Electrical Machines
Electrical Machines
Electrical Machines
OR
We can easily add rotor resistance by using Since the rotor bars are permanently shorted,
slip ring and brushes its not possible to add external resistance
Slip ring and brushes are present Slip ring and brushes are absent
Frequent maintenance is required due to
Less maintenance is required
presence of brushes
The construction is complicated and the The construction is simple and robust and it is
presence of brushes and slip ring makes the cheap as compared to slip ring induction
motor more costly motor
Slip ring or phase wound Squirrel cage induction motor
Induction motor
This motor is rarely used only 10% industry Due to its simple construction and low
cost. The squirrel cage induction motor is
uses slip ring induction motor
widely used
Rotor copper losses are high and hence Less rotor copper losses and hence high
less efficiency efficiency
SINGLE-PHASE INDUCTION MOTOR
• Single phase motors are very widely used in home, offices, workshops etc.
as power delivered to most of the houses and offices is single phase. In
addition to this, single phase motors are reliable, cheap in cost, simple in
construction and easy to repair.
• Single-phase motors are used in equipment and machines that are smaller
in size and require lower horsepower (for example, one horsepower). This
include equipment such as pumps, refrigerators, fans, compressors, and
portable drills.
Why Single Phase Induction Motor Is
Not Self Starting?
• The stator of a single phase induction motor is
wound with single phase winding.
• When the stator is fed with a single phase
supply, it produces alternating flux (which
alternates along one space axis only).
• Alternating flux acting on a squirrel cage rotor
can not produce rotation, only revolving flux
can.
• That is why a single phase induction motor is
not self starting.
Double Field Revolving Theory
How To Make Single Phase Induction Motor
Self Starting?
• As explained above, single phase induction motor is not self-starting. To make
it self-starting, it can be temporarily converted into a two-phase motor
while starting. This can be achieved by introducing an additional 'starting
winding' also called as auxillary winding.
• Hence, stator of a single phase motor has two windings: (i) Main winding
and (ii) Starting winding (auxillary winding).
• These two windings are connected in parallel across a single phase supply
and are spaced 90 electrical degrees apart. Phase difference of 90 degree can
be achieved by connecting a capacitor in series with the starting winding.
• Hence the motor behaves like a two-phase motor and the stator produces
revolving magnetic field which causes rotor to run. Once motor gathers
speed, say up to 80 or 90% of its normal speed, the starting winding gets
disconnected form the circuit by means of a centrifugal switch, and the
motor runs only on main winding.
SINGLE-PHASE INDUCTION MOTOR
• Construction of a single phase induction motor is similar to the
construction of three phase induction motor having squirrel cage
rotor, except that the stator is wound for single phase supply.
• Stator is also provided with a 'starting winding' which is used only
for starting purpose. This can be understood from the schematic of
single phase induction motor at the left.
Working Principle Of Single Phase Induction
Motor
• When the stator of a single phase motor is fed with single phase
supply, it produces alternating flux in the stator winding.
• The alternating current flowing through stator winding causes
induced current in the rotor bars (of the squirrel cage rotor)
according to Faraday's law of electromagnetic induction.
• This induced current in the rotor will also produce alternating flux.
• Even after both alternating fluxes are set up, the motor fails to start
(the reason is explained below).
• However, if the rotor is given a initial start by external force in either
direction, then motor accelerates to its final speed and keeps
running with its rated speed.
Working Principle Of A DC Generator
• According to Faraday’s laws of electromagnetic induction, whenever a
conductor is placed in a varying magnetic field (OR a conductor is
moved in a magnetic field), an emf (electromotive force) gets induced
in the conductor.
• The magnitude of induced emf can be calculated from the emf
equation of dc generator.
• If the conductor is provided with a closed path, the induced current
will circulate within the path. In a DC generator, field coils produce an
electromagnetic field and the armature conductors are rotated into
the field.
• Thus, an electromagnetically induced emf is generated in the
armature conductors. The direction of induced current is given by
Fleming’s right hand rule.
C
D
A
B
D
C
Separately excited DC motor
• A DC motor like we all know is a device that deals in the conversion of
electrical energy to mechanical energy and this is essentially brought
about by two major parts required for the construction of DC motor,
namely.
• Stator – The static part that houses the field windings and receives the
supply and,
• Rotor – The rotating part that brings about the mechanical rotations.
• Other than that there are several subsidiary parts namely the
– Yoke of DC motor.
– Poles of DC motor.
– Field winding of DC motor.
– Armature winding of DC motor.
– Commutator of DC motor.
– Brushes of DC motor.
Separately excited DC motor
YOKE OF DC MOTOR
• The magnetic frame or the yoke of DC
motor made up of cast iron or steel
and forms an integral part of the stator
or the static part of the motor.
• Its main function is to form a
protective covering over the
sophisticated inner parts of the motor
and provide support to the armature.
• It also supports the field system by
housing the magnetic poles and field
winding of the DC motor.
Poles of DC Motor
• The magnetic poles of DC motor are
structures fitted onto the inner wall of the
yoke with screws.
• The construction of magnetic poles basically
comprises of two parts. Namely, the pole
core and the pole shoe stacked together
under hydraulic pressure and then attached
to the yoke.
• These two structures are assigned for
different purposes, the pole core is of small
cross-sectional area and its function is to just
hold the pole shoe over the yoke, whereas
the pole shoe having a relatively larger cross-
sectional area spreads the flux produced over
the air gap between the stator and rotor to
reduce the loss due to reluctance.
