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Index current controlled PWM, current source, , , , L Load commutation, Load torques: active, 9 components, 4 passive, 9 Loss minimization,. M Machine tools, 65 Margin angle, , constant margin angle control, Mechanical power, Mechanical time constant, 11, 58 Mine winders, N Natural characteristic, 1, 39 Naturally commutated device, 32 No-load torque angle, constant no-load torque angle control, Normalised: speed, 86, , torque, 86, , Plugging: de motors, induction motor. R Rectifier see Controlled rectifiers Rectifier control of dc rnotors: closed loop speed control see Closed loop control of dc dri ves controlled f1ywheeling, Index unity power factor operation , Transier characteri,tic, of recrifiers.
Voltage source inverter. Voltage induced in the armature of a de motor or in the stator of an induction motor, V Frequency, Hz Base frequency, Hz Frequency of the carrier wave, Hz Average value of the armature current of a dc motor.
A lnstantaneous value of the armature current of a dc motor, A Ripple in armature current, A Average value of the armature current at critical speed Wonc, A Average value of the dc link current of a con verter.
A Fundamental component of rotor current referrcd to stator, A R. Kg-m" K. W Rotor circuit copper loss. Braking resistance, n Rotor resistance, n Rotor resistance referred to stator, n. Stator resistance , n Thevenins equivalen: resistance, n Slip per unit also Laplace operator kth harmonic slip pcr unit Slip at the maximum torque Time, S Torque developed, N-m also chopper period, S.
Source voltage, V Instantaneous value of de source voltage. V Average value of the dc motor armature voltage also rectitier output voltage , V lnstantaneous value of the de motor armature voltage, V Instantaneous inverter output voltage betwecn pha e A and the central point of the de ource. V Fundamental component in a non-sinusoidal voltage.
V Instanfneous value of de link voltage , V Excitation emf. V kth harrnonic component in a non-sinu. V Peak value of an ac source voltage , V Injected voltage , V. V Magnetising reactance , n Rotor leakage reactance. Cornrnutation lead angle of a load commutated inverter or the angle at which rhe de motor armature current drops to zero , rad Minimum value of commutation lead angle.
Pha e of tator current with rcspect to the source voltage V. Source frequency. This chapter describes the common features of electrical drives controlled by power semiconductor converters.
The block diagram of an electrical drive controlled by a power semiconductor converter is shown in figure 1. The load is usually a machine designed to accomplish a certain task. For examplevmachines like trains, rolling milIs, machine tools, paper mills, cranes, excavators, and so on constitute a load on the electrical drive. Usually the requirements of the load can be specified in terms of speed and torque demands.
As examples, some requirements of loads for transient operations and normal running are listed in section 1. The requirements of some specific loads are described in section 1. A motor having speed-torque and speed-current characteristics that suit the load requirements is choseno A motor will have characteristics compatible to the load if it satisfies the speed and torque requirements of the load without exceeding the current limitation imposed either by the motor rating or the source capacity.
The characteristic obtained when the supply is maintained at rated conditions is called the natural characteristic. For example, the speed-torque characteristic of an induction motor obtained at the rated terminal voltage and frequency will be its natural characteristic. Usually, the natural speed-torque characteristic is not compatible with the load requirements. Therefore, a power semiconductor converter is interposed between the source and the motor.
The power semiconductor convertor, which will from now on simply be called a con verter, controls the flow of power from the source to the motor in such a way that the motor speed-torque and speed-current characteristics become compatible with the load requirements. The controls for the converter are built in a control unit which operates at much lower voltage and power levels.
The control unit consists of linear and digital integrated circuits and transistors. It may also consist of a microprocessor when sophisticated control is required. The command signal, which adjusts the operating point of the drive, forrns an input to the control unit. The low voltage control unit is electrically isolated from the power circuit or converter-motor circuit for two reasons: In the absence of isolation, a malfunction may lead to the application of the power circuit voltage to the control unit.
This may damage the control unit and may be detrimental to the safety of the operator who applies the command signal. Converters generate a considerable amount of harmonics.
In the absence of isolation, the harmonics can enter the control unit and interfere with its operation. Sensingof certgin parameters - such as converter current, motor speed, and so on - is usually required either for protection or for closed-loop operation. This function is perforrned by the sensing unit. When sensing the power circuit's electrical parameters - such as the converter current, voltage, and so on - isolation is provided between the power circuit and the control circuit for the reasons just stated.
