ELECTRICAL

TECHNOLOGY (ME2112)
SY MECH
UNIT 1

MR. ATUL J. PATIL

M.TECH.(CONDITION MONITORING OF ELECTRICAL APPARATUS)
ASSISTANT PROFESSOR
ELECTRICAL ENGINEERING DEPT. RIT, SAKHRALE
Electrical machines
Motor Classification
 DC Motor
 DC Motor - A machine that converts DC electrical power into mechanical power
is known as DC motor. It works on the principle that when a current carrying
conductor is placdd in a magnetic field, then a force exerted on it, and torque
develops.
DC Motor construction

Parts of DC motor –
1. Yoke or Frame
2. Stator – Static part
3. Rotor – Rotating part
4. Poles
5. Field windings
6. Armature windings
7. Commutator
8. Brushes
DC Motor construction – Yoke or Frame

 Another name of a yoke is frame.

 The main function of the yoke in the machine is to offer
mechanical support for poles and protects the entire machine
from the moisture & dust.
 The materials used in the yoke are designed with cast iron, cast
steel or rolled steel.
 It not only provides mechanical strength to the whole assembly
but also carries the magnetic flux produced by the field
winding.
 Magnetic Flux - is defined as the number of magnetic field
lines passing through a given closed surface.
DC Motor construction – Stator

 The stator is the static part of the motor.

 The main function of the stator is to generate the rotating magnetic
field.
 The stator frame, stator core and stator winding are the three parts of
the stator.
 The stator core support and protect the winding of the stator.
 High-grade silicon steel stamping makes the core of the stator.

Stamping includes a variety of sheet-metal forming

manufacturing processes, such as punching using a
machine press or stamping press,
DC Motor construction – Rotor

 The rotating part of the motor is known as the rotor.

 The rotor core and the rotor winding are the part of the rotor.
 The winding of the rotor is excited by the DC supply.
 The core of the rotor is made of the cylindrical iron core.
 The core has a semi-circular slot on their outer surface on which the
copper or aluminium conductors are placed.
 Winding is one or more turns of wire that forms a continuous
coil through which an electric current can pass
DC Motor construction – Poles

 The magnetic poles of DC motor are structures fitted

onto the inner wall of the frame with screws.
 The pole has two parts, the pole core and the pole
shoe stacked together under hydraulic pressure and
then attached to the frame.
 These two structures are assigned for different
purposes, the pole core is of small area and its
function is to just hold the pole shoe over the Frame.
 The pole shoe having a relatively larger area spreads
the flux produced over the air gap between the stator
and rotor.
 The pole shoe also carries slots for the field windings
that produce the field flux.
DC Motor construction – Field winding

 The field winding of DC motor are made with field

coils (copper wire), they are wound in such a way
that, when energized, they form alternate North and
South poles.
 The field winding basically form an electromagnet,
that produces field flux within which the rotor
armature of the DC motor rotates.
 Field coils are former wound and placed on each pole
and are connected in series.
 They are usually made of copper.
Energize – supply electrical energy
DC Motor construction – Armature winding
 The armature winding of DC motor is attached to the rotor
and so it is subjected to changing magnetic field in the path
of its rotation which results in magnetic losses. For this
reason the rotor is made of armature core, that’s made with
several silicon steel lamination, to reduce the magnetic
losses like hysteresis and eddy current loss .
 These laminated steel sheets are stacked together to form
the cylindrical structure of the armature core.
 Thickness of these lamination’s kept low to reduce eddy
current losses.
 The materials used for this winding are conducting
material like copper.
Lamination is the technique/process of manufacturing a
material in multiple layers, so that the composite material
achieves improved strength
DC Motor construction – Commutator

 Physical connection of supply to the armature

winding (Rotor) is made through a commutator-
brush arrangement.
 The commutator is a cylindrical structure made
up of copper segments stacked together, but
insulated from each other by mica.
 Its main function is to supply or relay the supply
current from the mains to the armature winding
through the brushes.
 It also provides uni-directional torque for DC-
DC Motor construction – Brushes

 Brushes rest on the surface of the commutator.

