One of the attractive features the dc motor offers over all other types is the relative ease with which speed control can be achieved and, therefore, dc motors are indispensable for many adjustable speed drives. The various schemes available for speed control can be deduced from the expression of speed for a dc motor which is repeated here with one modification-
The modification involves the inclusion of an external resistance in the armature circuit.
The above expression reveals that the speed can be controlled by adjusting any one of the three factors appearing on the right hand side of the expression:
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(i) Applied voltage to the armature terminals, V
(ii) External resistance in the armature circuit, R and
(iii) Flux per pole, O.
The first two possibilities involve adjustment affecting the armature circuit, whereas the third involves change in the magnetic field.
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Therefore, speed control methods are broadly classified as armature control methods and field control methods. Sometimes a combination of the two methods is employed. With armature control the speed decreases as the voltage applied to the armature terminals is reduced, whereas with field control the speed increases as the flux is reduced.
1. Field Control Methods:
In case of dc series motors, the flux can be varied by a one of the following methods:
(i) Field Divertor Method:
The field flux can be reduced by shunting a portion of motor current around the series field, thus reducing the excitation mmf and weakening of field. This method is illustrated in Fig. 1.72 (a). This method provides speeds above normal because flux is reduced by this method. Lesser the divertor resistance less the field current, less flux and, therefore, more the speed. This method is convenient as well as economical and provides the speed control range usually not exceeding 2:1. This method of speed control is used in electric drives in which the speed should rise sharply as soon as the load falls.
(ii) Tapped Field Control:
This is another method of increasing the speed by reducing the flux and it is accomplished by reducing the number of turns of the field winding through which the current flows. In this method of speed control of dc series motors a number of tappings from the field winding are brought outside, as illustrated in Fig. 1.72(b). A number of series field turns can be short circuited according to the requirement. When all field turns are in circuit, the motor runs at the lowest speed and speed increases with cutting out some of the series field turns. This method is usually employed in electric traction.
(iii) Paralleling Field Coils Method:
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In this method, used in fan motors, several speeds can be obtained by regrouping of field coils, as illustrated in Fig. 1.72 (c).
In case of dc shunt motors the flux can be varied by inserting a variable resistance in series with the field winding (Refer to Fig. 1.73). Since this resistance has to carry only a small current, so it is made up of slide-wire type of resistor to have continuously variable speed over the range.
Since the flux can be only decreased (not increased) so the speeds only above normal one can be obtained by this method. The speed is minimum at the maximum value of flux and depends upon the design of the field and its saturation point. The speed is maximum at the minimum value of flux, which is governed by the demagnetizing effect of armature reaction on the field as at higher speeds the motor tends to be unstable and difficulties in commutation arise. The high speed is also restricted due to mechanical considerations as the centrifugal forces are set up at high speeds. Speed variation by this method is limited to ratio of 4 or 5 to 1.
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Creeping speeds cannot be obtained by this method. The power output being proportional to TN or VIa remains constant. This method is, therefore, suitable only where power of the load remains constant. This method of speed control is very simple, convenient and most economical and is, therefore, extensively used in electric drives. The power wasted in the controlling resistance is very little, field current being very small. This method of speed control is independent of load on the motor and permits the remote control of speed.
The speed-torque characteristics obtainable with this method are given in Fig. 1.74.
2. Armature Resistance Control Methods:
In this method of speed control, reduced speeds are obtained by inserting resistance in the armature circuit. It may be employed with series, shunt and compound wound motors. For the last two types, the series resistance is connected between the shunt field and armature, not between the line and motor.
In case of a dc shunt motor an increase in armature circuit resistance will cause more voltage drop in the armature circuit, so the speed will be reduced. Field current will remain unaffected, as the shunt field is directly connected across the supply voltage. For a constant torque load, the armature current remains the same so input to the motor remains the same but the output decreases in proportion to the speeds. Operating costs are, therefore, comparatively high for long time running at reduced speed.
In case of fans and centrifugal pumps where the load torque decreases with the decrease in speed, the losses are considerably low and because of its low initial cost and simplicity this method may be quite convenient and economical for short-time or intermittent slowdowns. Wide range of speed (below normal one) can be obtained by this method and at the same time motor will develop any desired torque over its operating range.
The main advantage of this method is that speeds below base speed down to creeping speeds of only a few rpm are easily available. This method of speed control, therefore, is employed where speeds lower than rated one are required for a short period only and also occasionally as in printing machines, cranes and hoists where the motor is frequently started and stopped. This method of speed control is also employed where the load drops off rapidly with the decrease in speed, as in fans and blowers.
