UNIT-4

ELECTRICAL MACHINES
1. Explain the working principle of transformer.
A. The basic principle on which the transformer works is Faraday’s Law
of Electromagnetic Induction or mutual induction between the two coils.
The working of the transformer is explained below. The transformer
consists of two separate windings placed over the laminated silicon steel
core.

The winding to which AC supply is connected is called primary winding

and to which load is connected is called secondary winding as shown in the
figure below. It works on the alternating current only because an
alternating flux is required for mutual induction between the two windings.

When the AC supply is given to the primary winding with a voltage of V1,
an alternating flux ϕ sets up in the core of the transformer, which links with
the secondary winding and as a result of it, an emf is induced in it
called Mutually Induced emf. The direction of this induced emf is
opposite to the applied voltage V1, this is because of the Lenz’s law shown
in the figure below:
 Physically,
there is no electrical connection between the two windings, but they are
magnetically connected. Therefore, the electrical power is transferred from
the primary circuit to the secondary circuit through mutual inductance.

The induced emf in the primary and secondary windings depends upon the
rate of change of flux linkage that is (N dϕ/dt).

dϕ/dt is the change of flux and is same for both the primary and secondary
windings. The induced emf E1 in the primary winding is proportional to the
number of turns N1 of the primary windings (E1 ∞ N1). Similarly induced
emf in the secondary winding is proportional to the number of turns on the
secondary side. (E2 ∞ N2).

2. Derive EMF equation of transformer.

A. EMF Equation of a Transformer
When a sinusoidal voltage is applied to the primary winding of a
transformer, alternating flux ϕm sets up in the iron core of the transformer.
This sinusoidal flux links with both primary and secondary winding. The
function of flux is a sine function.

The rate of change of flux with respect to time is derived mathematically.

The derivation of the EMF Equation of the transformer is shown below.

Let

● ϕm be the maximum value of flux in Weber

 ● f be the supply frequency in Hz
● N1 is the number of turns in the primary winding
● N2 is the number of turns in the secondary winding
Φ is the flux per turn in Weber

As shown in the above

figure that the flux changes from + ϕm to – ϕm in half a cycle of 1/2f
seconds.

By Faraday’s Law

Let E1 be the emf induced in the primary winding

Where Ψ = N1ϕ
Since ϕ is due to AC supply ϕ = ϕm Sinwt

So the induced emf lags flux by 90 degrees.

Maximum valve of emf

But w = 2πf

Root mean square RMS value is

Putting the value of E1max in equation (6) we get

Putting the value of π = 3.14 in the equation (7) we will get the value of
E1 as

Similarly
Now, equating the equation (8) and (9) we get

The above equation is called the turn ratio where K is known as the
transformation ratio.

The equation (8) and (9) can also be written as shown below using the
relation

(ϕm = Bm x Ai) where Ai is the iron area and Bm is the maximum value of
flux density.

For a sinusoidal wave

Here 1.11 is the form factor.

3. Write applications of transformer.

A.
o The transformer used for impedance matching.
o The transformer used for isolate two circuits electrically.
o It is used to increase or decrease the alternating voltages in electric
power applications.
o The transformer used in voltmeter, ammeters, protective relay etc.
o The transformer used for step up low voltage in case of measurement.
o The transformer used for step down high voltage for safety.
o The transformer used in rectifier.
o It is used in voltage regulators, voltage stabilizers, power supplies etc.

4.Explain operating principle of dc motor.

A. A DC motor is an electrical machine that converts electrical energy into
mechanical energy. In a DC motor, the input electrical energy is the direct
current which is transformed into the mechanical rotation.

DC Motor Parts
.

DC Motor Diagram

Different Parts of a DC motor

A DC motor is composed of the following main parts:

Armature or Rotor
The armature of a DC motor is a cylinder of magnetic laminations that are
insulated from one another. The armature is perpendicular to the axis of the
cylinder. The armature is a rotating part that rotates on its axis and is
separated from the field coil by an air gap.

Field Coil or Stator

A DC motor field coil is a non-moving part on which winding is wound to
produce a magnetic field. This electro-magnet has a cylindrical cavity
between its poles.

