Principles of operation of synchronous machines klempmer pdf download

The latter is rarely used in practice. Therefore, we shall discuss the characteristics of cumulatively- compounded generator. It may be noted that external characteristics of long and short shunt compound generators are almost identical.

External characteristic Fig. The series excitation aids the shunt excitation. In such a case, as the load current increases, the series field m. The increase in generated voltage is greater than the IaRa drop so that instead of decreasing, the terminal voltage increases as shown by curve A in Fig.

The series winding of such a machine has lesser number of turns than the one in over-compounded machine and, therefore, does not increase the flux as much for a given load current. Consequently, the full-load voltage is nearly equal to the no-load voltage as indicated by curve B in Fig 3. Such a machine is called under-compounded generator. Generators In a d. This is due to the following reasons: i Continuity of service If a single large generator is used in the power plant, then in case of its breakdown, the whole plant will be shut down.

However, if power is supplied from a number of small units operating in parallel, then in case of failure of one unit, the continuity of supply can be maintained by other healthy units. Electric power costs less per kWh when the generator producing it is efficiently loaded.

Therefore, when load demand on power plant decreases, one or more generators can be shut down and the remaining units can be efficiently loaded. Therefore, if generators are operated in parallel, the routine or emergency operations can be performed by isolating the affected generator while load is being supplied by other units. This leads to both safety and economy. When added capacity is required, the new unit can be simply paralleled with the old units.

In that case a number of smaller units can be operated in parallel to meet the load requirement. Generally a single large unit is more expensive. The positive terminals of the generators are.

When the load on the power plant increases beyond the capacity of this generator, the second shunt generator 2 is connected in parallel wish the first to meet the increased load demand. The procedure for paralleling generator 2 with generator 1 is as under: i The prime mover of generator 2 is brought up to the rated speed.

Now switch S4 in the field circuit of the generator 2 is closed. This is indicated by voltmeter V2. The main switch S3, is closed, thus putting generator 2 in parallel with generator 1. Note that generator 2 is not supplying any load because its generated e. By increasing the field current and hence induced e. E , the generator 2 can be made to supply proper amount of load.

Thus if generator 1 is to be shut down, the whole load can be shifted onto generator 2 provided it has the capacity to supply that load. In that case, reduce the current supplied by generator 1 to zero This will be indicated by ammeter A1 open C. The load may be shifted from one generator to another merely by adjusting the field excitation. Let us discuss the load sharing of two generators which have unequal no-load voltages. These values may be changed by field rheostats.

The common terminal voltage or bus-bars voltage will depend upon i the e. It is generally desired to keep the bus- bars voltage constant. This can be achieved by adjusting the field excitations of the generators operating in parallel. This is achieved by connecting two negative brushes together as shown in Fig.

The conductor used to connect these brushes is generally called equalizer bar. Suppose that an attempt is made to operate the two generators in Fig. If, for any reason, the current supplied by generator 1 increases slightly, the current in its series field will increase and raise the generated voltage.

This will cause generator 1 to take more load. Since this effect is cumulative, the generator 1 will take the entire load and drive generator 2 as a motor. Under such conditions, the current in the two machines will be in the direction shown in Fig. After machine 2 changes from a generator to a motor, the current in the shunt field will remain in the same direction, but the current in the armature and series field will reverse. Thus the magnetizing action, of the series field opposes that of the shunt field.

As the current taken by the machine 2 increases, the demagnetizing action of series field becomes greater and the resultant field becomes weaker. The resultant field will finally become zero and at that time machine 2 will short- circuit machine 1, opening the breaker of either or both machines.

To consider this, suppose that current delivered by generator 1 increases [See Fig. The increased current will not only pass through the series field of generator 1 but also through the equalizer bar and series field of generator 2. Therefore, the voltage of both the machines increases and the generator 2 will take a part of the load.

Chapter 4 D. Motors Introduction D. Therefore, it is not surprising to note that for industrial drives, d. Like d. The use of a particular motor depends upon the mechanical load it has to drive. Motor Principle A machine that converts d. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force.

The same d. Motor Consider a part of a multipolar d. When the terminals of the motor are connected to an external source of d. All conductors under N-pole carry currents in one direction while all the conductors under S-pole carry currents in the opposite direction. Suppose the conductors under N-pole carry currents into the plane of the paper and those under S-pole carry currents out of the plane of the paper as shown in Fig. All these forces add together to produce a driving torque which sets the armature rotating.

