This document provides information on drive motor characteristics and braking methods for DC motors. It includes: 1. Descriptions of speed-torque characteristics for DC shunt, series, and compound motors. 2. Explanations of different types of braking for DC motors including regenerative, dynamic, and plugging braking. Electrical braking methods convert kinetic energy to electrical energy to generate braking torque. 3. Questions and answers on topics like motor characteristics, braking methods, efficiency advantages of electrical braking over mechanical braking, and speed-torque analyses involving changes in voltage, current and motor parameters.

1. UNIT 2 - DRIVE MOTOR CHARACTERISTICS
PART A
1. What is meant by mechanical characteristics?
A curve is drawn between speed-torque. This characteristic is called mechanical
characteristics.
2. Draw the speed-torque characteristics of dc shunt motor and series motor.
(May2013) (May 2014)
3. A series motor should never be started without some mechanical load why?
When the load current Ia falls to a small value, speed becomes dangerously high.
Hence a series motor should never be started without some mechanical load.
4. What is rheostatic braking?
In rheostatic braking the armature is disconnected from the supply and is
connected as a variable resistance. The braking is controlled by varying the series
resistance.
5. What are the different types of dc motor?
I. DC series motor 2. Shunt motor
3. Compound motor 4. Separately excited de motor
6. What is meant by electrical characteristics?
A curve is drawn torque and armature current. It is known as electrical
characteristics.
7. What is the relation between speed and flux of a dc motor?
The speed of a dc motor is inversely proportional to field flux.
ELECTRICAL DRIVES AND CONTROL

2. 8. What is the application of dc motor?
DC shunt motor:-
1. For driving constant speed operations 2. Machine tools 3. Lathes 4. Blowers
and fans
5. Centrifugal pumps 6.Reciprocating pumps
DC series motor:-
1.Electric locomotives 2. Rapid transit systems3.Trolley cars4.Cranes and hoists5.
Conveyors
DC compound motor:-1. Elevators.2. Air compressors3. Rolling mills4. Heavy planers
9. A dc shunt motor is called as constant speed motor-why? ( May 2015)
The drop in speed from no-load full-load is small; hence the dc shunt motor is also
called as constant speed motor.
10. What is mean by braking?
Whenever an electric drive is disconnected from the supply, the speed of the
driving motor gradually decreases and becomes zero. Braking is a generic term
used to describe a set of operating conditions for electric drive systems. It includes
rapid stopping of the electric motor holding the motor shaft to a specific position,
maintaining the speed to a desired value of preventing the motor from over
speeding.
11. What are the two types of braking?
1. Mechanical braking 2. Electrical braking
12. What is meant by mechanical braking?
In mechanical braking, the frictional force between the rotating parts and brake
drums provide the required brake.
13. What is meant by electric braking?
In electric braking, the motor is made to work as generator. So it produces a
negative slip and negative torque (braking torque). This is achieved by suitably
changing the electrical connections of the motor.
14. What are the different types of electric braking?(Nov2013)(May 2013) ( May
2015)
1. Regenerative braking 2. Dynamic braking 3. Plugging
15. What are the advantages of electric braking?

3. 1. High efficient method 2. Low maintenance 3. Braking is very
smooth
16. What is meant by regenerative braking? (Dec 2011)
In the regenerative braking operation, the motor operators as a generator, while it
is still connected to the supply. Here, the motor speed is greater than the
synchronous speed. Mechanical energy is converted into electrical energy, part of
which is returned to the supply and rest of the energy is last as heat in the winding
and bearings.
17. What is meant by dynamic braking?
When an electric motor rotates, a kinetic energy of the motor is converted into
electric energy. This energy is dissipated in resistive elements.
18. What is meant by plugging? (Dec 2014)
The plugging operation can be obtained by changing the polarity of the motor. For
a machine, the phase sequence of the starter windings and dc machines the
polarities of the field or armature terminals.
19. What are the disadvantages of dc machine?
1. Higher cost 2. Higher rotor inertia 3. EMI problems 4. Maintains problems with
commutator and brushes 5. Do not permit a machine to operate in dirty and
explosive environments.
20. What are the advantages of squirrel cage induction motor?
1. Rugged 2. Cheaper 3.Lighter 4.More efficient 5. Less maintenance6.
Can operate in explosive and dirty environment.
21. What are the two types of rotors in three phase induction motors?
1. Squirrel cage rotor 2. Slip ring rotor
22.What is the necessity of braking?(Dec 2014)
The quickness and accuracy of braking techniques determine the productivity and
quality of the manufactured goods.Control the motor for our optimum requirement.
23.What are different methods of Braking of DC series motor? (Nov 2015)
1. Regenerative braking
2. Dynamic braking
3. Plugging