Commutator and Brushes

Commutator
The commutator of a DC motor is a cylindrical structure that is made of
copper segments stacked together but insulated from each other using
mica. The primary function of a commutator is to supply electrical current
to the armature winding.
Brushes
The brushes of a DC motor are made with graphite and carbon structure.
These brushes conduct electric current from the external circuit to the
rotating commutator. Hence, we come to understand that the commutator
and the brush unit are concerned with transmitting the power from
the static electrical circuit to the mechanically rotating region or the
rotor.

DC Motor Working
A magnetic field arises in the air gap when the field coil of the DC motor is
energized. The created magnetic field is in the direction of the radii of the
armature. The magnetic field enters the armature from the North pole side
of the field coil and “exits” the armature from the field coil’s South pole
side.
The conductors located on the other pole are subjected to a force of the
same intensity but in the opposite direction. These two opposing forces
create a torque that causes the motor armature to rotate.

5. Draw the characteristics of dc motors.

A. Characteristics Of DC Motors:
Generally, three characteristic curves are considered important for dc
motors which are, (i) Torque vs. armature current, (ii) Speed vs. armature
current and (iii) Speed vs. torque. These are explained below for each types
of dc motor. These characteristics are determined by keeping the following

Ta ∝ ɸ.Ia and N ∝ Eb/ɸ

two relations in mind.

These above equations can be studied at - emf and torque equation of

motor. For a DC motor, magnitude of the back emf is given by the same

and A are constant, therefore, N ∝ Eb/ɸ

emf equation of a dc generator i.e. Eb = PɸNZ / 60A. For a machine, P, Z

Characteristics Of DC Series Motors

Torque Vs. Armature Current (Ta-Ia):

This characteristic is also known as electrical characteristic. We know

field flux, Ta ∝ ɸ.Ia. In DC series motors, field winding is connected in

that torque is directly proportional to the product of armature current and

series with the armature, i.e., Ia = If. Therefore, before magnetic saturation
of the field, flux ɸ is directly proportional to Ia. Hence, before magnetic
saturation Ta α Ia2. Therefore, the Ta-Ia curve is parabola for smaller
values of Ia.
After magnetic saturation of the field poles, flux ɸ is independent of

T ∝ Ia. Therefore, after magnetic saturation, Ta-Ia curve becomes a straight

armature current Ia. Therefore, the torque varies proportionally to Ia only,

line.
The shaft torque (Tsh) is less than armature torque (Ta) due to stray losses.
Hence, the curve Tsh vs Ia lies slightly lower.
In DC series motors, (prior to magnetic saturation) torque increases as the
square of armature current, these motors are used where high starting
torque is required.
Speed Vs. Armature Current (N-Ia):
We know the relation, N ∝ Eb/ɸ
For small load current (and hence for small armature current) change in
back emf Eb is small and it may be neglected. Hence, for small currents
speed is inversely proportional to ɸ. As we know, flux is directly
proportional to Ia, speed is inversely proportional to Ia. Therefore, when
armature current is very small the speed becomes dangerously high. That
is why a series motor should never be started without some mechanical
load.
But, at heavy loads, armature current Ia is large. And hence, speed is low
which results in decreased back emf Eb. Due to decreased Eb, more
armature current is allowed.

Speed Vs. Torque (N-Ta)

This characteristic is also called as mechanical characteristic. From the
above two characteristics of DC series motor, it can be found that when
speed is high, torque is low and vice versa.
Characteristics Of DC Shunt Motors
Torque Vs. Armature Current (Ta-Ia)
In case of DC shunt motors, we can assume the field flux ɸ to be constant.
Though at heavy loads, ɸ decreases in a small amount due to
increased armature reaction. As we are neglecting the change in the flux ɸ,
we can say that torque is proportional to armature current. Hence, the Ta-Ia
characteristic for a dc shunt motor will be a straight line through the origin.
Since heavy starting load needs heavy starting current, shunt motor
should never be started on a heavy load.

