Electrical Technology(Very simple manner for ECE/ME/CE/EEE)

Author:Nanajee Karri M.Tech. MISTE, (Ph.D)Assistant Professor,Dept. of Electrical Engineering,Sasi Institute of Technology & Engineering,Tadepalligudem-534101,Andhra Pradesh, India.nanajee.karri@gmail.com

Contents:DC machines:

DC Generator DC Motor

AC machines: Transformers Alternator Induction Motor

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Principle of DC GeneratorThere are two types of generators, one is ac generator and other is DC generator. Whatever may be the types of generators, it always converts mechanical power to electrical power. An AC generator produces alternating power. A DC generator produces direct power. Both of these generators produce electrical power, based on same fundamental principle of Faraday's law of electromagnetic induction.

According to this law, when a conductor moves in a magnetic field it cuts magnetic lines of force, due to which an emf is induced in the conductor. The magnitude of this induced emf depends upon the rate of change of flux (magnetic line force) linkage with the conductor. This emf will cause a current to flow if the conductor circuit is closed.

Hence the most basic two essential parts of a generator are 1. Magnetic field2. Conductors which move inside that magnetic field.

Basic Construction and Working of A DC Generator.A dc generator is an electrical machine which converts mechanical energy into direct current electricity. This energy conversion is based on the principle of production of dynamically induced emf. This article outlines basic construction and working of a DC generator.

1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only provides mechanical strength to

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the whole assembly but also carries the magnetic flux produced by the field winding.

2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.

3. Field winding: They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such a way that, when energized, they form alternate North and South poles.

4. Armature core: Armature core is the rotor of the machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed to the shaft.

5. Armature winding: It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.

6. Commutator and brushes: Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft. Brushes are usually made from carbon or graphite. They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

Working Principle Of A DC MotorA motor is an electrical machine which converts electrical energy into mechanical energy. The principle of working of a DC motor is that "whenever a current carrying conductor is placed in a magnetic field, it experiences a mechanical force". 

The direction of this force is given by Fleming's left hand rule and it's magnitude is given by F = BIL.

Where,

B = magnetic flux density,

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I = current and

L = length of the conductor within the magnetic field.

Fleming's left hand rule: If we stretch the first finger, second finger and thumb of our left hand to be perpendicular to each other AND direction of magnetic field is represented by the first finger, direction of the current is represented by second finger then the thumb represents the direction of the force experienced by the current carrying conductor.

Classifications of DC Machines : (DC Motors and DC Generators)

Each DC machine can act as a generator or a motor. Hence, this classification is valid for both: DC

generators and DC motors. DC machines are usually classified on the basis of their field

excitation method. This makes two broad categories of dc machines;

(i) Separately excited and (ii) (ii) Self-excited.

Separately excited: In separately excited dc machines, the field winding is supplied from a separate power source. That means the field winding is electrically separated from the armature circuit. Separately excited DC generators are not commonly used because they are relatively expensive due to the requirement of an additional power source or circuitry. They are used in laboratories for research work, for accurate speed control of DC motors with Ward-Leonard system and in few other applications where self-excited DC generators are unsatisfactory. In this type, the stator field flux may also be provided with the help of permanent magnets (such as in the case of a permanent magnet DC motors). A PMDC motor may be used in a small toy car.

Self-excited: In this type, field winding and armature winding are interconnected in various ways to achieve a wide range of performance characteristics (for example, field winding in series or parallel with the armature winding).In self-excited type of DC generator, the field winding is energized by the current produced by themselves. A small amount of flux is always present in the poles due to the residual magnetism. So, initially, current induces in the armature conductors of a dc generator only due to the residual magnetism. The field flux gradually increases as the induced current starts flowing through the field winding.

Self-excited machines can be further classified as –

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Series wound – In this type, field winding is connected in series with the armature winding. Therefore, the field winding carries whole load current (armature current). That is why series winding is designed with few turns of thick wire and the resistance is kept very low (about 0.5 Ohm).