The complete drive system-c-consisting of the load, motor, converter, source, control unit, and sensing circuit-must be treated as an integrated system. The operating point of the motor may be disturbed due to a change in any of these elements of the drive.
Any such disturbance may produce changes which may affect the entire system. Choice of various elements is also interrelated. For example, the source may be deterrnined by the necessity of using a particular motor, or the motor may have to be selected to suit the source available. Usually the converter is selected to make the motor characteristics suitable to the load; however, it has to be chosen taking into account the capacity of the source and rules goveming its use.
For example, it may be necessary to limit the current inrush during starting or other transient operations, to avoid voltage fluctuations on the supply lines, even when such a limitation is not required for the safe and satisfactory operation of the motor or the loado Further, a converter capable of imparting necessary characteristics to the motor most economically may have to be abandoned because harmonics are generated in the supply lines or due to the power factor deteriorating beyond permissible limits.
This section is concemed with the presentation of the dynamic relations applicable to all types of motors and loads. A motor generally drives a load machine through some transmission system. While the motor always rotates, the load may rotate or may undergo a translational motion.
The load speed may be different from that of the motor; if the load has many parts, their speeds may be different, and while some may rotate, others may go through a translational motion. It is convenient, however, to represent the motor load system by an equivalent rotational system, as shown in figure 1.
The following notation is adapted: J. Equation 1. The drive accelerates or decelerates depending on whether T is greater or less than TL.
In applications having a load with large inertia, such as trains, the motor torque must exceed the load torque by a large amount to get an adequate amount of acceleration. Similarly, in applications requiring fast response, the motor torque should be main-: tained at the highest value and the motor-load system should be designed to have the lowest possible inertia.
When the speed increases, the kinetic energy of the drive given by! When fast response is required, the braking torque should be maintained at the highest value and the motorload system should be designed with the lowest possible inertia. When, for a short time, the load torque T L exceeds the maximum torque capability of the motor running at a given speed, deceleration occurs and the dynamic torque assists the motor torque in maintaining the motion.
In some applications, involving a large torque of relavtively short duration followed by a no-load or lightload period of sufficient duration, the dynarnic torque component is used so that a motor of smaller rating can be used. For example, in a pressing machine, a large torque of short duration is required during the pressing operation; otherwise the torque is nearly zero. A flywheel is mounted on the motor shaft to increase the equivalent inertia J. During the no-load period, the drive accelerates to store the kinetic energy.
During the pressing operation, the load torque is much higher compared to the motor torque. The deceleration occurs, producing a dynamic torque.
The dynarnic torque and the motor torque together are able to produce the torque required for the pressing operation. In the absence of the flywheel, the motor will be required to supply the entire torque required for the pressing operation, and therefore the motor rating has to be much higher. The load torque T L can be further divided into the following components: 1. Friction torque T F: The friction will be present at the motor shaft and also in the various parts of the loado The friction torque T F is the equivalent value of various friction torques referred to the motor shaft.
Windage torque Tw: When a motor runs, the wind generates a torque opposing the motion. This is known as the windage torque. Torque required to do the useful mechanical work, T M: The nature of this torque depends on the type of loado It may be constant and independent of speed, it may be some function of speed, it may be time invariant or time variant, and its nature may also vary with the change in the load's mode of operation.
The variation of friction torque with speed is shown in figure 1. Its value at standstill is much higher than its value at slightly above zero speed. Friction at zero speed is called stiction or static friction. For the drive to start, the motor torque should at least exceed stiction. The friction torque can be resolved into three components as shown in figure 1.
The component Tv which varies linearly with speed is called viscous friction and is given by the following equation: 1. The component Te which is independent of speed is known as coulomb friction.
A third component Ts accounts for the additional torque present at standstill. Since Ts is present only at standstill, it is not taken into account in the dynarnic analysis.
With this approximation, from equation 1. Otherwise, equation 1. Theoretically, the transients decay to zero in infinite time, which is not true in practice. To resolve this anomaly, the transient operation is considered over when a 95 percent change in speed has already taken place. Figure 1. In the case of centrifugal pumps, blowers, fans, and other loads involving the turbulent flow of fluid, the load-torque varies as the square of speed, as shown in figure l.
This is nothing but the windage torque given by equation 1. The windage is also a predominant component at high speeds for trains, cars, and so on. The variation of the traction load torque with speed, excluding the torque due to gravity, is shown in figure l. It is applicable to electric trains and road vehicles.
