Induction motors are used at various
places. Speed control of induction motors is quite difficult and that’s why
their use was restricted and DC motors had to be used as their speed regulation was possible. But
when induction motor drives were invented and implemented, they were given
preference because of many advantages over DC motors. Whenever controlling of
motors is done, braking is the most important term, so as with induction
motors. Induction motor braking can be done by different methods, which are-
1. Regenerative braking of
induction motor
2. Plugging Braking of
induction motor
3. Dynamic braking of
induction motoris further
categorized as
● AC dynamic breaking
● Self
excited braking
using capacitors
● DC dynamic braking
● Zero Sequence braking
Regenerative Braking of Induction Motor
We know
the power (input) of an induction motor is given as.
Pin =
3VIscosφs
Here, φs the phase angle
between stator phase voltage V and the stator phase current Is. Now, for motoring
operation φs < 90° and for
braking operation φs > 90°. When the
speed of the motor is more than the synchronous speed, relative speed between
the motor conductors and air gap rotating
field reverses, as a result the phase angle because greater than 90° and the
power flow reverse and thus regenerative braking takes place. The nature of the
speed torque curves are shown in the figure beside. It the source frequency is
fixed then the regenerative braking of induction motor can only take place if
the speed of the motor is greater than synchronous speed, but with a variable
frequency source regenerative braking of induction motor can occur for speeds
lower than synchronous speed. The main advantage of this kind of braking can be
said that the generated power is use fully employed and the main disadvantage of
this type of braking is that for fixed frequency sources, braking cannot happen
below synchronous speeds.
Plugging
induction motor braking is done by reversing the phase sequence of the motor.
Plugging braking of induction motor is done by interchanging connections of any
two phases of stator with respect of supply terminals. And with that the
operation of motoring shifts to plugging braking. During plugging the slip is
(2 - s), if the original slip of the running motor is s, then it can be shown
in the following way.
From the figure beside we can see that the torque is not zero at zero speed.
That’s why when the motor is needed to be stopped, it should be disconnected
from the supply at near zero speed. The motor is connected to rotate in the
reverse direction and the torque is not zero at zero or any other speed, and as
a result the motor first decelerates to zero and then smoothly accelerates in
the opposite direction.
There are four
type of dynamic braking of induction motor or rheostatic braking, we
will discuss about then.
AC Dynamic Braking-
This type of induction motor braking is obtained when the motor is made to run
on a single phase supply by disconnecting any one of the three phase from the
source, and it is either left open or it is connected with another phase. When
the disconnected phase is left open, it is called two lead connection and when
the disconnected phase is connected to another machine phase it is known as
three load connection. The braking operation can be understood easily. When the
motor is running on 1-phase supply, the motor is fed by positive and negative
sequence, net torque produced by the machine at that point of time is sum of torques
due to positive and negative sequence voltage. At high resistance the net torque is
found to be negative and braking occurs. From the figure below the two and
three load connections can be understood.
Self excited braking using capacitors
The figures
above shows the circuit diagram and various characteristics of self excited braking
using capacitors. As we can see from the figure, in this method there
capacitors are kept permanently connected across the source terminals of the
motor. The value of the capacitors are chosen depending upon their capability
to deliver enough reactive current to excite the motor and make it work as a
generator. So, that when the motor terminals are disconnected from the source
the motor works as a self
excited generator and the produced torque and field is in
the opposite direction and the induction motor braking operation occurs. In the
figure (b) the curve A represents the no load magnetization curve and line B is
the current through capacitors, which is given by
Here, E is the stator induced voltage per phase
The speed torque characteristics under self excited braking is
shown in the figure (c). To increase the braking torque and to utilize the
generated energy sometimes external electrical resistance are connected across the stator terminals.
DC Dynamic Braking
To obtain
this type
of braking the
stator of a running induction motor is connected to a DC supply. Two and three
load connections are the two common type of connections for star and delta
connected stators.
Another diagram
is shown below to illustrate how by diode bridge two load coonection can
be obtained within a circuit.
Two Loads DC Dynamic Braking Operation
Now coming to the method of operation, the moment when AC supply is
disconnected and DC supply is introduced across the terminals of the induction
motor, there is a stationery magnetic field generated due to the DC current flow and as the rotor of
the motor rotates in that field, there is a field induces in the rotor winding,
and as a result the machine works as a generator and the generated energy
dissipates in the rotor circuit resistance and dynamic braking of induction
motor occurs.
Zero
Sequence Braking
In this type of braking all the three stator phases are connected in series and
single phase AC or DC is connected across them (as shown in the figure). This
type of connection is called zero-sequence connection, because current in all
the stator windings are co-phasal.
When the connected supply is AC, resultant field is stationery in space and
pulsates at the frequency of supply, when the supply is DC, resultant field is
stationery and is of constant magnitude. The main advantage of this induction
motor braking is that all the stator phases are uniformly loaded. It does not
require large rotor resistance like AC dynamic braking, it does not require
large rotor resistance. The circuit diagram and the speed torque
characteristics are shown below.
The above discussion easily describes the concept of induction motor breaking.