Bracking of DC motors

Electric Braking
A motor and its load may be brought to rest quickly
by using either (i) Friction Braking or (ii) Electric
Braking. The commonly-used mechanical brake has
one drawback : it is difficult to achieve a smooth stop
because it depends on the condition of the braking surface
as well as on the skill of the operator.
The excellent electric braking methods are available
which eliminate the need of brake lining levers
and other mechanical gadgets. Electric braking, both
for shunt and series motors, is of the following three types: (i) rheostatic or dynamic braking
Strong electric brake for airwheels
1062 Electrical Technology
(ii) plugging i.e., reversal of torque so that armature tends to rotate in the opposite direction and
(iii) regenerative braking.
Obviously, friction brake is necessary for holding the motor even after it has been brought to rest


Electric Braking of Shunt Motors
(a) Rheostatic or Dynamic Braking
In this method, the armature of the shunt motor is disconnected from the supply and is connected
across a variable resistance R as shown
in Fig. 30.21 (b). The field winding is,
however, left connected across the supply
undisturbed. The braking effect is
controlled by varying the series resistance
R. Obviously, this method makes
use of generator action in a motor to
bring it to rest.* As seen from



Obviously, TB decreases as motor slows down and disappear
altogether when it comes to a stop.
(b) Plugging or Reverse Current Braking
This method is commonly used in controlling elevators,
rolling mills, printing presses and machine tools etc.
In this method, connections to the armature terminals are
reversed so that motor tends to run in the opposite direction
(Fig. 30.22). Due to the reversal of armature connections,
applied voltage V and Eb start acting in the same direction
around the circuit. In order to limit the armature current to a
reasonable value, it is necessary to insert a resistor in the circuit
while reversing armature connections.




Regenerative Braking

This method is used when the load on the motor has overhauling
characteristic as in the lowering of the cage of a hoist
or the downgrade motion of an electric train. Regeneration
takes place when Eb becomes grater than V. This happens when
the overhauling
load acts as a
prime mover
and so drives the
machines as a
generator. Consequently,
direction of Ia and hence of armature torque
is reversed and speed falls until Eb becomes lower than
V. It is obvious that during the slowing down of the
motor, power is returned to the line which may be used
for supplying another train on an upgrade, thereby relieving
the powerhouse of part of its load (Fig. 30.23).
For protective purposes, it is necessary to have some type of mechanical brake in order to
hold the load in the event of a power failure.


Electric Braking of Series Motor
The above-discussed three methods as applied to series motors are as follows :
(a) Rheostatic (or Dynamic) Braking
The motor is disconnected from the supply, the field connections are reversed and the motor is
connected in series with a variable resistance R as shown in Fig. 30.24. Obviously, now, the machine
is running as a generator.
The field connections are reversed
to make sure that current
through field winding
flows in the same direction as
before (i.e., from M to N ) in
order to assist residual magnetism.
In practice, the variable
resistance employed for starting
purpose is itself used for
braking purposes. As in the
case of shunt motors,
TB = k2 ?2 N = k3 Ia2 N ... prior to saturation
(b) Plugging or Reverse Current Braking
As in the case of shunt motors, in this case also
the connections of the armature are reversed and a
variable resistance R is put in series with the armature