Shunt wound – Here, field winding is connected in parallel with the armature winding. Hence, the full voltage is applied across the field winding. Shunt winding is made with a large number of turns and the resistance is kept very high (about 100 Ohm). It takes only small current which is less than 5% of the rated armature current.

Compound wound – In this type, there are two sets of field winding. One is connected in series and the other is connected in parallel with the armature winding. Compound wound machines are further divided as -

Short shunt – field winding is connected in parallel with only the armature winding

Long shunt – field winding is connected in parallel with the combination of series field winding and armature winding

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3 Point Starter | Working Principle and Construction of Three Point StarterA 3 point starter in simple words is a device that helps in the starting and running of a shunt wound DC motor or compound wound DC motor. Now the question is why these types of DC motors require the assistance of the starter in the first case. The only explanation to that is given by the presence of back emf Eb, which plays a critical role in governing the operation of the motor. The back emf, develops as the motor armature starts to rotate in presence of the magnetic field, by generating action and counters the supply voltage. This also essentially means, that the back emf at the starting is zero, and develops gradually as the motor gathers speed.

The general motor emf equation E = Eb + Ia.Ra,at starting is modified to E = Ia.Ra as at starting Eb = 0.

Thus we can well understand from the above equation that the current will be dangerously high at starting (as armature resistance Ra is small) and hence its important that we make use of a device like the 3 point starter to limit the starting current to an allowable lower value.

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Construction of 3 Point Starter Construction wise a starter is a variable resistance, integrated into number of sections as shown in the figure beside. The contact points of these sections are called studs and are shown separately as OFF, 1, 2, 3, 4, 5, RUN. Other than that there are 3 main points, referred to as

1. 'L' Line terminal. (Connected to positive of supply.)2. 'A' Armature terminal. (Connected to the armature winding.)3. 'F' Field terminal. (Connected to the field winding.)

And from there it gets the name 3 point starter. Now studying the construction of 3 point starter in further details reveals that, the point 'L' is connected to an electromagnet called overload release (OLR) as shown in the figure. The other end of 'OLR' is connected to the lower end of conducting lever of starter handle where a spring is also attached with it and the starter handle contains also a soft iron piece housed on it. This handle is free to move to the other side RUN against the force of the spring. This spring brings back the handle to its original OFF position under the influence of its own force. Another parallel path is derived from the stud '1', given to the another electromagnet called No Volt Coil

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(NVC) which is further connected to terminal 'F'. The starting resistance at starting is entirely in series with the armature. The OLR and NVC acts as the two protecting devices of the starter.

Working of Three Point StarterHaving studied its construction, let us now go into the working of

the 3 point starter. To start with the handle is in the OFF position when the supply to the DC motor is switched on. Then handle is slowly moved against the spring force to make a contact with stud No. 1. At this point, field winding of the shunt or the compound motor gets supply through the parallel path provided to starting resistance, through No Voltage Coil. While entire starting resistance comes in series with the armature. The high starting armature current thus gets limited as the current equation at this stage becomes Ia = E/(Ra+Rst). As the handle is moved further, it goes on making contact with studs 2, 3, 4 etc., thus gradually cutting off the series resistance from the armature circuit as the motor gathers speed. Finally when the starter handle is in 'RUN' position, the entire starting resistance is eliminated and the motor runs with normal speed. This is because back emf is developed consequently with speed to counter the supply voltage and reduce the armature current. So the external electrical resistance is not required anymore, and is removed for optimum operation. The handle is moved manually from OFF to the RUN position with development of speed.

Electrical Transformer - Basic Construction, Working And TypesElectrical transformer is a static electrical machine which transforms electrical power from one circuit to another circuit, without changing the frequency. Transformer can increase or decrease the voltage with corresponding decrease or increase in current.

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Working Principle of Transformer

 The basic principle behind working of a transformer is the phenomenon of mutual induction between two windings linked by common magnetic flux. Basically a transformer consists of two inductive coils; primary winding and secondary winding. The coils are electrically separated but magnetically linked to each other.

