UNIT 3: ELECTRICAL MACHINES
DC machines
• A DC machine is an electromechanical energy conversion device.
• The DC machines are classified into two types such as DC generator as well as DC motor.
• When electric current flows through a coil within a magnetic field, and then the magnetic force
generates a torque that rotates the DC motor.
• The main function of the DC generator is to convert mechanical power to DC electrical power,
whereas a DC motor converts DC power to mechanical power.
• The AC motor is frequently used in industrial applications for altering electrical energy to
mechanical energy.
• However, a DC motor is applicable where good speed regulation & an ample range of speeds
are necessary like in electric-transaction systems.
Construction of DC Machine
• The major construction parts of DC machine
are Yoke, Pole core & pole shoes, Pole coil &
field coil, Armature core, Armature winding
otherwise conductor, commutator, brushes &
bearings.
• Yoke :- Another name of a yoke is the frame.
The main function of the yoke in the machine
is to offer mechanical support intended for
poles and protects the entire machine from
moisture, dust, etc. The materials used in the
yoke are designed with cast iron, cast steel
otherwise rolled steel.
Cont…
• Pole and Pole Core :- The pole of the DC machine is an electromagnet and
the field winding is winding among pole. Whenever field winding is energized
then the pole gives magnetic flux. The materials used for this are cast steel,
cast iron otherwise pole core. It can be built with the annealed steel
laminations for reducing the power drop because of the eddy currents.
• Pole Shoe :- Pole shoe in the DC machine is an extensive part as well as to
enlarge the region of the pole. Because of this region, flux can be spread out
within the air-gap as well as extra flux can be passed through the air space
toward armature. The materials used to build pole shoe is cast iron otherwise
cast steed, and also used annealed steel lamination to reduce the loss of power
because of eddy currents.
Cont…
• Field Windings :- In this, the windings are wounded in the region of pole core &
named as field coil. Whenever current is supplied through field winding then it
generates required flux. The material used for field windings is copper.
• Armature Core :-Armature core includes a huge number of slots within its edge. The
armature conductor is located in these slots. It provides the low-reluctance path
toward the flux generated with field winding. The materials used in this core are
permeability low-reluctance materials like iron otherwise cast. The lamination is used
to decrease the loss because of the eddy current.
Cont…
• Armature Winding :- The armature winding can be formed by interconnecting the armature
conductor. Whenever an armature winding is turned with the help of prime mover then the
voltage, as well as magnetic flux, gets induced within it. This winding is allied to an exterior
circuit. The materials used for this winding are conducting material like copper.
• Commutator :-The main function of the commutator in the DC machine is to collect the
current from the armature conductor as well as supplies the current to the load using brushes.
And also provides uni-directional torque for DC-motor. The commutator can be built with a
huge number of segments in the edge form of hard drawn copper. The Segments in the
commutator are protected from the thin mica layer.
Cont…
• Brushes :- Brushes in the DC
machine gather the current from
the commutator and supply it to
the exterior load. Brushes wear
with time to inspect frequently.
The materials used in brushes are
graphite otherwise carbon which is
in rectangular form.
Application of DC Machine as Generator
• The DC generators are classified as Separately excited DC generator, Shunt-wound, and SeriesWound generator.
• Separately excited DC Generator :-This type of DC generator is used for testing in
laboratories. Because it has a wide range of voltage input. It is also used as a supply to DC
motor.
• Shunt-wound Generator :-This type of generator used to charge a battery and provide
excitation to the alternator. This type of generator also used for lighting purposes.
• Series-wound Generator :-Series-wound generators are used in locomotive for providing field
excitation current as well as for regenerative braking. In a distribution power system, it is used
as a booster.
Application of DC Machine as a Motor
• The DC motors are divided into three types; Series motor, Shunt motor,
and Compound motor.
• Series Motor :-The series motors are used in the application where high
starting torque is necessary and speed variation is possible.
• Example- Vacuum cleaner, Air Compressor, Cranes, Traction system, etc.
• Shunt Motor :-The shunt motor is used in the application where starting
torque is not needed more and running on the constant speed.
• Example- conveyer, Lift, Fans, Lathe machine, Spinning machine, centrifugal pump,
etc.
• Compound Motor :-The compound motors are used in applications where
higher starting torque with constant speed is required.
• Examples- Rolling mills, Elevators, Conveyer, Presses, etc.
Alternating-current (A. C.) machine
• A.C. machine is an electrical machine that
functions as
• a generator used to produce alternating current
• a motor that converts electrical energy into mechanical
work
• a converter that changes the voltage or frequency of an
electric current.
• AC machines are classified as synchronous or
asynchronous.
Cont…
• Synchronous AC machines are devices in which the basic magnetic field is produced by direct
current or by a permanent magnet and the rate of rotation of the rotor is directly related to the AC
frequency by the expression Ns = 120f/p, where n is the rate of rotation of the rotor in rpm, f is the
AC frequency in hertz, and p is the number of poles in the magnetic system.
• Synchronous machines are used chiefly as AC generators and as motors in electric drives, and
occasionally as DC-to-AC converters. They are also used as synchronous compensators for the
phase shift between current and voltage in electrical networks, automatic measuring instruments,
and automatic equipment that requires synchronization of the rate of rotation of control and
actuating devices.
