BASIC ELECTRICAL AND ELECTRONICS ENGINEERING UNIT - 2

UNIT 2

ELECTRICAL MECHANICS

Construction, Principle of Operation, Basic

Equations and Applications of

DC Generators,

DC Motors,

Single Phase Transformer,

Single phase induction Motor.

Dr.Anushya,SRREC,PADUR

CONSTRUCTIONMain parts of dc machine are:1. Field magnet frame or yoke2. Pole cores and pole shoes3. Pole coil or field coils4. Armature core4. Armature core5. Armature winding6. Commutator7. Brushes8. Brush holder9. Bearing 10.Shaft


1.It provides a mechanical support for the poles.2.It act as a protective cover against mechanical

damage3.It provide a passage for the magnetic flux 3.It provide a passage for the magnetic flux

produced by the poles.


POLE CORE AND POLE SHOESThe pole core itself may be made of solid piece of cast iron or cast steel, but pole shoe is laminated and is screwed to the pole face by means of counter sunk screw.The pole cores may be made of thin laminations of steel, riveted together. This laminations of steel, riveted together. This type of pole is held in position with the frame by means of bolts.The pole shoe serves the two purpose as under.1. It support the pole coils.2. Being of larger cross section, it spread

the flux and also reduces the reluctance of the magnetic path.


EMF equation Let,

= flux per pole in weber

Z = Total number of conductor

P = Number of poles P = Number of poles

A = Number of parallel paths

N =armature speed in rpm

Eg = emf generated in any on of the parallel path


EMF equation

Flux cut by 1 conductor

in 1 revolution = P *

Flux cut by 1 conductor in

60 sec = P N /60

Avg emf generated in 1Avg emf generated in 1

conductor = PN/60

Number of conductors in

each parallel path = Z /A

Eg = PNZ/60ADr.Anushya,SRREC,PADUR

Types of DC Generator

DC generators are generally classified

according to their method of excitation .

Separately excited DC generator Separately excited DC generator

Self excited D C generator


Further classification of DC Generator

Series wound generator

Shunt wound generator Shunt wound generator

Compound wound generator

Short shunt & Long shunt

Cumulatively compound

&

Differentially compoundDr.Anushya,SRREC,PADUR

Characteristics

No load saturation characteristic (Eo/If)

Internal or Total characteristic (E/ Ia)

External characteristic (V/I)


TYPES OF GENERATOR


Losses in DC Generators

1. Copper losses or variable losses

2. Stray losses or constant losses

Stray losses : consist of (a) iron losses or core

losses and (b) windage and friction losses .losses and (b) windage and friction losses .

Iron losses : occurs in the core of the machine

due to change of magnetic flux in the core .

Consist of hysteresis loss and eddy current

loss.

Hysteresis loss depends upon the frequency ,

Flux density , volume and type of the core .Dr.Anushya,SRREC,PADUR

Losses

Hysteresis loss depends upon the frequency ,

Flux density , volume and type of the core .

Eddy current losses : directly proportional to Eddy current losses : directly proportional to

the flux density , frequency , thickness of the

lamination .

Windage and friction losses are constant due to

the opposition of wind and friction .


Shunt Generators:

a. in electro plating

b. for battery recharging

Applications

b. for battery recharging

c. as exciters for AC generators.

Series Generators :

A. As boosters

B. As lighting arc lamps


DC Motors

Converts Electrical energy into Mechanical energy

Construction : Same for Generator and motormotor

Working principle : Whenever a current carrying conductor is placed in the magnetic field , a force is set up on the conductor.


Back emf

The induced emf in the rotating armature

conductors always acts in the opposite

direction of the supply voltage .

According to the Lenzs law, the direction of the According to the Lenzs law, the direction of the

induced emf is always so as to oppose the

cause producing it .

In a DC motor , the supply voltage is the cause

and hence this induced emf opposes the

supply voltage. Dr.Anushya,SRREC,PADUR

Classification of DC motors

DC motors are mainly classified into three types as listed below:

Shunt motor Shunt motor

Series motor

Compound motor

Differential compound

Cumulative compound Dr.Anushya,SRREC,PADUR

Torque

The turning or twisting force about an axis is called torque .

P = T * 2 piN/ 60 P = T * 2 piN/ 60

Eb Ia = Ta * 2 piN/ 60 T I a

Ta I2a


Characteristic of DC motors

T/ Ia characteristic

N/ I a characteristic

N/T characteristic


TYPES OF DC MOTORS


According to the speed equation of a dc motor

N Eb/

V- Ia Ra/

Thus speed can be controlled by-

Flux control method: By Changing the flux by controlling the current through the field

Speed control of DC motors

Flux control method: By Changing the flux by controlling the current through the field winding.

