Since In this method of speed control flux can be only reduced (not increased), the speed only above normal can be obtained. The other limitations and drawbacks of this method of speed control are given below:
1. Creeping speeds cannot be obtained by this method.
2. Top speeds are only obtained at a reduced torque owing to very weak field, so the advantage cannot be taken of the high speeds for increasing the power output of the motor.
3. The speed is maximum at the minimum value of flux, which is governed by the demagnetizing effect of armature reaction on the field.
The variable field resistance may be simple field rheostat or it may be combined with a starting rheostat. This method of speed control is also employed for dc compound motors, although for any speed setting, there is some variation in the speed with load, in contrast to dc shunt motor, where the speed is but slightly affected by the load. 2. Armature Resistance Control This method consists simply of a variable resistance connected in series with the armature. The speed at full load may be reduced to any desired value depending on the amount of resistance. With this method, the voltage across the armature drops as the current passes through the series resistance and the remaining voltage applied to the armature is lower than the line voltage. Thus the speed is reduced in direct proportion to this voltage drop at the armature terminals. Field current will remain unaffected as the shunt field is directly connected across the supply main. 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 speed. In case of fans and centrifugal pumps where the load torque decreases with the fall in speed, the losses are considerably low and because of its low initial cost and simplicity this. methods 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, since the torque depends only upon the armature current, flux remaining unchanged. The main advantage of this method is that speeds below base speed down to creeping speeds of only a few rpm are easily available.
The speed torque characteristics of a dc shunt motor with armature resistance control. A comparison of speed adjustment of a dc shunt motor by means of field control and be means of general use. In field control the adjustment can be obtained by means of a small rheostat and relatively good speed rgu1ation is obtained for all speeds. With the armature control method is bulky rheostat is required, a large amount of power is wasted in the controlling resistance and poor speed regulation results for the lower speeds. The field control method is much more efficient if the rated output is to be delivered by the motor at the different speeds for relatively long periods of time. In armature control method output is reduced in the ratio as the speed. This method of speed control is not desirable for continuous operation.
The armature resistance control method, therefore, is employed where speeds lower than rated one are required for a short period only and that also occasionally as in printing machines, cranes and hoists where the motor is frequently started and stopped. This method of speed control is also employed where the load drops off rapidly with decrease in speed, as in fans arid blowers. 3. Armature Voltage Control This method of speed control requires a variable source of voltage separate from the source supplying he 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. 4. 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 simply working the motor with a constant excitation and applying a variable voltage to its armature to give the required speed. The variable voltage supply is obtained from a motor-generator or converter set. 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. Vary 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 efficiencies 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 Higher System Which is a modified form of Ward Leonard system, incorporates a heavy flywheel mounted on the shaft coupling the driving motor M2 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 below:
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 store sup 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.
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