4. 24. A 220V DC shunt motor having the armature current of 10A, runs at 1500 rpm.
Find the armature current if the source voltage drops to 150V. Assume the load
torque is constant.(Nov 2015)
Given, V1=220V, Ia1=10A, V2=150V, Ia2=?.
For Shunt motor, Ia1Φ1= Ia2Φ2 (since torque is constant)
Φ2=(150/220) Φ1 -------> Φ2=0.68 Φ1.
Therefore,
Ia2=(10 Φ1/0.68 Φ1) = 14.67A.
PART – B
1. Explain about the speed-torque characteristics of a DC Compound Motor with
suitable graph and equations.(Dec 2013)
The DC motor that has both shunt as well as series field winding is known as DC
compound Motor. If the flux produced by series field winding helps (aids) the flux
produced by shunt field, then the total flux increases and DC compound motor is
known as Cumulative Compound Moor. On the other hand, if the series field flux
opposes the Shunt field flux, total flux decreases and the motor is known as
Differential compound Motor.
(N VsIa), (T VsIa) and (N Vs T) characteristics should be obtained.
2. Draw and explain the speed torque characteristics of dc series motor and three
phase induction motor.(Dec 2011)
The DC motor in which the field winding is connected in series with Armature
Conductors is known as DC series Motor. In series circuit the current flowing
through the armature is same as the current flowing through series field winding.
Since the full load armature current flow through series field, the magnetic
saturation takes place (Flux don’t increase with current after certain point ).(N
VsIa), (T VsIa) and (N Vs T) characteristics should be obtained.
3. Explain about the speed-torque characteristics of a DC Shunt Motor with suitable
graph and equations. (Dec 2014)
Speed / Torque Characteristics of DC shunt Motor
DC Shunt Motor
The DC Motor is which in Field Winding is connected in Parallel with Armature
conductors is known as DC Shunt motor since Field winding is connected in
parallel with both supply and the armature, the voltage across the field winding is
always constant. So the current flowing through the field winding and hence the
flux produced in the DC shunt Motor is Constant.

5. (i) Speed / Current Characteristics (N VsIa)
(ii) Torque / Current Characteistics (T VsIa )
(iii) Speed / Torque Characteristics ( NVs T)

6. 4. Explain about the quadrant diagram of speed-torque characteristics for a motor
driving hoist load.(Nov 2015) (May 2015)
Theory : Explanation in detail for the 4 quadrant is needed.

8. 5. Explain how an induction motor is brought to stop by (i) Plugging and (ii)
dynamic braking.
(Dec 2014)
(a) Plugging or Reverse Current Braking of 3 Induction Motor
Plugging incase of 3 induction motor is obtained by interchanging any of the two
phases, out of the three phases. There are three phase terminals in 3 induction motor
and it is represented by R, Y and B. The order in which, these phases progresses is
known as Phase sequence. The phase sequence determines the direction of rotation and
torque. The normal working induction motor is shown in Fig. The phase sequence applied
is RYB and the motor rotates in Clockwise direction. The Plugging or Reverse Current
Braking is obtained by interchanging supply phase terminals Y and B. Now the phase
sequence is changed to RBY. The change is phase sequence from RYB to RYB changes
the direction of current and hence the direction of rotating magnetic field. Due to this,
direction of torque is reversed and this torque is known as Braking torque. The Braking
torque reduced the speed of the motor to standstill. If the supply terminal is not
disconnected at this stage, then the braking torque accelerates the motor in the anit
clockwise direction. So plugging is accomplished by changing the supply terminals.
The reverse current braking characteristic is shown in Fig.. It has two part namely
normaly motoring zone before the application of plugging. The Slip = 0 corresponds to
Synchronous speed Ns. The induction motor always runs at speed less than synchronous
speed. When reverse current braking is applied the braking torque is generated and is
shown as braking zone in Fig. The Speed reduced to zero i.e., n = 0 and the
corresponding Slip, S = 1

