SPEED CONTROL OF DC SHUNT MOTORS
The speed of a dc shunt motor can be controlled by following methods:-
l FIELD CONTROL METHOD
l ARMATURE RESISTANCE CONTROL METHOD
l ARMATURE VOLTAGE CONTROL METHOD
1. FIELD CONTROL METHOD:-
In this method, speed variation is accomplished by means of a variable resistance inserted in series with the shunt field, as shown in fig.
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| FIELD CONTROL METHOD OF DC SHUNT MOTOR |
An increases in controlling resistance reduces the field current with a consequent reduction in flux and an increases in speed. This method is speed control is very simple, convenient and most economical and is, therefore extensively used in modern electric drives.
Since in this method of speed control flux can be only reduced, the speed only above normal can be obtained. The 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 advantages 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 speed torque characteristics is shown in fig.
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| SPEED AND TORQUE CHARAC. |
2. ARMATURE RESISTANCE CONTROL:-
The method consists simply of a variable resistance connected In series with the armature, as shown in fig. 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 reduced in direct proportion to this voltage drop at the armature terminal.
With the resistance control method a bulky rheostat is required, a large amount of power is wasted in the controlling resistance and poor speed regulations results for the lower speeds.
The armature- resistance control method, therefore, is employed where speeds lower than rating one are required for a short period only an 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 decreases in speed, as in fans and blowers
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| ARMATURE RESISTANCE CONTROL METHOD |
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| SPEED AND TORQUE CHARACTERISTICS| LEARN AS TOPPER |
figure
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3. 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 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 form 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 decreases in applied voltage and a corresponding decrease in speed.
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keywords:-DC shunt motor, speed control of dc series motor, electrical catalogues, determine the cost of labour, learnastopper, Systems of wiring
COMMENTS BELOW YOUR DOUBTS.
ALSO READ:- CATALOGUE
ALSO READ:- ELECTRICAL SCHEDULE
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