Lesson 23 Speed Control of DC Motor
A regulator system is one which normally provides output power in its steady-state
For example, a motor speed regulator maintains the motor speed at a constant value despite
variations in load torque.  Even if the load torque is removed, the motor must provide sufficient
torque to overcome the viscous friction effect of the bearings. Other forms of regulator also
provide output power; A temperature regulator must maintain the temperature of, say, an oven
constant despite the heat loss in the oven. A voltage regulator must also maintain the output
voltage constant despite variation in the load current. For any system to provide an output, e.g.,
speed, temperature, voltage, etc., an error signal must exist under steady-state conditions.
In many speed control systems, e.g., rolling mills, mine winders, etc., the load has to be
frequently brought to a standstill and reversed.  The rate at which the speed reduces following a
reduced speed demand is dependent on the stored energy and the braking system used. A small
speed control system (sometimes known as a velodyne) can employ mechanical braking, but this
is not feasible with large speed controllers since it is difficult and costly to remove the heat
The various methods of electrical braking available are:
(1) Regenerative braking.
(2) Eddy current braking.
(3) Dynamic braking.
(4) Reverse current braking (plugging) .
Regenerative braking is the best method, though not necessarily the most economic.  The
stored energy in the load is converted into electrical energy by the work motor (acting
temporarily as a generator) and is returned to the power supply system. The supply system thus
acts as a “sink” into which the unwanted energy is delivered. Providing the supply system has
adequate capacity, the consequent rise in terminal voltage will be small during the short periods
of regeneration.  In the Ward-Leonard method of speed