9. b.Dynamic or Rheostatic braking (3 Slip Ring Induction Motor)
Rheostatic braking is not possible in case of squirrel cage induction motor and is
only possible in the case of slip ring induction motor. Here braking is obtained by exciting
any of the two stator phase with DC supply and rotor circuit is connected to star connected
external rheostat. The braking is accomplished by disconnecting the stator form 3 supply
and connecting any of the two phases with DC supply. Simultaneously external star
connected rheostat is connected to the rotor circuit through slip rings. Now the start
produced constant flux due to DC supply and the rotor is still rotating due to inertia. So
emf is induced in the rotor and current flows in such a direction to oppose the cause
(speed). So braking torque is produced that results in, stoppage of the motor. The
principle of rheostatic braking can also arrived by the fact that the kinetic energy of rotating

10. mass is converted into electrical energy in rotor and it is wasted in star connected rheostat
and the motor stops, because the kinetic energy is consumed.
From the braking characteristics it is clear that braking torque become zero, when
the motor is stopped i.e there is no holding torque. The magnitude of braking torque is
varied by changing the resistance of the star connected rheostat.
6. Explain the various methods of braking of induction motors. (June 2014)
Electrical braking Methods of AC motor (Induction motor)
The braking methods that are applied to DC motors are also applied to AC motors,
particularly 3 induction motor. The braking methods are
1. Plugging or Reverse Current Braking
2. Dynamic or Rheostatic Braking and
3. Regenerative Braking
7. Describe the speed Torque characteristics of DC Dynamic braking of three
phase induction motor.
(May 2014)

11. Rheostatic braking is not possible in case of squirrel cage induction motor and is
only possible in the case of slip ring induction motor. Here braking is obtained by exciting
any of the two stator phase with DC supply and rotor circuit is connected to star connected
external rheostat. The braking is accomplished by disconnecting the stator form 3 supply
and connecting any of the two phases with DC supply. Simultaneously external star
connected rheostat is connected to the rotor circuit through slip rings. Now the start
produced constant flux due to DC supply and the rotor is still rotating due to inertia. So
emf is induced in the rotor and current flows in such a direction to oppose the cause
(speed). So braking torque is produced that results in, stoppage of the motor. The
principle of rheostatic braking can also arrived by the fact that the kinetic energy of rotating
mass is converted into electrical energy in rotor and it is wasted in star connected rheostat
and the motor stops, because the kinetic energy is consumed.
From the braking characteristics it is clear that braking torque become zero, when
the motor is stopped i.e there is no holding torque. The magnitude of braking torque is
varied by changing the resistance of the star connected rheostat.