It is comprised of the windage, viscous friction, coulomb friction, and stiction. When deciding about the torque requirements of the driving motor, the torque components that are needed to provide acceleration and to overcome gravity must also be. An important factor known as the coefficient of adhesion is defined as follows: Coefficient of adhesion.
The tractive effort is nothing but the driving force at the wheel rim, and, therefore, it is proportional to the motor torque. The wheels coupled to the motor are called driving wheels.
The foregoing equation suggests that for a given value of the coefficient of adhesion, there is a maximum value of torque which can be exerted without the driving wheels slipping. Slipping is always to be avoided as it damages the track and reduces its life.
The value of the coefficient of adhesion depends on the conditions of the rail and wheel surface. In long distance trains, the distance between the consecutive stations is generally great. Acceleration and deceleration times form only a very small proportion of the total time of travel between the two stations. Therefore, the average speed mainly depends on the maximum speed and the acceleration is allowed to be low to suit the passengers' convenience. In this case, locomotives are used to drive the trains.
Then the maximum weight on the driving wheels can be at the most equal to the weight of the locomotive. Therefore, the maximum torque that can be applied without the wheels slipping is small. In the case of suburban trains, the distance between the consecutive stations is usually very small. The acceleration and deceleration times form a major proportion of the total traveling time.
To get a high average speed, it is necessary to reduce the acceleration time. For a given value of the coefficient of adhesion, the acceleration can be increased only by increasing the weight on the driving wheels.
Hence, instead of a locomotive, motorized train cars are used. Each motorized train car has its own driving motor. The usual pattem is to use the motorized train car and trailer cars in a ratio of 1: 2. This allows a much higher weight to come on the driving wheels,. The coefficient of adhesion also depends on the speed-torque characteristic of the motor. When a wheel slips, the speed of the driving motor increases. If the torque drops by a large amount, for a given increase in speed, the wheel regains its grip immediately.
Thus a motor speed-torque characteristic with a low regulation of speed is preferred to prevent wheel slippage. A smooth change of torque is also desirable to prevent wheel slippage. This is easily achieved when power semiconductor converters are used.
Figure l. One such example is a coiler drive which is used in steel strip, paper, and plastic mills. In a reversible cold rolling steel strip mill, the strip to be rolled is received by the mill in the form of a reel wound on a mandrel. The mandrel is mounted on one side of the rolling stand s. The purpose of the rolling stand is to reduce the cross section and to improve the surface finish. Another mandrel is mounted on the other side of the rolling stands.
After rolling, the. As the rolling process progresses, the strip uncoils from one mandrel and coils on the other mandrel. To maintain even, good quality roUing and coiling, it is necessary to keep the strip's tension constant on both sides.
Since the rolling stands receive and give out the strip at a fixed linear speed, the coiling and uncoiling is done at a constant power. Thus, the motor driving the coiling reel must develop a constant motoring power, and the motor coupled to the uncoiling reel must develop a constant braking power. The amount of power changes with the cross section and the material of the strip.
A diesel electric loco motive employs a de motor fed by a dc generator driven by a diesel engine. Instead of a dc generator, an altemator followed by a diode rectifier may also be used. Since the speed-torque curves of the diesel engine are not compatible with the traction requirements, it cannot be directly coupled to the driving wheels.
By interposing the de generator-rnotor set, speed-torque curves compatible with the traction requirements are obtained. The generator-rnotor system essentially acts as a torque converter, similar to a gear system in a car, but of course with a much superior performance.
The diesel engine runs at full speed developing constant power. When the fuel injection is set at maxirnum, it develops full power. To make full use of the diesel engine power, the de motor should develop constant power over the range of speed. Care is also taken to ensure that the diesel engine is not overloaded, otherwise it will simply stall. This is achieved by the constant power characteristic Be. The part CD is obtained by imposing a limit on the maximum motor torque.
This is done for three reasons: 1 to limit the rnotor-generator current within a safe limit, 2 to prevent wheel slippage when the coefficient of adhesion is low, and 3 to prevent too higha motor torque which may lead to a draw-bar fracture when the coefficient of adhesion is high.
For lower fuel injections, we get the inner characteristics, for example B'C' shownin the figure. The different speed and torque limits, A'B and C'D', are' imposed to take care of varying conditions of the track related to the gradient and the adhesive coefficient.