In case of a dc series motor an increase in armature circuit resistance will cause more voltage drop in the armature circuit, so speed will be reduced. The torque which is proportional to the product of the flux and armature current will also be reduced with the increase in armature circuit resistance as increase in armature circuit resistance will reduce the armature current and the flux which also depends upon the armature current.
In order to obtain different speeds for constant load torque, the armature current is kept constant so the flux. The resistance in the armature circuit is increased to reduce the speed. The drawbacks of armature resistance control for machines with shunt fields are not as important in the speed control of dc series motors.
The poor speed regulation that is inherent in this method has no significance for the control of dc series motors, since the speed characteristic of a dc series motor is a rapidly drooping curve. The power loss in the control resistance for many applications of dc series motors is not too serious, since in these applications the control is utilized for a large portion of time for reducing the speed under light-load conditions and is only employed intermittently when the motor is carrying full load.
The speed-torque characteristics of dc series motors with resistance control are shown in Fig. 1.76 (b). The maximum range of speed control of about 3 : 1 will be available depending on the load. This method of speed control is most economical for constant torque drives. This method of speed control is employed chiefly for dc series motors driving cranes, hoists, trains etc. because such drives operate on intermittent duty.
The speed-torque characteristics of armature resistance control as applied to dc shunt and series motors are shown in Figs. 1.76(a) and 1.76(b) respectively.
3. Shunted Armature Control:
The series resistance arrangement described above suffers from a drawback that the speed varies with the load, i.e., the speed increases with the fall in load. This drawback is overcome by shunting the armature with a variable resistance, as shown in Fig. 1.77.
In effect resistors, R1 and R2 act as a voltage divider applying a reduced voltage to the armature. Greater flexibility is possible because two resistors may now be adjusted to provide the desired performance. For series motors, no-load speed may be adjusted to a finite, reasonable value, and the scheme is, therefore, applicable to the production of low speeds at light loads. For shunt motors the speed regulation in the low speed range is appreciably improved because the no-load speed is definitely lower than the value with no controlling resistor. The main disadvantages of this method are its low efficiency and the fact that a heavy current is drawn from the supply under certain load conditions.
In Fig. 1.78, natural speed-torque characteristic, speed- torque characteristic with conventional series resistance and shunted armature control are represented by curves I, II and III respectively.
The shunted armature control is generally employed in drives of comparatively small power rating. This method is also not economical for continuous operation as there is a relatively large power loss in the speed controlling resistances. It is usually employed as a means for sharply reducing the speed so as to achieve accurate stopping.
4. Armature Voltage Control:
This method of speed control requires a variable source of voltage separate from the source supplying the field current. This method avoids the disadvantages of poor speed regulation and low efficiency which are characteristics of the armature-resistance control method but it is more expensive in initial cost. The adjustable voltage for the armature is obtained from an adjustable voltage generator or from an adjustable electronic rectifier. This method gives a large speed range with any desired number of speed points. It is essentially a constant-torque system, because the output delivered by the motor decreases with a decrease in applied voltage and a corresponding decrease in speed.
This particular system has a further advantage that can be employed to provide excellent starting characteristics by bringing the generator voltage gradually up from zero, starting and bringing the motor up to speed with a comparatively slowly increasing voltage. Because of the excellent starting characteristics, this system is used largely for modern high-speed elevators, and on account of the combination of excellent starting characteristics and the wide speed range available, it is used to some extent for reversing planer installations. This method is not applied to any great extent, generally on account of higher initial cost of the generating equipment.
Another drawback of this system is that only the speed below the base speed can be had as the voltage applied to the armature can be reduced from its rated value. If the motor is operated with a voltage across armature terminals higher than the rated value for a long duration of time, the armature winding insulation may get damaged.
5. Ward-Leonard Method of Speed Control:
The basic adjustable-voltage armature control method of speed control accomplished by means of an adjustable-voltage generator is called the Ward-Leonard system. This system consists of simply working the motor with a constant excitation and applying a variable voltage to its armature to provide the required speed. The variable voltage supply is obtained from a motor-generator or convener set. The scheme is illustrated in Fig. 1.79 (a). M1 is the work motor, powered by the generator G, which is driven by a synchronous or induction motor M2.
The excitation current for the work motor M1 and the generator G is obtained from the exciter E mounted on the same shaft as the generator. The Ward-Leonard set is started by starting the driving motor. The field rheostat R of the generator is gradually brought out of the circuit as the generator picks up the speed and the work motor begins to rotate.
