# Important Question in Electrical Traction

In this post, some of important questions of electrical traction is discuss which include efficiency of traction motor, regenerative braking system, passenger services in traction system, speed – time curve, terminologies in traction system, schedule speed, trolley collectors, catenary and types of current collectors are discussed.

## Efficiency of Traction Motor

Describe the effect on the efficiency of the traction motors due to series – parallel control.

Effect on efficiency due to series – parallel control

• When two traction motors employ for series – parallel control, the efficiency of the motor is increased from 50% to 66.7%.
• The efficiency is increased almost 16.7% thereby 15-20% saving of energy is achieved.

What is efficiency of the traction motor at starting?

• The efficiency of the traction motor is approximately 50% at starting because half of electrical energy is wasted in the starting resistor.

## Most efficient Braking

Why the regenerative braking is most efficient than any other electrical braking system?

The regenerative braking is most efficient than other method because in

• Plugging: Energy is drawn from the supply source.
• Rheostatic braking: The kinetic energy of the rotor is wasted into external resistor.
• Regenerative braking: No electrical energy is drawn from the supply but some energy is fed back to the supply source.

## Types of Passenger Services in Traction

What are the different types of passenger services in the traction system?

Types of passenger services

• Main line service: Distance between two stops is more than 10 km.
• Urban service: Distance between two stops is more than 1 km.
• Sub-urban service: Distance between two stops is approximately 1 km to 8 km.

## Periods of Speed – Time Curve

Describe the different periods of speed – time curve for the main line service.

Speed – time periods of the main line service

• Constant acceleration or notching period
• Acceleration
• Free running period
• Coasting period
• Braking period

Terminologies in Traction System

Explain the following terms related to the electrical traction system: ( a ) Notch up period ( b ) Free running period ( c ) Coasting period

Notch up period

• During this period, current taken by the traction motor is approximately constant and voltage is gradually increased by cutting out starting resistance.
• As the current taken by the motor is constant, acceleration is also constant.
• Therefore, this period is known as constant acceleration or notch up period.

Free running period

• The traction motor draws constant power from the supply lines during this period therefore the speed of the train remains constant.

Coasting period

• The supply of the traction motor will be cut off resulting motor speed falls due to friction during this period.
• The rate of decrease of the traction motor is known as coasting period.

In which type of service system there is no free running period?

The distance between two stops is less than 1 km in the urban service therefore there is no free running period.

Explain the following terms regarding to electrical traction: ( a ) Crest speed ( b ) Average speed ( c ) Schedule speed ( d ) Schedule time

Crest speed

• The maximum speed attained by the train during running is known as crest speed.

Average speed

• It is defined as ratio of the distance covered by the train between start and stop station to the time during this distance.

Schedule speed

• It is defined as the ratio of the distance covered by the train between two stations to the time required for the run and stop of the train.

Schedule time

• It is defined as the time required by the train for actual the run and stop.

Explain the following terms : ( a ) Tractive effort ( b ) Specific energy consumption ( c ) Dead weight ( d ) Adhesive weight and ( e ) Co – efficient  of adhesive

Tractive effort

• It is defined as the effective force acting on the locomotive to propel the train.

Specific energy consumption

• It is defined as the ratio of the total energy consumption by the traction motor to the product of the weight of train and distance covered by the train.
• The unit of specific energy consumption is watt - hour / ton / km.

• It is defined as the total weight of train to be propelled by the locomotive.

• It is defined as the total weight to be carried out on the wheel of a locomotive.

• It is the ratio of the tractive effort required to propel the train to the adhesive Weight.

Schedule Speed

What are the factors affecting schedule speed of the train?

Factors affecting schedule speed of the train

• Distance between two stops
• Time duration of the stop
• Crest speed
• Acceleration
• Braking retardation

Types of Current Collectors

State the different types of current collector used in electrical locomotives.

Different types of current collectors

• Trolley collector
• Bow collector and
• Pantograph collector

Trolley Collectors

What are limitations of trolley collectors?

Limitations of trolley collectors

• The trolley collectors are used only for speed limit of 22 – 30 kmph.
• The main drawback of the trolley collector is that it has to rotate through 1800 whenever vehicle turns into reverse direction.

Catenary in Traction System

Explain : Catenary

Catenary

• The trolley wire is supported by another wire for short distance ( which is made of seven strands of steel ) is known as catenary.

Why the small sag is provided in overhead contact wire for high-speed traction system?

• The small sag is provided in order to achieve current collection without any interruption in high-speed traction system.

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