The characteristics shown in figure l. The motor speed-torque requirements for excavators are shown in figure l. The purpose of the excavator is to dig earth. While digging, it may come across a rock. The motor will then simply stop. In such a situation, the motor torque must be limited to prevent mechanical damage to the excavator, and the motor current should also be restricted within the converter rating. This explains why the portion Be has the nature shown in the figure.
The characteristics AB, A'B', A"B", and so on are provided to take care of the varying nature of the material to be excavated. The crane-hoist characteristics are shown in figure 1. In a low speed hoist, the torque is mainly due to gravity, which is constant and independent of speed.
In high speed hoists, the viscous friction and windage also form an appreciable proportion of the load torque. Active load torques. Passive load torques. Load torques which have the potential to drive the motor under equilibrium conditions are called active load torques.
Such load torques usually retain their sign when the direction of the drive rotation is changed. Torque due to the force of gravity and torques due to tension, compression, and torsion undergone by an elastic body come under this category. Let us consider the example of an electric train. When the train climbs up, the active torque due to gravity opposes the motion. Therefore, the driving motor has to generate extra torque to overcome the torque due to gravity. On the other hand, when the train goes down a steep grade, it is driven by the torque due to gravity.
The motor produces braking torque to limit the speed within the safe values. This confirms the features of the active load torque just stated.
Load torques which always oppose the motion and change their sign on the reversal of motion are called passive load torques.
Torques due to friction, cutting, and so on are in this category. The electric braking may be required due to the following reasons: 1.
If a motor running at some speed is disconnected from the supply, the only opposing torque will be' the load torque T L. The motor will stop only after the kinetic energy stored in its inertia is dissipated.
When either the load torque is small or the inertia is large, the motor takes a long time to stop. In applications requiring frequent stops, the stopping time must be reduced by introducing additional opposing torque by the use of electric braking. In some applications, such as traction, rapid emergency stops are essential to prevent accidents.
The electric braking helps in achieving quick and smooth stops. There are applications where accurate stops are required, such as in lifts, machine tools, ingot buggy control, and the screwdown mechanism in rolling mills.
Electric braking allows accurate steps without subjecting mechanical parts to unduly large stress. In certain applications involving active loads, the drive speed will reach dangerous values if the braking force is not provided by the motor.
For example, in a hoist application when a loaded hoist is being lowered, the motor should provide a braking force to hold the speed within safe limits. Similarly in traetion, when a train goes down a steep gradient, a braking force is required to hold the train speed within safe limits. The braking force can also be obtained by using mechanical brakes.
However, this leads to wear and tear of the mechanical parts. It is not as smooth as electric braking. Furthermore, by using regenerative braking explained in later chapters , the energy is usefully employed instead of being wasted as with mechanical brakes. In electrical machines, the torque is usually expressed as a function of speed or slip , and therefore the speed is treated as an independent variable and the torque as a dependent variable. Accordingly, the speed is plotted on the 'X' axis and the torque on the 'Y' axis.
In drives, the torque is considered the independent variable, and therefore is plotted on the 'X' axis. The speed, which is now the dependent variable, is plotted on the 'Y' axis. The reason for this convention is as follows. When selecting a drive, the first thing to be noted is the torque the motor should produce for the drive to be operable. The question of finding the drive speed arises only if the drive operates. Hence the torque is regarded as the independent variable.
For the proper use of equation 1. The motor speed is considered positive when rotating in the forward direction. For drives which operate only in one direction, the forward speed will be their normal speed. For reversible drives the forward speed is chosen arbitrarily. Then the rotation in the opposite direction gives the reverse speed which is assigned the negative signo The positive motor torque is defined as the torque which produces acceleration or a positive rate of change of speed in the forward direction.
It tends to drive the motor in the forward direction. According to equation 1. Motor torque in the direction opposite to the positive motor torque will produce --deceleration or a positive rate of change of speed in the reverse direction. It tends to drive the motor in the reverse direction. A motor operates in two modes-motoring and braking. In motoring, it converts electrical energy to mechanical energy, which supports its motion.
In braking, it works as a generator converting mechanical energy to electrical energy, which is consumed in some part of the circuit; and therefore it opposes the motion. The motor can provide motoring and braking operations in both forward and reverse directions. In quadrant 1, the power, which is the product of speed and torque is positive. Hence, the machine works as a motor supplying the mechanical energy.
