Beee Unit III

Basic Electrical & Electronics Engineering

Prepared By: Deependra Singh 1

Unit III ROTATING ELECTRIC MACHINES CONSTRUCTIONAL DETAILS OF DC MACHINE

As pointed out earlier, D.C machines were first developed and used extensively in spite of its

complexities in the construction. The generated voltage in a coil when rotated relative to a magnetic

field is alternating in nature. To convert this A.C. voltage into a D.C. voltage we therefore need a

unit after the coil terminals. This unit comprises of a number of commutator segments attached to the

shaft of the rotor and a pair of suitably placed stationary carbon brushes touching the commutator

segments. Commutator segments together with the fixed brushes do the necessary rectification from

A.C. to D.C. and hence sometimes called mechanical rectifier.

A D.C. machine is an electro-mechanical energy conversion device. When it converts mechanical power into D.C. electrical power, it is known as a D.C. generator. On the other hand, when it converts D.C. electrical power into mechanical power it is known as a D.C. motor.

The complete assembly of various parts in a scattered form of D.C. machine is shown.

The essential parts of a D.C. machine are described below:

1. Magnetic Frame or Yoke: The outer cylindrical frame

to which main poles and inter-poles are fixed to the foundation is called the yoke. It serves two purposes: a) It provides mechanical protection to the inner parts of

the machine. b) It provides a low reluctance path for the magnetic flux.



2. Pole-Core and Pole Shoes: The pole core and pole shoes are fixed to the magnetic frame

or yoke by bolts. They serve the following purposes: a) They support the field or exciting coils. b) They spread out the magnetic flux over the armature periphery more uniformly. c) Since pole shoes have larger cross-section, the reluctance of magnetic path is reduced. Usually, the pole core and pole shoes are made of thin cast steel or wrought iron laminations which are riveted together under hydraulic pressure as shown in figure (b).

3. Field or Exciting Coils: Enameled copper wire is used for the construction of field or

exciting coils. The coils are wound on the former [figure (c)] and then placed around the pole core. When direct current is passed through the field winding, it magnetizes the poles which produce the required flux. The field coils of all the poles are connected in series in such a way that when current flows through them, the adjacent poles attain opposite polarity as shown in figure (d).

4. Armature Core: It is cylindrical in shape and keyed to the rotating shaft. At the outer

periphery slots are cut as shown in fig., which accommodate the armature winding. The armature core serves the following purposes: a) It houses the conductors in the slots. b) It provides an easy path for magnetic flux.

Since armature is a rotating part of the machine, reversal of flux takes place in the core, hence hysteresis losses are produced. To minimize these losses silicon steel material is used for its construction. The rotating armature cuts across the magnetic field which induces an e.m.f. in it. The e.m.f. circulates eddy currents which results in eddy current loss in it. To reduce these losses, armature core is laminated, in other words we can say that about 0.3 to 0.5 mm thick stampings are used for its construction. Each lamination or stamping is insulated from the other by varnish layer.

5. Armature winding: The insulated conductors housed in the armature slots are suitably

connected. This is known as armature winding. The armature winding is the heart of D.C. machine. It is a place where conversion of power takes place i.e. in case of generator, mechanical power is converted into electrical power and in case of motor, electrical power is converted into mechanical power. On the basis of connections, there are two types of armature windings named as (i) Lap Winding and (ii) Wave Winding. a) Lap Winding: In lap winding, conductors are connected in such a way that number of

parallel paths is equal to the number of poles. Thus, if machine has P poles and Z



armature conductors, then there will be P parallel paths; each path will have Z/P conductors in series. In this case, the number of brushes is equal to the number of parallel paths. Out of which half the brushes are positive and remaining half are negative

b) Wave Winding: In wave winding, the conductors are so connected that they are divided into two parallel paths irrespective of the number of poles of the machine. Thus, if machine has Z armature conductors, there will be only two parallel paths each having Z/2 conductors in series. In this case, the number of brushes is equal to two i.e. number of parallel paths.