Armature control method: By Changing the armature resistance which in turn changes the voltage applied across the armature


Applications:

Shunt Motor:

Blowers and fans

Centrifugal and reciprocating pumps

Lathe machinesLathe machines

Machine tools

Milling machines

Drilling machines


Applications:

Series Motor:

Cranes

Hoists , Elevators

TrolleysTrolleys

Conveyors

Electric locomotives


Applications:

Cumulative compound Motor:

Rolling mills

Punches

ShearsShears

Heavy planers

Elevators


TransformerAn A.C. device used to change high voltage low

current A.C. into low voltage high current A.C. and vice-

versa without changing the frequency

In brief,

1. Transfers electric power from one circuit to another1. Transfers electric power from one circuit to another

2. It does so without a change of frequency

3. It accomplishes this by electromagnetic induction

4. Where the two electric circuits are in mutual inductive

influence of each other.


The primary winding is connected to the

incoming power supply.

The secondary winding is connected to the

driven load.


This is an isolation transformer. The secondary

winding is physically and electrically isolated

from the primary winding.


Each set of windings (primary and

secondary) is formed from loops of wire

wrapped around the core.

Each loop of wire is called a turn.

The ratio of the primary and secondary

voltages is determined by the ratio of the voltages is determined by the ratio of the

number of turns in the primary and

secondary windings.

The volts-per-turn ratio is the same on

both the primary and secondary windings.


Working of a transformer1. When current in the

primary coil changes being alternating in nature, a changing magnetic field is produced

2. This changing magnetic field gets associated with field gets associated with the secondary through the soft iron core

3. Hence magnetic flux linked with the secondary coil changes.

4. Which induces e.m.f. in the secondary.


Principle of operation

It is based on

principle of MUTUAL

INDUCTION.

According to whichAccording to which

an e.m.f. is induced

in a coil when

current in the

neighbouring coil

changes.


Constructional detail : Shell type

Windings are wrapped around the center leg of a laminated core.


Core type

Windings are wrapped around two sides of a laminated square core.


Construction of transformer from

stampings


Cut view of transformer


Transformer with conservator and

breather


Ideal Transformers Zero leakage flux:

-Fluxes produced by the primary and secondary currents are confined within the core

The windings have no resistance:- Induced voltages equal applied voltages

The core has infinite permeability The core has infinite permeability- Reluctance of the core is zero- Negligible current is required to establish magnetic

flux Loss-less magnetic core

- No hysteresis or eddy currents


Ideal transformer

V1 supply voltage ; I1- noload input current ; V2- output voltgae; I2- output currentIm- magnetising current; E1-self induced emf ; E2- mutually induced emf


EMF equation of a transformer

Refer pdf file: emf-equation-of-tranformer


Phasor diagram: Transformer on No-

load


All day efficiency

hours) 24 (kWhin output

in wattsinput in wattsput out

efficiency commercialordinary

for=

=

hours) 24 (kWhin Input day

forall =

All day efficiency is always less than the commercial efficiency


Construction of Single Phase Induction Motor

Like any other electrical motor asynchronous motor

also have two main parts namely rotor and stator.

Stator: As its name indicates stator is a stationary

part of induction motor. A single phase ac supply is

given to the stator of single phase induction motor.given to the stator of single phase induction motor.

Rotor: The rotor is a rotating part of induction

motor. The rotor is connected to the mechanical

load through the shaft. The rotor in single phase

induction motor is of squirrel cage rotor type.


Working Principle of Single Phase Induction Motor

We know that for the working of any electrical

motor whether its ac or dc motor, we require two

fluxes as, the interact of these two fluxes produced

the required torque, which is desired parameter

for any motor to rotate.


Stator of Single Phase Induction Motor

The stator of the single phase induction motor has

laminated stamping to reduce eddy current losses

on its periphery.

The slots are provided on its stamping to carry

stator or main winding. stator or main winding.

In order to reduce the hysteresis losses, stamping

are made up of silicon steel.

When the stator winding is given a single phase ac

supply, the magnetic field is produced and the

motor rotates at a speed slightly less than the

synchronous speed Ns which is given by Dr.Anushya,SRREC,PADUR

The construction of the stator of asynchronous motor is similar to that of three phase induction motor except there are two dissimilarity in the winding part of the single phase induction motor.Firstly the single phase induction motors are mostly provided with concentric coils. As the number of turns per coil can be easily adjusted with the help of concentric coils, the mmf distribution is almost concentric coils, the mmf distribution is almost sinusoidal.

