Electric Traction MCQ Questions and Answers

Answer.1. High mechanical strength

Explanation:-

Special features of traction Motor

The general features of the electric motors used for traction purpose are:

1. Mechanical features
2. Electrical features

Mechanical features

A traction motor must be mechanically strong and robust and it should be capable of withstanding severe mechanical vibrations.

The traction motor should be completely enclosed type when placed beneath the locomotive to protect against dirt, dust, mud, etc.

In overall dimensions, the traction motor must have a small diameter, to arrange easily beneath the motor coach. A traction motor must have minimum weight so the weight of locomotive will decrease. Hence, the load carrying capability of the motor will increase.

Electrical features

High-starting torque:- A traction motor must have high-starting torque, which is required to start the motor on load during the starting conditions in urban and suburban services.

Speed control:- The speed control of the traction motor must be simple and easy. This is necessary for the frequent starting and stopping of the motor in traction purpose

Dynamic and regenerative braking:- Traction motors should be able to provide easy simple rheostatic and regenerative braking subjected to higher voltages so that the system must have the capability of withstanding voltage fluctuations.

Temperature:- The traction motor should have the capability of withstanding high temperatures during transient conditions.

Overload capacity:- The traction motor should have the capability of handling excessive overloads.

Parallel running:- In traction work, more number of motors need to run in parallel to carry more load. Therefore, the traction motor should have such speed-torque and current-torque characteristics and those motors may share the total load almost equally.

Answer.1. Coasting

Explanation:-

The above diagram represents the speed-time curve of suburban service. The AB represent the coasting period.

In suburban service, the distance between two adjacent stops for an electric train is lying between 1 and 8 km. In this service, the distance between stops is more than the urban service and smaller than the mainline service. The typical speed—time curve for suburban service is shown in Fig.

The speed—time curve for urban service consists of three distinct periods they are:

(i) Acceleration.

(ii) Coasting.

(iii) Retardation.

For this service, there is no free-running period. The coasting period is comparatively longer since the distance between two stops is more. Braking or retardation period is comparatively small. It requires relatively high values of acceleration and retardation. Typical acceleration and retardation values are Iying between 1.5 and 4 kmph and 3 and 4 kmph, respectively.

Answer.4. No free running period

Explanation:-

In suburban service, the distance between stops is more than the urban service and smaller than the mainline service. In this case, the distance between stops averages from 3 to 5 km over a distance of 25 to 30 km from the city terminus. Here, also, high rates of acceleration and retardation are necessary. 

Since the distance between the two stations is small therefore there is no running period.

Answer.4. Distance traveled

Explanation:-

The area under the speed time curve gives the distance traveled during, given time internally and slope at any point on the curve toward abscissa gives the acceleration and retardation at the instance, out of the two speed—time curve is more important.

Answer.3. t₂ – t₃

Explanation:-

The duration of braking is represented by t₂ – t₃. In suburban service braking or retardation, the period is comparatively small. It requires relatively high values of acceleration and retardation. Typical acceleration and retardation values are Iying between 1.5 and 4 kmph and 3 and 4 kmph, respectively.

Answer.1. Plugging

Explanation:-

Plugging

• Plugging It is also called reverse current braking.
• The connections of the motor are reversed, reversing the direction of torque, thereby bring it to a quick stop. For this, either the direction of field current or the direction of armature current is reversed.

• The field circuit usually has a large time constant due to the high value of inductance, so the time taken to bring the field current to zero is large. Hence, it is usual to reverse the armature current.
• On reversing the armature current the supply voltage to the armature and back emf developed in it are additive. So, this condition is worse than the starting condition as the applied voltage to the armature is approximately double (V+E = 2V) to the supply voltage.
• Hence, an external resistance is inserted in the armature circuit simultaneously with the reversal of the armature current. Braking torque can be regulated by varying the magnitude of this resistance.

The disadvantages of this method are:

1. The kinetic energy of the motor is dissipated in the external resistance in the form of heat. So, this method of braking is inefficient.

2. The braking in this method fails in case of failure of the supply.

Answer.2. Rheostatic

Explanation:-

Rheostatic braking

• Rheostatic braking is also called dynamic braking.
• 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.
• In rheostatic braking, the armature is cut off and the armature and the motor are connected in series with variable resistance. The field is still energized.
• The motor, during braking, works as a generator feeding the resistance.

• The kinetic energy of the motor is converted to electrical energy during the period of braking until finally, the motor stops.
• The electrical energy is dissipated in the resistance in the form of heat.
• In the case of a dc series motor, the connections are as shown in Fig. The supply is disconnected and the motor is connected to a resistance.
• Care must be taken that the direction of current through the field winding does not change.
• The external resistance should be of a value less than that of the critical resistance, which is a condition for a self-excited generator to build up.
• The magnitude of the braking torque can be regulated by varying the value of external resistance.

The disadvantage of the method is that the kinetic energy of the motor is dissipated as heat in the resistance and it is a wastage. The armature current and so the magnetic field changes with the change in speed. Therefore, the speed-torque curve is not linear in case of the series motor.