 The brushes of DC motor are made with carbon
or graphite structures, making sliding contact
over the rotating commutator.
 The brushes are used to relay the current from
supply to the rotating commutator from where it
flows to the armature winding.
 The commutator and brush unit of the DC motor
is concerned with transmitting the power from the
static electrical circuit to the mechanically
rotating region or the rotor.
 Brushes have wear & tear with time so we need
Working of DC Motor
 DC Motor working

 The DC motor is the device which converts the direct current into the mechanical work.
 It works on the principle of Lorentz Law, which states that “when a current-carrying
conductor is placed in a magnetic field it experience a force”. The experienced force is
called as Lorentz force.
 DC motor contains a current carrying conductors (armature windings), connected to the
DC supply through commutator and brushes.
 The armature (rotor) is placed in between north pole and south pole of a permanent or an
electromagnet (field winding)
 As soon as we supply direct current in the armature, a mechanical force acts on it due to
the electromagnetic effect of the magnet on armature conductors.
 The Flemming left-hand rule gives the direction of the force (rotation).
DC Motor working

 The Flemming left-hand rule gives the direction of

the force (rotation).
 If we stretch the first finger, second finger and
thumb of our left hand to be perpendicular to each
other, and the direction of magnetic field is
represented by the first finger, direction of the
current is represented by the second finger, then
the thumb represents direction of the force
experienced by the current carrying conductor.
F = BIL.
 Where, F = Force, B = magnetic flux density, I =
current and L = length of the conductor within the
DC Motor working
 DC Motor working
BACK EMF OR COUNTER EMF –
 When the armature of a motor is rotating, the conductors (windings) are cutting the
magnetic field lines and hence according to the Faraday's law of electromagnetic induction,
an emf (voltage) induces in the armature (rotor) conductors. The direction of this induced
emf is such that it opposes the armature current.

Faraday's First Law of Electromagnetic Induction (Generator working)

 When a conductor is placed in a varying magnetic field, an emf is induced (generated). If
the conductor circuit is closed, a current is induced which is called induced current.
OR
 When a varying conductor is placed in a steady magnetic field, an emf is induced
(generated). If the conductor circuit is closed, a current is induced which is called induced
current.
 Faradays law
When a varying conductor is placed in a steady magnetic field, an
emf is induced (generated). If the conductor circuit is closed, a
current is induced which is called induced current.
 DC Motor working
BACK EMF OR COUNTER EMF –
 When the armature of a motor is rotating, the conductors (windings) are also cutting the
magnetic field lines and hence according to the Faraday's law of electromagnetic induction, an
emf (voltage) induces in the armature (rotor) conductors. The direction of this induced emf is
such that it opposes the armature current.
 Magnitude of back emf is directly proportional to speed of the motor.
 If a dc motor is suddenly loaded, the load will cause decrease in the speed. Due to decrease in
speed, back emf will also decrease allowing more armature current. Increased armature current
will increase the torque (armature current is directly proportional to torque) to satisfy the load
requirement. Hence, presence of the back emf makes a dc motor ‘self-regulating’.
 Self-regulating machine means motor draws as much armature current as is just sufficient to
develop the torque required by the load.
 Back e.m.f. in a d.c. motor regulates the flow of armature current i.e., it automatically changes
the armature current (hence torque) to meet the load requirement.
Back EMF:  Eb = 
DC Motor working
BACK EMF OR COUNTER EMF –
 Average emf generated per conductor is given by
dΦ/dt (Volts) i.e. change of magnetic flux with respect to change in time ……..eq. 1
 Flux cut by one conductor in one revolution = dΦ = PΦ (Weber),
 Number of revolutions per second (speed in RPS) =N/60 ….RPS – rotations per second
 Therefore, time for one revolution =dt = 60/N (Seconds)
 From eq. 1, emf generated per conductor = dΦ/dt = PΦN/60 (Volts) ……………………eq. 2
 Above equation-2 gives the emf generated in one conductor of the generator. The conductors are connected in
series per parallel path, and the emf across the generator terminals is equal to the generated emf across any
parallel path.