Consequently, the direction of force on the conductor remains the same. When the armature of a d. 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: i The armature current under this condition is very large—much excess of rated current of the machine. 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: i If a motor does not have commutating poles compoles , the brushes must be given a negative lead i.

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: i Apart from armature Cu loss, field Cu loss and brush contact loss, Cu losses also occur in interpoles commutating poles and compensating windings.

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.

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: i Cumulative-compound motors in which series field aids the shunt field. 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.

Note: 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. 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 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: i Field divertcrs. 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. Share This Paper. Background Citations. Methods Citations. Figures from this paper. Citation Type. Has PDF. Publication Type. More Filters.

Validation of sequence circuits useful for split-phase current signature analysis SPCSA and diagnosis of eccentric-rotor traction cage motors. A squirrel-cage three-phase induction motor with eccentric rotor and parallel connections in the stator can be represented by five equivalent sequence-circuits of virtually centered-rotor machines, … Expand.

Highly Influenced. The armature, normally containing a three-phase winding, is mounted on the shaft. The armature winding is fed through three sliprings collectors and a set of brushes sliding on them. This arrangement can be found in machines up to about 5 kVA in rating. For larger machines—all those covered in this book—the typical arrangement used is the rotating magnetic field. The rotating magnetic field also known as revolving-field synchronous machine has the field-winding wound on the rotating member the rotor , and the armature wound on the stationary member the stator.

A dc current, creating a magnetic field that must be rotated at synchronous speed, energizes the rotating field-winding. The rotating field winding can be energized through a set of slip rings and brushes external excitation , or from a diode-bridge mounted on the rotor self-excited. The rectifier-bridge is fed from a shaft-mounted alternator, which is itself excited by the pilot exciter. In externally fed fields, the source can be a shaft-driven dc generator, a separately excited dc generator, or a solid-state rectifier.

Several variations to these arrangements exist. The stator core is made of insulated steel laminations. The thickness of the lam- inations and the type of steel are chosen to minimize eddy current and hysteresis losses, while maintaining required effective core length and minimizing costs.

The core is mounted directly onto the frame or in large two-pole machines through spring bars. The core is slotted normally open slots , and the coils making the winding are placed in the slots. There are several types of armature windings, such as concentric windings of several types, cranked coils, split windings of various types, wave windings, and lap windings of various types.

Modern large machines typically are wound with double-layer lap windings more about these winding types in Chapter 2. Non-salient-pole cylindrical rotors are utilized in two- or four-pole machines, and, very seldom, in six-pole machines.

These are typically driven by steam or combustion turbines. The vast majority of salient-pole machines have six or more poles. They include all synchronous hydrogenerators, almost every synchronous condenser, and the overwhelming majority of synchronous motors.

Non-salient-pole rotors are typically machined out of a solid steel forging. The winding is placed in slots machined out of the rotor body and retained against the large centrifugal forces by metallic wedges, normally made of aluminum or steel. The retaining rings restrain the end part of the windings end-windings.

In the case of large machines, the retaining rings are made out of steel. Large salient-pole rotors are made of laminated poles retaining the winding under the pole head.

The poles are keyed onto the shaft or spider-and-wheel Fig. Schematic cross section of a salient-pole synchronous machine. In a large generator, the rotor is magnetized by a coil wrapped around it.

The figure shows a two-pole rotor. Salient-pole rotors normally have many more than two poles. When designed as a generator, large salient-pole machines are driven by water turbines. The bottom part of the figure shows the three-phase voltages obtained at the terminals of the generator, and the equation relates the speed of the machine, its number of poles, and the frequency of the resulting voltage. This is the typical design for all large turbogenerators. Here both the stator and rotor windings are installed in slots, distributed around the periphery of the machine.

The lower part shows the resulting waveforms of a pair of conductors, and that of a distributed winding. The formula giving the magneto-motive force mmf created by the windings. Salient-pole machines have an additional winding in the rotating mem- ber. This winding, made of copper bars short-circuited at both ends, is embedded in the head of the pole, close to the face of the pole.

The winding also serves to damp the oscillations of the rotor around the synchronous speed, and is therefore named the damping-winding also known as amortisseurs or damper-windings. This book focuses on large turbine-driven generators. These are always two- or four-pole machines, having cylindrical rotors. The discussion of salient-pole machines can be found in other books. See the Additional Reading section at the end of this chapter. In self-excited generators, shaft-mounted exciter and rectifier diodes generate the required field current.

