The insulated wires housed in armature slots are suitably connected. This is called the armature winding. Armature winding plays vital role in a dc machine. It is a place where conversion of power takes place i.e. conversion of mechanical power into electrical one in case of a generator and conversion of electrical power into mechanical one in case of moto
In lap winding finish end of one coil is connected to a commutator segment and to the start end of one coils is connected to a commutator segment and to the start end of the adjacent coil under the same pole and similarly all coils are connected. The winding is knowna lap winding because the sides of successive coils overlap each other.
Wave winding is also sometimes known as series winding. In wave winding finish end of one coils is connected to the start of another coils Thus in wave winding, the winding progresses, passing every N pole and S pole till it returns to the coils side from where it was started, As the winding is wavy, the winding is, therefore, called wave winding.
ARMATURE REACTION AND COMMUTATION IN DC MACHINES
When the generator is supplying no load and the field winding is energised, there exists in it only the mmf of the main poles which creates the main flux. When the generator is loaded, current flows through the armature winding and a magnetizing effect or mmf that acts at right angles to the main field flux, is set up. The effect of magnetic field set up by the armature current on the distribution of the flux under main poles is known as the armature reaction. The effect of armature reaction is to weaken the field strength in the gap under the leading pole tips and strengthen under the trailing pole tips. The magnetic field of the machine is distorted and the physical neutral line is shifted in the direction of rotation. The displacement of the neutral line depends on the magnitude of the armature flux, which in turn depends on the armature or load current. As a result, the brushes have to be shifted in the direction of rotationto avoid sparking.
The armature mmf can be resolved into two components with one of the component in phase opposition to the main field mmf and is called the demagnetising component and the other component is at right angles to the main field mmf cand is called the cross-magnetising component. Demagnetising ampere -turns per pole are given as
And cross-magnetizing ampere- turns per pole are given as
Z is the total number of armature conductors, IC is the current flowing through the armature conductors, P is the number current flowing through
the armature conductors, P is the number of poles and q is forward lead in mechanical or angular degrees.
Armature reaction can be neutralized by (i) increasing the air gap length, (ii) reducing the cross section of the pole-pieces, (iii) providing interpoles
(wound in such a way so as to oppose the cross- field) between the main poles and (iv) by providing the machine with a compensating winding.
The commutation and its accompanying brushes are very important parts of a dc machine. Two essential actions take place here, namely, the passage of a current from (or to) a moving armature to (or from) external circuit and the commutation process. The commutation process involves the change from a generated alternating current to an externally available direct current. The transfer of current from the rotating
armature to the stationary brushes (and hence to the load) involves a continuously moving contact. Both these actions are to be carefully controlled by the use of suitable materials, good design and proper adjustments, for otherwise there would be serious arching and possible breakdown of the machine.
This reversal of current in the armature coil by means of brush and commutator bars, is called the commutation process and the period during which the coils remains short circuited is called the commutation period (represented by TC).
Good commutation means no sparking at the brushes and with commutator surface remaining unaffected during continuous operation of the dc machine. A machine is said to have poor commutation if there is sparking at the brushes and the commutator surface gets damaged during the machine operation.
The poor commutation may be caused by mechanical or electrical conditions, The mechanical conditions include uneven commutator surface, non - uniform brush pressure, vibration of brushes in the holders, etc. The electrical conditions include an increase in the voltage between the commutator segments, an increase in the current density at the trailing edge of the brush etc.
OPERATING CHARACTERISTICS AND APPLICATIONS OF DC GENERATORS
Equivalent circuit of a DC Generator Armature
Types of DC Generators
When the field coils are excited from a storage battery or from a separate dc source, the generator is called a separately-excited generator.
When the field coils are excited by the generator itself, it is called a self-excited generator. Self-excited generators are further subdivided into
Characteristics and Applications of DC Generators
There are three important characteristics of a dc generator:
1. Magnetic or Open-Circuit Characteristic. This characteristic is also known as no-load characteristic and represents the relation between the generated emf, Eg and field current, If at a constant speed. The shape of the curve is practically the same for all types of generator whether they are separately excited or selfexcited.
