Browsing: DC Machines

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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Electro-mechanical Energy Conversion Devices

We all know that energy exists in many forms, and we use numerous devices on a daily basis that convert one form of energy into another. When we speak of electromechanical energy conversion, however, we mean either the conversion of electric energy into mechanical energy or vice versa.

Definition

A device which makes possible the conversion of energy from electrical to mechanical form or from mechanical energy to electrical form is known as an Electromechanical energy conversion device.

Depending upon the conversion of energy from one form to another, the electromechanical device can be named as a motor or generator.

  1. Motor : An electro-mechanical device which converts electrical energy into mechanical energy is called motor.

Electromechanical energy Conversion Devices Motor

Applications of Motor

Electric Motors are used for driving Industrial machines:

  • Drilling Machines
  • Hammer presses
  • Lathe Machines
  • Shapers etc.
  1. Generator : An electro-mechanical device which converts mechanical energy into electrical energy is called generator.

Electromechanical energy Conversion Devices Motor generator

Applications of generator

Electric generators are used for generating electrical energy. Electric generators are installed at Hydro-power generating stations, Steam power generating stations, Nuclear power generating stations etc.

Electromechanical energy conversion is a reversible process except for the losses in the system. The term “reversible” implies that the energy can be transferred back and forth between the electrical and the mechanical systems. However, each time we go through an energy conversion process, some of the energy is converted into heat and is lost from the system forever.

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EMF Equation of DC Generator

An electrical generator is an electrical device that converts mechanical energy (or power) into electrical energy (or power). The working principle of dc generator is based on dynamically induced emf. According to the Faraday’s law of electromagnetic induction whenever a conductor cuts by the magnetic flux or field, dynamically induced e.m.f. produced in it.

Let,

Φ = flux per pole in Weber (Wb)

Z = total number of armature conductors

N = speed of armature in r.p.m.

P = number of poles

A = number of parallel paths in armature winding

Eg = generated emf in volts

Flux cut by one conductor in one revolution = Wb

Time taken to complete one revolution, dt = 60/N

The number of conductors conducted in series in each parallel path = Z/A

Average induced e.m.f. across each parallel path or across armature terminals,

For Wave Winding

Number of parallel paths, A = 2

For Lap Winding

Number of parallel paths, A = Number of poles

Conclusion

Thus, we conclude that the induced e.m.f. is directly proportional flux per pole (Φ) and speed (N).

 

 

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Construction of DC Machine

Construction of DC Machine

DC machine consists of two main parts. One of them is armature and other is field system. The field system is stationary part of the dc machine and armature is rotating part. The DC machine consists of following main parts.

YOKE

It is a outer body of the machine. It is cylindrical in shape. It serves the following purpose in machine:

It supports inner part of the machines e.g. poles are fixed on it.

It provides mechanical protection to the inner parts of the machine.

It provides low reluctance part for the magnetic flux.

Cast Iron yokes are made for small machines and for large machine. It is made up of fabricated steel.

Construction of DC Machine

POLES

Pole consists of two main parts.

  • Pole Core
  • Pole shoe

POLE CORE

The Pole Core is made of thin cast steel or wrought iron laminations which are riveted together. The pole core is circular in section and filed coil is wound over it.

POLE SHOES

Pole Shoes are also made of cast steel or wrought iron laminations and it screwed to the pole face. Pole shoes have larger cross section area. The poles sever following purposes.

  • It supports the filed coils.
  • It spreads out the magnetic flux in the air gap.

FIELD OR EXCITING COILS

Fields coils are made of enameled copper. The coils are wound on the former then placed around the pole core as the field coils of all the poles are connected in series in, such as way that when current flows through them, the adjacent poles attain opposite polarity. When direct current passed through the field coils, the required magnetic flux produced.

ARMATURE

The armature is the rotating part of the dc machine. The conversion of energy takes place in the armature. The armature core is of laminated silicon steel. Laminations are used to reduce the eddy current losses and silicon steel is used to reduced the hysteresis losses. Armature is cylindrical in shape and keyed to the rotating shaft. At the outer periphery slots are cut, which accommodate the armature winding.

ARMATURE WINDING

Enameled copper is used for the construction of armature winding. The armature winding is housed in armature slots, which is suitably connected. The armature winding is heart of the dc machine.

On the basis of connections, there are two types of armature winding.

  • Lap Winding
  • Wave Winding

COMMUTATOR

It is the most important part of the DC machine. It is just a reversing switch. Commutator connects the rotating armature conductors to the stationary external circuit through brushes.

In generating action, it converts alternating voltage into direct voltage and in motoring action it converts unidirectional torque into alternating torque.

The commutator is a form of rotating switch placed between armature and external circuit and so arranged it will reverse the connections with the external circuit at the instant of each reversal of current in the armature.

The commutator is of cylindrical shape and is made up of wedge shape hard drawn copper segments. The segments are insulated from each other by a thin sheet of mica.

BRUSHES

The Brushes are pressed upon the commutator and make the connecting link between the armature winding and external circuit. Carbon is used for the construction of Brushes because it is conducting material and good lubricating material.

BRUSH ROCKER

It holds the spindles of the brush holders. It is fitted on to the stationary frame of the machine with nut and bolts.

END HOUSING

End housings are attached to the ends of the main frame and support bearings. The front housing supports the bearings and the brush assemblies whereas the rear housing usually support the bearing only.

BEARINGS

The function of Bearings is to reduce friction between the rotating and stationary parts of the machine. Usually ball or roller bearing are used.

SHAFT

The Shaft is made of mild steel. The shaft is used to transfer mechanical poser from or to the machine.

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