6. Commutator: It is the most important part of a D.C. machine and serves the following

purposes: a) It connects the rotating armature conductors to the stationary external circuit through

brushes. b) It converts the alternating current induced in the armature conductors into unidirectional

current in the external load circuit in generator action, whereas, it converts the alternating torque into unidirectional (continuous) torque produced in the armature in motor action. The commutator is of cylindrical shape and is made up of wedge shaped hard drawn copper segments. The segments are insulated from each other by a thin sheet of mica. The segments are held together by means of two wedge shaped rings that fit into the V-grooves cut into the segments. Each armature coil is connected through riser. The sectional view of the commutator is shown.

7. Brushes: The brushes are pressed upon the commutator and form the connecting link between the armature winding and the external circuit. They are usually made of high grade carbon because carbon is conducting material and at the same time in powdered form provides lubricating effect on the commutator surface. The brushes are held in particular position around the commutator by brush holders.

8. End Housings: End housings are attached to the ends of the main frame and support

bearings. The front housing supports the bearing and the brush assemblies whereas the rear housing usually supports the bearing only.

9. Bearings: The ball or roller bearings are fitted in the end housings. The function of the

bearings is to reduce friction between the rotating and stationary parts of the machine. Mostly high carbon steel is used for the construction of bearings as it is very hard material.

10. Shaft: The shaft is made of mild steel with a maximum breaking strength. The shaft is used

to transfer mechanical power from or to the machine. The rotating parts like armature core, commutator, cooling fans etc. are keyed to the shaft.



CONSTRUCTIONAL DETAILS INDUCTION MACHINE

THREE-PHASE AC INDUCTION SQUIRREL CAGE MOTOR

For industrial and mining applications, 3-phase AC induction motors are the prime movers

for the vast majority of machines. These motors can be operated either directly from the mains or

from adjustable variable frequency drives. In modern industrialized countries, more than half the

total electrical energy used in those countries is converted to mechanical energy through AC

induction motors. The applications for these motors cover almost every stage of manufacturing and

processing.

Applications also extend to commercial buildings and the domestic environment. They are

used to drive pumps, fans, compressors, mixers, agitators, mills, conveyors, crushers, machine tools,

cranes, etc, etc.

It is not surprising to find that this type of electric motor is so popular, when one considers its

simplicity, reliability and low cost. In the last decade, it has become increasingly common practice to

use 3-phase squirrel cage AC induction motors with variable voltage variable frequency (VVVF)

converters for variable speed drive (VSD) applications. To clearly understand how the VSD system

works, it is necessary to understand the principles of operation of this type of motor. Although the

basic design of induction motors has not changed very much in the last 50 years, modern insulation

materials, computer based design optimization techniques and automated manufacturing methods

have resulted in motors of smaller physical size and lower cost per kW. International standardization

of physical dimensions and frame sizes means that motors from most manufacturers are physically

interchangeable and they have similar performance characteristics.

The reliability of squirrel cage AC induction motors, compared to DC motors, is high. The

only parts of the squirrel cage motor that can wear are the bearings. Slip rings and brushes are not

required for this type of construction. Improvements in modern pre-lubricated bearing design have

extended the life of these motors. Although single-phase AC induction motors are quite popular and



common for low power applications up to approx 2.2 kW, these are seldom used in industrial and

mining applications. Single-phase motors are more often used for domestic applications.

The information in this chapter applies mainly to 3-phase squirrel cage AC induction motors,

which is the type most commonly used with VVVF converters.

BASIC CONSTRUCTION The AC induction motor comprises 2 electromagnetic parts:

Stationary part called the stator

Rotating part called the rotor, supported at each end on bearings

The stator and the rotor are each made up of:

An electric circuit, usually made of insulated copper or aluminum, to carry current

A magnetic circuit, usually made from laminated steel, to carry magnetic flux

THE STATOR: The stator is the outer stationary part of the motor, which consists of:

The outer cylindrical frame of the motor, which is made either of welded sheet steel, cast iron

or cast aluminum alloy. This may include feet or a flange for mounting.