2. External Characteristic. This is a curve which represents the relation between the terminal voltage V and the load current IL.
3.Internal or Total Characteristic. This is a curve which represents the relation between the generated emf, Eg and armature current Ia.
Characteristics of Separately-Excited Generators.
1. Characteristics of a Seperately-Excited DC Generator
The seperately-excited generator has a decided advantage over the self-excited generator that it will operate in a stable condition with any field excitation. Thus a wide range of output voltage may be obtained
2. Characteristics of Series Wound Generators
In series wound dc generator, the armature winding, field winding and external load circuit, all are connected in series with each other, therefore, the same current flows through all parts of the circuit. From external characteristic it is observed that the terminal voltage first increase in load current, reaches the maximum value and finally decreases, with the increase in load current, reached the maximum value and finally decreases. If the load circuit resistance is reduced sufficiently, the terminal voltage may fall to zero, So if the series generator is operated on initial straight portion of the characteristic, it gives voltage approximately proportional to the current and if it is operated on drooping portion of characteristic, it give approximately constant current irrespective of load circuit resistance
Critical Load Resistance. If a straight line OB passing through the origin is drawn, then its point of intersection with the external characteristic give terminal voltage on Y-axis and current on X-axis when the resistance of the external load circuit is equal to the resistance represented by the slope of this line i.e. when load resistance = gradient of line OB. Thus if series of such lines are drawn, the points of intersection with the external characteristic will give terminal voltage and load current corresponding to their resistances in the external load circuit. If a line tangential to the external characteristic and passing through origin O, neglecting the initial ordinate due to residual magnetism, such as OC is drawn the resistance represented by the slope of this line is known as critical resistance, since it is the maximum resistance with which the generator will be able to excite.
Characteristics of Series Wound DC Generators
3. Characteristics of shunt Wound Generators
The curve plotted between the generated emf and shunt field current will be similar to that shown in fig. The generator excites itself due to residual magnetism and develops the voltage as illustrated in fig.
The maximum voltage which can be generated by a generator is given by the the point of intersection of field resistance line with the characteristic. The maximum resistance with which the generator can excite is called the critical field resistance
Building-up of voltage of a shunt Generator At No Load
The internal and external (or load) characteristics for a shunt wound generator ar illustrated in Fig. With the increase in load on the shunt generator, the terminal voltage falls. This is due to (i) increase in voltage drop across armature winding (ii) increased effect of armature reaction and (iii) decrease in field current owing to decrease in terminal voltage as a result of first two factors. In these generators when the load current reaches a certain value (much higher than full load value) the characteristic turns back, a shown in Fig.
Load Characteristics of Shunt Wound Generator
When the terminals are short circuited, there is no terminal voltage and thus the shunt winding becomes inactive but a small current is established due to a small voltage induced by the residual magnetism. This external characteristic meets the current axis at point B. The shunt generators are, therefore, self-protective against accidental short circuit.
Critical Load Resistance. If a line from the origin O is drawn tangential to internal characteristic curve, the gradient of this line will give the minimum value of the external load resistance for which the generator will excite on load. This resistance is known as critical load resistance. If the resistance of the external load circuit is less than this value, the generator will fail to build upits voltage.
4. Characteristics of Compound Wound Generators.
A generator with both series and shunt field windings is called a compound wound generator. When the series field is so connected that its ampere-turns act in the same direction as those of shunt field, the generator is said to be a cumulatively compounded generator.
If the series excitation is such that the terminal voltage on full load is the same as that on no load, the generator is said to be level or flat compounded. If the series excitation becomes more prominent than that of the shunt the terminal voltage rises with the increase of load and the generator is said to be over compounded. Similarly when shunt excitation plays the prominent part and full-load terminal voltage is less than the no-load voltage the generator is said to be undercompounded. The external characteristics of over, under and flat level compound generators are shown in Fig.