Therefore, Eb = PΦNZ / 60A

For lap winding A=P

For wave winding A=2
 DC Motor working
Torque in motor
 Mech Power = Pmech = Pa = Ta × ω
 Elect Power = Pele = V*I = Eb*Ia (Eb = back emf, Ia = Armature current)
 The power developed in the armature can be given as, Pa = Ta × ω = Ta × 2πN/60
Where ω (omega) = 2πN/60
 The mechanical power developed in the armature is converted from the electrical power,
Therefore, mechanical power = electrical power (Pmech = Pele)
That means, Ta × 2πN/60 = Eb.Ia
 We know, back emf Eb = PΦZN / 60A
 Therefore, Ta × 2πN/60 = (PΦNZ / 60A) × Ia
 Rearranging the above equation,
Armature torque = Ta = (PZ / 2πA) × Φ.Ia (N-m)
 The term (PZ / 2πA) is practically constant for a DC machine. Thus, armature torque is
QUIZ

 https://docs.google.com/forms/d/1p0GpaBUIUNJHHbJGN7FK5rGIV4TjvsOvec2Dm_dc
DPQ/
DC Motor Equivalent
Ckt
Equivalent Circuit of a DC Motor Armature

 The armature of a DC Motor can be represented by an

equivalent circuit.
 It can be represented by three series-connected elements E,
Ra and Vb.
 The element E is the back emf, Ra is the armature winding
resistance and Vb is the brush contact voltage drop
(generally neglected).
 In a DC motor, the current flows from the line into the
armature, against the generated voltage (Eb). By applying
KVL,
V = Eb + IaRa (vtg drop at arm)
 V – Motor supply voltage, Eb – Back EMF, Ia – Armature current, Ra –
Armature circuit resistance (winding).
 This is called the fundamental motor equation. It is seen that
the Back EMF of the motor is always less than its terminal
voltage V.
Types of DC Motor
Types of DC Motor
Permanent magnet Motor
 The permanent magnet DC motor (also
known as a PMDC motor) consists of an
armature winding as in case of an usual
motor, but does not contain the field
windings.
 The construction of these types of DC
motor are such that, radially magnetized
permanent magnets are mounted on the
inner periphery of the stator core to
produce the field.
 The rotor on the other hand has a DC
armature winding with commutator
segments and brushes.
Separately Excited Motor

 As the name suggests, in case of a separately

excited DC motor the supply is given separately to
the field winding and armature windings.
 The main fact is that, the armature current does not
flow through the field windings, as the field
winding is energized from a separate external
source of DC.
 From the torque equation of DC motor we know
Ta = Ka φ Ia
So the torque in this case can be varied by varying
field flux φ.
 Self Excited Motor

 In case of self excited DC motor, the field winding is connected either in series or in
parallel or partly in series, partly in parallel to the armature winding. Based on this, self
excited DC Motors can be classified as:

1. Shunt (parallel) excited

2. Series excited
3. Compound excited
Shunt (parallel) Excited Motor

 Here the field winding is connected in

parallel with the armature winding.
 By applying KCL at junction, The sum of
the incoming currents at Junction = Sum of
the outgoing currents at Junction.
I = Ia + Ish
 I is the input line current, Ia is the armature current, Ish is
the shunt field current
 The voltage equation can be obtained by
applying KVL
V = Eb + IaRa
Series Excited Motor

 In the series motor, the field winding is

connected in series with the armature
winding.
 By applying the KCL
I = Ise = Ia
 I – Supply current, Ise – Series winding current, Ia
- armature winding current
 The voltage equation can be obtained by
applying KVL
V = Eb + Ia (Rse + Ra)
Compound Excited Motor
 A DC Motor having both shunt and series
field windings is called a Compound
Motor
 The compound motor is further subdivided
as Cumulative Compound Motor and
Differential Compound Motor.
 In a cumulative compound motor the flux
produced by both the windings is in the
same direction
 In differential compound motor, the flux
produced by the series field windings is
opposite to the flux produced by the shunt
field winding
Characteristics of DC
Motor
Characteristics of DC Motor

