• Types of electric drives
• Choice of motor
• Starting characteristics
• Running characteristics
• Speed control
• Temperature rise
• Particular Applications of Electric drives
• Types of industrial loads: continuous, intermittent
and variable loads, load Equalization.
• The system which is used for controlling the motion
of an electrical machine, such type of system is called
an electrical drive.
• In other words, the drive which uses the electric
motor is called electrical drive.
• The electrical drive uses any of the prime movers like
diesel or a petrol engine, gas or steam turbines, steam
engines, hydraulic motors and electrical motors as a
primary source of energy. This prime mover supplies
the mechanical energy to the drive for motion control.
Power Modulator :
• The power modulator regulates the output
power of the source.
• It controls the power from the source to the
motor in such a manner that motor transmits
the speed torque characteristic required by the
• During the transient operations like starting, braking and
speed reversing the excessive current drawnfrom the
• This excessive current drawn from the source may
overload it or may cause a voltage drop. Hence the
power modulator restricts the sourceand motor
• e.g. if the sourceis DC and an induction motor is
used then power modulator convert DC into AC. It also
selects the mode of operation of the motor, i.e., motoring
• The control unit controls the power modulator which
operates at small voltage and power levels. The
control unit also operates the power modulator as
• It also generates the commands for the protection of
power modulator and motor. An input command
signal which adjusts the operating point of the drive,
from an input to the control unit.
• It senses the certain drive parameter like motor
current and speed.
• It mainly required either for protection or for closed
ADVANTAGE OF ELECTRIC DRIVES
1. The electrical drives are available in a wide
range of torque, speed and power.
2. They are adaptable to almost any operating
conditions such as explosive and radioactive
environment, submerged in liquids, vertical
mounting and so on.
3.The electrical drive does not pollute the
ADVANTAGE OF ELECTRIC DRIVES
4. It can operate in all four quadrants of speed
5. They can be started instantly and can
immediately be fully loaded. i.e., there is no
need to refuel or warm up the motor.
6. They have flexible control characteristic and
can be employed to automatically control the
ADVANTAGE OF ELECTRIC DRIVES
7. They have flexible control characteristic and can be
employed to automatically control the drive.
8. Because of the following advantages, the mechanical
energy already available from a non-electrical prime
mover is sometimes first converted into electrical
energy by a generator and back to a mechanical
energy of an electrical motor.
9. Electrical link thus provides between the non-
electrical prime mover and the load impact to the
drive flexible control characteristic.
Disadvantage Of Electric Drives
• The power failure completely disabled the
whole of the system.
• The application of the drive is limited because
it cannot use in a place where the power
supply is not available.
• It can cause noise pollution.
• The initial cost of the system is high.
• 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
• Advantage :
• 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
• Not safe to operate. Also Noise level at the
working spot is high.
• In individual drive a single electric motor is used to drive one
individual machine. i.e In individual drive each working
machine has the individual main machine.
• Example: single-spindle drilling machines and various types of
electrical hand tools and simple types of metal working tools.
1. Installation of individual drive is easy.
2. If a fault is occurred in one main machine then the
whole operations are not effected because it has
individual main machines.
3.Each main machine can be effectively utilized at rated
4. Full control and desired operation of each machine is
obtained because of different machines are driven
with their respective individual drive.
5. Machines can be located at convenient places
Cost is high because in this type of drive the
number of machines required is high.
More space is required because of each
working machine has its individual main
It consists of several individual drives each of
which serves to operate one of many working
Multi-motor drives means the number of
operations are required to perform a task.
Example: The operation of CRANE.
FACTORS GOVERNING SELECTION OF ELECTRIC MOTORS
CHOICE OF MOTOR
• Basic Elements are electric motor, the
transmission and the electrical control system.
• Here the electric motor is very important one
in the drive equipment.
• So we have to select the motor as per our
requirements. Some factors are to be considers
while selecting the motor in the drives. Those
factors are as fallows.
1. Nature of Supply
• 3-phase a.c. or single phase a.c.
• In case of three phase a.c. supply is available,
polyphase induction motors, squirrel cage type for
small ratings and slip ring type for higher ratings may
• Pole changing motors with stepped pulleys are used
where large speed variation is required.
• Use of single phase motors is limited to small loads
only because of their limited outputs.
D.C. motors are not used so widely used
Additional equipment is required for converting
existing a.c. supply into d.c. supply.
D.C. motors have commutators that are subject to
trouble and resulting in sparking, brush wear, arc over
and the presence of moisture and destructive fumes in
the surrounding air.
D.C. motors are generally more expensive than a.c.
machines for similar working conditions.
1.Loads required constant torque at all speeds, as shown
by the horizontal line 1 in the fallowing figure. Such
loads are cranes during hoisting, cranes
2. Loads requiring torque which may increase in direct
proportion to the speed as shown by straight line 2 in
3. Loads requiring, which may increase with the square
of speed. As shown by the curve3 in the above figure.
Such loads are blowers, fans, centrifugal pumps, ship
4. Loads requiring torque which may decrease with the
increase in the speed as shown by the curve 4 in the
above figure. Boring machines, milling machines and
other types of metal cutting machines are examples of
Operating or Running characteristics.
• Speed-current characteristics
• Torque current characteristics
• Torque speed characteristics
case 1: For dc shunt motor
• In shunt motor, is maintained to be constant since DC
shunt motor is a constant speed motor. If V is also
maintained to be constant then the speed the speed-
current characteristics is obtained as shown in the
For Dc Compound Motor
The cumulative compound motors are used in the
driving machines. Which are subjected to the sudden
application of heavy loads. These motors are used
where a large starting torque is required.