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. But practically, ɸ
as well as Eb decreases with increase in load. Back emf Eb decreases
slightly more than ɸ, therefore, the speed decreases slightly. Generally, the
speed decreases only by 5 to 15% of full load speed. Therefore, a shunt
motor can be assumed as a constant speed motor. In speed vs. armature
current characteristic in the following figure, the straight horizontal line
represents the ideal characteristic and the actual characteristic is shown by
the dotted line.
Characteristics Of DC Compound Motor
DC compound motors have both series as well as shunt winding. In a
compound motor, if series and shunt windings are connected such that
series flux is in direction as that of the shunt flux then the motor is said to
be cumulatively compounded. And if the series flux is opposite to the
direction of the shunt flux, then the motor is said to be differentially
compounded. Characteristics of both these compound motors are explained
below.
(a) Cumulative compound motor
Cumulative compound motors are used where series characteristics are
required but the load is likely to be removed completely. Series winding
takes care of the heavy load, whereas the shunt winding prevents the motor
from running at dangerously high speed when the load is suddenly
removed. These motors have generally employed a flywheel, where sudden
and temporary loads are applied like in rolling mills.
(b) Differential compound motor
Since in differential field motors, series flux opposes shunt flux, the total

constant or even it may increase slightly with increase in load (N ∝ Eb/ɸ).

flux decreases with increase in load. Due to this, the speed remains almost

Differential compound motors are not commonly used, but they find
limited applications in experimental and research work.
6. Explain the operating principle of servo motor.
A:
Servo implies an error sensing feedback control mechanism which is
utilized to correct the performance of a system, which a general motor does
not have. Servomotor makes use of a regular motor and pair of the sensor
for position feedback. This motor can easily rotate in both direction and
also hold the position at any angle with the use of control mechanism. This
motor is basically used in automation industries where we want desired
motion by preprogramed input.
Now one question arises in mind that how a simple motor performs at that
accuracy level. The answer hidden in a sophisticated controller, usually a
devoted module designs particularly for use with the servomotor. This
servo circuit is assembled right inside the motor chamber and has a shaft
which can be positioned, that is usually fitted with gear mechanisms. This
motor is controlled with an electric signal that determines the amount of
movement of the shaft. It is the closed loop mechanism that uses position
feedback in order to control its rotation speed and position.
A servomotor consists four main parts.
1. Induction Motor
2. Gear assembly
3. Position Sensor
4. Controller
Advantages
1. Easily available in all sizes.
2. Work well for velocity control
3. Gives high intermittent torque.
4. High efficiency.
5. The operation is possible even at high speed.
6. Vibration free operations.
Disadvantages
1. Not suitable in vacuum.
2. More expensive than stepper motor.
3. Tuning is required to stabilize feedback loop.
4. The Design is complex as the microcontroller, microprocessor is used.
5. The Motor cannot sustain overload.
Applications
1. Robotics to actuate the robot arms
2. Conveyor belts to carrying the products along to various stages.
3. Camera autofocus to sharpen out of focus images.
4. Automatic door opener.
5. Antenna positioning.
 6. CNC Machining

7. Explain the working principle of stepper motor.

A: Stepper Motor Working Principle

A stepper motor is a D.C. motor with field placed on the rotor in the form
of permanent magnets with two, three or four sets of coils called phases,
placed in the stator around the rotor. The windings are connected to an
external logic driver which delivers voltage pulses to the windings
sequentially. The motor responds to these pulses and performs start, stop,
and reverse operations under command.

Both the rotor and stator have a definite number of teeth to suit the
designed step angle. The step angle is defined as the angular displacement
of the rotor in response to each pulse.

The rotor position depends upon the step angle and the number of pulses.
The speed of rotation depends upon the rate of pulses (and not supply
voltage) are precisely controlled; thus making the stepper motor an ideal
drive for operations involving precise positioning. Unlike control and servo
motors, no feedback control winding is required to close the loop and
monitor the position and speed of the rotor.

The stepper motor working principle is explained in Figure. The rotor

takes a position as per excitation of winding:
● In position (a) only winding A is energized.
● In position (b) both the windings, A and B are energized.
 ● In position (c), winding B is energized and so on.

From the above illustration, you can understand easily that we can rotate
the rotor in steps by supplying current to stator coils in a specific sequence.
This is what we do in stepper motors.
stepper motor system must accelerate and deaccelerate at a rate that allows
the motor to overcome the system inertia. For this reason, rotors are
constructed with less diameter and longer lengths. If a stepper motor is
dynamically overloaded, it will slip phase. These motors are best suited for
applications where loads are well within the capacity of the motor.