When, primary winding is connected to a source of alternating voltage, alternating magnetic flux is produced around the winding. The core provides magnetic path for the flux, to get linked with the secondary winding. Most of the flux gets linked with the secondary winding which is called as 'useful flux' or main 'flux', and the flux which does not get linked with secondary winding is called as 'leakage flux'.  As the flux produced is

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alternating (the direction of it is continuously changing), EMF gets induced in the secondary winding according to Faraday's law of electromagnetic induction. This emf is called 'mutually induced emf', and the frequency of mutually induced emf is same as that of supplied emf. If the secondary winding is closed circuit, then mutually induced current flows through it, and hence the electrical energy is transferred from one circuit (primary) to another circuit (secondary)

Types Of TransformersTransformers can be classified on different basis, like types of construction, types of cooling etc.

(A) On the basis of construction

[1] Core type transformer [2] Shell type transformer

(B) On the basis of their purpose

[1] Step up transformer: Voltage increases at secondary.[2] Step down transformer: Voltage at secondary.

(C) On the basis of type of supply

[1] Single phase transformer[2] Three phase transformer

(D) On the basis of their use

[1] Power transformer: Used [2] Distribution transformer: [3] Instrument transformer:

 Current transformer (CT) Potential transformer (PT)

(E) On the basis of cooling employed 

[1] Oil-filled self cooled type[2] Oil-filled water cooled type[3] Air blast type (air cooled)

Losses in Transformer(I) Core Losses or Iron LossesEddy current loss and hysteresis loss depend upon the magnetic properties of the material used for the construction of core. Hence these losses are also known as core losses or iron losses.

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Hysteresis loss in transformer: Hysteresis loss is due to reversal of magnetization in the transformer core. This loss depends upon the volume and grade of the iron, frequency of magnetic reversals and value of flux density.

It can be given by, Steinmetz formula:Wh= Kh Bmax1.6 f V (watts)where,   Kh = Steinmetz hysteresis constant             V = volume of the core in m3

Eddy current loss in transformer: In transformer, AC current is supplied to the primary winding which sets up alternating magnetizing flux. When this flux links with secondary winding, it produces induced emf in it. But some part of this flux also gets linked with other conducting parts like steel core or iron body or the transformer, which will result in induced emf in those parts, causing small circulating current in them. This current is called as eddy current. Due to these eddy currents, some energy will be dissipated in the form of heat.

 (II) Copper Loss in TransformerCopper loss is due to ohmic resistance of the transformer windings.  Copper loss for the primary winding is I12R1 and for secondary winding is I22R2. Where, I1 and I2 are current in primary and secondary winding respectively, R1 and R2 are the resistances of primary and secondary winding respectively. It is clear that Cu loss is proportional to square of the current, and current depends on the load. Hence copper loss in transformer varies with the load.

Efficiency of TransformerJust like any other electrical machine, efficiency of a transformer can be defined as the output power divided by the input power.

That is  Efficiency = Output / Input

Transformers are the most highly efficient electrical devices.

Most of the transformers have full load efficiency between 95% to 98.5%.

As a transformer being highly efficient, output and input are having nearly same value, and hence it is impractical to measure the efficiency of transformer by using output / input.

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A better method to find efficiency of a transformer is using, 

Efficiency = (Input - Losses) / Input = 1 - (Losses / Input).

All day efficiency of a transformer is always less than ordinary efficiency of it.

Condition For Maximum Efficiency

Hence, efficiency of a transformer will be maximum when copper loss and iron losses are equal.

That is Copper loss = Iron loss.

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Constructional details of Alternator(AC Generator)Construction of Alternators: An alternator has 3,-phase winding on the stator and a d.c. field winding on the rotor.

a. Stator It is the stationary part of the machine and is built up of silicon steel laminations having slots on its inner periphery. A 3-phase winding is placed in these slots and serves as the armature winding of the alternator. The armature winding is always connected in star and the neutral is connected to ground. b. Rotor The rotor carries a field winding which is supplied with direct current through two slip rings by a separate d.c. source. This d.c. source (called exciter) is generally a small d.c. shunt or compound generator mounted on the shaft of the alternator. Rotor construction is of two types, namely; 1. Salient (or projecting) pole type 2. Non-salient (or cylindrical) pole type

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Salient pole type:

In this type, salient or projecting poles are mounted on a large circular steel frame which is fixed to the shaft of the alternator as shown in Fig. (1). The individual field pole windings are connected in series in such a way that when the field winding is energized by the d.c. exciter, adjacent poles have opposite polarities.