 Generally, three characteristic curves are considered important for DC motors

(i) Torque vs. armature current,
(ii) Speed vs. armature current
(iii) Speed vs. torque.
 These characteristics are determined by keeping the following two relations in mind.
Ta ∝ ɸ.Ia and N ∝ Eb/ɸ
Characteristics of DC shunt motors
 Torque vs. armature current (Ta-Ia)
 Torque is proportional to armature current. Hence, the Ta-Ia characteristic for a dc shunt motor will be a
straight line through the origin.
 Speed vs. armature current (N-Ia)
 As flux ɸ is assumed to be constant, we can say N ∝ Eb. But, as back emf is also almost constant, the
speed should remain constant ideally. But practically, ɸ as well as Eb decreases with increase in load.
 Speed vs. torque (N-Ta)
 This characteristic is also called as mechanical characteristic. It can be found that when speed is high,
torque is low and vice versa (Inversely proportional)
 Characteristics of DC series motors
 Torque vs. armature current (Ta-Ia)
 This characteristic is also known as electrical characteristic. We know that torque is directly proportional
to the product of armature current and field flux, Ta ∝ ɸ.Ia.
 Speed vs. armature current (N-Ia)
 Speed is inversely proportional to Ia
 Speed vs. torque (N-Ta)
 This characteristic is also called as mechanical characteristic. It can be found that when speed is high,
torque is low and vice versa(Inversely proportional)
DC Motor - Speed control
 Speed control of DC motors
V = Eb + IaRa
V is the supplied voltage, Eb is the back EMF, Ia is the armature current, and Ra is the armature resistance.
Eb = (PøZN)/60A.
P – number of poles,
A – constant
Z – number of conductors
N- speed of the motor
Substituting the value of Eb in the voltage equation, we get
V = ((PøNZ)/60A) + IaRa
Or, V – IaRa = (PøNZ)/60A
i.e., N = (PZ/60A) (V – IaRa)/ ø
The above equation can also be written as:

N = K (V – IaRa)/ ø, K is a constant
Speed control of DC motors

N = K (V – IaRa)/ ø, K is a constant
The speed is dependent upon the supply voltage V, the armature circuit resistance Ra and the field
flux ϕ, which is produced by the field current.

DC motor speed control methods-

1. Armature Resistance control
2. Field Flux Control Method
3. Armature (terminal) voltage control
1. Armature resistance
control
(for shunt motor)
Armature resistance control
Shunt Motor
 In this method, a variable external resistor Re
is put in the armature circuit.
 Change in Re affects the speed
 If Re is increased then there is more voltage
drop (acco to V=IR) so less voltage means
less speed.
 The variation in the variable resistance does
not effect the flux as the field is directly
connected to the supply mains
Armature resistance control
Shunt Motor
 Armature resistance control

Disadvantages of Armature Resistance Control Method

 A large amount of power is wasted in the external resistance Re.
 Armature resistance control is restricted to keep the speed below the rated speed of the
motor and increase in the speed above rated speed is not possible by this method.
 For a given value of variable resistance Re, the speed reduction is not constant but varies
with the motor load (speed affected by load change)
 This speed control method is used only for small motors.

Rated Speed - Rated Speed is the speed of the motor (stated in R.P.M) at which it produces its
rated (maximum) power, when the specified (rated) voltage is given at its rated Load.
2. Field flux control
(for shunt motor)
Field flux control
Shunt Motor
 The speed control by this method is achieved by
control of the field current (stator side).
 In a Shunt Motor, the variable resistor Rc is
connected in series with the shunt field windings.
 This resistor Rc is known as a Shunt Field
Regulator.
I = Ia + Ish
Ish = (as V = IR)
 The connection (addition) of Rc in the field
reduces the field current, and hence the flux is
also reduced. This reduction in flux increases the
speed, and thus, the motor runs at speed higher
than the normal speed.
Field flux control
Shunt Motor
Field flux control
Advantages of Field Flux Control Method
 This method is easy and convenient.
 As the shunt field is very small, the power loss in the shunt field is also small.

Note-
 The flux cannot usually be increased beyond its normal values because of the saturation of the iron.
 This method is applicable over only to a limited range because if the field is weakened too much, there is a loss
of stability.
3. Armature voltage control
Armature (terminal) voltage control method
 In armature voltage control method the speed control is achieved by varying the applied
voltage to the motor.
 This speed control method is also known as Ward Leonard Method
Ward Leonard Method Of Speed Control Or Armature Voltage Control

 M is the main DC motor whose

speed is to be controlled
 G is a separately excited DC
Generator.
 The generator G is driven by a 3
phase driving motor
 The combination of AC driving
motor and the DC generator is called
the Motor-Generator (M-G) set.
Ward Leonard Method Of Speed Control Or Armature Voltage Control
 The voltage of the generator (which is applied to motor) is changed by changing the
generator field current.
 This voltage when directly applied to the main DC motor, the speed of the motor M changes
(armature voltage control)
 The motor field current Ifm is kept constant so that the motor field flux ϕm also remains
constant (no field control)
 The generated field current Ifg is varied such that the armature voltage Vt changes from zero
to its rated value so that the motor speed will change from zero to the rated speed.
Ward Leonard Method Of Speed Control Or Armature Voltage Control
Advantages of Ward Leonard Drives
 Smooth speed control of DC motor over a wide range in both the direction is possible.
 It has an inherent braking capacity.
 Overall power factor improves.