If the series field of the generator is connected so that its ampere-turns oppose those of shunt field, the generator is said to be differential compounded generator. In such generators the terminal voltage falls very rapidly with the increase in load current and therefore it is interesting to note that a short circuit can not cause any damage to the machine.
External Characteristics of DC Compoound
Conditions For Self excitation And Causes Of Failure To Build Up Voltage.
The condition required to be fulfilled before a series or/and shunt generator excites itself are that
Remedy: In case the generator is started up for the first time, it may be that no voltage will be built up either because the poles have no residual magnetism or the poles have retained some residual magnetism but the field winding connections are reversed so that the magnetism developed by the field winding on starts has destroyed the residual magnetism and the machine can not “Build up”. In both the cases, the field coils must be connected to a dc source (a storage battery) for a short while to magnetise the poles.
5. DC MOTORS
The equivalent circuit of a motor is shown in fig. The armature circuit is equivalent to a source of emf Eb in series with a resistance, Ra put across a dc supply mains of V volts.
V=Eb +Ia Ra
Ia is the armature current and Ra is the armature resistance.
Power Relationship in a motor.
Multiplying each tem of the voltage expression by Ia we get VIa = Eb I2a + Ia Ra
The Term VIa represents the power supplied to the motor armature and the tem I2a Ra represents the power lost in the armature and, therefore, the term, Eb Ia must represent the power developed by the motor armature causing rotation of the armature. The power developed Eb Ia is not all available at the shaft since some of its is used to overcome the mechanical or frictional losses of the motor. The power developed by the motor will be maximum when
Direction of Rotation of Dc Motors
The direction of rotation of motor can be reversed by reversing the current through either the armature winding or the field coils. if the current through both is reversed, the motor will continue to rotate in the same direction as before.
Types of DC Motors
Similar to dc generators, the dc Motors can also be classified a
Operating Characteristics of DC Motors.
The important characteristics of dc motors are:
|Types of Motor||Connection Diagram||Important Relations|
|1. Permanent Magnet Motor|
|2. Separetely Excited DC Motor||i) Ia = IL = I (say) |
li) Eb = V - IRa
iii) Pin = VI
iv) Pmech = Ebl = (V -IRa)I
|3. Series Worm DC Motor||i) Ia = IL = I (say) |
ii) Eb = V-I(Ra -Rae)
iii) Pin = VI
iv) Pmech = Ebl = VI - I2 (Ra + Rse)
|4. Shunt Wound DC Motor||i) IL = Ia + Ish |
ii) Ish = V/Rsh
ii) Eb = V - IaRa
iv) Pin = VIL
v) Power Developed
Pmech = EbIa = VIL - VIsh - I2aRa
|5. Cumulative Comp omit|
|Wound DC Motor|
3. Speed Torque Characteristic. This characteristic curve gives relation between speed, N and mechanical torque, T. This is also known as mechanical characteristic. This curve can be derived from the above two cases.
1. Characteristics of DC Series Motors
The flux first increased following a linear law with the increase in load current, becomes maximum at saturation point and finally becomes constant, as shown in Fig.
The speed- current characteristic is a rectangular hyperbola in shape, as shown. It is obvious from the speed-current characteristic that on no load the speed is dangerously high, which will result in heavy certrifugal force resulting in damage to the motor. So these motors are not suitable for the services
(i) where the load may be entirely removed or
(ii) for driving by means of belts because mishap to the belt would cause the motor to run on no load. These motors are suitable for gear drive.
The torque current characteristic up to saturation point is a parabola and a straight line after saturation point i.e. series motor develops very high starting torque. Hence series motors are used where large starting torque is required such as in hoists, electric railways, trolleys and electric vehicles.
From speed -current characteristic it is evident that speed falls as the load increases, so series motor is automatically relieved from heavy excessive load. Hence dc series motor is best suited for electric traction work.
The speed- torque characteristic is shown in fig. Which shows that with the increase in torque the speed decreases. Hence dc series motors are best suited for services where the motor is directly coupled to the load such as fans whose speed falls with the increase in torques.