Applications: Rolling mills
In differential compound motor the speed remains
constant and sometimes increase with increase in the
load. This motor may rotate in opposite direction at
high loads that is why this motor is seldom used
Case 3: for dc compound motor
• In cumulative compound motor flux is more, hence torque developed is
more. Where as in differential compound motor flux is less and hence
torque developed is less.
• Cumulative compound motors are used in driving machines.
Which are subjected to sudden application of heavy loads such
as in rolling mills. This type of motor is also used, when high
starting torque is required such as in cranes.
• Case 1: For dc shunt motor
In a dc shunt motor when the supply voltage is constant the
field flux and armature flux is also constant and speed of the
motor mainly depends upon the armature current. The speed
decrease with the increase in armature current.
Case 2: For dc series motor
• As the current increases the torque also increases, where as the
speed falls. Hence it is observed from the characteristics
during starting the torque is less and the speed is dangerously
high. The motor must always be started on full load.
Torque Slip Characteristics of Three
Phase Induction Motor
• The torque slip curve for an induction motor gives us
the running characteristics of induction motor.
• The torque-slip characteristic curve can be divided
roughly into three regions:
• Low slip region
• Medium slip region
• High slip region
• Motoring Mode:
The induction motor torque varies from zero to
full load torque as the slip varies. The slip varies from
zero to one. It is zero at no load and one at standstill.
From the curve it is seen that the torque is directly
proportional to the slip.
That is, more is the slip, more will be the torque
produced and vice-versa. The linear relationship
simplifies the calculation of motor parameter to great
• In this mode of operation induction motor runs above the
synchronous speed and it should be driven by a prime mover.
• The stator winding is connected to a three phase supply in
which it supplies electrical energy. Actually, in this case, the
torque and slip both are negative so the motor receives
mechanical energy and delivers electrical energy.
• Induction motor is not much used as generator because it
requires reactive power for its operation.
Why reactive power is needed for
• An induction generator essentially needs a lagging current
(inductive current) to magnetise the core. Therefore it draws
reactive power from the grid (to which it is connected to feed
real power). The reactive power drawn by induction generators
(particularly of squirrel cage type) has to be compensated by
• However, this problem can be overcome by using doubly fed
induction generator or wound rotor induction generator. In
these machines, both stator and rotor have three phase AC
windings. Stator is directly connected to grid AC supply.
• In the Braking mode, the two leads or the polarity of the
supply voltage is changed so that the motor starts to rotate in
the reverse direction and as a result the motor stops.
• This method of braking is known as plugging. This method is
used when it is required to stop the motor within a very short
period of time.
• The kinetic energy stored in the revolving load is dissipated as
Applications of SCIM
• They are particularly suited for applications where the
motor must maintain a constant speed, be self-
starting, or there is a desire for low maintenance.
• Centrifugal pumps
• Industrial drives (e.g. to run conveyor belts)
• Large blowers and fans
• Machine tools
• Lathes and other turning equipment
• These motors are used where higher torque
and low starting current are required.
• Used in applications like elevators,
compressors, cranes, conveyors, hoists, and
Starting and Running characteristics of
Single phase induction motors
• From the figure, we see that at a slip of unity, both
forward and backward field develops equal torque but
the direction of which are opposite to each other so
the net torque produced is zero hence the motor fails
• There must be some means to provide the starting
torque. If by some means, we can increase the
forward speed of the machine due to which the
forward slip decreases the forward torque will
increase and the reverse torque will decrease as a
result of which motor will start.
Torque speed characteristics of
• To get a better starting torque the damper winding must have a
high resistance. However, this inhibits their primary function
of damping the oscillations, since a low resistance damper is
more effective for this task. A judicious choice of resistance is
required, depending upon the application of the motor.
Applied Voltage Control
The voltage obtained from the generator is controlled by a
field divertor resistance connected across the series field
winding of the generator
Ward-Leonard System of Speed Control
Thissystemiscommonlyemployedfor elevators, hoists andmaindrive in steel mills
ThemodifiedWard-Leonard systemiscalledWard-Leonard – Ilgner systeminwhich a
flywheelisused in addition to motor generator set.
Why starter required for IM
• From the torque slip characteristic, it is clear that at the slip
equals to one we have some positive starting torque hence we
can say that the three-phase induction motor is self starting
machine, then why there is a need of starters for three phase
• we can see the motor behaves like an electrical transformer
with short circuited secondary winding, because at the time of
starting, the rotor is stationary and the back emf due to the
rotation is not developed yet hence the motor draws the high
By using primary resistors
• Consider, the starting voltage is reduced by 50%.
Then according to the Ohm's law (V=I/Z), the starting
current will also be reduced by the same percentage.
From the torque equation of a three phase induction
motor, the starting torque is approximately
proportional to the square of the applied voltage.
That means, if the applied voltage is 50% of the rated
value, the starting torque will be only 25% of its
normal voltage value.
• It is basically a three phase step down transformer
with different taps provided that permit the user to
start the motor at, say, 50%, 65% or 80% of line
• For example, when a motor is started on a 65% tap, the
applied voltage to the motor will be 65% of the line voltage
and the applied current will be 65% of the line voltage starting
value, while the line current will be 65% of 65% (i.e. 42%) of
the line voltage starting value. This difference between the
line current and the motor current is due to transformer
• The stator phases are first connected to the star by the help of triple pole
double throw switch (TPDT switch) in the diagram the position is marked
as 1 then after this when the steady state speed is reached the switch is
thrown to position 2 as shown in the above diagram.