The shaft-mounted exciter is itself excited from a stationary winding. The fact that unlike the stator, the rotor field is fed from a relatively low power, low voltage circuit has been the main reason why these machines have the field mounted on the rotating member and not the other way around. Moving high currents and high power through the collector rings and brushes with a rotating armature would represent a serious technical challenge, making the machine that much more complex and expensive.

Older generators have field supplies of volts dc. Later ones have supplies of volts and higher. Excitation voltages of volts or higher are common in newer machines. A much more elaborated discussion of rotor winding design and construction can be found in Chapter 2. An actual description of individual coil design and construction, as well as how the completed winding is distributed around the stator, is meticulously described in Chapter 2.

In this section a very elementary description of the winding arrangement is presented to facilitate the understanding of the basic operation of the machine. As stated above, coils are distributed in the stator in a number of forms. Each has its own advantages and disadvantages. The basic goal is to obtain three balanced and sinusoidal voltages having very little harmonic content harmonic voltages and currents are detrimental to the machine and other equipment in a number of ways.

To achieve a desired voltage and MVA rating, the designer may vary the number of slots, and the manner in which individual coils are connected, producing different winding patterns.

The most common winding arrangement is the lap winding, and it shown in Figure 1. In this chapter the most elementary principles of operation of synchronous machines will be presented. The section shown is of one of the three phases. It can be readily seen that the winding runs clockwise under a north pole, and counterclockwise under a south pole.

This pattern repeats itself until the winding covers the four poles. A similar pattern is followed by the other two phases, but located at electrical degrees apart.

On the right, the three phases are indicated by different tones. Note that, some slots only have coils belonging to the same phase, while in others, coils belonging to two phases share the slot. Thus the best place to start describing the operation of a three-phase synchronous machine is a description of its magnetic field.

Earlier we described how a current flowing through a conductor produces a magnetic field associated with that current. Recall that if the three phases of the winding are distributed at electrical degrees apart, three balanced voltages are generated, creating a three-phase system. Now a new element can be brought into the picture. By a simple mathematical analysis it can be shown that if three balanced currents equal magnitudes and electrical degrees apart flow in a balanced three-phase winding, a magnetic field of constant magnitude is produced in the airgap of the machine.

This mag- netic field revolves around the machine at a frequency equal to the frequency of the currents flowing through the winding see Fig. The importance of a three-phase system creating a constant field cannot be stressed enough. The con- stant magnitude flux allows hundred of megawatts of power to be transformed Fig. A constant magnitude and constant rota- tional speed magnetic flux is created when three-phase balanced currents flow through a three-phase symmetrical winding. In a two-pole winding, however, any the same result applies for any number of pairs of poles.

It is important to remember that a constant- magnitude flux produces a constant-magnitude torque. Now try to imagine the same type of power being transformed under a pulsating flux and therefore pulsating torque , which is tremendously difficult to achieve.

It is convenient to introduce the fundamental principles describing the oper- ation of a synchronous machine in terms of an ideal cylindrical-rotor machine connected to an infinite bus.

The infinite bus represents a busbar of constant voltage, which can deliver or absorb active and reactive power without any lim- itations. The ideal machine has zero resistance and leakage reactance, infinite permeability, and no saturation, as well as zero reluctance torque. The production of torque in the synchronous machine results from the natural tendency of two magnetic fields to align themselves.

Bold symbols indicate vector quantities. When the torque applied to the shaft equals zero, the magnetic fields of the rotor and the stator become perfectly aligned. The instant torque is introduced to the shaft, either in a generating mode or in a motoring mode, a small angle is created between the stator and rotor fields.

As described above, it can be shown that a three-phase balanced voltage applied to a three-phase wind- ing evenly distributed around the core of an armature will produce a rotating revolving magneto-motive force mmf of constant magnitude Fs. The speed at which this field revolves around the center of the machine is related to the supply frequency and the number of poles, by the following expression:!

This situa- tion represents the underexcited condition shown in condition no load a in Figure 1. When operating under this condition, the machine is said to behave as a lagging condenser, meanings it absorbs reactive power from the network. If the field excitation is increased over the value required to produce E1 , the stator currents generate a flux that counteracts the field-generated flux.

The machine is behaving as a leading condenser; that is, it is delivering reactive power to the network. One must be aware that in many texts the name torque angle is used to indicate the load angle.

The name torque angle is also sometimes given to indicate the angle between the terminal voltage V1 and the excitation voltage E1.