Holding torque is the maximum load torque that can be overcome by the
motor without causing the rotor to slip from its stable equilibrium position.

The operation of a stepper motor is accurate and precise over a wide range
of speed. Accuracy tolerance is the maximum deviation from nominal
values of each rotor displacement in response to input pulse under no-load
condition. The accuracy tolerance is usually in the range of 3 to 5% and
this error is non-cumulative.

Applications of Stepper Motors

The most familiar application of stepper motors is in quartz analog

watches. Because of the simplicity of logic controls, precision, and
reliability, stepper motors are extensively used in computers peripherals, in
CNC machine tools, remote controls, instrumentation, etc.

In computers, card readers, punched paper tape readers, teleprinters, digital

X-ray plotters, dot matrix printers, the reading/writing heads of floppy and
Winchester discs all use stepper motors as positioning drives. Two-phase
motors are used for very light torque applications. Mostly higher power
stepper motors are of the 3 or 4-phase types.

8. Explain about driver circuit

A. In electronics, a driver is a circuit or component used to control another
circuit or component, such as a high-power transistor, liquid crystal
display (LCD), stepper motors, and numerous others.
They are usually used to regulate current flowing through a circuit or to
control other factors such as other components, some devices in the circuit.
The term is often used, for example, for a specialized integrated circuit that
controls high-power switches in switched-mode power converters. An
amplifier can also be considered a driver for loudspeaker, or a voltage
regulator that keeps an attached component operating within a broad range
of input voltages.
Typically, the driver stage(s) of a circuit requires different characteristics
to other circuit stages. For example, in a transistor power amplifier circuit,
typically the driver circuit requires current gain, often the ability to
discharge the following transistor bases rapidly, and low output impedance
to avoid or minimize distortion.

LED circuit

Simple LED (Light Emitting Diode) circuit diagram

In electronics, an LED circuit or LED driver is an electrical circuit used
to power a light-emitting diode (LED). The circuit must provide sufficient
current to light the LED at the required brightness, but must limit the
current to prevent damaging the LED. The voltage drop across an LED is
approximately constant over a wide range of operating current; therefore, a
small increase in applied voltage greatly increases the current. Very simple
circuits are used for low-power indicator LEDs. More complex, current
source circuits are required when driving high-power LEDs for
illumination to achieve correct current regulation.

What Is Motor Driver IC?

A motor driver IC is an integrated circuit chip which is usually used to
control motors in autonomous robots. Motor driver ICs act as an interface
between microprocessors in robots and the motors in the robot. The most
commonly used motor driver ICs are from the L293 series such as L293D,
L293NE, etc. These ICs are designed to control 2 DC motors
simultaneously. L293D consist of two H-bridge. H-bridge is the simplest
circuit for controlling a low current rated motor. For this tutorial we will be
referring the motor driver IC as L293D only. L293D has 16 pins, they are
comprised as follows:
Ground Pins - 4
Input Pins - 4
Output Pins - 4
Enable pins - 2
Voltage Pins - 2
Why We Need Motor Driver IC?
Motor Driver ICs are primarily used in autonomous robotics only. Also,
most microprocessors operate at low voltages and require a small amount
of current to operate while the motors require a relatively higher voltages
and current. Thus, current cannot be supplied to the motors from the
microprocessor. This is the primary need for the motor driver IC.

How Motor Driver Operates?

The L293D IC receives signals from the microprocessor and transmits the
relative signal to the motors. It has two voltage pins, one of which is used
to draw current for the working of the L293D and the other is used to apply
voltage to the motors. The L293D switches it output signal according to the
input received from the microprocessor.
For Example: If the microprocessor sends a 1(digital high) to the Input Pin
of L293D, then the L293D transmits a 1(digital high) to the motor from its
Output Pin. An important thing to note is that the L293D simply transmits
the signal it receives. It does not change the signal in any case.
9. Derive the relation between torque, speed of dc motor
A. Torque Equation of a DC Motor
When a DC machine is loaded either as a motor or as a generator, the rotor
conductors carry current. These conductors lie in the magnetic field of the
air gap.

