Working Principle and Application of Electrical Machines Part-I

 

04/09/2023

Some Basic Questions and Answers of Electrical Machines.

 

 

Answer all the question:

1. Draw a distribution transformer showing all the parts with neat sketch.
2. What is transformer , generator and motor? Write down the law or principle by which it operates.
3. Explain the working principle of dc generator, dc motor and transformer.
4. Mention the application of dc generator, dc motor and transformer.

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Ans.1

 

You may draw another picture also.

 

Ans. 2

Transformer:

 

A transformer is a static device which consists of two or more stationary electric circuits interlinked by a common magnetic circuit for the purpose of transferring electrical energy between them. The transfer of energy from one circuit to another takes place without a change in frequency.

Or,

 

Transformer is an ac machine that (i) transfers electrical en- ergy from one electric circuit to another (ii) does so without a change of frequency (iii) does so by the principle of electromag- netic induction and (iv) has electric circuits that are linked by a common magnetic circuit

 

So,

Transformer is an AC machine or static device which transfer electrical energy from one circuit to another circuit which are electrically separated but connected by a common magnetic circuit. The energy transfer is done by without changing the frequency and generally changing the voltage level.

 

Transformer works on the fairies law of electromagnetic induction.

Actually transformer operates on mutual induction.

 

Generator:

A DC generator is an electrical  machine that converts mechanical energy supply to it into electrical energy.

The generator operates on the principle of the production of dynamically induced EMF.

It can be explained by Faraday’s law.

 

Motor:

A DC motor is an electrical machine that converts electrical energy supplied to eat into mechanical energy.

 

The principle of on which a DC Motor works is very simple.  If a current carrying conductor is placed in a magnetic field mechanical force is experienced on the conductor due to these force motor rotates in a specified direction.

 

The direction of the motor can be explained by Fleming’s left hand rule or motor rule.

 

Ans. 3

Working principle of DC Generator:

 

The generator operates on the principle of the production of dynamically induced emf i.e. whenever flux is cut by the conductor, dynamically induced emf is produced in it according to the laws of electromagnetic induction, which will cause a flow of current in the conductor if the circuit is closed.

 

For production of dynamically induced emf, three things are necessary a magnetic field, a conductor and motion of the conductor with respect to the field. In de generators the field is produced by the field magnets which are stationary. Permanent magnets are used for very small capacity machines and electromagnets are used for large machines to create magnetic flux. The conductors are situated on the periphery of the armature being rotated by the prime mover.

 

 

WORKING PRINCIPLE OF DC MOTOR

 

The principle upon which a de motor works is very simple. If a current carrying conductor is placed in a magnetic field, mechanical force is experienced on the conductor, the direction of which is given by Fleming’s left hand rule (also called motor rule) and hence the conductor moves in the direction of force. The magnitude of the mechanical force experienced on the conductor is given by

 

F= BIL newtons

 

Where B is the field strength in teslas (Wb/m²), I is the current flowing through the conductor in amperes and L is the length of conductor in meters.

 

When the motor is connected to the de supply mains, a direct current passes through the brushes and commutator to the the armature winding; while it passes through the commutator it is converted into ac so that the group of conductors under successive field poles carry currents in the opposite directions, as shown in Fig. 2 Also the direction of current in the indi- vidual conductors reverses as they pass away from the influence of one pole to that of the next.

 

Fig-2

 

 

In Fig. 2, a 4-pole de motor is shown when the field and armature circuits are connected across de supply mains. Let the current in armature conductors be outwards under the N-poles (shown by dots) and inwards under S-poles (shown by crosses). By applying Fleming’s left hand rule, the direction of force on each conductor can be determined, which has been illustrated in Fig.2 . From Fig.2  it is observed that each conductor experiences a force which tends to rotate the motor armature in clockwise direction. These forces collectively produce a driving torque.