The magnetic path, which comprises a set of slotted steel laminations pressed into the

cylindrical space inside the outer frame. The magnetic path is laminated to reduce eddy currents,

lower losses and lower heating.

A set of insulated electrical windings, which are placed inside the slots of laminated magnetic

path. The cross-sectional area of these windings must be large enough for the power rating of the

motor. For a 3-phase motor, 3 sets of windings are required, one for each phase.

Stator and rotor laminations

THE ROTOR: This is the rotating part of the motor. As with the stator above, the rotor consists of a

set of slotted steel laminations pressed together in the form of a cylindrical magnetic path and the

electrical circuit. The electrical circuit of the rotor can be either:



WOUND ROTOR TYPE, which comprises 3 sets of insulated windings with connections

brought out to 3 slip-rings mounted on the shaft. The external connections to the rotating part

are made via brushes onto the slip rings. Consequently, this type of motor is often referred to

as a slip ring motor.

SQUIRREL CAGE ROTOR TYPE, which comprises a set of copper or aluminum bars

installed into the slots, which are connected to an end-ring at each end of the rotor. The

construction of these rotor windings resembles a squirrel cage. Aluminum rotor bars are

usually die-cast into the rotor slots, which results in a very rugged construction. Even though

the aluminum rotor bars are in direct contact with the steel laminations, practically all the

rotor current flows through the aluminum bars and not in the laminations.

THE OTHER PARTS

The other parts, which are required to complete the induction motor, are:

Two end-flanges to support the two bearings, one at the drive-end (DE) and the other at the

non drive-end (NDE)

Two bearings to support the rotating shaft, at DE and NDE

Steel shaft for transmitting the torque to the load

Cooling fan located at the NDE to provide forced cooling for the stator and rotor

Terminal box on top or either side to receive the external electrical connections

Assembly details of a typical AC induction motor



CONSTRUCTIONAL DETAILS SYNCHRONOUS MACHINE

This chapter includes a description of the design criteria leading to the construction of a

modern turbo-generator, as well as contains a detailed description of all components most commonly

found in such a machine. This section is limited to the presentation of the basic components

comprising a synchronous machine, with the purpose of describing its basic operating theory.

Synchronous machines come in all sizes and shapes, from the miniature permanent magnet

synchronous motor in wall-clocks, to the largest steam-turbine-driven generators of up to about 1500

MVA. Synchronous machines are one of two types: the stationary field or the rotating dc magnetic

field. The stationary field synchronous machine has salient poles mounted on the stator the

stationary member. The poles are magnetized either by permanent magnets or by a dc current. The

armature, normally containing a three-phase winding, is mounted on the shaft. The armature winding

is fed through three slip rings (collectors) and a set of brushes sliding on them. This arrangement can

be found in machines up to about 5 kVA in rating. For larger machines all those covered in this

book the typical arrangement used is the rotating magnetic field.

The rotating magnetic field (also known as revolving-field) synchronous machine has the

field-winding wound on the rotating member (the rotor), and the armature wound on the stationary

member (the stator). A dc current, creating a magnetic field that must be rotated at synchronous

speed, energizes the rotating field-winding. The rotating field winding can be energized through a set

of slip rings and brushes (external excitation), or from a diode-bridge mounted on the rotor (self-

excited). The rectifier-bridge is fed from a shaft-mounted alternator, which is itself excited by the

pilot exciter. In externally fed fields, the source can be a shaft-driven dc generator, a separately

excited dc generator, or a solid-state rectifier. Several variations to these arrangements exist.