The variable voltage across the terminals of the generator or across the motor is obtained by varying the exciting current of the generator G by means of shunt regulator R. The direction of rotation of motor armature can be reversed by reversing the direction of exciting current of the generator G with the help of reversing switch RS. The converter set runs always in the same direction.
Braking of motor M1 may be carried out by reducing the generator excitation so that its emf is less than the counter emf of motor M1. Under these conditions, motor M1 begins to operate momentarily as a generator, generator G as a motor and ac driving machine M2 as a generator. As a result kinetic energy of motor M1 and its load is returned to the supply mains and braking action on the motor M1 takes place.
Advantages:
1. Very fine speed control over the whole range from zero to normal speed in both directions can be obtained.
2. Uniform acceleration can be obtained.
3. Speed regulation is good.
Disadvantages:
1. Two extra machines are required, so arrangement is costly.
2. Low overall efficiency of the system, especially at light loads.
Applications:
This system of speed control is best suited where almost unlimited speed control in either direction of rotation is required such as in steel rolling mills, paper machines, elevators, cranes, mine hoists, diesel-electric propulsion of ships etc.
Ward Leonard Ilgner System, which is a modified form of Ward Leonard system, incorporates a heavy flywheel mounted on the shaft coupling the driving motor M, and generator G. In operation with a flywheel, the driving motor has to have a drooping speed-load characteristic i.e., its speed must drop with the increase in load on the shaft.
The function of the flywheel is to reduce the fluctuations in the power demand from the supply mains as explained- an increase in the load on the shaft causes the work motor to draw more current from the generator so more power is required to drive the later and if there were no flywheel, the driving motor would take all the additional power from the supply line, thus causing sharp fluctuations in it. The heavy flywheel, however, stores a large amount of kinetic energy. When an increase in the generator load causes the driving motor to slow down, some of the kinetic energy of the flywheel goes to sustain the peak load on the shaft of the work motor. When the load on the work motor decreases, the driving motor picks up speed and the flywheel stores up kinetic energy.
This system of speed control is employed where the load on the motor shaft sharply varies such as in mine hoists, rolling mills etc.
6. Booster Speed Control Method:
A booster in series with the armature of the main motor may be employed for varying the voltage across the driving motor. The booster is a separately excited dc generator, which adds or subtracts the voltage in the motor feeder. The adding (boosting) or subtracting (bucking) depends upon the field current and its direction, which is achieved by parallel resistance arrangement. By this method the speed can be varied over a wide range depending upon the size of the booster.
Since booster generator is to provide only the variation in the voltage for boosting and bucking its output, therefore, need not to be as much as that of the main motor inspite of the fact that it has to carry the main motor current. The exact size of the booster however, depends upon the range of speed control required. The chief disadvantages of this method is that it requires two extra machines which is expensive, Generally this method is employed where only a small variation in speed is required.
7. Grid-Controlled Mercury-Arc Rectifier Method:
In this method, grid-controlled mercury-arc rectifier consisting of a multi-anode steel tank or glass-tube unit or a group of single-anode (thyratron or ignitron) tubes is employed to supply variable voltage to the driving motor.
The output voltage can be varied from zero to maximum value by means of grid control and direction of rotation can only be changed by reversing the connection at the motor armature. The advantage of this method is that the control equipment is static, has a higher operating efficiency and occupies less floor area. The disadvantages of this method are that the power factor on the input side of the rectifier decreases approximately in proportion to speed of the main motor, the control and auxiliary equipment are complicated and it has got no storage capacity so it cannot be employed where large fluctuations of load are involved.
8. Constant Current System Method:
For certain services, where there is a possibility of the motors being stalled such as with excavators, ship’s windlasses etc. the constant current system is advantageous and is used. In this system a dc generator delivering a constant current by means of a special exciter under all load conditions is employed. The current remains the same throughout and the voltage varies with the variation in load. The connections are shown in Fig. 1.81.
The field of the main motor is excited from tappings across the resistance R and field winding of the regulating machine so that variable voltage in either direction can be applied to the field system. The regulating machine is mounted on the motor shaft and is so connected in series with the motor field that the emf of the regulating machine opposes the main field current.
Any decrease in speed of the motor which may occur due to increase in load, would reduce opposite emf, (emf induced in the regulating machine) and thus field current and torque would increase. The torque would reach maximum when the motor stalls. Speed-torque characteristics for the various positions of the potential divider are shown in Fig. 1.82.
A full-range of speed control can be had with the maximum torque when the motor stalls without any excessive current.
Any number of motors can be connected in series with each other as the current is independent of load. Thus cable cost and copper losses are reduced. The motor can be stopped simply by closing switch S.