Principle of Electrical Machines. V K Mehta & Rohit Mehta About the Book; Table of Content Principles of Electrical Engineering for U.P and U.K, 2/e. Principles of Electrical Machines by V. K. Mehta, , available at Book Depository with free delivery worldwide. , V.K. Mehta, Rohit Mehta. All rights reserved. The general response to the first edition of the book was very encouraging. Authors feel that sion, Circle Diagrams and Special-purpose Electric Machines have been added. Secondly.
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Chapter (1) D.C. Generators Introduction Although a far greater percentage of the electrical machines in service are a.c. machines, the d.c. machines are of. principle of electrical machines vk mehta download, principles of electrical _SX_BO1,,,_ Electrical Installation, Free Pdf Books, Authors, Jun. Principle of Electrical Machines by VK Mehta PDF. Books Type PDF Electric Power Systems [PDF, ePub, Mobi] by Alexandra von Meier Books Online for Read.
Skip to main content. Log In Sign Up. Principles of Electrical Machines by V. Suresh Muthusamy. Chapter 1 D. Generators Introduction Although a far greater percentage of the electrical machines in service are a.
The back e. Consider a shunt wound motor shown in Fig. When d. Therefore, driving torque acts on the armature which begins to rotate. As the armature rotates, back e.
Eb is induced which opposes the applied voltage V. The applied voltage V has to Fig. The electric work done in overcoming and causing the current to flow against Eb is converted into mechanical energy developed in the armature. It follows, therefore, that energy conversion in a d. If the speed of the motor is high, then back e.
The presence of back e. Therefore, the armature current Ia is small and the back e. Therefore, the speed at which the armature conductors move through the field is reduced and hence the back e.
Eb falls. The decreased back e. Thus, the driving torque increases as the motor slows down. The motor will stop slowing down when the armature current is just sufficient to produce the increased torque required by the load. As the armature speed increases, the back e.
Eb also increases and causes the armature current Ia to decrease. The motor will stop accelerating when the armature current is just sufficient to produce the reduced torque required by the load. It follows, therefore, that back e. Motor Let in a d. Eb acts in opposition to the Fig. Limitations In practice, we never aim at achieving maximum power due to the following reasons: Motors Like generators, there are three types of d. The current through the shunt field winding is not the same as the armature current.
Shunt field windings are designed to produce the necessary m. Therefore, shunt field current is relatively small compared with the armature current. Therefore, series field winding carries the armature current.
Since the current passing through a series field winding is the same as the armature current, series field windings must be designed with much fewer turns than shunt field windings for the same m. Therefore, a series field winding has a relatively small number of turns of thick wire and, therefore, will possess a low resistance.
There are two types of compound motor connections like generators. When the shunt field winding is directly connected across the armature terminals [See Fig. Therefore, shunt field in compound machines is the basic dominant factor in the production of the magnetic field in the machine.
Motor Torque is the turning moment of a force about an axis and is measured by the product of force F and radius r at right angle to which the force acts i. Therefore, each conductor exerts a torque, tending to rotate the armature. The sum of the torques due to all armature conductors is known as gross or armature torque Ta. Let in a d. It is represented by Tsh. The total or gross torque Ta developed in the armature of a motor is not available Fig.
Therefore, shaft torque Tsh is somewhat less than the armature torque Ta. If the speed of the motor is N r. The horse power developed by the shaft torque is known as brake horsepower B. If the motor is running at N r. Motor For any motor, the torque and speed are very important factors. When the torque increases, the speed of a motor increases and vice-versa. We have seen that for a d. This is not possible because the increase in motor speed must be the result of increased torque. Indeed, it is so in this case.
When the flux decreases slightly, the armature current increases to a large value. As a result, in spite of the weakened field, the torque is momentarily increased to a high value and will exceed considerably the value corresponding to the load. The surplus torque available causes the motor to accelerate and back e. Steady conditions of speed will ultimately be achieved when back e. Illustration Let us illustrate the above point with a numerical example.
Suppose a V shunt motor is running at r. The armature resistance is 0. This will result in the production of high value of torque. However, soon the steady conditions will prevail.
This will depend on the system inertia; the more rapidly the motor can alter the speed, the sooner the e. Motors As in a d. This is expected because when current flows through the armature conductors of a d. For a motor with the same polarity and direction of rotation as is for generator, the direction of armature reaction field is reversed.