The stator core is made of insulated steel laminations. The thickness of the laminations and the type

of steel are chosen to minimize eddy current and hysteresis losses, while maintaining required

effective core length and minimizing costs. The core is mounted directly onto the frame or (in large

two-pole machines) through spring bars. The core is slotted (normally open slots), and the coils

making the winding are placed in the slots. There are several types of armature windings, such as

concentric windings of several types, cranked coils, split windings of various types, wave windings,

and lap windings of various types. Modern large machines typically are wound with double-layer lap

windings.

The rotor field is either of salient-pole (Fig. 1.19) or non-salient-pole construction, also

known as round rotor or cylindrical rotor (Fig. 1.20). Non-salient-pole (cylindrical) rotors are utilized

in two- or four-pole machines, and, very seldom, in six-pole machines. These are typically driven by

steam or combustion turbines. The vast majority of salient-pole machines have six or more poles.

They include all synchronous hydro-generators, almost every synchronous condenser, and the



overwhelming majority of synchronous motors. Non-salient-pole rotors are typically machined out of

a solid steel forging. The winding is placed in slots machined out of the rotor body and retained

against the large centrifugal forces by metallic wedges, normally made of aluminum or steel. The

retaining rings restrain the end part of the windings (end-windings). In the case of large machines,

the retaining rings are made out of steel. Large salient-pole rotors are made of laminated poles

retaining the winding under the pole head. The poles are keyed onto the shaft or spider-and-wheel

structure. Salient-pole machines have an additional winding in the rotating member. This winding,

made of copper bars short-circuited at both ends, is embedded in the head of the pole, close to the

face of the pole. The purpose of this winding is to start the motor or condenser under its own power

as an induction motor, and take it unloaded to almost synchronous speed, when the rotor is pulled

in by the synchronous torque. The winding also serves to damp the oscillations of the rotor around

the synchronous speed, and is therefore named the damping-winding (also known as amortisseurs or

damper-windings).

Fig. 1.19: Synchronous machine construction. Schematic cross section of a salient-pole synchronous machine. In a large generator, the rotor is magnetized by a coil wrapped around it. The figure shows a two-pole rotor. Salient-pole rotors normally have many more than two poles. When designed as a generator, large salient-pole machines are driven by water turbines. The bottom part of the figure shows the three-phase voltages obtained at the terminals of the generator, and the equation relates the speed

of the machine, its number of poles, and the frequency of the resulting voltage.



Fig. 1.20: Schematic cross section of a synchronous machine with a cylindrical round-rotor

(turbogenerator). This is the typical design for all large turbogenerators. Here both the stator and rotor windings are installed in slots, distributed around the periphery of the machine. The lower part shows the resulting waveforms of a pair of conductors, and that of a distributed winding. The formula giving the magneto-motive force (mmf) created by the windings.

WORKING PRINCIPLE OF 3-PHASE INDUCTION MOTOR

The balanced three-phase winding of the stator is supplied with a balanced three-phase

voltage. The current in the stator winding produces a rotating magnetic field, the magnitude of which

remains constant. The axis of the magnetic field rotates at a synchronous speed (ns = 2f/p), a function

of the supply frequency (f), and number of poles (p) in the stator winding. The magnetic flux lines in

the air gap cut both stator and rotor (being stationary, as the motor speed is zero) conductors at the

same speed. The e.m.f. in both stator and rotor conductors are induced at the same frequency, i.e. line

or supply frequency, with No. of poles for both stator and rotor windings (assuming wound one)

being same. The stator conductors are always stationary, with the frequency in the stator winding

being same as line frequency. As the rotor winding is short-circuited at the slip-rings, current flows

in the rotor windings. The electromagnetic torque in the motor is in the same direction as that of the

rotating magnetic field, due to the interaction between the rotating flux produced in the air gap by the

current in the stator winding, and the current in the rotor winding. This is as per Lenzs law, as the

developed torque is in such direction that it will oppose the cause, which results in the current

flowing in the rotor winding. This is irrespective of the rotor type used cage or wound one, with



the cage rotor, with the bars short-circuited by two end-rings, is considered equivalent to a wound

one The current in the rotor bars interacts with the air-gap flux to develop the torque, irrespective of