Except for shaded pole motor, the asynchronous motor has two stator windings namely the main winding and the auxiliary winding. These two windings are placed in space quadrature with respect to each other.


Rotor of Single Phase Induction Motor

The construction of the rotor of the single phase induction motor is similar to the squirrel cage three phase induction motor.

The rotor is cylindrical in shape and has slots all over its periphery. The slots are not made parallel to each other but are bit skewed as the skewing prevents magnetic locking of stator and rotor teeth prevents magnetic locking of stator and rotor teeth and makes the working of induction motor more smooth and quieter.

The squirrel cage rotor consists of aluminium, brass or copper bars. These aluminium or copper bars are called rotor conductors and are placed in the slots on the periphery of the rotor.


The rotor conductors are permanently shorted by

the copper or aluminium rings called the end

rings. In order to provide mechanical strength

these rotor conductor are braced to the end ring

and hence form a complete closed circuit

resembling like a cage and hence got its name as

squirrel cage induction motor. squirrel cage induction motor.

As the bars are permanently shorted by end rings,

the rotor electrical resistance is very small and it

is not possible to add external resistance as the

bars are permanently shorted. The absence of slip

ring and brushes make the construction of single

phase induction motor very simple and robust.Dr.Anushya,SRREC,PADUR

Methods for Making Single Phase

Induction as Self Starting Motor

Split phase induction motor,

Capacitor start inductor motor,

Capacitor start capacitor run induction motor,

Shaded pole induction motor. Shaded pole induction motor.


Split Phase Induction Motor


In addition to the main winding or running winding, the stator of single phase induction motor carries another winding called auxiliary winding or starting winding.

A centrifugal switch is connected in series with auxiliary winding . The purpose of this switch is to disconnect the auxiliary winding from the main circuit when the motor attains a speed up to 75 to circuit when the motor attains a speed up to 75 to 80% of the synchronous speed.

We know that the running winding is inductive in nature. Our aim is to create the phase difference between the two winding and this is possible if the starting winding carries high resistance. Let us say


Irun is the current flowing through the main or

running winding,

I is the current flowing in starting winding, Istart is the current flowing in starting winding,

& VT is the supply voltage.


We know that for highly resistive windingthe current is almost in phase with thevoltage and forhighly inductive winding the current lag behindthe voltage by large angle.

The starting winding is highly resistive so,the current flowing in the starting winding lags behindthe applied voltage by very small angle and therunning winding is highly inductive in nature so,the current flowing in running winding lags behindthe current flowing in running winding lags behindapplied voltage by large angle.

The resultant of these two current is IT. The resultantof these two current produce rotating magneticfield which rotates in one direction. In split phaseinduction motor the starting and main current getsplitted from each other by some angle so this motorgot its name as split phase induction motor.Dr.Anushya,SRREC,PADUR

Applications of Split Phase Induction Motor

Split phase induction motors have low

starting current and moderate starting torque. So

these motors are used in fans, blowers, centrifugal

pumps, washing machine grinder etc. These motors

are available in the size ranging from 1 / 20 to 1 / 2

KW.KW.


Capacitor Start IM


We already know that single phase induction motor is

not self starting because the magnetic fieldproduced is

not rotating type.

In order to produce rotating magnetic field there must

be some phase difference. In case of split

phase induction motor we use resistancefor creating

phase difference but here we use capacitor for this phase difference but here we use capacitor for this

purpose.

We are familiar with this fact that the current flowing

through the capacitor leads the voltage.

So, in capacitor start inductor motor and capacitor

start capacitor run induction motor we are using two

winding, the main winding and the starting winding. Dr.Anushya,SRREC,PADUR

With starting winding we connect a capacitor so the current flowing in thecapacitor i.e Ist leads the applied voltage by some angle, st.

The running winding is inductive in nature so, the current flowing in running winding lags behind applied voltage by an angle, m.

Now there occur large phase angle differences between these two currents which produce an resultant current, these two currents which produce an resultant current, I and this will produce a rotating magnetic field.

Since the torque produced by these motors depends upon the phase angle difference, which is almost 90. So, these motors produce very high starting torque.