2. Characteristics of DC Shunt Motors.
In case of a dc shunt motor, if applied voltage is kept constant, the flux also remains almost constant (neglecting the armature reaction effect). The speed from no load to full load also remains almost constant (neglecting armature voltage drop). For all practical purposes the shunt motor is taken as constant speed motor.
Shunt motors can be used for the loads which are totally and suddenly thrown off without resulting excessive speed. Shunt motors being constant speed motors are best suited for driving of line shafts, machine lathes, milling machines, conveyors, fans and for all purposes where constant speed is required. It is not suitable for use with flywheel or with fluctuating loads or for parallel operation due to its constant speed characteristic.
3. Characteristic of DC Compound Motors
(i) Cumulative Compound Wound Motor Characteristics. The Characteristics of the cumulative compound wound motor are combination of shunt and series characteristics. As the load is increased, the flux due to series field winding increases and caused the torque greater than it would have with shunt field winding alone for a given machine and for a given current .The increase in flux due to series field winding on account of increase in load causes the speed to fall more rapidly than it would have done in shunt motor.
The cumulative compound motor develops a high torque with increase of load. It also has a definite speed at no load, so does not run away when the load is removed.
Cumulatively compound wound motors are used in driving machines which are subject to sudden application of heavy loads, such as occur in rolling mills, shears or punches. This type of motor is used also where a large starting torque is required.
Another advantage of the cumulative compound wound motor for suddenly applied loads is that the motor automatically undergoes a substantial drop in speed when the load is applied Accordingly much of its stored kinetic energy becomes available for supplying a part of the increased load thus reducing the electrical load on the motor as well as the peaks on the power system. The available kinetic energy is frequently increased by the use of a flywheel, particularly with rolling mill motors
(ii) Differential Compound Wound Motor Characteristics.
In differential compound wound motors, the series field winding is connected in such a way that the series field opposes the shunt field while in cumulative compound wound motor series field helps the shunt field.
Since the flux decreases with the increase in load so the speed remains nearly constant as the load is increased and in some cases the speed will increase in load caused the torque to be less than that of a shunt motor. The characteristics are similar to those of a shunt motor.
Startin of DC Motor
When the motor is at rest, there is no back emf. therefore, if the motor is connected directly across the supply mains, a heavy current will flow through the armature conductors and will damage it since resistance of motor armature is very low. Hence for the protection of the motor against the flow of excessive starting current, it is necessary that a high resistance be connected in series with the armature of the motor gains speed and develops back emf and ultimately .When the motor attains its normal speed, the additional resistance from the armature circuit is totally disconnected.
There are two standard types of motor starters for shunt and compound motors namely three point and four point starter.
Speed Control of DC Motors
Speed control means intentional change of the drive speed to a value required for performing the specific process. The various schemes available for speed control can be deduced from the expression of the speed for a dc motor which is repeated here with one modification
Speed control methods are broadly classified as armature control methods and field control methods, Sometimes a combination of the two methods is employed. With armature control the speed decreases as the voltage applied to the armature terminals is reduced, whereas with field control the speed increase as the flux is reduced.
1. Speed Control of DC Shunt Motors. I. Field Control Methods.
Speed adjustment of dc shunt motors by field rheostat control may be obtained by any one of the methods
(i) field reheostat control
(ii) reluctance control and
(iii) field voltage control.
(i) Field Rheostat Control
In this method speed variation is accomplished by means of variable resistance inserted in series with the shunt field. Since controlling resistance has to carry only a small current so it is made up of slide wire type of resistor to provide continuously variable speed over the range. The power wasted in the controlling resistance is very small as the field current is very small. By this method the speed only above normal can be obtained. The speed is maximum at the minimum value of flux, which is governed by the demagnetizing effect of armature reaction on the field as at higher speeds the motor tends to be unstable and difficulties in commutation arise. The High speed limit is also restricted due to mechanical considerations as the centrifugal forces are set up at high speeds.