12. 8. Explain speed- torque characteristics of different types of load with graph.(May
2013)
N Vs T characteristics of shunt, series and compound motor should be explained.
9. A 220 V dc series motor runs at1200 rpm (clockwise) and takes an armature
current of 80 A when driving a load with constant torque. Armature resistance is
0.05Ω and field resistance is 0.05Ω. Find the magnitude and direction of motor
speed and armature current if the motor terminal voltage is reversed and number
of turns in field winding is reduced to 80%. Assume linear magnetic circuit.
(Dec 2013)
Answer: Direction of current is anticlockwise.
Eb1 = 212V;
Ia2 = 100A; Eb2 = 210V;
N2 = 1485 rpm.
10. Explain various methods of braking of DC Series Motors with neat
diagrams.(May 2013)
The DC motor in which the field winding is connected in series with Armature Conductors
is known as DC series Motor. In series circuit the current flowing through the armature is
same as the current flowing through series field winding. Since the full load armature
current flow through series field, the magnetic saturation takes place (Flux don’t increase
with current after certain point ). (N VsIa), (T VsIa) and (N Vs T) characteristics should be
obtained.
11.Discuss the dynamic braking of DC shunt motor. (Nov 2015)
Rheostatic or Dynamic Braking
Rheostatic braking of DC motor is obtained by disconnecting the armature
terminals from the supply and connecting it to the external resistance R. The field winding
is not disturbed.
Rheostatic Braking of DC shunt Motor
The armature terminals are disconnected from the supply and connected to
external resistance R. now the armature current direction is reversed. Due to this reversed
current direction the torque is produced in the opposite direction. Which is known as
braking torque. Now the armature generates voltage and hence acts as a separately
exited generator. The kinetic energy of the moving system is converted into electrical

13. energy, which is dissipated in the external resistance R. Here the motor is braked by
generated action. The connection diagram for Rheostatic braking of DC shunt motor
shown below.
12. List the advantages and disadvantages of Electrical braking over
mechanical braking. Discuss any one method of electrical braking of DC
machines.(May 2015)

14. Electrical Braking methods for DC motor.
The Electrical braking is applied to DC motor by generating the electrical braking
torque. The braking torque is obtained by just changing some of the electrical
connection of the electric motor. In electric braking the drive motor itself methods
available, they are
1. Plugging or reverse current or counter current
2. Rheostatic braking
3.Regenerative braking
Plugging in DC Series motor
The reverse current braking of series motor is obtained by reversing the armature
terminals. Due to reversal of armature terminals, the direction of the armature current
is reversed. Enough care should be taken to ensure that the field current is not reversed.
This is because in series motor armature and series field are in the same circuit. The
braking torque is produced due to reversal of armature current. To limit the over current
during braking an external resistance R is included in the armature current. The
Plugging is also done in series motor by separately existing the series field winding.

15. Rheostatic braking of DC series
Motor
The Rheostatic braking of DC series motor is shown. Here armature and field winding
present in the same circuit. Rheostatic Braking is obtained by disconnecting the
armature circuit form the supply and connected to external resistance R. Now
armature current is reversed. Since this is series motor, the field current is also
reversed (armature current is also the field current). This leads to demagnetization of
field circuit, as a result generator action do not takes place. To avoid this, field terminals
(f1, f2) are reversed when armature circuit is connected to the external resistance R.
Now motor acts as a generator and as the a result, kinetic energy of the moving system
is converted to the electrical energy and wasted in the resistance R. Since the kinetic
energy of the moving system is wasted, Braking torque is produced.
Regenerative braking of DC series
Motor
In series motor the armature and the field, both present in the san circuit. Here
the armature current is equal to the field current. When the over hauling load increase
the speed of the DC motor, the back emf also increase. So the current also flow through
the field windings the flux is also decreases. Due to decrease in flux, it is clear that the
back emf never exceeds the supply voltage. So there is no possibility of motor entering
into the generating mode. So there is no chance of regenerative braking torque. It is
clear from the above discussion that Regenerative braking is not possible in case of DC
series motor.
The DC series motor in practice is subjected to over hauling loads, as in the case of
hoist (lowering load) and traction (when descending the mountain). So the
regenerative braking can be obtained in the DC series motor by separately exciting
the series field winding. This method of separately exciting DC series motor to obtain

16. Regenerative braking. Now the Back emf (Eb) exceeds the supply voltage (V), as a
result the machine enters into generating mode. The current direction is reversed now
(towards the supply mains), as a result regenerative braking torque is produced,
due to magnetic drag associated with generating mode. The speed of the machine
is reduced till it reaches the rated no load speed. When the speed of the machine
reaches rated no load speed, back emf is no longer greater than supply voltage and
the machine reenters the motoring mode.