Eg whereas in a motor, the armature current flows against the induced e. Therefore, it should be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in the opposite direction to that of the generator. Hence instead of the main flux being distorted in the direction of rotation as in a generator, it is distorted opposite to the direction of rotation. We can conclude that: Armature reaction in a d.
However, in case of a d. With no commutating poles used, the brushes are given a forward lead in a d. Since commutating poles windings carry the armature current, then, when a machine changes from generator to motor with consequent reversal of current , the polarities of commutating poles must be of opposite sign. Therefore, in a d. This is the opposite of the corresponding relation in a d. Motors Since the armature of a motor is the same as that of a generator, the current from the supply line must divide and pass through the paths of the armature windings.
In order to produce unidirectional force or torque on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times. In this case, the current flows away from the observer in the conductors under the N-pole and towards the observer in the conductors under the S-pole.
Therefore, when a conductor moves from the influence of N-pole to that of S-pole, the direction of current in the conductor must be reversed. This is termed as commutation.
The function of the commutator and the brush gear in a d. For good commutation, the following points may be noted: For a d.
When the operation of a d. Since commutating poles winding carries armature current, the polarity of commutating pole reverses automatically to the correct polarity. These are [See Fig. The following points may be noted: Since d. Motor Like a d. Motor Characteristics There are three principal types of d. Both shunt and series types have only one field winding wound on the core of each pole of the motor. The compound type has two separate field windings wound on the core of each pole.
The performance of a d. It is also known as electrical characteristic of the motor. It is very important characteristic as it is often the deciding factor in the selection of the motor for a particular application. It is also known as mechanical characteristic. The field current Ish is constant since the field winding is directly connected to the supply voltage V which is assumed to be constant. Hence, the flux in a shunt motor is approximately constant.
We know that in a d. The shaft torque Tsh is less than Ta and is shown by a dotted line. It is clear from the curve that a very large current is required to start a heavy load. Therefore, a shunt motor should not be started on heavy load. The speed N of a.
Eb in a shunt motor are almost constant under normal conditions. Therefore, speed of a shunt motor will remain constant as the armature current varies dotted line AB in Fig. The curve is obtained by plotting the values of N and Ta for various armature currents See Fig. It may be seen that speed falls somewhat as the load torque increases. Hence, it is essentially a constant-speed motor. Note that current passing through the field winding is the same as that in the armature.
If the mechanical load on the motor increases, the armature current also increases. Hence, the flux in a series motor increases with the increase in armature current and vice-versa. We know that: If Ia is doubled, Ta is almost quadrupled. However, after magnetic saturation, torque is directly proportional to the armature current. It may be seen that in the initial portion of the curve i. This means that starting torque of a d.
After saturation, the flux becomes constant and so does the speed. It is clear that series motor develops high torque at low speed and vice-versa.
It is because an increase in torque requires an increase in armature current, which is also the field current. Reverse happens should the torque be low. Thus if the load decreases, its speed is automatically raised and vice-versa. This is dangerous for the machine which may be destroyed due to centrifugal forces set up in the rotating parts.
Therefore, a series motor should never be started on no-load. However, to start a series motor, mechanical load is first put and then the motor is started. The minimum load on a d. If the speed becomes dangerously high, then motor must be disconnected from the supply. The shunt field is always stronger than the series field. Compound motors are of two types: Differential compound motors are rarely used due to their poor torque characteristics at heavy loads.
Each pole carries a series as well as shunt field winding; the series field aiding the shunt field. As the load increases, the series field increases but shunt field strength remains constant. It may be noted that torque of a cumulative-compound motor is greater than that of shunt motor for a given armature current due to series field [See Fig. As explained above, as the lead increases, the flux per pole also increases.
It may be noted that as the load is added, the increased amount of flux causes the speed to decrease more than does the speed of a shunt motor. Thus the speed regulation of a cumulative compound motor is poorer than that of a shunt motor. Due to shunt field, the motor has a definite no load speed and can be operated safely at no-load. For a given armature current, the torque of a cumulative compound motor is more than that of a shunt motor but less than that of a series motor.
