16
|
At
which condition linear induction motor causes regenerative braking?
|
|
The regenerative braking in the linear induction motor occurs when
rotor plate rotates at a speed higher than the linear synchronous speed.
|
17
|
What
are the main disadvantages of linear induction motor?
|
|
Disadvantages of Linear induction
motor
·
Low
power factor and poor efficiency due to large air-gap
·
Higher
capital cost for providing collector for the traction purpose.
|
18
|
What
are the applications of linear induction motor?
|
|
Applications of the Linear
induction motor
·
High
speed railway traction
·
Metallic
belt conveyors
·
Trolley
cars
|
19
|
What
are the different types of braking system?
|
|
Types of
braking system
·
Mechanical
braking and
·
Electrical
braking
( 1 ) Plugging
( 2 ) Rheostatic
braking and
( 3 ) Regenerative
braking
|
20
|
What
are the advantages of electrical braking over mechanical braking?
|
|
Advantages of electrical braking
over mechanical braking
·
The
electrical braking is smooth and fast.
·
No wear
tear
·
Less
maintenance
·
Less
heat is produced as compared to mechanical braking
·
It is
possible to fed back electrical energy back to the supply in the regenerative
braking therefore it will reduce the operating cost.
|
21
|
What
is main disadvantage of electrical braking over the mechanical braking in the
traction?
|
|
Disadvantages of electrical
braking
·
The
electrical motor acts as a generator and electrical brake stop the motor but
it cannot be hold stationary therefore to stop the traction motor in addition
mechanical braking must be necessary.
|
22
|
What
do you mean by the Plugging?
|
|
Plugging
·
It is
one of the methods for applying braking to the electrical motor.
·
The
motor is reconnected such that it will produce torque in the opposite
direction resulting the motor will decelerates until speed becomes zero and
again it accelerates in the reverse direction.
·
The
brake is applied immediately when the motor speed becomes zero and it try to accelerate
in the reverse direction.
|
23
|
How
the plugging is achieved in the ( a ) DC shunt motor ( b ) DC series motor ( c
)Three phase induction motor
|
|
DC shunt motor
·
Reversing
either armature or field connection
DC series motor
·
Reversing
either armature or field connection
Three phase induction motor
·
Reversing
any two phases of the three phase stator winding. The brake is applied when
induction motor produces torque in the reverse direction.
|
24
|
What
is slip during plugging period in the three phase induction motor? How the
rotor winding is protected during plugging?
|
|
·
The slip
during braking
s = [ ( – Ns )
– Ns ] / Ns
= (– 2 )
·
The
rotor induced emf is directly proportional to product of the slip and voltage
E2 α sV
·
The
rotor induced emf is twice the supply voltage ( E2 α 2V
) therefore the rotor winding must be protected to avoid damage it. The rotor
winding is protected by providing additional insulation during braking.
|
25
|
How
the plugging can be achieved in the synchronous motor?
OR
How
the damper winding assists to achieve braking torque in the synchronous
motor?
|
|
Plugging in the Synchronous motor
·
The
stator magnetic field and rotor both rotates at the synchronous speed in the
synchronous speed.
·
Therefore
interchanging any two supply leads does not produce braking action.
·
The
field winding is provided with damper winding in order to stop synchronous
motor. There will not any induced emf in the damper winding at synchronous
speed.
·
Whenever
the direction of the stator magnetic field change, there will be induced emf
in the damper winding. It will produce torque in the opposite direction (known
as braking torque) which helps to rest the motor.
|
26
|
What
is dynamic or Rheostatic braking?
|
|
Dynamic braking
·
The
electric motor is disconnected from the supply during braking period and it
is reconnected to electrical resistance in the dynamic braking.
·
The
kinetic energy of the rotor is dissipated in the resistance in this way.
|
27
|
How
the rheostatic braking applied to the ( a ) DC shunt motor ( b ) DC series
motor and ( c ) three phase induction motor?
|
|
DC shunt motor
·
The
armature is disconnected from the supply and connected to the external
resistance during braking period. The DC shunt motor acts as generator and
kinetic energy of the rotor is dissipated in the resistance.
DC series motor
·
The
armature and field winding of DC series motor are disconnected from the
supply and connected to the external resistance.
·
Only
armature winding connections are reversed in order to achieve braking torque.
Three phase induction motor
·
If the
induction motor is disconnected from the supply, there is no rotor emf, no
rotor current and therefore no braking torque.
·
The DC
supply is given to the stator of the induction motor in order to avoid above
difficulty. It will produce constant air gap flux which is cut by rotor
conductors.
·
The
braking torque is produced by the rotor conductors. The braking torque is
controlled by controlling either DC supply or by varying rotor resistance by
suitable arrangement.
|
28
|
Which
parameters affect the rheostatic braking torque in the DC shunt motor?
|
|
Rheostatic braking
·
The
rheostatic braking torque is directly proportional to field current and
armature current. The field current remains constant in the DC shunt motor
therefore the braking torque is directly proportional to the armature
current.
|
29
|
What
is Regenerative braking?
|
|
Regenerative braking
·
The
regenerative braking is achieved when the back emf is more than the supply
voltage.
·
The DC
motor acts as generator and the direction of the armature current is reversed
therefore the torque developed by the armature is reversed.
·
No
energy is drawn from the supply during regenerative braking period but some
energy is fed back to the supply side.
|
30
|
Describe
the purpose of series – parallel traction motor control.
|
|
Series – parallel control
·
The
series – parallel control employs in two traction motors.
·
The two
DC motors are connected in series in order to achieve high starting torque
and low speed during starting condition.
·
The two
DC motors are connected in parallel in order to achieve high speed and low
torque during running condition.
|
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