Electrical Traction Interview Question Answer - 1

Important Questions in Electrical Traction

In this post, some of important questions of electrical traction is discuss which include types of traction system, meaning of tramways and trolley buses, supply of track electrification, first electrical traction in India, direct voltage in traction, ac voltage in traction, features of electrical traction motors, dc series motor for traction, linear synchronous speed are discussed.

Meaning of Electrical Traction

What is meant by Electrical traction?

Electrical traction : It is a system that causes propulsion of a vehicle in which the driving force is obtained by steam engine drive, diesel engine drive, traction motor etc. 

Types of Traction Systems

What are the different types of traction systems in the India?

Types of traction system

( 1 ) Electrical traction system: Steam electric drive and Diesel electrical drive and ( 2 ) Non – electrical traction system : Direct steam engine drive and Direct internal combustion engine drive

What do you mean by the Tramways and Trolley buses?

Tramway : The tramway is ordinary buses or vehicle but the only difference is that these buses are run only on the designed track. The tramway is operated by single phase 500V DC supply.  The single overhead conductor acts as a positive which is fed from suitable substation / power station and track acts as a return path.

Trolley bus : The main drawback of the tramway is that it runs along the designed track path. This difficulty is overcome by using two overhead collectors. The trolley buses are electrically operated vehicle and the operating voltage is 600V DC.

Supply for Electrical Traction

What are the supply systems for track electrification?

Supply system for track electrification : DC system, Single phase AC, three phase AC and Composite supply system

Which is first electrical traction in India?

The electrical traction in India was first introduced between Bombay VT to Kurla Hharbour in 1925 on 1.5 kV DC system.

What is the operating Direct voltage in the traction system?

DC Voltage in the Traction : The operating direct voltage for traction system is 600 V, 750 V, 1500 V and 3000 V. The direct voltage of 600 V – 750 V is employed for tramways in the urban areas and1500 V – 3000 V for main line system.

What are the operating voltage and frequency for single phase AC and three phase AC traction system?        

( 1 ) Single Phase : 15 kV – 20 kV at 16.7 Hz and ( 2 ) Three phase : 3000 V – 3600 V at normal operating frequency or reduced frequency at 16.7 Hz

Features of Electrical Traction Motor

What are the features required for electrical traction motor?

Features of the electrical traction motors : 

  • High starting torque
  • Easy speed control
  • Overload capacity
  • Better commutation
  • Series-parallel control
  • Capability to withstand higher temperature under transient conditions

Why Induction Motor is not used in Traction?

Why the three-phase induction motor is not suitable in electrical traction system?

The induction motor suffers from

  • Low starting torque
  • High starting current and
  • Absence of speed control
  • The track electrification for the three-phase induction motor requires high capital cost because the distribution system consists of two overhead phase wire and track rail acts as third phase.

Why DC Series Motor is suitable for Traction?

Which characteristics of DC series motor is suitable for traction system than DC shunt motor?

DC series motor

  • The output power in the DC series motor is directly proportional to √ torque whereas in it is directly proportional to the torque in the DC shunt motor therefore for the same amount of torque DC shunt motor draws more current from the supply than in the DC series motor.
  • The torque in the DC shunt motor is directly proportional to flux per pole and armature current. 
  • But the field flux depends upon the supply voltage therefore small variation in the supply voltage resulting torque is affected. 
  • When the two DC shunt motor are running in parallel, the torque – speed and speed – armature current characteristics must be same otherwise both motors draw unequal current from the supply and resulting unequal sharing of load.

Working of DC Series Motor on AC Supply

Why the DC series motor does not work satisfactory when it is operated at AC supply?

When the DC series motor is operated on AC supply it will rotates but the operation of the motor is not satisfactory due to following reasons.

  • Unidirectional torque is developed at double frequency.
  • The alternating flux causes excessive eddy current loss in the yoke and field core resulting excessive heating of the motor.
  • Poor power factor due to inductance of the armature and field coil.
  • Sparking occurs at the brushes due to high voltage and current induced in the short-circuited armature coil during commutation period.

What are the design modifications made to operate DC series motor on AC supply?

The following modifications are made to operate DC series motor on the AC supply

  • To reduce eddy current, yoke and field cores must be laminated.
  • Power factor is improved by reducing the reactance of armature and field winding.
  • To reduce inductive reactance, the field winding is designed with few turns. This will reduce air - gap flux resulting speed of the motor increase but torque decrease. To obtain same torque, numbers of armature turn increase proportionally. This increase in armature turns increase inductance, therefore net inductance.
  • The inductive reactance of the armature is neutralized by providing compensating winding in series with armature.

Why the AC series motor is not employed for sub – urban services?

AC Series motor

  • The AC series motor develops low torque at the time of starting for a given current as compared to the DC motor due to low power factor.
  • Therefore, AC series motor is not suitable in sub – urban a service in which frequent stop is necessary.

Linear Synchronous Speed

Explain the term : Linear synchronous speed

Linear synchronous speed

  • When the primary (stator) winding is energized by the three-phase alternating supply, magnetic field is produced which is rotate at linear synchronous speed. 
  • Therefore, the linear synchronous speed is given by

            Vs = 2τf meter / sec

              τ = pole pitch and

              f = frequency

Which parameters greatly affect the linear synchronous speed?

Linear synchronous speed

  • The linear synchronous speed does not depend upon numbers of poles, but its value depends upon pole pitch and frequency.
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