Braking scheme of Traction

Motor

Prepared by----------
SURAJ BHUSHAN (04096404915)
NEHA CHAUHAN (63214804915)
BATCH: EE6
Semester: VI
AIM: Braking scheme of traction motor.

THEORY:
Braking of induction motors can be classified mainly in three types--
1) Regenerative braking
2) Plugging or reverse voltage braking
3) Dynamic braking which can be further classified as
A) AC dynamic braking
B) DC dynamic braking
C) Self-excited braking using capacitors
D) Zero sequence braking

1.Regenerative braking: - To explain that regeneration braking for induction motor, we can
take help of the equation Pin = 3 N Is Cosθs here θs is the phase angle between the stator
voltage and stator
current, the simple words whenever this phase angle exceeds 90° (i.e θs>90°) regenerative
braking can
take place. To explain this more clearly and easily we can say that whenever the speed of the
rotor exceeds
synchronous speed, regeneration braking occurs. That is because whenever the rotor rotates at a
speed
more than synchronous speed there is a reverse field occurs which opposes the normal rotation of
the
motor and therefore braking takes place. Main disadvantage of this type of braking is that the
speed
of the motor has to exceed synchronous speed which may not be possible every time. To acquire
regenerative braking at a lower speed than synchronous speed, variable frequency source be
used.
2.Plugging braking: - 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.

3.Dynamic Braking: -There are four types of dynamic braking of induction motor

A) 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 connections. 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.
 B) 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.

C) Self excited braking using capacitors: - 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 are 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.
 D) 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. This type of connection is called zero
sequence connection, because current in all the stator windings are co-phase. 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.

PROCEDURE:
1. Make the connections as per wiring sequence.
2. Check the interconnections as per above circuit diagram
3. Make MCB ‘ON’ on the EMT1 panel
4. Press the start button on EMT1 panel, now motor start running continuously.
5. First measure the nominal time to stop the motor without applying any braking
6. Press the stop button on EMT1 panel, measure nominal time to stop motor.
7. Set the timer manually for 1.2 Sec. time on EMT71B panel.
8. Press the start button on EMT1 panel, now motor start running continuously
9. Now to apply AC dynamic braking press the start button on EMT71A panel, the R phase
of AC immediately disconnected and resistors are connected to rotor, simultaneously the
counter will start to count time.
10. After 1.2 Sec. the rotor will stops the running & simultaneously counter will stops the
counting.
11. After 1.2 Sec. EMT71 also trips EMT1 panel, 3Φ AC supply disconnected.
12.Reset the elapsed counter for each new reading to measure breaking time.

OBSERVATION:
SN Applied voltage to stator Nominal time to stop Elapsed time measured
(Vac) motor without braking with braking (Seconds)
(Seconds)
1. 415 6

RESULT: From the above observations it is observed that by using dynamic braking method
3 phase slip ring induction motor brakes in 1.0 seconds.

VIVA VOCE QUESTIONS

Q1. Explain the Pluggin braking in traction motor?

Ans. 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.

Q2. What is meant by electrical braking of a motor?

Ans. Dynamic braking is the use of an electric traction motor as a generator when slowing a
vehicle such as an electric or diesel-electric locomotive. It is termed "rheostatic" if the generated
electrical power is dissipated as heat in brake grid resistors, and "regenerative" if the power is
returned to the supply line. Dynamic braking reduces wear on friction-based braking
components, and regeneration lowers net energy consumption. Dynamic braking may also be
used on railcars.

Q3. To save energy during braking-----------------braking is used?

Ans. Regenerative

Q4. What are the application of regenerative braking?

Ans. (a). Regenerative braking is used especially where frequent braking and slowing of drives
is required.
(b). It is most useful in holding a descending load of high potential energy at a constant speed.
(c). Regenerative braking is used to control the speed of motors driving loads such as in electric
locomotives, elevators, cranes and hoists.
(d). Regenerative braking cannot be used for stopping the motor. It is used for controlling the
speed above the no-load speed of the motor driving.