Hello Electrical people, you already know
about the different types of motors that are available today and in this article, I shall discuss
about the various techniques used to stop a DC Motor or to bring it
to rest as we cut off the supply. Kindly see that the braking preferred to stop
a DC Motor is Electrical Braking and not Mechanical Braking. In other
words, the motor is stopped by the voltage and current action in the
circuit rather than the mechanical friction brakes on the rotor.Basically, there are
three types
of electrical braking done in a DC Motor:-
1. Regenerative Braking
2. Dynamic Braking
3. Plugging
It is a form of braking in which the kinetic energy of the motor is returned to the power supply system. This type of braking is possible when the driven load forces the motor to run at a speed higher than its no-load speed with a constant excitation.
The motor back emf Eb is greater
than the supply voltage V, which reverses the direction of the motor armature
current. The motor begins to operate as an electric generator.
It is very interesting to note that regenerative braking cannot be used to stop
a motor but to control its speed above the no-load speed of the motor driving
the descending loads.
It is also known as Rheostatic
braking. In this type of braking, the DC motor is disconnected from the supply
and a braking resistor Rb
is immediately connected across the armature. The motor will now work as a
generator, and produces the braking torque.
During electric braking when the motor works as a generator, the kinetic energy
stored in the rotating parts of the motor and a connected load is converted
into electrical energy. It is dissipated as heat in the braking resistance Rb
and armature circuit resistance Ra.
Dynamic
Braking
is an inefficient method of braking as all the generated energy is dissipated
as heat in resistances.
It is also known as reverse current braking.
The armature terminals or supply polarity of a separately excited DC motor or shunt DC motor when running
are reversed. Therefore, the supply Voltage V and the induced voltage Eb
i.e. back emf
will act in the same direction. The effective voltage across the armature will
be V + Eb
which is almost twice the supply voltage.
Thus, the armature current is reversed and a high braking torque is produced.
Plugging is a highly inefficient method of braking because, in addition to the
power supplied by the load, power supplied by the source is wasted in
resistances.
It is used in elevators, printing press etc.
These were the main three types of braking techniques preferred to stop a DC
motor and used widely in industrial applications.