The operation in quadrant 1 is called forward motoring. In quadrant ll, the power is negative. Hence, the machine works under braking opposing the motion. The operation in quadrant II is called forward braking.
When coupled to a passive load, the deceleration will be caused both by the machine and the loado Consequently, the drive cannot have an equilibrium speed in this quadrant.
Operation in this quadrant will take place only under transients. The same will happen when coupled to an active load torque with a positive signo However, if the active load torque is negative, the drive can have an equi-. Then the drive will also operate in steady-state in this quadrant. For example, when an electric train moves down a steep gradient, the torque due to gravity supports the motion and the braking opposes the motion. By the adjustment of the braking torque, the train is made to run at a desired speed.
Following the preceding arguments, it can be noted that quadrant III gives reverse motoring and quadrant IV provides reverse braking. This is the speed at which the drive will normally operate in steady-state provided it is a speed of stable equi. Equilibrium speed will be viewed as the stable speed provided that the operation will be restored to this speed after any smaIl departure from it due. The stability of an equilibrium point can be readily investigated by using the concept of steady-state stability.
In this concept, the stability of an equilibrium point is evaluated from the steady-state speed-torque curves of the motor and the loado It is assumed that any departure from the equilibrium point, due to any disturbance, will be along these curves. This in effect means that the motor is assumed to be in electrical equilibrium for all operating points. The basis of this assumption is that the electrical time constant of a motor is usuaIly negligible compared to its mechanical time constant.
At a given equilibrium operating point, let the motor torque, load torque, and drive speed be denoted by Te' T Le, and Wme respectively. A disturbance in the supply, load, or any part of the drive will cause perturbations in the motor torque, load torque, and drive speed. Now from equation 1. This differential equation provides the relation between small perturbations around an equilibrium point.
For small perturbations, the speed-torque curves of the motor and load can be assumed to be straight lines. Substituting from equations 1. This is a first-order linear differential equation. For this to happen, the exponent in equation 1. Equation 0. Similarly, for a decrease in speed, the motor torque must exceed the load torque so that acceleration occurs and the operation retums to the equilibrium speed.
Let us examine the equilibrium points A and B, which are obtained when an induction motor dri ves the load T Ll, as shown in figure 1. Let us first examine point A for the steady-state stability. A small increase in speed makes the load torque greater than the motor torque.
Deceleration occurs and the operation is restored to point A. Sirnilarly, a small decrease in speed causes the motor torque to exceed the load torque.
Acceleration occurs and the operation is restored to point A. Thus, A is a stable equilibrium point. Let us next examine the stability of the equilibrium point B. A smalI increase in speed causes the motor torque to exceed the load. Acceleration takes place and the operating point moves away from B.
Similarly, a small decrease in speed malees the load torque greater than the motor torque, causing deceleration and the operating point to drift away from B. Thus B is an unstable equilibrium point. Let us now consider the equilibrium point e which is obtained when the motor drives the load-torque T L2. The load T L2 has a characteristic similar to the fan load fig. Examination of point e shows that it is a stable equilibrium point. Note that points B and e lie on the same part of the motor speed-torque curve.
However, e provides the stable operation but not B. This shows that the stability of an equilibrium point depends not on either the motor characteristic alone or the load characteristic alone but on the relative nature of the two. Notice that the part of the motor characteristic on which points B and e are situated has a positive slope. Such a motor characteristic gives unstable operation with most loads, and therefore, it is sometirnes called a statically unstable characteristic.
This section describes types of converters and features common to all variable speed drives controlled by power semiconductor converters. For the control of induction motors, a fixed frequency ac supply with variable voltage or a variable frequency ac supply with variable voltage or current is required.
Synchronous motors need a variable frequency supply with variable voltage or current. For the control of de motors, a variable voltage de supply is required. The variable voltage de supply is also used for the control of induction and synchronous motors. The supply that is generally available is the fixed voltage and fixed frequency ac supply. Sometimes a fixed voltage de supply may be available. To satisfy the supply needs of various motors, a number of power semiconductor converters have been.
Fixed voltage de to variable voltage de Fixed voltage ae to variable voltage ae at same frequeney DC to fixed or variable voltage and frequeney ae, voltage or eurrent sources. AC voltage eontrollers 4.