Thus, each conductor experiences a force. The conductors lie near the
surface of the rotor at a common radius from its centre. Hence, a torque is
produced around the circumference of the rotor, and the rotor starts
rotating.

When the machine operates as a generator at a constant speed, this torque

is equal and opposite to that provided by the prime mover.

When the machine is operating as a motor, the torque is transferred to the

shaft of the rotor and drives the mechanical load. The expression is the
same for the generator and motor.

When the current-carrying current is placed in the magnetic field, a force is

exerted or it which exerts turning moment or torque F x r. This torque is
produced due to the electromagnetic effect, hence is
called Electromagnetic torque.

The torque which is produced in the armature is not fully used at the shaft
for doing the useful work. Some part of it gets lost due to mechanical
losses. The torque which is used for doing useful work in known as
the shaft torque.

Since,

Multiplying the equation (1) by Ia we get

Where,

VIa is the electrical power input to the armature.

I2aRa is the copper loss in the armature.

We know that,
Total electrical power supplied to the armature = Mechanical power
developed by the armature + losses due to armature resistance

Now, the mechanical power developed by the armature is Pm,

Also, the mechanical power that rotates the armature can be given
regarding torque T and speed n.

Where n is in revolution per seconds (rps) and T is in Newton-Meter.

Hence,

But,

Where N is the speed in revolution per minute (rpm) and

Where n is the speed in (rps).

Therefore,
So, the torque equation is given as:

For a particular DC Motor, the number of poles (P) and the number of
conductors per parallel path (Z/A) are constant.

Where

Thus, from the above equation (5) it is clear that the torque produced in the
armature is directly proportional to the flux per pole and the armature
current.

Moreover, the direction of electromagnetic torque developed in the

armature depends upon the current in armature conductors. If either of the
two is reversed the direction of torque produced is reversed and hence the
direction of rotation. But when both are reversed, and direction of torque
does not change.

10. Explain working principle of brushless dc motor.

A. Brushless DC electric motors also known as electronically commutated
motors (ECMs, EC motors).
Primary efficiency is a most important feature for BLDC motors. Because
the rotor is the sole bearer of the magnets and it doesn't require any power.
i.e., no connections, no commutator and no brushes. In place of these, the
motor employs control circuitry. To detect where the rotor is at certain
times, BLDC motors employ along with controllers, rotary encoders or a
Hall sensor.

Construction of Brushless DC motor

In this motor, the permanent magnets attach to the rotor. The current-
carrying conductors or armature windings are located on the stator. They
use electrical commutation to convert electrical energy into mechanical
energy.

The main design difference between a brushed and brushless motors is the
replacement of mechanical commutator with an electric switch circuit. A
BLDC Motor is a type of synchronous motor in the sense that the magnetic
field generated by the stator and the rotor revolve at the same frequency.
Brushless motor does not have any current carrying commutators. The field
inside a brushless motor is switched through an amplifier which is
triggered by the commutating device like an optical encoder.

The layout of a DC brushless motor can vary depending on whether it is in

“Out runner” style or “In runner” style.

● Outrunner – The field magnet is a drum rotor which rotates around

the stator. This style is preferred for applications that require high
torque and where high rpm isn’t a requirement.
● In runner – The stator is a fixed drum in which the field magnet
rotates. This motor is known for producing less torque than the out-
runner style, but is capable of spinning at very high rpm.

Working Principle of Brushless DC motor

BLDC motor works on the principle similar to that of a Brushed DC motor.
The Lorentz force law which states that whenever a current carrying
conductor placed in a magnetic field it experiences a force. As a
consequence of reaction force, the magnet will experience an equal and
opposite force. In the BLDC motor, the current carrying conductor is
stationary and the permanent magnet is moving.
When the stator coils get a supply from source, it becomes electromagnet
and starts producing the uniform field in the air gap. Though the source of
supply is DC, switching makes to generate an AC voltage waveform with
trapezoidal shape. Due to the force of interaction between electromagnet
stator and permanent magnet rotor, the rotor continues to rotate.

With the switching of windings as High and Low signals, corresponding

winding energized as North and South poles. The permanent magnet rotor
with North and South poles aligns with stator poles which causes the motor
to rotate.