Conclusions A cumulative compound motor has characteristics intermediate between series and shunt motors. However, the starting torque of a cumulative compound motor lies between series and shunt motors See Fig. However, a series motor has dangerously high speed at no-load. Motors 1. Shunt motors The characteristics of a shunt motor reveal that it is an approximately constant speed motor. It is, therefore, used i where the speed is required to remain almost constant from no-load to full-load ii where the load has 10 be driven at a number of speeds and any one of which is required to remain nearly constant Industrial use: Lathes, drills, boring mills, shapers, spinning and weaving machines etc.
Series motors It is a variable speed motor i. 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 i where large starting torque is required e. Electric traction, cranes, elevators, air compressors, vacuum cleaners, hair drier, sewing machines etc.
Compound 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. Industrial use: Presses, shears, reciprocating machines etc. Motors Several troubles may arise in a d. Failure to start This may be due to i ground fault ii open or short-circuit fault iii wrong connections iv too low supply voltage v frozen bearing or vi excessive load.
Sparking at brushes This may be due to i troubles in brushes ii troubles in commutator iii troubles in armature or iv excessive load. An open armature coil will cause sparking each time the open coil passes the brush. The location of this open coil is noticeable by a burnt line between segments connecting the coil.
Vibrations and pounding noises These maybe due to i worn bearings ii loose parts iii rotating parts hitting stationary parts iv armature unbalanced v misalignment of machine vi loose coupling etc. Overheating The overheating of motor may be due to i overloads ii sparking at the brushes iii short-circuited armature or field coils iv too frequent starts or reversals v poor ventilation vi incorrect voltage. Chapter 5 Speed Control of D. Motors Introduction Although a far greater percentage of electric motors in service are a.
The principal advantage of a d. Such a fine speed control is generally not possible with a. In fact, fine speed control is one of the reasons for the strong competitive position of d.
In this chapter, we shall discuss the various methods of-speed control of d. Motors The speed of a d. This is known as flux control method. This is known as armature control method. This is known as voltage control method. Shunt Motors The speed of a shunt motor can be changed by i flux control method ii armature control method iii voltage control method. The first method i. In this method, a variable resistance known as shunt field rheostat is placed in series with shunt field winding as shown in Fig.
Therefore, we can only raise the speed of the motor above the normal speed See Fig. Generally, this method permits to increase the speed in the ratio 3: Wider speed ranges tend to produce instability and poor commutation. Advantages i This is an easy and convenient method. Disadvantages i Only speeds higher than the normal speed can be obtained since the total field circuit resistance cannot be reduced below Rsh—the shunt field winding resistance.
It is because if the flux is too much weakened, commutation becomes poorer. The field of a shunt motor in operation should never be opened because its speed will increase to an extremely high value.
Armature control method This method is based on the fact that by varying the voltage available across the armature, the back e.
Eb is decreased. Hence, this method can only provide speeds below the normal speed See Fig. Disadvantages i A large amount of power is wasted in the controller resistance since it carries full armature current Ia.
Due to above disadvantages, this method is seldom used to control tie speed of shunt motors. The armature control method is a very common method for the speed control of d.
The disadvantage of poor speed regulation is not important in a series motor which is used only where varying speed service is required. Voltage control method In this method, the voltage source supplying the field current is different from that which supplies the armature. This method avoids the disadvantages of poor speed regulation and low efficiency as in armature control method.
Therefore, this method of speed control is employed for large size motors where efficiency is of great importance. In this method, the shunt field of the motor is connected permanently across a-fixed voltage source. The armature can be connected across several different voltages through a suitable switchgear.
In this way, voltage applied across the armature can be changed. The speed will be approximately proportional to the voltage applied across the armature. Intermediate speeds can be obtained by means of a shunt field regulator.
In this method, the adjustable voltage for the armature is obtained from an adjustable-voltage generator while the field circuit is supplied from a separate source. The armature of the shunt motor M whose speed is to be controlled is connected directly to a d. The field of the shunt motor is supplied from a constant-voltage exciter E. The field of the generator G is also supplied from the exciter E.
The voltage of the generator G can be varied by means of its field regulator. By reversing the field current of generator G by controller FC, the voltage applied to the motor may be reversed. Sometimes, a field regulator is included in the field circuit of shunt motor M for additional speed adjustment. With this method, the motor may be operated at any speed upto its maximum speed.
When the generator voltage is reduced below the back e. The disadvantage of the method is that a special motor-generator set is required for each motor and the losses in this set are high if the motor is operating under light loads for long periods.
Series Motors The speed control of d. The latter method is mostly used.
