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Applications of DC Generators

DC generators are rarely used. There are various applications of dc generators that are given below:

Separately excited generators

Separately excited generator are required, separate supply to excite its field. Due to this, the use is restricted to some special applications such as electroplating, electrorefining of materials, etc.

DC shunt generators

These generators are commonly used in battery charging and ordinary lighting purposes.

DC series generators

These are commonly  used as boosters on DC feeders, as a constant current generators for welding generators and arc lamps.

Cumulatively compound generators

These are used for domestic lightning purposes and to transmit energy over long distances.

Differential compound generators

The use of differential compound generators is very rare and it is used for special applications like electric arc welding.

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Working Principle of DC Motor

The working of dc motor depends upon the principle that when a current –carrying conductor is kept in a magnetic field, a force acts on the conductor which tends to rotate it. How this force acts and produces a continuous motion is explained below:

Also Read: Working Principle of DC Generator

A sectional view of a loop of wire is shown under north and south poles. The current in conductor A shown inward while in conductor B it is outward. A force will act on both conductors A and B which will tend to rotate the loop in the anticlockwise direction. This direction of force is given by the Fleming’s left hand rule. This force will act till the conductors are in the magnetic field. Therefore, to obtain a continuous motion, many conductors are placed such that if one leaves the magnetic field, another enters it.Working Principle of DC MotorWhen loop AB starts rotating, after a very short time, conductor B will come under the north pole and conductor A under the south pole. Now the current in conductor B is inward, while in conductor A it is outward as it is automatically changed with the help of the commutator. Again an anticlockwise force will act on the loop of wire and thus a continuous rotation is the anticlockwise direction will be obtained in this case.

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DC Generator

A dc generator is an electrical machine that takes mechanical energy and converts it into electrical energy. A generator does not produce electrical energy by magic, it simply converts mechanical energy into electrical energy. This mechanical energy rotates the armature of a generator and thus electrical energy is produced in the armature conductor. For producing electricity from a generator, the following three requirements are essential:

Also Read :- Working Principle of DC Generator

  • Conductors
  • Magnetic field, and
  • Mechanical energy

Conductors

Insulated conductors are placed in the armature slots. These conductors may be copper conductors or aluminium conductors.

Magnetic Field

The magnetic field is produced by the use of either a permanent magnet or an electromagnet. However, in all generators the electromagnet is mostly used. The reason is that the magnetic field can be easily controlled. These field poles are provided on the pole shoe of the generator.

Mechanical Energy

For the rotation of the conductor, mechanical energy is obtained from the prime mover (diesel engine, steam turbine, water turbine, etc). The mechanical energy given to the generator rotates the armature conductors which cuts the magnetic lines of force, and emf is induced.

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Working Principle of DC Generator

A DC generator works on the principle of Faraday’s law of electromagnetic induction which says whenever a conductor is moved in the magnetic field or flux, an emf is generated in the conductors and the magnitude of the induced emf is directly proportional to the rate of change of flux linkage.

The emf generated in the conductor is of alternating nature and the generator which gives out electrical energy in the form of alternating current is called a synchronous generator or alternator.

The generator, that gives out electrical energy in the form of direct current is called a dc generator. The essential difference between the alternator and dc generator is that is an alternator slip rings are used to collect the supply, while in the dc generator, the commutator is used for this purpose. This commutator converts the alternating current induced in the conductors into direct current for the external circuit.

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Three-Point Starter

The dc shunt motor is started by a three-point starter consisting of the resistance, handle, studs no-volt release coil and overload release coil.

When armature is connected to the supply, the current to the armature goes through the starting resistance and the field winding is connected directly to the mains through the no-volt release coil. When the motor attains speed, some back emf is induced in the armature. The starting current is gradually cut off and thus the starting current is reduced to a low value.

three-point starter

Parts of Three-Point Starter and their Function

Starter Resistance

The resistance is put in series of the armature to protect the motor from the heavy rush of current at the time of starting. The tapping is taken at several places and connected to studs. The resistance is reduced by gradually moving the handle from left to right from one stud to another.

Handle

The handle is hand operated and insulated. It carries a soft iron piece which is directly attached to No-volt release coil when it reaches running position.

Brass Arc

The brass arc is provided below and along the series resistance stud. It is provided to supply full voltage to shunt field from the process of starting to running position.