Advantages of Brushless DC motor

● Less overall maintenance due to absence of brushes

● Reduced size with far superior thermal characteristics
● Higher speed range and lower electric noise generation.
● It has no mechanical commutator and associated problems
● High efficiency and high output power to size ratio due to the use of
permanent magnet rotor
● High speed of operation even in loaded and unloaded conditions due
to the absence of brushes that limits the speed
● Smaller motor geometry and lighter in weight than both brushed
type DC and induction AC motors.
● Long life as no inspection and maintenance is required for
commutator system
 ● Higher dynamic response due to low inertia and carrying windings
in the stator
● Less electromagnetic interference
● Low noise due to absence of brushes

Limitations of Brushless DC motor

● These motors are costly

● Electronic controller required control this motor is expensive
● Requires complex drive circuitry
● Need of additional sensors

Applications of Brushless DC motor

Brushless DC motors (BLDC) use for a wide variety of application
requirements such as varying loads, constant loads and positioning
applications in the fields of industrial control, automotive, aviation,
automation systems, health care equipments etc.

● Computer hard drives and DVD/CD players

● Electric vehicles, hybrid vehicles, and electric bicycles
● Industrial robots, CNC machine tools, and simple belt driven
systems
● Washing machines, compressors and dryers
● Fans, pumps and blowers.

11. Explain relay driver circuit?

A. Relay Driver:
In a low power circuit or an output from a Microprocessor is very low. It
is sufficient for a LED to glow but to drive a high load you will need a
Relay (Electromagnet Switch), and to give proper voltage or current to a
relay you will need a relay driver. Many times, one transistor with a
resistance is enough to make a Relay Driver. In this type of circuit
Transistor is use as current amplifier and Relay does two things (a) they
isolate current (flow of electron) this is important because high load
appliances run at different voltage (potential difference) thus Relay
protects your sensitive electronics parts. (b) Relay is an electromagnetic
switch. It is a type of mechanical Switch which is pulled by an electro-
magnet so its resistance is very low and thus it can control large power
appliances.

Now days in market Relay Driver Module is available which is generally

combination of relay and a transistor. In many modules LED's is also
placed to indicate status of a Relay Switch. In Market Relay module can
purchased by telling how many Channels Relay is Required and operating
voltage.
Advantage: You will be able to control various appliances, and other
equipment with large current. It can be controlled directly by
Microcontroller.

12. Draw the characteristics of ac servo motor and mention its applications.
A. An AC servo motor is basically a two-phase induction motor except for
certain special design features. A two-phase induction motor consisting of
two stator windings oriented 90 degrees electrically apart in space and
excited by ac voltage which differ in time phase by 90 degrees. Generally,
voltages of equal magnitude and 90 degrees phase difference are applied to
the two stator phases thus making their respective fields 90 degrees apart in
both time and space, at synchronous speed. As the field sweeps over the
rotor, voltages are induced in it producing current in the short-circuited
rotor. The rotating magnetic field interacts with these currents producing a
torque on the rotor in the direction of field orientation. The shape of the
characteristics depends upon ratio of the rotor reactance (X) to the rotor
resistance (R). In normal induction motors X/R ratio is generally kept high
so as to obtain the maximum torque close to the operating region which is
usually around 5% slip. A two-phase servo motor differs in two ways from
normal induction motor.
1. The rotor of the servo motor is built with high resistance so that its X/R
ratio is small and the torque speed characteristics are as shown in fig 1.

2. In servo applications, the voltage applied to the two stator windings are
seldom balanced one. One of phase is known as reference phase is excited
by constant voltage and the other known as the control phase w.r.t. the
voltage supplied to the reference winding and it has a variable magnitude
and polarity. The control winding is supplied from a servo amplifier. Fig 2.
 Fig2. Schematic Diagram of Two-phase motor

Features

● These are low weight devices.

● It offers reliability as well as stability in operation.
● There is not much noise generated at the time of operation.
● It offers almost linear torque-speed characteristics.
● As brushes and slip rings are not present here thus it reduces
maintenance cost.
Applications of AC Servomotors

Due to the various advantages offered by the AC servomotors, these

majorly finds applications in the instruments that operate on
servomechanism, in position controlling devices, computers. Along with
this these also find applications in tracking systems, machine tools and
robotics machinery.