Flux control method In this method, the flux produced by the series motor is varied and hence the speed. The variation of flux can be achieved in the following ways: In this method, a variable resistance called field diverter is connected in parallel with series field winding as shown in Fig.
The lowest speed obtainable is that corresponding to Fig. Obviously, the lowest speed obtainable is the normal speed of the motor. Consequently, this method can only provide speeds above the normal speed. The series field diverter method is often employed in traction work. In order to obtain speeds below the normal speed, a variable resistance called armature diverter is connected in parallel with the armature as shown in Fig. The diverter shunts some of the line current, thus reducing the armature current.
By adjusting the armature diverter, any speed lower than the normal speed can be obtained. In this method, the flux is reduced and hence speed is increased by decreasing the number of turns of the series field winding as shown in Fig. With full turns of the field winding, the motor runs at normal speed and as the field turns are cut out, speeds higher than normal speed are achieved. This method is usually employed in the case of fan motors. By regrouping the field coils as shown in Fig.
Armature-resistance control In this method, a variable resistance is directly connected in series with the supply to the complete motor as shown in Fig. This reduces the voltage available across the armature and hence the speed falls. By changing the value of variable resistance, any speed below the normal speed can be obtained. This Fig. Although this method has poor speed regulation, this has no significance for series motors because they are used in varying speed applications.
In this system which is widely used in traction system, two or more similar d. Therefore, the speed will be low. When the motors are connected in parallel, each motor armature receives the normal voltage and the speed is high [See Fig. Thus we can obtain two speeds. Note that for the same load on the pair of motors, the system would run approximately four times the speed when the machines are in parallel as when they are in series.
Series-parallel and resistance control In electric traction, series-parallel method is usually combined with resistance method of control. In the simplest case, two d. The motors are started up in series with each other and starting resistance is cut out step by step to increase the speed. When all the resistance is cut out See Fig. The speed is then about one-half of what it would be if the full line voltage were applied to each motor. The starting resistance is again cut out step by step until full speed is attained.
Then field control is introduced. This may be necessary in case of emergency or to save time if the motor is being used for frequently repeated operations. The motor and its load may be brought to rest by using either i mechanical friction braking or ii electric braking. In mechanical braking, the motor is stopped due to the friction between the moving parts of the motor and the brake shoe i. Mechanical braking has several disadvantages including non-smooth stop and greater stopping time.
In electric braking, the kinetic energy of the moving parts i. For d. However, the main advantage of using electric braking is that it reduces the wear and tear of mechanical brakes and cuts down the stopping time considerably due to high braking retardation.
However, the field winding is left connected to the supply. The armature, while slowing down, rotates in a strong magnetic field and, therefore, operates as a generator, sending a large current through resistance R. This causes the energy possessed by the rotating armature to be dissipated quickly as heat in the resistance. As a result, the motor is brought to standstill quickly. The braking torque can be controlled by varying the resistance R.
If the value of R is decreased as the motor speed decreases, the braking torque may be maintained at a high value. At a low value of speed, the braking torque becomes small and the final stopping of the motor is due to friction. This type of braking is used extensively in connection with the control of elevators and hoists and in other applications in which motors must be started, stopped and reversed frequently. When the motor comes to rest, the supply must be cut off otherwise the motor will start rotating in the opposite direction.
Note that armature connections are reversed while the connections of the field winding are kept the same. As a result the current in the armature reverses.
During the normal running of the motor [See Fig. Eb opposes the applied voltage V. However, when armature connections are reversed, back e.
Eb and V act in the same direction around the circuit. In order 10 limit the current to safe value, a variable resistance R is inserted in the circuit at the time of changing armature connections. We now investigate how braking torque depends upon the speed of the motor.
As a result, the kinetic energy of the motor is converted into electrical energy and returned to the supply. As a result, induced e.
E exceeds the supply voltage V and the machine feeds energy into the supply. Thus braking torque is provided upto the speed at which induced e. As the machine slows down, it is not possible to maintain induced e. Therefore, this method is possible only for a limited range of speed. As a result, the induced e. E becomes greater than the supply voltage V [See Fig.
The direction of armature current I, therefore, reverses but the direction of shunt field current If remains unaltered. Hence the torque is reversed and the speed falls until E becomes less than V. Speed control cannot be obtained through adjustment of the series field since such adjustment would radically change the performance characteristics of the motor. Motor Starter At starting, when the motor is stationary, there is no back e.