No-volt Release Coil

No-volt release coil is safety device provided in the starter. The no-volt coil consists of a thin wire of many turns and is connected in series with the field winding of the motor. This coil is magnetized when current flows through the shunt field winding. Its function is to attract the handle of the starter and keep it in on position. It releases the handle and goes to the OFF position by the spring tension due to failure of supply and in this way disconnects the motor from the supply.

Overload Release Coil

Its function is to demagnetize the no-volt release coil in the case abnormal condition (such as a fault or overload) of the machine. It consists of a few turns of thick wire and is connected in series to the armature. This coil will only be sufficiently magnetized when the excessive current flows through the armature due to overload of some fault. Now coil attract the tripping plunger which short circuited the terminal of the no-volt coil. The no-volt release coil will be demagnetized and release the starting handle which will come to the OFF position at once due to spring tension. Thus motor stops

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Types of DC Generators

DC generators are generally classified according to the methods of their field excitation. Based upon the method of excitation, dc generators can be divided into;

  1. Separately excited DC generator
  2. Self-excited DC generator 
  1. Separately excited DC generator

A DC generator in which current is supplied to the field winding from an independent external DC source (e.g. a battery) is called separately excited DC generator.  The flux produced by the poles depends upon the field current. The greater the speed and field current, greater is the generated e.m.f. It may be noted that separately excited d.c. generators are rarely used in practice.

separately excited generator

Important Relation;

Here, Ia = IL = I

Ia = armature current

IL = Line current

Ra = armature resistance

Vb = brush drop per brush

Terminal Voltage, V  = Eg – IaRa

Terminal voltage, V  = Eg – IaRa – 2vb

Power developed = Eg Ia

Power output = VIL = VIa

  1. Self – excited generators

Separately – excited generators are those whose field magnets are energized by the current produced by the generators themselves. Due to residual magnetism, there is always present some flux is the poles. When the armature is rotated, some e.m.f. and hence some induced current is produced which is partly or fully passed through the coils, thereby strengthening the residual  There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely;

  1. Series wound DC generators
  2. Shunt wound DC generators
  3. Compound wound DC generators
  1. Series wound DC generators

In series wound DC generator, the field winding is connected in series with the armature winding forming a series circuit. Therefore, full line current IL or armature current Ia flows through it. As they carry full load current, they consists of relatively few turns of thick wire or strip.

Important Relation;

Here, Ia = IL = Ise

Ia = armature current

IL = Line current

Ra = armature resistance

Rse = series resistance

Vb = brush drop per brush

Terminal Voltage, V  = Eg – IaRa – IseRse

Or

Terminal Voltage, V  = Eg – Ia (  Ra – Rse)- 2vb

Power developed = Eg Ia

Power output = VIL = VIa

2. Shunt wound DC generators

in a shunt wound DC generators, the field winding is connected across the armature winding forming a parallel or shunt circuit. Therefore, full terminal voltage is applied across them. As they carry very small load current, they consists of many turns of fine wire.

Important Relation;

Here, Ia = IL = Ish

Ia = armature current

IL = Line current

Ra = armature resistance

Rsh = series resistance

Vb = brush drop per brush

Ish = shunt current

Ish = V/Rsh

Ia = IL + Ish

Terminal Voltage, V  = Eg – IaRa

Or

Terminal Voltage, V  = Eg – Ia Ra– 2vb

Power developed = Eg Ia

Power output = VIL = VIa

3. Compound wound DC generators

In compound wound DC generator, there are two sets of field windings on each pole. One of them (having many turns of fine wire ) is connected across the armature and the other winding (having few turns of thick wires ) is connected in series with the armature winding. A compound wound DC generator may be; Long Shunt and Short Shunt.

Long shunt in which the shunt field winding is connected in parallel with combination of both armature and series field winding.

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

Compound Short Shunt Generator

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BACK EMF

When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator The direction of this induced e.m.f. acts in opposite to the applied voltage V, (according to the Lenz’s law) and it is referred as back or counter e.m.f. Eb. The back e.m.f. Eb(= P φ ZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions.

The rotating armature generating the back emf Eb is like a battery of e.m.f. Eb put across a supply mains of V volts. Obviously, V has to drive Ia  against the opposition of Eb . The power required to overcome this opposition is EbIa.