Drawbacks of Classical Ward Leonard System

 The Initial cost of the system is high as there is a motor generator set installed, of the same rating as that of the
main DC motor.
 Larger size and weight.
 Requires large floor area
 Costly foundation
 Maintenance of the system is frequent.
 Higher losses.
 Lower efficiency.
Static Ward Leonard System
 Now a days Static Ward Leonard system is mostly used.
 In this system, the rotating motor-generator (M-G) set is replaced by a converter to control
the voltage or flux of motor and so the speed of the DC motor.
 Controlled Rectifiers and choppers are used as a converter.
 In the case of an AC supply, controlled rectifiers are used to convert fixed AC supply voltage
into a variable DC supply voltage.
 In the case of DC supply, choppers are used to obtain variable DC voltage from the fixed DC
voltage.
DC Motor – Starters
DC motor starter

 A starter is a device to start and accelerate a motor.

 While starting the DC motor, it takes heavy current which can damage the motor.
 The starter reduces the heavy current and protects the system from damage.
DC motor starter
Need of Starters for DC Motors
 The dc motor has no back emf at the starting of the motor (standstill). As The resistance of
the armature winding (Ra) is low, and when the full voltage is applied at the standstill
condition, the armature current becomes very high which can damage the parts of the motor.
 The armature current of a motor is given by, Ia = (V = Eb+IaRa)
 When the motor is first switched ON, the armature (rotor) is stationary. Hence, the back
EMF Eb is also zero. The initial starting armature current Ias is given by, Ias = =
 Since, the armature resistance of a motor is very small, generally less than 1 ohm. Therefore,
the starting armature current Ias would be very large.
 For example – if a motor with the armature resistance of 0.5 ohms is connected directly to a
230 V supply, then we get, Ias = = = 460 A.
 This large current would damage the brushes, commutator and windings.
DC motor starter
Need of Starters for DC Motors
 As the motor speed increases, the back EMF increases(directly proportional) and the
difference (V – Eb) go on decreasing.
 This results in a gradual decrease of armature current until the motor attains its stable speed.
 The back EMF helps armature resistance (Ra) in limiting the current through the armature
 At starting of all DC Motors(except for very small motors) an extra resistance must be
connected in series with the armature. This extra resistance is added to limit the starting
current until the motor has attained its stable speed.
 The series resistance is divided into sections which are cut out one by one, as the speed of
the motor rises and the back EMF builds up.
 The extra resistance is cut out when the speed of the motor builds up to its normal value.
DC motor starter