Non-salient pole type:

In this type, the rotor is made of smooth solid forged-steel radial cylinder having a number of slots along the outer periphery. The field windings are embedded in these slots and are connected in series to the slip rings through which they are energized by the d.c. exciter. The regions forming the poles are usually left un slotted as shown in Fig. (2). It is clear that the poles formed are non-salient i.e., they do not project out from the rotor surface.

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Induction Motor | Working Principle | Types of Induction MotorOne of the most common electrical motor used in most applications which is known as induction motor. This motor is also called as asynchronous motor because it runs at a speed less than its synchronous speed(Ns).

Synchronous speed is the speed of rotation of the magnetic field in a rotary machine and it depends upon the frequency and number poles of the machine.

An induction motor always runs at a speed less than synchronous speed because the rotating magnetic field which is produced in the stator will generate flux in the rotor which will make the rotor to rotate, but due to the lagging of flux current in the rotor with flux current in the stator, the rotor will never reach to its rotating magnetic field speed i.e. the synchronous speed.

Working Principle of Induction MotorWe need to give double excitation to make a machine to rotate. For example if we consider a DC motor, we will give one supply to the stator and another to the rotor through brush arrangement. But in induction motor we give only one supply. It is very simple, from the name itself we can understand that induction process is involved. Actually when we are giving the supply to the stator winding, flux will generate in the coil due to flow of current in the coil. Now the rotor winding is arranged in such a way that it becomes short circuited in the rotor itself. The flux from the stator will cut the coil in the rotor and since the rotor coils are short circuited, according to Faraday's law of electromagnetic induction, current will start flowing in the coil of the rotor. When the current will flow, another flux will get generated in the rotor. Now there will be two flux, one is stator flux

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and another is rotor flux and the rotor flux will be lagging w.r.t to the stator flux. Due to this, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic flux. So the speed of the rotor will be depending upon the ac supply and the speed can be controlled by varying the input supply.

There are basically two types of induction motor that depend upon the input supply –

[1] Single phase induction motor a. Split phase induction motorb. Capacitor start induction motorc. Capacitor start capacitor run induction motord. Shaded pole induction motor

[2] Three phase induction motor a. Squirrel cage induction motorb. Slip ring induction motor

Single phase induction motor is not a self starting and three phase induction motor is a self-starting motor.

A three phase induction motor runs on a three phase AC supply. 3 phase induction motors are extensively used for various industrial applications.

Advantages:

They have very simple and rugged (almost unbreakable) construction

They are very reliable and having low cost They have high efficiency and good power factor Minimum maintenance required 3 phase induction motor is self starting hence extra starting motor

or any special starting arrangement is not required

Disadvantages:

Speed decreases with increase in load, just like a DC shunt motor If speed is to be varied, we have sacrifice some of its efficiency

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Torque-Slip Characteristics of Induction MotorAs the induction motor is located from no load to full load, its speed decreases hence slip increases. Due to the increased. load, motor has to produce more torque to satisfy load demand. The torque ultimately depends on slip as explained earlier. The behaviour of motor can be easily judged by sketching a curve obtained by plotting torque produced against slip of induction motor.

The curve obtained by plotting torque against slip from s = 1 (at start) to s = 0 (at synchronous speed) is called torque-slip characteristics of the induction motor. It is very interesting to study the nature of torque-slip characteristics.       We have seen that for a constant supply voltage, E2 is also constant. So we can write torque equations as,

Now to judge the nature of torque-slip characteristics let us divide the slip range (s = 0 to s = 1) into two parts and analyse them independently.

i) Low slip region :

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       In low slip region, 's' is very very small. Due to this, the term (s X2)2 is so small as compared to R22 that it can be neglected.