As an example, 5 H. This high starting current may result in: 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 inserted in series with the armature circuit. This resistance is gradually reduced as the motor gains speed 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. Motor Starters The stalling operation of a d.
It is very important and desirable to provide the starter with protective devices to enable the starter arm to return to OFF position i when the supply fails, thus preventing the armature being directly across the mains when this voltage is restored. For this purpose, we use no-volt release coil. For this purpose, we use overload release coil.
There are two principal types of d. As we shall see, the two types of starters differ only in the manner in which the no-volt release coil is connected. Schematic diagram Fig. It is so called because it has three terminals L, Z and A. The starter consists of starting resistance divided into several sections and connected in series with the armature.
The tapping points of the starting resistance are brought out to a number of studs. The three terminals L, Z and A of the starter are connected respectively to the positive line terminal, shunt field terminal and armature terminal. The other terminals of the armature and shunt field windings are connected to the negative terminal of the supply. The no-volt release coil is connected in the shunt field circuit.
One end of the handle is connected to the terminal L through the over-load release coil. The other end of the handle moves against a spiral spring and makes contact with each stud during starting operation, cutting out more and more starting resistance as it passes over each stud in clockwise direction. Operation i To start with, the d. As soon as it comes in contact with the first stud, the shunt field winding is directly connected across the supply, while the whole starting resistance is inserted in series with the armature circuit.
If no-volt release coil were not used, then in case of failure of supply, the handle would remain on the final stud. If then supply is restored, the motor will be directly connected across the supply, resulting in an excessive armature current. This current will increase the ampere-turns of the over-load release coil and pull the armature C, thus short-circuiting the no- volt release coil.
The no-volt coil is demagnetized and the handle is pulled to the OFF position by the spring. Thus, the motor is automatically disconnected from the supply.
While exercising speed control through field regulator, the field current may be weakened to such an extent that the no-volt release coil may not be able to keep the starter arm in the ON position. This may disconnect the motor from the supply when it is not desired. This drawback is overcome in the four point starter. Now the no-volt release coil circuit is independent of the shunt field circuit. Therefore, proper speed control can be exercised without affecting the operation of no- volt release coil.
Note that the only difference between a three-point starter and a four-point starter is the manner in which no-volt release coil is connected. However, the working of the two starters is the same. It may be noted that the three- point starter also provides protection against an open- field circuit. This protection is not provided by the four-point Fig.
The upper limit is that value established as the maximum permissible for the motor; it is generally 1. The lower limit is the value set as a minimum for starting operation; it may be equal to full-load current of the motor or some predetermined value. When the current has fallen to I, arm A is moved over to stud 2, cutting out sufficient resistance to allow the current to rise to Im again. This operation is repeated until the arm A is on stud 4 and the whole of the starting resistance is cut out of the armature circuit.
Shunt Motor Fig. As the armature accelerates, the induced e. Let the value of back e. When the current has fallen to the predetermined value I, the starter arm is moved over to stud 3. Let Eb2 be the value of back e. Chapter 6 Testing of D. Machines Introduction There are several tests that are conducted upon a d. One important test is performed to measure the efficiency of a d.
The efficiency of a d. The smaller the losses, the greater is the efficiency of the machine and vice-versa. The consideration of losses in a d. First, losses determine the efficiency of the machine and appreciably influence its operating cost. Secondly, losses determine the heating of the machine and hence the power output that may be obtained without undue deterioration of the insulation.
In this chapter, we shall focus our attention on the various methods for the determination of the efficiency of a d. Machine The power that a d. Therefore, the efficiency of a d. Then we can use Eq. This method suffers from three main drawbacks. First, this method requires the application of load on the machine. Thirdly, even 'fit is possible to provide such loads, large power will be dissipated, making it an expensive method.
The most common method of measuring the efficiency of a d. The course aims at exposing the students to the inter-disciplinary nature in which scientific research is done in many upcoming fields, and comes at a time when the importance of science education at the undergraduate level is emphasised and several incentives are provided by the Government to promote the same.
Apart from an intensive training in sciences, courses in Engineering and Humanities are prescribed to empower the student with technical skills required for a scientist, to appreciate the social context as well as constraints of doing science. Students enrolled in the program take courses in Biology, Chemistry, Engineering, Humanities, Mathematics, and Physics for the first three semesters which are common and compulsory to all.