 

Working principle of Transformer:

 

When the primary winding is connected to an ac supply mains, a current flows through it. Since this winding links with an iron core, current flowing through this winding produces an alternating flux in the core. Since this flux is alternating and links with the secondary winding also, induces an emf in the secondary winding. The frequency of induced emf in secondary winding is the same as that of the flux or that of the supply voltage. The induced emf in the secondary winding enables it to deliver current to an external load connected across it. Thus the energy is transformed from primary winding to the secondary winding by means of electromagnetic induction without any change in frequency.

 

 

 

Extra Note:

The flux of the iron core links not only with the secondary winding but also with the primary winding, so produces self-induced emf in the primary winding. This induced emf in the primary winding opposes the applied voltage and, therefore, sometimes it is known as back emf of the primary. In fact the induced emf in the primary winding limits the primary current in much the same way as the back emf in a de motor limits the armature current.

 

Ans 4.

Application of DC Generator:

 

Separately excited DC generator:

 

These are generally used higher self excited generators would be relatively on satisfactory. These are used in Ward leonard system of Speed control.

 

Series Wound DC Generators:

 

They can be employed as a constant current source by making use of the dropping portion of the voltage current characteristic.

It is used for supplying field current in regenerative breaking in DC locomotives.

They are also used in series Arc lighting.

It is used as a series booster in the feeder.

 

Sound Wound DC Generators:

It can be used as a constant output voltage source.

Sun generators with field regulators are used for light and power supply purpose.

These are also used for charging of batteries.

Cumulative Compound Wound DC Generators:

Over compounding type dc generators are generally used for lighting and power services.

Differential Compound Wound DC Generator:

It is a constant current generator.

It is used in arc welding as a arc welding generator.

 

Application of DC Motor:

Separately excited DC Motors:

 

Used for paper machines, diesel electric propulsion of ships, in steel rolling mills etc.

DC Deries Motors:

For drives recovering very high starting torque and fire exhaustible wedding speed is satisfactory.

It is used for-

Hoists

Cranes

 Trolley cars

 Conveyors

Electric locomotive etc

 

Loads must be positively connected not belted. To prevent over speed lightest load should not be much less than 15 to 20% of full load torque.

DC shunt Motors:

Essentially for constant speed applications recovering medium starting torque.

Used for-

Lathes

Centrifugal pumps

Reciprocating pumps

Fans

Blowers

Conveyors

Wood working machines

Machine tools

Printing presses

Spinning and weaving machines etc.

 

Cumulative compound wound DC motor:

For drives requiring high starting torque and only fairly constant speed, pulsating loads with fly wheel action.

Used for-

Shears

Conveyors

Crushers

Bending rolls

Punch presses

Hoists

Elevators

Heavy planners

Ice making machines

Air compressors

Rolling Mills

Printing presses etc .

Differentially compound wound DC Motors:

 Used for experimental and research work.

 

Application of Transformer:

 

Transformers are extensively used like

The instrument transformers for metering purposes.

One-to-one ratio transformers (i.e. the transformers having equal primary and secondary voltages) are used to electrically isolate the two parts of an electrical circuit.

 In high-voltage laboratories the transformers are used to provide very high voltages for testing purposes, termed as testing transformers.

 In electric communication circuits, transformers are employed for a variety of purposes e.g.,

As an impedance transformation device to allow maximum transfer of power from the input circuit to the output device.

In radio and television circuits input transformers, interstage transformers and output transformers are widely employed.

In power electronics, the transformers find wide use (i) in gate pulse triggering and (ii) in synchronising the pulse gating signals with the ac supply provided to the main power circuit.

The transformers, specificallycalled the pulse transformers, are widely employed in radar, television and digital computers.

 Transformers are also employed in telephone circuits,

Instrumentation circuits and

Control circuits etc.

Source: A Course in Electrical Technology ( Electrical Machines) Voll-II by J. B. Gupta

Electric Machines by Ashfaq Husain