the no. of poles for which the winding in the stator is designed. Thus, the cage rotor may be termed

as universal one. The induced emf and the current in the rotor are due to the relative velocity

between the rotor conductors and the rotating flux in the air-gap, which is maximum, when the rotor

is stationary (nr = 0). As the rotor starts rotating in the same direction, as that of the rotating

magnetic field due to production of the torque as stated earlier, the relative velocity decreases, along

with lower values of induced emf and current in the rotor. If the rotor speed is equal that of the

rotating magnetic field, which is termed as synchronous speed, and also in the same direction, the

relative velocity is zero, which causes both the induced emf and current in the rotor to be reduced to

zero. Under this condition, torque will not be produced. So, for production of positive (motoring)

torque, the rotor speed must always be lower than the synchronous speed. The rotor speed is never

equal to the synchronous speed in an IM. The rotor speed is determined by the mechanical load on

the shaft and the total rotor losses, mainly comprising of copper loss.

The difference between the synchronous speed and rotor speed, expressed as a ratio of the

synchronous speed, is termed as slip in an IM. So, slip (s) in pu is

s

r

s

rs

n

n

n

nns

1 or, sr nsn 1

Where, ns and nr are synchronous and rotor speeds in rev/s.

In terms of ss nN 60 and rr nN 60 , both in rev/min (rpm), slip is

s

rs

N

NNs

If the slip is expressed in %, then 100/ srs NNNs

Normally, for torques varying from no-load ( zero) to full load value, the slip is proportional

to torque. The slip at full load is 4-5% (0.04-0.05).



ALTERNATE METHOD:

An alternative explanation for the production of torque in a three-phase induction motor is

given here, using two rules (right hand and left hand) of Fleming. The stator and rotor, along with

air-gap, is shown in Fig. 30.4a. Both stator and rotor is shown there as surfaces. Also shown is the

path of the flux in the air gap. This is for a section, which is under North Pole, as the flux lines move

from stator to rotor. The rotor conductor shown in the figure is at rest, i.e., zero speed (standstill).

The rotating magnetic field moves past the conductor at synchronous speed in the clockwise

direction. Thus, there is relative movement between the flux and the rotor conductor. Now, if the

magnetic field, which is rotating, is assumed to be at standstill as shown in Fig. 30.4b, the conductor

will move in the direction shown. So, an emf is induced in the rotor conductor as per Faradays law,

due to change in flux linkage. The direction of the induced emf as shown in the figure can be

determined using Flemings right hand rule.

The rotor bars in the cage rotor are short circuited via end rings. Similarly, in the wound

rotor, the rotor windings are normally short-circuited externally via the slip rings. In both cases, as

emf is induced in the rotor conductor (bar), current flows there, as it is short circuited. The flux in the

air gap, due to the current in the rotor conductor is shown in Fig. 30.4c. The flux pattern in the air

gap, due to the magnetic fields produced by the stator windings and the current carrying rotor

conductor, is shown in Fig. 304d. The flux lines bend as shown there. The property of the flux lines

is to travel via shortest path as shown in Fig. 30.4a. If the flux lines try to move to form straight line,

then the rotor conductor has to move in the direction of the rotating magnetic field, but not at the



same speed, as explained earlier. The current carrying rotor conductor and the direction of flux are

shown in Fig. 30.4e. It is known that force is produced on the conductor carrying current, when it is

placed in a magnetic field. The direction of the force on the rotor conductor is obtained by using

Flemings left hand rule, being same as that of the rotating magnetic field. Thus, the rotor

experiences a motoring torque in the same direction as that of the rotating magnetic field. This

briefly describes how torque is produced in a three-phase induction motor.

THE FREQUENCY OF THE INDUCED EMF AND CURRENT IN THE ROTOR

As given earlier, both the induced emf and the current in the rotor are due to the relative

velocity between the rotor conductors and the rotating flux in the air-gap, the speed of which is the

synchronous speed (Ns = 2f/p). The rotor speed is

Nr = (1-s) Ns

The frequency of the induced emf and current in the rotor is

fsnpsnnpf srsr .)..().(

For normal values of slip, the above frequency is small.

