24-11-2012, 06:20 PM
Application of d.c. motors
1Application of d.c..pdf (Size: 1.11 MB / Downloads: 467)
1. Starting
2. Speed control
3. Braking
The speed of the machine has to be increased from zero and brought to the op-
erating speed. This is called starting of the motor. The operating speed itself should be
varied as per the requirements of the load. This is called speed control. Finally, the running
machine has to be brought to rest, by decelerating the same. This is called braking. The
torque speed characteristics of the machine is modified to achieve these as it is assumed
that the variation in the characteristics of the load is either not feasible or desirable. Hence
the methods that are available for modifying the torque speed characteristics and the actual
variations in the performance that these methods bring about are of great importance. When
more than one method is available for achieving the same objective then other criteria like,
initial cost, running cost, efficiency and ease operation are also applied for the evaluation of
the methods. Due to the absence of equipment like transformer, d.c. machine operation in general is assumed to be off a constant voltage d.c. supply.
Starting of d.c. machines
For the machine to start, the torque developed by the motor at zero speed must
exceed that demanded by the load. Then TM − TL will be positive so also is d!/dt, and the
machine accelerates. The induced emf at starting point is zero as the ! = 0 The armature
current with rated applied voltage is given by V/Ra where Ra is armature circuit resistance.
Normally the armature resistance of a d.c. machine is such as to cause 1 to 5 percent drop
at full load current. Hence the starting current tends to rise to several times the full load
current. The same can be told of the torque if full flux is already established. The machine
instantly picks up the speed. As the speed increases the induced emf appears across the
terminals opposing the applied voltage. The current drawn from the mains thus decreases,
so also the torque. This continues till the load torque and the motor torque are equal to
each other. Machine tends to run continuously at this speed as the acceleration is zero at
this point of operation.
DC shunt motor
If armature and field of d.c. shunt motor are energized together, large current is
drawn at start but the torque builds up gradually as the field flux increases gradually. To
improve the torque per ampere of line current drawn it is advisable to energize the field
first. The starting current is given by V/Ra and hence to reduce the starting current to a
safe value, the voltage V can be reduced or armature circuit resistance Ra can be increased.
Variable voltage V can be obtained from a motor generator set. This arrangement is called
Ward-Leonard arrangement. A schematic diagram of Ward-Leonard arrangement is shown
in Fig. 45. By controlling the field of the Ward-Leonard generator one can get a variable
voltage at its terminals which is used for starting the motor.