In the next 4 semesters, they choose a major discipline of study and take a handful of other science courses and a stipulated number of engineering and humanities courses. The last semester is devoted to a final project. Besides, all the students, either KVPY or DST-Inspire scholars, spend a couple of months in various research institutes across the country exploring a topic or a research problem of their interest.
Research —the admission is through the GATE. Mgt , which has a ceiling strength of only 25 seats across India, the admission is through Common Admission Test. IISc does not conduct any entrance test for providing admissions. It consists of a cylindrical metal ring cut into two halves or segments C1 and C2 respectively separated by a thin sheet of mica. The commutator is mounted on but insulated from the rotor shaft.
Two stationary carbon brushes rest on the commutator and lead current to the external load. Also note the direction of current through load. It is from Q to P. Note that commutator has reversed the coil connections to the load i. Also note the direction of current through the load.
It is again from Q to P. The reader may note that e. It is by the use of commutator that we convert the generated alternating e. The purpose of brushes is simply to lead current from the rotating loop or winding to the external stationary load. This is not a steady direct voltage but has a pulsating character. It is because the voltage appearing across the brushes varies from zero to maximum value and back to zero twice for each revolution of the loop. A pulsating direct voltage such as is produced by a single loop is not suitable for many commercial uses.
What we require is the steady direct voltage. This can be achieved by using a large number of coils connected in series.
The resulting arrangement is known as armature winding. Generator The d. In fact, when the machine is being assembled, the workmen usually do not know whether it is a d. Any d. All d. It consists of a number of salient poles of course, even number bolted to the inside of circular frame generally called yoke.
The 5. Field coils are mounted on the poles and carry the d. The field coils are connected in such a way that adjacent poles have opposite polarity. The m. Practical d. Since armature and field systems are composed of materials that have high permeability, most of the m.
By reducing the length of air gap, we can reduce the size of field coils i. It consists of slotted soft-iron laminations about 0. The laminations See Fig. The purpose of laminating the core is to reduce the eddy current loss. This is known as armature winding. The armature conductors are connected in series-parallel; the conductors being connected in series so as to increase the 6.
The armature winding of a d. The commutator is made of copper segments insulated from each other by mica sheets and mounted on the shaft of the machine See Fig 1. The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding. Depending upon the manner in which the armature conductors are connected to the commutator segments, there are two types of armature winding in a d. Great care is taken in building the commutator because any eccentricity will cause the brushes to bounce, producing unacceptable sparking.
The sparks may bum the brushes and overheat and carbonise the commutator. The brushes are made of carbon and rest on the commutator. The brush pressure is adjusted by means of adjustable springs See Fig. If the brush pressure is very large, the friction produces heating of the commutator and the brushes. On the other hand, if it is too weak, the imperfect contact with the commutator may produce sparking. For example, a 4- pole machine has 4 brushes. As we go round the commutator, the successive brushes have positive and negative polarities.
Brushes having the same polarity 7. Armature Windings i A d. Each conductor lies at right angles to the magnetic flux and to the direction of its movement Therefore, the induced e. The basic component of all types of armature windings is the armature coil.
It has two conductors or coil sides connected at the back of the armature. Consequently the e. If the e. For the same flux and speed, the e. One coil side of a coil lies at the top of a slot and the other coil side lies at the bottom of some other slot. The coil ends will then lie side by side. In two-layer winding, it is desirable to number the coil sides rather than the slots. The coil sides are numbered as indicated in Fig. The coil sides at the top of slots are given odd numbers and those at the bottom are given even numbers.
The coil sides are numbered in order round the armature. As discussed above, each coil has one side at the top of a slot and the other side at the bottom of another slot; the coil sides are nearly a pole pitch apart. In connecting the coils, it is ensured that top coil side is joined to the bottom coil side and vice-versa. This is illustrated in Fig. The coil side 1 at the top of a slot is joined to coil side 10 at the bottom of another slot about a pole pitch apart. The coil side 12 at the bottom of a slot is joined to coil side 3 at the top of another slot.
How coils are connected at the back of the armature and at the front commutator end will be discussed in later sections. It may be noted that as far as connecting the coils is concerned, the number of turns per coil is immaterial. For simplicity, then, the coils in winding diagrams will be represented as having only one turn i. It may be noted here that in the conventional way of representing a developed armature winding, full lines represent top coil sides i. In such a winding, if one starts at some point in the winding and traces through the winding, one will come back to the starting point without passing through any external connection.