Starters for DC Motors

1. Three point starter

2. Four point starter
1. Three point starter
DC motor starter – Three point starter
 3 point starter connects the resistance in series with the
circuit which reduces the high starting current and
hence protects the machines from damage.
 Three main points in 3 point starter of DC motor – L.
A, F
 L is known as Line terminal, which is connected to the positive
supply.
 A is known as the armature terminal and is connected to the
armature windings.
 F is known as the field terminal and is connected to the field
terminal windings.
 The handle H is kept in the OFF position by a spring S.
 The handle H is manually moved, for starting the motor
and when it makes contact with resistance one the
motor is said to be in the START position.
 In this initial position, the field winding of the motor
receives the full supply voltage, and the armature
DC motor starter – Three point starter
 The starter handle is moved from stud to stud, and this
builds up the speed of the motor until it reaches the RUN
position.
 In the RUN position, three main points are considered,
 The motor attains the full speed.
 The supply is direct across both the windings of the motor.
 The resistance R is completely cut out.
 The handle is held in RUN position by an electromagnet
energised by a no volt trip coil (NVC).
 This no volt trip coil is connected in series with the field
winding of the motor.
 In the event of when the supply voltage falls below a
CERTAIN value, or the complete failure of supply NVC is
energised. The handle is released and pulled back to the
OFF position by the action of the spring. The current to
the motor is cut off, and the motor is stopped
 The no voltage coil also provides protection against an
DC motor starter – Three point starter
 The other protective device incorporated in the starter is the overload protection.
 The Over Load Trip Coil (OLC) provides the overload protection of the motor. The overload
coil is made up of a small electromagnet, which carries the armature current.
 The magnetic pull of the Overload trip coil is insufficient to attract the strip P, for the normal
values of the armature current
 When the motor is overloaded, armature current exceeds the normal rated value, P is
attracted by the electromagnet of the OLC and closes the contact thus, the No Voltage Coil is
short-circuited, as a result, the handle H is released, which returns to the OFF position, and
the motor supply is cut off.
 To stop the motor, the starter handle should never be pulled back as this would result in
burning the starter contacts. Thus, to stop the motor, the main switch of the motor should be
opened.
DC motor starter – Three point starter
Drawbacks of a 3 Point Starter
 The 3 point starter suffers from a serious drawback for motors with a large variation of
speed by adjustment of the field rheostat (field flux control method).
 To increase the speed of the motor, the field resistance should be increased. Therefore, the
current through the shunt field is reduced. The field current may become very low because
of the addition of resistance to obtain high speed. A very low field current will make the
holding electromagnet too weak to overcome the force exerted by the spring.
 The holding magnet may release the arm of the starter during the normal operation of the
motor and thus, disconnect the motor from the line. This is not a desirable action.

Hence, to overcome this difficulty, the 4 Point Starter is used.

2. Four point starter
DC motor starter – Four point starter

 A 4 Point Starter is almost similar in functional

characteristics like 3 Point Starter.
 4 Point Starter also acts a protecting device.
 The basic difference in 4 Point Starter with 3
Point Starter is that in this a holding coil is
removed from the shunt field circuit.
 This coil is connected across the line in series
with a current limiting resistance R.
 The studs are the contact points of the
resistance represented by 1, 2, 3, 4, 5.
DC motor starter – Four point starter

 The arrangement forms three parallel circuits.

 Armature, starting the resistance and the shunt field
winding.
 A variable resistance and the shunt field winding.
 Holding coil and the current limiting resistance
 With three arrangements of the circuit, there
will be no effect on the current through the
holding coil if there is any variation in speed
of the motor or any change in field current.
This is because the two circuits are
independent of each other.
DC motor starter
EXTRA INFORMATION
 Nowadays automatic push button starters are also used.
 In the automatic starters, the ON push button is pressed to connect the starting resistors in
series with the armature circuit. As soon as the full line voltage is available to the armature
circuit, this resistor is gradually disconnected by an automatic controlling arrangement.
 The circuit is disconnected when the OFF button is pressed. Automatic starter circuits have
been developed using electromagnetic contactors and time delay relays.
DC motor – Braking
DC motor braking
 Electrical braking is used in applications where frequent, quick, accurate or emergency
stops are required. Electrical Braking allows smooth stops without any inconvenience to
passengers.
 When a loaded crane is lowered, electric braking keeps the speed within safe limits.
Otherwise, the machine or drive speed will reach the dangerous values.
 When a train goes down a steep gradient, electric braking is employed to hold the train
speed within the prescribed safe limits.
 Electrical Braking is more commonly used where active loads are applicable. In spite of
electric braking, the braking force can also be obtained by using mechanical brakes.

Disadvantages of Mechanical Braking

1. It requires frequent maintenance and replacement of brake shoes.
2. Braking power is wasted in the form of heat.
DC motor braking

Types of electrical braking –

1. Regenerative Braking
2. Dynamic Braking or Rheostatic Braking of DC Motor
3. Plugging or Reverse Current Braking
1. Regenerative Braking
Regenerative braking

 In Regenerative Braking, the power or energy of the driven machinery is returned back
to the power supply mains.
 This type of braking is possible when the driven load forces the motor to run at a speed
higher than no load speed.
 Under this condition, the back emf Eb of the motor is greater than the supply voltage V,
which reverses the direction of motor armature current.
 The machine now begins to operate as a generator and the energy generated is supplied
back to the source.

