made to rotate at a speed more than the synchronous
speed, the slip becomes negative. A rotor
current is generated in the opposite direction,
due to the rotor conductors cutting stator
magnetic field.
 Working of Induction Generators
This generated rotor current produces a rotating magnetic field in the
rotor which pushes (forces in opposite way) onto the stator field. This
causes a stator voltage which pushes current flowing out of the stator
winding against the applied voltage. Thus, the machine is now
working as an induction generator (asynchronous generator).
 Working of Induction Generators
Induction generator is not a self-excited machine.
Therefore, when running as a generator, the machine takes reactive
power from the AC power line and supplies active power back into the
line.
Reactive power is needed for producing rotating magnetic field.
The active power supplied back in the line is proportional to slip
above the synchronous speed.
 Self-Excited Induction Generator
It is clear that, an induction machine needs reactive power for
excitation, regardless whether it is operating as a generator or a motor.
When an induction generator is connected to a grid, it takes reactive
power from the grid.
But what if we want to use an induction generator to supply a load
without using an external source (e.g. grid)?
A capacitor bank can be connected across the stator terminals to supply
reactive power to the machine as well as to the
load.
 Self-Excited Induction Generator
When the rotor is rotated at an enough speed, a small voltage is
generated across the stator terminals due to residual magnetism. Due to
this small generated voltage, capacitor current is produced which
provides further reactive power for magnetization.
 Applications of SEIG
Induction generators produce useful power even at varying rotor
speeds. Hence, they are suitable in wind turbines.
 Advantages of SEIG
Induction or asynchronous generators are more rugged and require
no commutator and brush arrangement (as it is needed in case of
synchronous generators).
 Disadvantage of SEIG
One of the major disadvantage of induction generators is that they take
quite large amount of reactive power.
Thank
You
Department : Electrical & Electronic Engineering
Name : Dr. Surendra Kumar Tripathi
Designation : Associate Professor
Semester : VIth
Subject : Special Electrical Machines
Subject Code : KEE-061
Topic : DFIG Operating Principle, Equivalent Circuit,
Characteristics, Applications
 Doubly Fed Electric Machine
Doubly-fed electric machines also slip-ring generators are electric
motors or electric generators, where both the field magnet windings
and armature windings are separately connected to equipment outside
the machine.
By feeding adjustable frequency AC power to the field windings, the
magnetic field can be made to rotate, allowing variation in motor or
generator speed. This is useful, for instance, for generators used in
wind turbines.
DFIG-based wind turbines, because of their
flexibility and ability to control active and reactive
power, are almost the most interesting wind turbine
technology.
Doubly Fed Induction Generator
Doubly fed induction generators are similar to AC generators, but
have additional features which allow them to run at speeds slightly
above or below their natural synchronous speed. This is useful for
large variable speed wind turbines, because wind speed can change
suddenly.
When a gust of wind hits a wind turbine, the blades try to speed up,
but a synchronous generator is locked to the speed of the power grid
and cannot speed up. So large forces are developed in the hub,
gearbox, and generator as the power grid pushes
back.
Doubly Fed Induction Generator
This causes wear and damage to the mechanism. If the turbine is
allowed to speed up immediately when hit by a wind gust, the
stresses are lower with the power from the wind gust still being
converted to useful electricity.
 Working of DFIG
The principle of the DFIG is that stator windings are connected to the
grid and rotor winding are connected to the converter via slip rings
and back-to-back voltage source converter that controls both the rotor
and the grid currents.
Thus rotor frequency can freely differ from the grid frequency (50 or
60 Hz). By using the converter to control the rotor currents, it is
possible to adjust the active and reactive power fed to the grid from
the stator independently of the generator's turning speed.
 Working of DFIG
The control principle used is either the two-axis current vector control
or direct torque control (DTC). DTC has turned out to have better
stability than current vector control especially when high reactive
currents are required from the generator.
The doubly-fed generator rotors are typically wound with 2 to 3 times
the number of turns of the stator. This means that the rotor voltages
will be higher and currents respectively lower. Thus in the typical
±30% operational speed range around the synchronous speed, the
rated current of the converter is accordingly
lower which leads to a lower cost of the
converter.
 Working of DFIG
The drawback is that controlled operation outside the operational
speed range is impossible because of the higher than rated rotor
voltage.