It is denoted by YC. In Fig. Therefore, the number of commutator segments spanned by the coil is 1 i. The commutator pitch of a winding is always a whole number. Since each coil has two ends and as two coil connections are joined at each commutator segment, Fig. Therefore, number of commutator segments is also Note that commutator pitch is the most important factor in determining the type of d. Thus if the coil span is 9 slots, it means one side of the coil is in slot 1 and the other side in slot In this case, the e.
Therefore, e. Therefore, coil span should always be one pole pitch unless there is a good reason for making it shorter. Fractional pitched coil. If the coil span or coil pitch is less than the pole pitch, then it is called fractional pitched coil See Fig.
In this case, the phase difference between the e. Fractional pitch winding requires less copper but if the pitch is too small, an appreciable reduction in the generated e.
Armature Windings The different armature coils in a d. Two basic methods of making these end connections are: Simplex lap winding 2. Simplex wave winding 1. Simplex lap winding. Thus the ends of any coil are brought out to adjacent commutator segments and the result of this method of connection is that all the coils of the armature. Consequently, closed circuit winding results.
Only two coils are shown for simplicity. The name lap comes from the way in which successive coils overlap the preceding one. The result is that the coils under consecutive pole pairs will be joined together in series thereby adding together their e. After passing once around the armature, the winding falls in a slot to the left or Continuing in this way, all the conductors will be connected in a single closed winding.
This winding is called wave winding from the appearance wavy of the end connections. It is denoted by YR. Therefore, the resultant pitch is the algebraic sum of the back and front pitches. A retrogressive winding is seldom used because it requires more copper.
Armature Windings In the design of d. This will result in increased e. This will permit all end connections back as well as front connection between a conductor at the top of a slot and one at the bottom of a slot.
Developed diagram Developed diagram is obtained by imagining the cylindrical surface of the armature to be cut by an axial plane and then flattened out. Note that full lines represent the top coil sides or conductors and dotted lines represent the bottom coil sides or conductors. The winding goes from commutator segment 1 by conductor 1 across the back to conductor 12 and at the front to commutator segment 2, thus forming a coil.
Then from commutator segment 2, through conductors 3 and 14 back to commutator segment 3 and so on till the winding returns to commutator segment 1 after using all the 40 conductors. Position and number of brushes We now turn to find the position and the number of brushes required.
The brushes, like field poles, remain fixed in space as the commutator and winding revolve. It is very important that brushes are in correct position relative to the field poles. By right-hand rule, the direction of e. In order to find the position of brushes, the ring diagram shown in Fig. A positive brush will be placed on that commutator segment where the currents in the coils are meeting to flow out of the segment.
A negative brush will be placed on that commutator segment where the currents in the coils are meeting to flow in. Referring to Fig. Therefore, we arrive at a very important conclusion that in a simplex lap winding, the number of brushes is equal to the number of poles.
If the brushes of the same polarity are connected together, then all the armature conductors are connected in four parallel paths; each path containing an equal number of conductors in series. Since segments 6 and 16 are connected together through positive brushes and segments 11 and 1 are connected together through negative brushes, there are four parallel paths, each containing 10 conductors in series. Therefore, in a simplex lap winding, the number of parallel paths is equal to the number of poles.
Therefore, the armature winding has 4 parallel paths, each consisting of 10 conductors in series. As a result, the coil voltages add. The simplex wave winding must not close after it passes once around the armature but it must connect to a commutator segment adjacent to the first and the next coil must be adjacent to the first as indicated in Fig. This is repeated each time around until connections are made to all the commutator segments and all the slots are occupied after which the winding automatically returns to the starting point.
If, after passing once around the armature, the winding connects to a segment to the left of the starting point, the winding is retrogressive [See Fig. If it connects to a segment to the right of the starting point, it is progressive [See Fig. This type of winding is called wave winding because it passes around the armature in a wave-like form. The YB must be an odd integer so that a top conductor and a bottom conductor will be joined. When one tour of armature has been completed, the winding should connect to the next top conductor progressive or to the preceding top conductor retrogressive.
In either case, the difference will be of 2 conductors or one slot. In Eq. If they differ by 2, they are one more and one less than YA. The two conductors which lie in the same slot are drawn nearer to each other than to those in the other slots.