(ii) Field Voltage Control
The variable voltage supply for the field, which is separate from the main power supply to the armature, can be obtained by means of control generator or an adjustable electronic rectifier.
2. Armature Control Methods
Speed variation of shunt motors by armature control requires that voltage applied to the armature terminal shall be changed, without altering the field current. Speed adjustment of dc shunt motors by armature control may be obtained by any one of the methods
(i) armature resistance control.
(ii) Shunt armature control, and
(iii) armature terminal voltage control
(i) Armature Resistance Control
This method consists simply of a variable resistance connected in series with the armature as shown in fig. The speed at full load may be reduced to any desired value, depending on the amount of resistance. With this method, the voltage across the armature drops as the current passes through the series resistance, and the remaining voltage applied to the armature is lower than the line voltage. Thus, the speed is reduced in direct proportion to this voltage drop at the armature terminals.
Wide range of speed (below normal one) can be obtained by this method and at the same time motor will develop any desired torque over its operating range, since the torque depends only upon the armature current, flux remaining unchanged
The armature resistance control method is employed where speeds lower than rated one are required for a short period only and that also occasionally as in printing machines, cranes and hoists where the mortor is continually started and stopped. This method of speed control is also employed where the load drops of rapidly with decrease in speed, as in fans and blowers.
(ii) Shunt Armature Control.
In armature resistance control, speed also changes with every change in load. The double dependence makes is impossible to keep the speed sensibly constant on rapidly changing load. A more stable operation can be obtained by using a divertor across the armature in addition to series resistance as shown in Fig.
(iii) Armature Voltage Control.
This method of speed control requires a variable source of voltage separate from the source supplying the field current.
The adjustable voltage for the armature is obtained from an adjustable voltage generator or from an adjustable electronic rectifier. This method gives a large speed range with any desired number of speed points. It is essentially a constant- torque system because the output delivered by the motor decrease with a decrease in applied voltage and a corresponding decrease in speed.
(iv) Ward Leonard Control
This system consists of simply running the motor with a constant excitation and applying a variable voltage to its armature to give required speed. The variable voltage is obtained from a dc generator forming part of a constant speed motor-generator set. The supply voltage is varied by controlling the excitation to the generator. The advantages and disadvantages of this method of speed control are given below.
(i) Very fine speed control over the whole range from zero to normal speeds in both directions
(ii) rapid and instant reversal without excessively high armature current
(iii) starting without series armature resistance
(iv) stepless control
(v) uniform acceleration and
(vi) extremely good speed regulation.
(i) High initial cost- two extra machines are required and
(ii) low overall efficiency of the system, specially at light loads.
This system of speed control is used where speed adjustment and smooth acceleration is required as in colliary winders and elevators.
3. Speed Control of DC Series Motors
Speed control of dc series motors may be obtained through either armature of field control Armatureresistance control is the most common method employed. control of the armature voltage for the series motor is the same as control of the applied voltage to the complete motor.
The poor speed regulation that is inherent in this method has no significance for the control of dc series motors, since the speed characteristic of dc series motor is a rapidly drooping curve. The power loss in the control resistance for many application of a dc series motors is not too serious. The combination of a rheostat in series with the armature is used to give slow speeds at light loads. With the combination the no load speed can effectively be adjusted to any desired low value.
The speed of a dc series motor carrying a particular load may be increased in three ways. The field flux can be reduced by shunting a portion of the motor current around the series field, thus decreasing the excitation mmf and weakening the flux. This is illustrated in fig.This method is convenient as well as economical. Speed can be adjusted by varying the resistance of the field divertor.
Another method of increasing the speed by reducing the flux is to reduce the number of turns of the field winding through which the current flows. In this method of speed control of dc series motors a number of tappings from the field winding are brought outside, a illustrated in Fig. A number of series field turns can be short circuited according to the requirement. When all field turns are in the circuit. the motor runs at lowest speed and speed increases with cutting out some of the series field turns. This method is usually employed in electric traction.