Inverters voltage souree or eurrent souree 5. Applieations Control of de motors, induetion motors and synehronous motors Control of de motors and induetion motors Controlofinduetion motors Control of induetion motors and synehronous motors. These converters along with their conversion functions and applications are listed in table 1. A variable speed drive may use a single converter or more than one con verter.
Further, each converter listed in the table may be realized using different circuits. These circuits may not only differ in performance but also in terms of their capability for motor control in various quadrants. Usually, the lesser the number of quadrants covered, the lower the cost and the simpler the converter circuito While various features of individual converters and their circuits will be discussed in later chapters, some features cornmon to all converters are presented in the following sections.
Some of the features cannot be explained at this. Therefore, they would be simply stated. The detailed explanations will be given in later chapters. During transient operations such as starting, braking, speed reversal, speed changing, and SO on, the motor current can be allowed to be higher than its rated current, due to its large thermal capacity.
For example, depending on the design of the motor and the duration of the transient operation, a de motor may be allowed to carry 2 to 3. The flow of current equal to the maximum permissible value allows full use of the motor torque capability both during motoring and braking. Consequently, the transients are completed in the shortest posssible period, and the motor does not stall due to short-time overloads. When fast response during transient operations is not necessary, then the motor current need not be allowed to exceed the rated current.
Converters use semiconductor elements which do not have any capacity for overload, due to their low thermal capacity. Therefore, their current rating is chosen as equal to the maximum current that may be required to flow through the motor. Consequently, the converter cost, and hence the drive cost, increases substantially. The increase in the cost of the drive is well accepted as a price to be paid for the increase in the total work done by the drive, which may ultimately lead to an increase in production or financial retum.
Almost all converter drives are provided with some kind of current control, the purpose of which is to prevent the current from exceeding a perrnissible value. When the motor current is allowed to be K times its rated current, the current control will fail to protect the motor against sustained overloads. In this case, additional thermal protection will be required to protect against sustained overloads. When fast response is not necessary during transient operations, the motor current is restricted to its rated value.
This minimizes the cost of the con verter and the drive. The current control now provides protection against the sustained overloads as well. The continuous torque and power limitations of a drive in the four quadrants of operation are shown by the solid lines in figure 1.
From standstill to base speed, both for motoring and braking operations and for rotation in either direction, operation at the rated current imposes a limitation on the maximum available torque. The available power increases linearly with speed and generally reaches maximum value-equal to the continuous power rating of the motor-at base speed.
Usually the motor is operated at a reduced voltage below base speed. Above base speed, the motor terminal voltage is maintained at the rated value.
Motor operation at the rated current and rated volt-. One of the factors which imposes a limitation on the maximum speed is the mechanical strength of the motorload system.
Other factors, which are applicable to specific motors, are described in the relevant chapters. As explained in the previous section, when fast response is desired, the motor current is allowed to exceed the rated current during transient operations.
When the converter rating is chosen to match the motor transient current rating, the torque and power limitations shown by the dotted lines are obtained fig. During transient operation, the current may be forced to reach maximum permissible value, thus causing the drive to operate on the dotted curves. This is done to make fuIl use of the motor torque and power capabilities.
All power semiconductor converters have harmonics in their output voltage and current. The contribution of these harmonics to the power developed by the motor is negligible.
The harmonic currents increase the rms value of the motor current and distort the flux. Consequently, the copper and core losses are increased. To prevent the motor temperature from exceeding a safe value, the load on the motor must be less than rated.
In other words, the motor has to be derated in the presence of harmonics. All power semiconductor converters also have harmonics in their input currents. These harmonics cause line voltage fluctuations, adversely affecting other loads connected to the same lines.
They also produce radio frequency interference through conduction and radiation. Some converters such as the controlled rectifier, the cycloconverter, and the ac voltage controller suffer from a poor power factor, particularly at low output voltages.
The discussion of the previous section has revealed some drawbacks of power semiconductor converters. To present a balanced picture, it is essential that we consider their advantages as weIl. The main advantages are high efficiency, fast response, control flexibility, easy maintenance, reliability, low weight and volume, less noise, long life, and so on.
Because of these advantages, and in spite of the disadvantages mentioned in the previous section, power semiconductor converters have replaced the conventional power controllers such as magnetic amplifiers, mercury-arc rectifiers, and so on in virtually all applications.
In certain drive applications, the change in speed is not required. The driving motor then operates at the rated conditions of supply on the natural speed-torque curve.
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