This means that the number of commutator segments spanned between the start end and finish end of any coil is 11 segments. Position and number of brushes We now turn to find the position and the number of brushes.
By right hand rule, the direction of e. It is clear that only two brushes—one positive and one negative—are required though two positive and two negative brushes can also be used. We find that there are two parallel paths between the positive brush and the negative brush. Thus is illustrated in Fig. Therefore, we arrive at a very important conclusion that in a simplex wave winding, the number of parallel paths is two irrespective of the number of poles.
Note that the first parallel path has 11 coils or 22 conductors while the second parallel path has 10 coils or 20 conductors. This fact is not important as it may appear at first glance.
The coils m the smaller group should supply less current to the external circuit. But the identity of the coils in either parallel path is rapidly changing from moment to moment. Therefore, the average value of current through any particular coil is the same.
If the total number of armature conductors is Z and P is the number of poles, then, Sometimes the standard armature punchings available in the market have slots that do not satisfy the above requirement so that more coils usually only one more are provided than can be utilized.
These extra coils are called dummy or dead coils. The dummy coil is inserted into the slots in the same way as the others to make the armature dynamically balanced but it is not a part of the armature winding. Let us illustrate the use of dummy coils with a numerical example. Suppose the number of slots is 22 and each slot contains 2 conductors.
The number of poles is 4. The extra coil or dummy coil is put in the slot. One end of this coil is taped and the other end connected to the unused commutator segment segment 22 for the sake of appearance. Since only 21 segments are required, the two 21 and 22 segments are connected together and considered as one. On the other hand, the lap winding carries more current than a wave winding because it has more parallel paths.
In small machines, the current-carrying capacity of the armature conductors is not critical and in order to achieve suitable voltages, wave windings are used.
On the other hand, in large machines suitable voltages are easily obtained because of the availability of large number of armature conductors and the current carrying capacity is more critical. Hence in large machines, lap windings are used. In general, a high-current armature is lap-wound to provide a large number of parallel paths and a low-current armature is wave-wound to provide a small number of parallel paths.
A simplex wave-wound armature has two parallel paths irrespective of the number of poles. In case of a pole machine, using simplex windings, the designer is restricted to either two parallel circuits wave or ten parallel circuits lap. Sometimes it is desirable to increase the number of parallel paths. For this purpose, multiplex windings are used. The sole purpose of multiplex windings is to increase the number of parallel paths enabling the armature to carry a large total current.
The degree of multiplicity or plex determines the number of parallel paths in the following manner: If an armature is changed from simplex lap to duplex lap without making any other change, the number of parallel paths is doubled and each path has half as many coils.
The armature will then supply twice as much current at half the voltage. Thus a duplex wave winding has 4 parallel paths, triplex wave winding has 6 parallel paths and so on. The commutator and brushes cause the alternating e. In lap as well as wave winding, it will be observed that currents Further, the direction of current in coil reverses as it passes the brush. Thus when the coil approaches the contact with the brush, the current through the coil is in one direction; when the coil leaves the contact with the brush, the current has been reversed.
This reversal of current in the coil as the coil passes a brush is called commutation and fakes place while the coil is short-circuited by the brush.
These changes occur in every coil in turn. If, at the instant when the brush breaks contact with the commutator segment connected to the coil undergoing commutation, the current in the coil has not been reversed, the result will be sparking between the commutator segments and the brush.
The criterion of good commutation is that it should be sparkless. In order to have sparkless commutation, the brushes on the commutator should be placed at points known as neutral point where no voltage exists between adjacent segments. The conductors connected to these segments lie between the poles in position of zero magnetic flux which is termed as magnetic neutral axis M.
Equation of a D. Generator We shall now derive an expression for the e. Generators The magnetic field in a d. Generators are generally classified according to their methods of field excitation. On this basis, d. Generators A d. The greater the speed and field current, greater is the generated e. It may be noted that separately excited d.
The d. There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely; i Series generator; ii Shunt generator; iii Compound generator i Series generator In a series wound generator, the field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load. Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance.
Series generators are rarely used except for special purposes e. The shunt field winding has many turns of fine wire having high resistance. Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load.
A compound wound generator may be: Obviously, its value will depend upon the amount of current flowing and the value of contact resistance.
This drop is generally small. Machine The losses in a d.