In case of capacitor start induction motor, the centrifugal switch is provided so as to disconnect the starting winding when the motor attains a speed up to 75 to 80% of the synchronous speedDr.Anushya,SRREC,PADUR

Capacitor Start Capacitor Run IM

Capacitor Start Capacitor Run Induction Motor:-

This motor is identical to a capacitor start motor except that starting winding is not opened after starting. after starting.

So that both the windings remain connected to the supply when running as well as at starting.

There are two designs. Constructions of both the designs are same as Capacitor start but difference is as follows.


Application:-

Permanent Capacitor Induction Motors are

used Where low torque is required Ceiling

Fan, Table fan etcFan, Table fan etc


Capacitor Start Capacitor Run

Induction Motor)


Two capacitor 1 C and 2 C are used in the starting

winding.

1 C is very small capacitor and permanently

connected in the circuit.

1 C helps for optimum running conditions.

2 C is very large capacitor connected in parallel with

1 C1 C

2 C helps for optimum running conditions and

remains in the circuit during starting.

When motor reaches about 75% of synchronous

speed then 2 C will disconnected

Motor runs continue till power is ON.Dr.Anushya,SRREC,PADUR

Application of Capacitor Start IM and

Capacitor Start Capacitor Run IM

These motors have high starting torque hence

they are used in conveyors, grinder, air they are used in conveyors, grinder, air

conditioners etc. They are available up to 6

KW.


Shaded Pole Single Phase Induction

Motors


The stator of the shaded pole single phase induction motor has salient or projected poles. These poles are shaded by copper band or ring which is inductive in nature. The poles are divided into two unequal halves. The smaller portion carries the copper band and is called as shaded portion of the pole.

ACTION: When a single phase supply is given to the stator of shaded pole induction motor an alternating flux is produced . This change of flux induces emf in the flux is produced . This change of flux induces emf in the shaded coil. Since this shaded portion is short circuited, the current is produced in it in such a direction to oppose the main flux. The flux in shaded pole lags behind the flux in the unshaded pole. The phase difference between these two fluxes produces resultant rotating flux.


We know that the stator winding current is alternating

in nature and so is the flux produced by the stator

current. In order to clearly understand the working of

shaded pole induction motor consider three regions-

When the flux changes its value from

zero to nearly maximum positive value.zero to nearly maximum positive value.

When the flux remains almost constant

at its maximum value.

When the flux decreases from maximum

positive value to zero.


REGION 1: When the flux changes its value from zeroto nearly maximum positive value In this region therate of rise of flux and hence current is very high.According to Faradays law whenever there is changein flux emf gets induced. Since the copper band isshort circuit the current starts flowing in the copperband due to this induced emf. This current in copperband produces its own flux. Now according to Lenzslawthe direction of this current in copper band is suchlawthe direction of this current in copper band is suchthat it opposes its own cause i.e rise in current. So theshaded ring flux opposes the main flux, which leads tothe crowding of flux in non shaded part of stator andthe flux weaken in shaded part. This non uniformdistribution of flux causes magnetic axis to shift in themiddle of the non shaded part.


REGION 2: When the flux remains almost constant

at its maximum value In this region the rate of

rise of current and hence flux remains almost

constant. Hence there is very little induced emf in

the shaded portion. The flux produced by this

induced emf has no effect on the main flux and

hence distribution of flux remains uniform and thehence distribution of flux remains uniform and the

magnetic axis lies at the center of the pole.


REGION 3: When the flux decreases from maximum positive value to zero In this region the rate of decrease in the flux and hence current is very high. According toFaradayslaw whenever there is change in flux emf gets induced. Since the copper band is short circuit the current starts flowing in the copper band due to this induced emf. This current in copper band produces its own flux. Now according to Lenzs lawthe direction of the current in copper band is such that it opposes its own cause i.e decrease in current. So the shaded ring flux aids the main flux, which leads to the crowding of flux in shaded part of stator and the flux weaken in non flux in shaded part of stator and the flux weaken in non shaded part. This non uniform distribution of flux causes magnetic axis to shift in the middle of the shaded part of the pole.

This shifting of magnetic axis continues for negative cycle also and leads to the production of rotating magnetic field. The direction of this field is from non shaded part of the pole to the shaded part of the pole.


Applications of Shaded Pole Motor

Applications of Shaded pole motors induction

motor are-

Due to their low starting torques and reasonable

cost these motors are mostly employed in small

instruments, hair dryers, toys, record players, instruments, hair dryers, toys, record players,

small fans, electric clocks etc. These motors are

usually available in a range of 1/300 to 1/20 KW.


Documents

BASIC ELECTRICAL AND ELECTRONICS ENGINEERING UNIT - 2