• In asynchronous AC machines, the basic magnetic field is produced by an alternating current,
and the frequency of rotation of the rotor is not strictly related to the frequency of the current in
the stator winding and varies with the load. The most common type is the asynchronous machine
without a commutator, which is used chiefly as an electrical motor. Asynchronous commutator
motors are seldom used because they are more costly and less reliable than machines without
commutators.
Single phase induction motor
• A Single Phase Induction Motor consists of a
single phase winding which is mounted on the stator
of the motor and a cage winding placed on the rotor.
• A pulsating magnetic field is produced, when the
stator winding of the single-phase induction motor is
energized by a single phase supply.
Double Revolving Field Theory of Single Phase Induction Motor
• The double revolving field theory of a single phase induction motor states that a
pulsating magnetic field is resolved into two rotating magnetic fields.
• They are equal in magnitude but opposite in directions. The induction motor
responds to each of the magnetic fields separately.
• The net torque in the motor is equal to the sum of the torque due to each of the
two magnetic fields.
• The first term of the right-hand side of the equation (2) represents the revolving
field moving in the positive α direction. It is known as a Forward Rotating field.
Similarly, the second term shows the revolving field moving in the negative α
direction and is known as the Backward Rotating field.
Equivalent Circuit of a Single Phase Induction Motor
• The equivalent circuit of a Single Phase Induction
Motor can be obtained by two methods named the
Double Revolving Field Theory and Cross Field
Theory.
• Firstly the equivalent circuit is developed on the basis
of double revolving field theory when only its main
winding is energized.
• Considering the case when the rotor is stationary and
only the main winding is excited. The motor behaves as
a single-phase transformer with its secondary short
circuit.
No-Load and Blocked Rotor Test
• The efficiency of small motors can be determined by directly loading them
and by measuring the input and output powers. But in the case of large
motors, it is difficult to arrange that much load for them. The power loss
will be large if we directly test the load. Therefore indirect methods are
used to determine the efficiency of 3-phase induction motors.
• We can perform the following test on the motor to find the efficiency:
• No-Load test.
• Blocked-rotor test.
No-Load test or Open-Circuit Test
• In the no-load test of the motor, the motor is run without
load at the rated voltage and rated frequency.
• The voltage, current and power input are measure using
voltmeter, ammeter and wattmeter respectively.
Blocked Rotor Test on Single-phase Induction Motor
• In the blocked rotor test, the rotor of the motor is
blocked, i.e., remains at rest.
• A reduced voltage is applied to the stator so that the
rated current flows in the main winding.
• Vsc, Isc and Psc are the voltage, current and power
under the blocked rotor conditions.
• Zeq=Vsc/Isc…(1)
Starting Methods of a Single Phase Induction Motor
• The Single Phase Motor is not self-starting and hence needs an auxiliary means
or equipment to start the single-phase induction motor.
• Mechanical methods are impractical and, therefore the motor is started temporarily
converting into a two-phase motor.
• Single-phase Induction motors are usually classified according to the auxiliary
means used to start the motor.
• They are classified according to the starting methods.
Various starting methods
• The various starting methods of a Single Phase Induction motor.
Split Phase Induction Motor
• The Split Phase Motor is also known as a Resistance
Start Motor.
• It has a single cage rotor, and its stator has two windings
known as main winding and starting winding.
• Both the windings are displaced 90 degrees in space.
The main winding has very low resistance and a high
inductive reactance whereas the starting winding has
high resistance and low inductive reactance.
Capacitor Start Induction Motor
• Capacitor Start Motors are single-phase
Induction Motors that employ a capacitor in the
auxiliary winding circuit to produce a greater
phase difference between the current in the
main and the auxiliary windings.
• The name capacitor starts itself shows that the
motor uses a capacitor for the purpose of
starting.
Characteristics of the Capacitor Start Motor
• The capacitor starts motor develops a much higher
starting torque of about 3 to 4.5 times the full load
torque. To obtain a high starting torque, the two
conditions are essential. They are as follows:• The Starting capacitor value must be large.
• The valve of the starting winding resistance must be
low.
Applications of the Capacitor Start Motor
• These motors are used for the loads of higher inertia where frequent starting is
required.
• Used in pumps and compressors
• Used in the refrigerator and air conditioner compressors.
• They are also used for conveyors and machine tools.
Capacitor Start Capacitor Run Motor
• The Capacitor Start Capacitor Run Motor has a
cage rotor, and its stator has two windings known as
Main and Auxiliary Windings. The two windings
are displaced 90 degrees in space.
• There are two capacitors in this method one is used
at the time of the starting and is known as starting
capacitor. The other one is used for continuous
running of the motor and is known as RUN
capacitor.
Characteristics of the Capacitor Start Capacitor run
Motor
• This type of motor is quiet and smooth
running.
• They have higher efficiency than the motors
that run on the main windings only.
• They are used for loads of higher inertia
requiring frequent starts where the maximum
pull-out torque and efficiency required are
higher.
• The two value capacitor motors are used in
pumping equipment, refrigeration, air
compressors, etc.
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