Taking an example, with full load slip as 4% (0.04), and supply (line) frequency as 50 Hz, the

frequency (Hz) of the rotor induced emf and current, fr is 0.0450.0 = 2.0, which is very small,

whereas the frequency (f) of the stator induced emf and current is 50 Hz, i.e. line frequency. At

standstill, i.e. rotor stationary (nr = 0.0), the rotor frequency is same as line frequency, as shown

earlier, with slip [s = 1.0 (100%)].



EMF EQUATION OF 3-PHASE INDUCTION MOTOR An emf is induced in rotor by the rotating magnetic field produced by the stator ampere-turns.

This induced emf depends upon the magnitude of the rotating flux and the speed at which this flux

cuts the rotor conductors.

Consider the following two points:

(i) When the rotor is at standstill, the stator flux cuts the rotor conductors at a speed NS

and an emf, E20 is induced.

(ii) When the rotor is rotating at speed Nr, the rotating field cuts the rotor conductors at a

speed (NS - Nr) r.p.m. Let the induced emf in the rotor be E2.

Now, sN

NN

S

rS

So SrS NsNN .

As the emf induced at speed NS is E20 so the emf induced at speed s.Ns will be s.E20.

Therefore, 202 .EsE (i)

Let us derive an expression for E20 when the rotor is at standstill.

Let flux passing through stator coil at any time t be cost.

For T-turn armature coil, flux linkage is given by

= Tcost

According to Faradays Law

tTdt

de sin

Considering to be constant, induced e.m.f. is given by

tEe sinmax

Where Emax is the maximum value of e.m.f. generated.

Now, )2(max fTTE mm

The r.m.s. value of generated e.m.f. is

mfTE

E 22

max

Or mfTE 44.4

Where f is frequency of generated e.m.f.

In a practical machine, the armature winding is distributed with fractional pitch coils and few

armature slots are left empty, leading to a distribution factor Kd. If Kp be the pitch factor, then

mdp fTKKE 44.4



When rotor is at standstill, induced emf in rotor is given by

220 44.4 fTKKE dp volt per phase

Where, f = supply frequency in Hertz.

= flux per pole produced by the stator

T2 = No. of rotor winding turns in series per phase

Kp = pitch factor of winding

Kd = distribution factor of winding

When rotor is rotating at a slip s, then rotor induced emf is given by phdp fTKKsEsE .44.4. 202 volt per phase

As the rotor frequency fr = s.fs

So, 22 44.4 TfKKE rdp volt per phase

CONCEPT OF SLIP IN 3-PHASE INDUCTION MOTOR

The difference between the synchronous speed and rotor speed, expressed as a ratio of the

synchronous speed, is termed as slip in an IM. So, slip (s) in pu is

s

r

s

rs

n

n

n

nns

1 or, sr nsn 1

Where, ns and nr are synchronous and rotor speeds in rev/s.

In terms of ss nN 60 and rr nN 60 , both in rev/min (rpm), slip is

s

rs

N

NNs

If the slip is expressed in %, then 100/ srs NNNs

Normally, for torques varying from no-load ( zero) to full load value, the slip is proportional

to torque. The slip at full load is 4-5% (0.04-0.05).