The current flowing through the armature is given by the relation:

When the mechanical load applied on the motor decreases, its speed decreases which reduces the value of Eb. As a result the value (V – Eb) increases which consequently increases Ia. Hence, motor draws extra current from the supply mains.

So, we find that Eb acts like a governor i.e. it makes the motor self-regulating so that it draws as much current as is just necessary.

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Ward Leonard System

Ward-Leonard system is used where an unusually wide (upto 10:1) and very sensitive speed control is required.

This system is used to supply variable to the motor. M1 is the main motor whose speed control is required. A d.c. generator G is mechanically coupled with either a d.c. shunt motor or an a.c. motor M2. The motor M2 runs at an approximately constant speed. The field winding of the d.c. generator is connected to a constant voltage d.c. supply line through a field regulator. The d.c. motor M1 is fed from the generator G and its field connected directly to a d.c. supply line.

Ward Leonard System

The voltage of the generator can be varied from zero upto its maximum value by means of its field regulator. By reversing the direction of the field current of generator G by means of the reversing switch, generated voltage can be reversed and hence the direction of rotation of motor M1 . The direction rotation of the generator – motor set is changed.

Advantages

  • The speed and direction of rotation both can be controlled very accurately.

Disadvantages

  • The capital investment in this method is very high as two extra machines (generator G and Motor M2) are required.
  • A large output machine must be used for the motor generator set.
  • This system has a low overall efficiency especially at light loads.

Applications

Ward-Leonard system is extensively used for in followings :

  • Elevators
  • Hoists
  • Steel rolling mills
  • Paper mills
  • Cranes
  • Diesel-electric propulsions etc.

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Necessity of Starter for a D.C. Motor

The armature current is given by the relation ;

When the motor is at rest, there is no back e.m.f. Eb in the armature. Consequently, if the motor is directly switched on to the mains, the armature will draw a heavy current (Ia = V/Ra) because of small armature resistance.

For instance , 10 H.P., 220 V shunt motor has a full-load current of 40 A and an armature resistance of about 0.2 Ω. If this motor is directly switched on to supply, it would take an armature current of 220/0.2 = 440 A which is 27.5 times the full-load current.

This high starting current may result in:

  • burning of armature due to excessive heating effect,
  • damaging the commutator and brushes due to heavy sparking,
  • excessive voltage drop in the line to which the motor is connected. The result is that the operation of other appliances connected to the line may be impaired and in particular cases, they may refuse to work.

In order to avoid excessive current at starting, a variable resistance (known as starting resistance) is connected in series with the armature circuit. This resistance is gradually reduced as the speed of the motor increases  (and hence Eb increases) and eventually it is cut out completely when the motor has attained full speed. The value of starting resistance is generally such that starting current is limited to 1.25 to 2.5 times the full-load current.

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Applications of DC Motors

DC motors are employed in various industrial applications. As per the characteristics of d.c. motors, different types of d.c. motors are applied for various applications. These applications are explained below:-

Separately Excited Motors

Very accurate speeds can be obtained by these motors. Moreover, these motors are best suited where speed variation is required from very low value to high value. These motors are used for paper machines, diesel-electric propulsion of ships, in steel rolling mills etc.

DC Series Motors

The characteristics of a series motor reveals that it is variable speed motor i.e. the speed is low at higher torques ( upto 500%) and vice versa. However, at light or no-load, the motor tends to attain dangerously high speed. The motor has a high starting torque. It is, therefore, used

  • Where large starting torque is required.
  • Where the load is subjected to heavy fluctuations and the speed is automatically required to reduce at high torques and vice-versa.

As such the dc series motors are most suitable for electric traction, hoists, cranes, elevators, vacuum cleaners, sewing machine etc.

DC Shunt Motors

The characteristics of a shunt motor reveal that it is an approximately constant speed motor. It is, therefore, used

  • Where the speed is required to remain almost constant from no-load to full-load.
  • Where the load has 10 be driven at a number of speeds and any one of which is required to remain nearly constant.

As such the dc shunt motors are most suitable for industrial drives such as lathes, centrifugal pumps, reciprocating pumps, fans blowers, drills grinders etc.

DC Compond Motors

Differential-compound motors are rarely used because of their poor torque characteristics. However, cumulative-compound motors are used where a fairly constant speed is required with irregular loads or suddenly applied heavy loads.

As such the dc compound motors are most suitable for Presses, shears, reciprocating machines etc.

 

 

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