At Braking V= Eb – IaRa, Eb>V (Generator equation)

At Motoring V= Eb – IaRa, Eb < V (Motor equation)
Regenerative braking
2. Dynamic Braking or
Rheostatic Braking of DC
Motor
Dynamic Braking
 In this, a braking resistor Rb is connected across the armature as soon as the DC motor is
disconnected from the supply mains.
 The motor now works as a generator, producing the braking torque.
 For the braking operation in Dynamic Braking, the motor is connected in two ways.
 Firstly the separately excited or shunt motor can be connected as a separately excited
generator, where the flux is kept constant. The second way is that it can be connected to a
self-excited shunt generator, with the field winding in parallel with the armature.
 This method is also known as Rheostatic Braking because an external braking resistance Rb
is connected across the armature terminals.
 The energy is dissipated as heat in the braking resistance Rb and armature circuit resistance
Ra
 The Dynamic or Rheostatic Braking is an insufficient method of braking because all the
energy which is generated is dissipated in the form of heat in the resistance.
Dynamic Braking
Self-excited
3. Plugging
Plugging
 In Plugging or Reverse Current Braking the armature terminals or the supply polarity of a
separately excited or shunt motors are reversed.
 Therefore the supply voltage V and the induced voltage Eb (back EMF) will act in the same
direction. (usually while motoring V and Eb are in opposite direction)
 During plugging the effective voltage across the armature will be (V + Eb) which is almost
twice the supply voltage.
 An external current limiting resistor is connected in series with the armature to limit the
armature current to a safe value.
 For braking, a series motor either the armature terminals or field terminals are reversed. But
both terminals are not reversed together. Reversing of both the terminals will give only
normal working operation.
 At the zero speed, the braking torque is not zero. The motor must be disconnected from the
supply at zero speed. If the motor is not disconnected from the supply mains, the motor will
speed up in the reverse direction.
Plugging
Losses in DC Motor
Losses in DC Motor
 The losses that occur in a DC Machine is divided into five basic categories. The various
losses are Electrical or Copper losses (I2R losses), Core losses or Iron losses, Brush losses,
Mechanical losses, Stray load losses.
Losses in DC Motor
1. Electrical or Copper Losses in dc machine
 These losses are also known as winding losses as the copper loss occurs because of the
resistance of the windings(wire). The ohmic loss is produced by the current flowing in the
windings.

 Armature copper losses = Ia2Ra where Ia is armature current, and Ra is the armature
resistance. These losses are about 30 per cent of the total full load losses.

 In shunt machine, the copper loss in the shunt field is I2shRsh where Ish is the current in the
shunt field, and Rsh is the resistance of the shunt field windings.
 In a series machine, the copper loss in the series windings is I2seRse where, Ise is the current
through the series field windings, and Rse is the resistance of the series field windings.
 In a Compound machine, both the shunt and the series field losses occur. These losses are
almost 20 per cent of the full load losses.
Losses in DC Motor
2. Core Losses or Iron Losses in dc machine
 The core losses are the hysteresis and eddy current losses.
 These losses are considered almost constant as the machines are usually operated at
constant flux density and constant speed.
 These losses are about 20 per cent of the full load losses.
Losses in DC Motor
3. Brush Losses in dc machine
 Brush losses are the losses taking place between the commutator and the carbon brushes.
 It is the power loss at the brush contact point.
 The brush drop depends upon the brush contact voltage drop and the armature current Ia.
It is given by the equation, Pbd = Vbd * Ia

 The voltage drop occurring over a large range of armature currents, across a set of brushes
is approximately constant. If the value of the brush voltage drop is not given then it is
usually assumed to be about 2 volts. Thus, the brush drop loss is taken as 2Ia.
Losses in DC Motor
4. Mechanical Losses in dc machine
 The losses that take place because of the mechanical effects of the machines are known as
mechanical losses.
 Mechanical losses are divided into bearing friction loss and windage loss.
 The losses occurring in the moving parts of the machine and the air present in the machine
is known as Windage losses. These losses are very small.
Losses in DC Motor
5. Stray Losses in dc machine
These losses are the miscellaneous type of losses. The following factors are considered in stray
load losses.
 The distortion of flux because of the armature reaction.
 Short circuit currents in the coil, undergoing commutation.
 These losses are very difficult to determine. Therefore, it is necessary to assign the
reasonable value of the stray loss. For most machines, stray losses are taken by convention
to be one per cent of the full load output power.
Efficiency of DC Motor
Efficiency of DC Motor