Hence in low slip region torque is directly proportional to slip. So as load increases, speed decreases, increasing the slip. This increases the torque which satisfies the load demand.       Hence the graph is straight line in nature.       At N = Ns , s = 0 hence T = 0. As no torque is generated at N = Ns, motor stops if it tries to achieve the synchronous speed. Torque increases linearly in this region, of low slip values.

ii) High slip region :        In this region, slip is high i.e. slip value is approaching to 1. Here it can be assumed that the term R22   is very very small as compared to (s X2) 2 . Hence neglecting from the denominator, we get

So in high slip region torque is inversely proportional to the slip. Hence its nature is like rectangular hyperbola.       Now when load increases, load demand increases but speed decreases. As speed decreases, slip increases. In high slip region as T α1/s, torque decreases as slip increases.       But torque must increases to satisfy the load demand. As torque decreases, due to extra loading effect, speed further decreases and slip further increases. Again torque decreases as T  α1/s hence same load acts as an extra load due to reduction in torque produced. Hence speed further drops. Eventually motor comes to standstill condition. The motor can not continue to rotate at any point in this high slip region. Hence this region is called unstable region of operation.

 So torque - slip characteristics has two parts,1. Straight line called stable region of operation2. Rectangular hyperbola called unstable region of operation.

       In low slip region, as load increases, slip increases and torque also increases linearly. Every motor has its own limit to produce a torque. The maximum torque, the  motor can produces as load increases is Tm which occurs at s = sm. So linear behaviour continues till s = sm.

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       If load is increased beyond this limit, motor slip acts dominantly pushing motor into high slip region. Due to unstable conditions, motor comes to standstill condition at such a load. Hence i.e. maximum torque which motor can produce is also called breakdown torque or pull out torque. So range s = 0 to s = sm is called low slip region, known as stable region of operation. Motor always operates at a point in this region. And range s = sm to s = 1 is called high slip region which is rectangular hyperbola, called unstable region of operation. Motor can not continue to rotate at any point in this region.       At s = 1, N = 0 i.e. start, motor produces a torque called starting torque denoted as Tst.       The entire torque - slip characteristics is shown in the Fig. 1.

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Applications of Three Phase Wound Rotor Induction Motors

[1] Wound rotor motors are suitable for loads requiring high starting torque and where a lower starting current is required.

[2] The Wound rotor induction motors are also used for loads having high inertia, which results in higher energy losses.

[3] Used for the loads which require a gradual build up of torque.[4] Used for the loads that requires speed control.[5] The wound rotor induction motors are used in conveyors, cranes,

pumps, elevators and compressors.[6] The maximum torque is above 200 percent of the full load value

while the full load slip may be as low as 3 percent. The efficiency is about 90 %.

Applications of Three Phase Cage Rotor Induction MotorsMany polyphase cage induction motors are available in the market to meet the demand of the several industrial applications and various starting and running condition requirement. They are classified according to the Class.

Class A MotorsClass A motors have normal starting torque, high starting current and low operating slip (0.005-0.015). The design has low resistance single cage rotor. The efficiency of the motor is high at full load. Applications of Class A motors are fans, blowers, centrifugal pumps, etc.

Class B MotorsClass B motors have normal starting torque, low starting current and low starting current and low operating slip. The motor is designed, in such a way to withstand the high leakage reactance; as a result, the starting

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current is reduced. The starting torque is maintained by use of a double cage or deep bar rotor.

The Class B motors are most commonly used motor and used for full voltage starting. The applications and the starting torque are same as that of Class A motors.

Class C MotorsThe class C motors have high starting torque and low starting current. Such motors are of the double cage and deep bar and has higher rotor resistance. The loads are compressors, conveyors, reciprocating pumps, crushers, etc.

Class D MotorsClass D motors have the highest starting torque as compared to all the other class of motors. The bars of the rotor cage are made up of brass. These types of motors have low starting current and high operating slip. The value of full load operating slip varies between 8 to 15%. Thus, the efficiency of the motor is low.

These motors are suitable for driving intermittent loads which require frequent acceleration and high loads. For example – punch presses, bulldozers and die stamping machines. When the motor is driving the high impact loads, it is coupled to a flywheel to provide kinetic energy.

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