30/06/2020

Electrical Traction Interview Question Answer - 2

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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