It may be broadly classified as electromechanical braking and electrical braking. Electromechanical or friction brakes are operated thyristors. The drawback of electromechanical brakes is the sudden application of braking force and accompanying shock to the machine. Electric braking are of three types namely
(i) plugging or counter-current braking
(ii) rheostatic or dynamic braking and
(iii) regenerative braking.’
(i) Plugging as Applied to DC motors
In a dc motor a reversed torque is obtained by reversing either the current in the armature or in the field (not in both). It is usually convenient to reverse the connections to armature. In order to limit the armature current to a reasonable value. it is necessary to insert a resistor in the circuit, while reversing the armature connections.
This method is wasteful of energy although it is efficient for braking purpose. This method is commonly used in controlling
(i) rolling mills (ii) elevators (iii) Printing presses and (iv) machine tools.
(ii) Rheostatic Braking With DC Motors
In this method of electric braking of shunt motors, the armature of shunt motor is disconnected from the supply and is connected across a braking resistance (variable resistance), the field winding is, however, left connected across the supply undisturbed. The braking effect is controlled by varying the braking resistance R. In this method of braking with dc series motor, the motor is disconnected from the supply, the field connections are reversed and the motor is connected in series with the braking resistance R. The field connections are reversed to make sure that current through the field winding flows in the same direction as before in order to assist residual magnetism. The resistance inserted in the circuit must be less than the critical resistance other wise the machine will not be self exciting.
(iii) Regenerative Braking With DC Motors
Regenerative braking is an inherent characteristic of dc shunt motors and does not require changing of connections. Regenerative braking can be easily applied to dc shunt motors, particularly in cases where it is required to hold a load at a certain speed for instance lowering a hoist. In dc series motors regenerative braking is not possible without modifications because reversal of armature current would also mean reversal of field current and hence back emf Eb. This method, however, is employed with special arrangements in traction motors.
Determination of Losses and Efficiency of DC machines.
The losses and efficiency of a dc machine can be experimentally determined by the following methods:
(i) Direct method
By this method the efficiency and losses of only small machines can be determined. In this method full load is applied to the machine and output is directly measured.
(ii) Indirect Method
By this method, the efficiency of shunt and level compound dc machines can be determined. This method enables the determination of losses without actually loading the machine. The power is required to supply the losses only, so there is no difficulty in applying this method even to very large machines. Although the efficiency can be calculated with fair accuracy from the results obtained with this method, the disadvantage of this method is that the machine is run light during the test which gives no indication as to the temperature rise on load or to the commutating qualities of the machine.
The simplest of the indirect tests is Swinburne’s test. In this test machine under test is run on no load at rated voltage and rated speed and no- load losses are determined. The main advantages of this test are that it is convenient and economical method of testing of dc machines and the efficiency of the machine can be predetermined at no load, stray losses being known. This test cannot be preformed with dc series motors, since on no load seires motor will attain such a high speed that it will get damaged and secondly this test is only applicable to those machines in which flux and speed remain constant.
(iii) Regenerative Method
It requires two identical machines, one of them works as a motor and drives the other, which is mechanically coupled to it. The other machine works as a generator and feeds back power to the supply. Thus the total power drawn from the supply is only for supplying the internal losses of the two machines. Thus even very large machines may be tested as the power required is small. The machines can also be tested under full-load condition and for long duration to study thier performance regarding commutaion, temperature rise etc. conveniently. Hopkinson’s test (or back to back test) is a regenerative test.
(iv) Retardation (or runningdown) test
This test is applicable to dc shunt machines and is used for determination of stray losses. In this test, machine under test is speeded up slightly above the normal speed and supply to the armature is cut off, maintaining field excitation and kinetic energy of slowing down armature is evaluated, which is a measure of stray losses.
(v) Field Test
This test is Performed on dc series motors. In this test two series machines are machanically coupled together and their fields are connected in series so as to make iron losses of both the machines equal. One of the machine runs as a motor and drives the other machine running as a separately excited generator.