Field winding of DC motor
• The field winding of DC motor are made with
field coils (copper wire) wound over the slots of
the pole shoes in such a manner that when field
current flows through it, then adjacent poles
have opposite polarity are produced. The field
winding basically form an electromagnet, that
produces field flux within which the rotor
armature of the DC motor rotates, and results in
the effective flux cutting.
Armature of DC Motor
• The armature winding of DC motor is attached to the rotor, or the
rotating part of the machine, and as a result is subjected to
altering magnetic field in the path of its rotation.
• For this reason the rotor is made of armature core, that’s made
with several low-hysteresis silicon steel lamination, to reduce the
magnetic losses like hysteresis and eddy current loss respectively.
• These laminated steel sheets are stacked together to form the
cylindrical structure of the armature core.
• The armature core are provided with slots made of the same
material as the core to which the armature winding made with
several turns of copper wire distributed uniformly over the entire
periphery of the core.
Commutator of DC Motor
• The commutator of DC motor is a cylindrical
structure made up of copper segments
stacked together, but insulated from each
other by mica.
• Its main function to convert ac to dc.
Brushes of DC Motor
• The brushes of DC motor are made with carbon or graphite structures,
making sliding contact over the rotating commutator.
• The brushes are used to extract the current from external circuit to the
rotating commutator form where it flows into the armature winding.
• So, 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.
Alternator
• An alternator is such a machine which
converts mechanical energy from a prime
mover to AC electric power at specific voltage
and frequency. It is also known as
synchronous generator.
Construction Of AC Generator (Alternator)
• Main parts of the alternator,
obviously, consists of stator and
rotor. But, the unlike other
machines, in most of the
alternators, field exciters are
rotating and the armature coil is
stationary.
• Stator: Unlike in DC machine stator
of an alternator is not meant to
serve path for magnetic flux. Instead,
the stator is used for holding
armature winding. The stator core is
made up of lamination of steel alloys
or magnetic iron, to minimize the
eddy current losses.
Why Armature Winding Is Stationary In An
Alternator?
• At high voltages, it easier to insulate stationary armature
winding, which may be as high as 30 kV or more.
• The high voltage output can be directly taken out from the
stationary armature.
• Whereas, for a rotary armature, there will be large brush
contact drop at higher voltages, also the sparking at the
brush surface will occur.
• Field exciter winding is placed in rotor, and the low dc
voltage can be transferred safely.
• The armature winding can be braced well, so as to prevent
deformation caused by the high centrifugal force.
• There are mainly two types of rotor used in
construction of alternator,
• Salient pole type.
• Cylindrical rotor type.
Salient Pole Type
1. Salient pole type: Salient pole type rotor is used in low and
medium speed alternators.
– Construction of AC generator of salient pole type rotor is shown in the
figure above.
– This type of rotor consists of large number of projected poles (called
salient poles), bolted on a magnetic wheel.
– These poles are also laminated to minimize the eddy current losses.
Alternators featuring this type of rotor are large in diameters and short
in axial length.
2. Cylindrical type: Cylindrical type rotors are used in high speed
alternators, especially in turbo alternators.
– This type of rotor consists of a smooth and solid steel cylinder having
slots along its outer periphery. Field windings are placed in these slots.
Salient Pole Type
• The term salient means protruding or projecting. The salient pole type of
rotor is generally used for slow speed machines having large diameters
and relatively small axial lengths. The poles, in this case, are made of
thick laminated steel sections riveted together and attached to a rotor
with the help of joint.
• An alternator as mentioned earlier is mostly responsible for generation
of very high electrical power. To enable that, the mechanical input given
to the machine in terms of rotating torque must also be very high.
• This high torque value results in oscillation or hunting effect of the
alternator or synchronous generator. To prevent these oscillations from
going beyond bounds the damper winding is provided in the pole faces
as shown in the figure. The damper windings are basically copper bars
short-circuited at both ends are placed in the holes made in the pole axis
Salient Pole Type
• When the alternator is driven at a steady speed, the relative
velocity of the damping winding with respect to the main field
will be zero.
• But as soon as it departs from the synchronous speed there
will be relative motion between the damper winding and the
main field which is always rotating at synchronous speed.
• This relative difference will induce the current in them which
will exert a torque on the field poles in such a way as to bring
the alternator back to synchronous speed operation.
• The salient feature of pole field structure has the
following special feature-
– They have a large horizontal diameter compared to a shorter
axial length.
– The pole shoes covers only about 2/3rd of pole pitch.
– Poles are laminated to reduce eddy current loss.
– The salient pole type motor is generally used for low-speed
operations of around 100 to 400 rpm, and they are used in
power stations with hydraulic turbines or diesel engines.
– Salient pole alternators driven by water turbines are called
hydro-alternators or hydro generators.
Cylindrical Rotor Type
• The cylindrical rotor is generally used for very high speed
operation and employed in steam turbine driven
alternators like turbo-generators.
• The machines are built in a number of ratings from 10
MVA to over 1500 MVA.
• The cylindrical rotor type machine has a uniform length in
all directions, giving a cylindrical shape to the rotor thus
providing uniform flux cutting in all directions.
• The rotor, in this case, consists of a smooth solid steel
cylinder, having a number of slots along its outer periphery
for hosting the field coils.
• The cylindrical rotor alternators are generally
designed for 2-pole type giving very high
speed of