Further, the voltage transients due to the grid disturbances (three- and
two-phase voltage dips, especially) will also be magnified.
In order to prevent high rotor voltages (and high currents resulting
from these voltages) from destroying the insulated-gate bipolar
transistors and diodes of the converter, a protection circuit
(called crowbar) is used.
 Applications of DFIG
DFIG produce useful power even at varying rotor speeds. Hence, they
are suitable in wind turbines.
 Advantages of DFIG
DFIG is a wound-rotor doubly-fed electric machine and has several
advantages over a conventional induction machine in wind power
applications.
First, as the rotor circuit is controlled by a power electronics
converter, the induction generator is able to both import and export
reactive power. This has important consequences for power system
stability and allows the machine to support the grid during severe
voltage disturbances (low-voltage ride-through; LVRT).
 Advantages of SEIG
Second, the control of the rotor voltages and currents enables the
induction machine to remain synchronized with the grid while the
wind turbine speed varies. A variable speed wind turbine utilizes the
available wind resource more efficiently than a fixed speed wind
turbine, especially during light wind conditions.
Third, the cost of the converter is low when compared with other
variable speed solutions because only a fraction of the mechanical
power, typically 25–30%, is fed to the grid through the converter, the
rest being fed to grid directly from the stator. The efficiency of the
DFIG is very good for the same reason.
Thank
You
Department : Electrical & Electronic Engineering
Name : Dr. Surendra Kumar Tripathi
Designation : Associate Professor
Semester : VIth
Subject : Special Electrical Machines
Subject Code : KEE-061
Topic : Linear Induction Motors. Construction, principle of
operation, Linear force, and applications.
 Linear Induction Motor
A Linear Induction Motor (LIM) is an advanced version of rotary
induction motor which gives a linear translational motion instead of
the rotational motion.
The stator is cut axially and spread out flat.
In this type of motor, the stator and rotor are called primary and
secondary respectively.
The secondary of the linear induction motor consists of a flat
aluminium conductor with a ferromagnetic core.
Types of Linear Induction Motor
(The linear induction motors are classified as Single Sided Linear
Induction Motor (SLIM) and Double Sided Linear Induction Motor
(DLIM) as shown. DLIM has primaries on both sides of the
secondary. The speed torque characteristic of these linear motors is
similar to that of rotary induction motors.)
Doubly Fed Induction Generator
Linear Induction Motor Polyphase Rotatory Induction Mot
 Operation of Linear Induction
Motor
If a three phase supply is connected to the stator of an induction motor,
a rotating flux is produced.
This flux rotates at a synchronous speed in the air gap.
Similarly, if the primary of the linear induction motor is connected to
the three phase supply, a flux is produced which travel across the
length of the primary.
A current is generated in the conductor which is made of the
aluminium material.
 Operation of Linear Induction
Motor
A current is generated in the conductor which is made of the
aluminium material.
The current, which is induced in the linear induction motor interacts
with the travelling flux produces a linear force.
If secondary of the linear induction motor is fixed and the primary is
free to move, the force will move the primary in the direction of the
travelling wave.
 Performance of the Linear
Induction Motor
The linear synchronous speed of the travelling wave is given by the
equation-

Where f is the supply frequency in hertz.

In the rotary induction motor, the speed of the secondary in the linear
induction motor is less than the synchronous speed vs and is given as
 Performance of the Linear
Induction Motor
Where s is the slip of the linear induction motor and is given as

The linear force is given by the equation shown below

Thrust Speed Curve of Linear
Induction Motor
• The thrust velocity curve of the linear
induction motor is similar to that of the
speed torque curve of the rotary induction
motor as shown in the figure.
• If a rotary induction motor is compared
with the linear induction motor, the LIM
requires a larger air gap and hence, the
magnetizing current is greater and the
power factor and efficiency of the motor
are lower.
Thrust Speed Curve of Linear
Induction Motor
• In the rotary induction motor the stator
and the rotor area are same whereas in the
LIM the one of the two is shorter than the
other.
• At the steady speed, the shorter part will
be passing continuously over a new part
of the other member.
 Applications of the Linear
Induction Motor
The various applications of the LIM are as follows:-
1. The main application of the LIM is in transportation and in electric
traction system. The primary is mounted on the vehicle and the
secondary is laid on the track.
2. It is used in the cranes
3. Pumping of liquid metals
4. Actuators for the movement of doors
5. Used in High voltage circuit breakers and
also in accelerators.
Thank
You
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