EXPLANATION OF TORQUE-SLIP CHARACTERISTICS OF 3-PHASE INDUCTION MOTOR

The current per phase in the rotor winding (the equivalent circuit of the rotor, per phase is shown in

Fig. 31.1)

In a similar way, the output power (gross) developed (W) is the loss in the fictitious resistance in the

equivalent circuit as shown earlier, which is

The motor speed in rps is nr = (1-s)ns

The motor speed (angular) in rad/s is r = (1-s)s

The gross torque developed in N-m is

The synchronous speed (angular) is s = 2.ns

The input power to the rotor (or the power transferred from the stator via air gap) is the loss in the

total resistance (r2/s ), which is

The relationship between the input power and the gross torque developed is Pi = s.T0. So, the input

power is also called as torque in synchronous watts, or the torque is:

T0 = (Pi / s)

The Torque slip characteristic is shown in figure 32.1. The slip is

s = (s r)/ s = (ns nr)/ ns = 1-(nr/ns)

The range of speed nr is between 0.0(standstill) and ns(synchronous speed). The range of slip is

between 0.0(nr = ns) and 1.0(nr = 0.0).



For low values of slip, r2 >> (s.x2). So, Torque is

This shows that T0 s, the characteristic being linear. The output torque developed is zero (0.0), at

s = 0.0, or if the motor is rotated at synchronous speed (nr = ns). Also, the slip at full load (output

torque = (T0)fl) is normally 4-5% (sfl = 0.04 0.05), the full load speed of IM being 95-96% of

synchronous speed ((nr)fl) = (1-sfl)ns = (0.05 - 0.96)ns).

For large values of slip, r2



CLASSIFICATION OF DC MACHINES

DC Machines can be classified as Motors & Generators

1. DC Motors

a. Series Motor

When the field winding of motor is connected in series with armature winding, then the

motor is called as series motor. A series motor has been shown

(a) Series DC Motor

Let Ra is the armature resistance, RSe is series field resistance, I is the armature current. In

case of series motor

I = Ia = ISe (current equation) (i)

V = Eb + Ia(Ra + RSe) (voltage equation) (ii)

And, Pm = EbIa = VIa Ia2(Ra + RSe) (iii)

Where Pm is mechanical power developed.

b. Shunt Motor

If the motor field winding is connected in parallel shunt across armature winding, then the

motor is called as shunt motor. A shunt motor is shown

(b) Shunt DC Motor

In shunt motor,

I = Ia + ISn (current equation) (i)

V = Eb + IaRa (voltage equation) (ii)



And, Pm = EbIa = VIa Ia2Ra (iii)

c. Compound Motor

This motor has both (series and shunt) winding.

i. Cumulative Compound Motor: In this type of motor, the series-field flux

aids the shunt-field flux. The cumulative compound motor has been shown.

ii. Differential Compound Motor: In this type of motor, the series-field flux

opposes the shunt-field flux. The differential compound motor has been

shown.

(c) Cumulative Compound DC Motor (d) Differential Compound DC Motor

Different Types of Motors

Comparison and Applications of Different Types of Motors

Types of Motors Characteristics Applications

1. Shunt Motor Approximately constant speed,

Medium starting torque

Used for Medium torque and

constant speed applications like

Lathe, pumps, fans, blowers etc.

2. Series Motor High Starting Torque, Speed is

not constant

Used for high starting torque

applications like tractor, cranes,

conveyors etc.

3. Compound Motor Reasonably high starting

torque, Adjustable speed

For intermittent high torque loads

like shears, punches, planner,

rolling mills etc.

2. DC Generators

A DC generator is classified according to the method of field excitation. There are two

methods of exciting DC generator, namely self-excitation & separately-excitation. Accordingly,

the generators are called Separately-Excited & Self-Excited Generator.



When the field winding is excited by the current supplied by the generator, the generator is

said to be self excited. The classification is as follows:

a. Series Excitation Generator: In series excitation generator, the field winding is

connected in series with the armature winding as shown

(e) Series Generator

b. Shunt Excitation Generator: In shunt excitation generator, the field winding is

connected in parallel (shunt) with the armature winding as shown

(f) Shunt Generator

c. Compound Excitation Generator: In compound excitation, there is a combination

of series and shunt winding. It can be either long shunt or short shunt. Depending

upon the combination the generators are classified as:

i. Long Shunt Excitation Generator



(g) Long Shunt Generator

ii. Short Shunt Excitation Generator

Beee Unit III

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