Answer.4. Regenerative braking

Explanation:-

Regenerative braking

In regenerative braking, the motor is not disconnected from the supply but is made to run as a generator by utilizing the Kinetic energy of the moving train. Electrical energy is feedback to the supply. The magnetic drag produced on account of generator action offers the braking torque. It is the most efficient method of braking.

• In regenerative braking, the motor, instead of being disconnected from the supply, remains connected to the supply and returns the braking energy to the supply line. So, the wastage of kinetic energy in the plugging and rheostatic braking methods is prevented in the regenerative braking. This method the drives cannot be brought to the standstill. It reduces the speed to a minimum permissible value.
• In case of a d.c. series motor, increase in excitation results decrease in speed. As such it is not possible to get e.m.f. more than voltage. It is not possible to make field current more than the armature current. Hence regeneration braking with series motors is not possible. But can be used with traction motors with some special arrangements.
• In one method of regenerative braking with a single series motor, the motor is provided with a main series field winding and a few auxiliary field windings.

• For normal running, all the field windings are connected in parallel as shown in above Fig.
• During braking period the auxiliary windings are connected in series with each other and this series connected windings are connected in parallel with the series motor, and then, connected across the supply.
• The machine acts as a compound generator, slightly differential. Such an arrangement is quite stable. A change in the line voltage causes a corresponding change in excitation which produces a change in the induced emf. Thus, the change in line voltage is compensated.

Advantages:-

Following are the advantages of regenerative braking:

• Reduced energy consumption.
• Less wear and tear of brake blacks, wheels and, track, consequently less maintenance required by these items.
• A relatively small amount of brake block dust formed which increases the life of bearings.
•  Ease and safety with which heavy loads can be hauled over steep gradients.
• The higher value of braking retardation is obtained 60 that the vehicle can be brought to rest quickly and running time is considerably reduced.
• Higher speeds are possible while going down the gradients.
• It is the most efficient method of braking.

Disadvantages:

• Increased capital cost as motors of heavier size is employed.
• Additional cost on control equipment with consequent complications in methods of operation and control.
• Mechanical braking is required at a speed of 6.5 Km/h to bring the locomotive to rest.
• Due to recuperated energy, the operation of the sub-stations become complicated and difficult.

Answer.4. All of the above

Explanation:-

The process of bringing the motor to rest within the predetermined time is known as braking.

A good braking system must have the following features:

1. Braking should be fast and reliable.
2. Force exerted by it should be controllable
3. The equipment to stop the motor should be in such a way that the kinetic energy of the rotating parts of the motor should be dissipated as soon as I brakes are applied.

Braking applied to bring the motor to rest position is of two types and they are:

1. Electric braking.
2. Mechanical braking.

Mechanical braking

In this process of braking, the kinetic energy of the rotating parts is dissipated in the form of heat by the brake shoes of the brake lining that rubs on a wheel of vehicle or brake drum.

Electric braking

In this process of braking, the kinetic energy of the rotating parts of the motor is converted into electrical energy which in turn is dissipated as heat energy in resistance or in sometimes, electrical energy is returned to the supply. Here, no energy is dissipated in brake shoes.

Advantages of the electric braking over the mechanical braking

Saving; no replacement cost: In case of mechanical braking, due to friction there is excessive wear on brake lining which needs frequent and costly replacement. But in electric braking, no such replacement is required hence saves money.

1. Small maintenance cost: In case of mechanical braking there is more wear and tear which increases the maintenance cost. There is no wear and tear in case of electric braking which reduces the maintenance cost to very low value.
2. Energy saving: In regenerative braking, a part of the energy is returned to the supply this reduces the operating cost. This is only possible in electric braking.
3. Reliability: In case of mechanical braking, the heat produced at brake blocks may result in failure of brakes. But in the case of electric braking, the heat produced does not affect the braking system this improves its reliability.
4. Smooth operation: Mechanical braking may produce jerks but electric braking never produces jerks.
5. System Capacity: By employing electric braking the capacity of the system can be increased by way of higher speeds and handling of heavy loads.

Disadvantages of the electric braking

• It cannot hold the machine or load stationary: In the case of electric braking, the driving motor starts working as a generator during the braking period. But at stand-still, the machine (motor) cannot work as a generator. Therefore, an electric brake can reduce the speed of the machine or load and bring it to almost at stand-still condition but it cannot hold it stationary. Hence, electric brake alone cannot fulfill the purpose, friction (mechanical) brake is always provided in addition to it.
• Cost: Addition equipment employed for electric braking increases the cost.
• Choice of the motor is limited: In the case of an electric braking system, the motor has to function as a generator and must have suitable braking characteristics. Thus, the choice of motor is limited.

Answer.1. The track is connected to negative of the supply

Explanation:-

We use 25kv ac supply which runs in overhead lines. Now pentagraph is the device which is used to give that supply to train. The overhead catenary wire acts as a positive supply. The tracks below are earthed via each of the electric poles on the side of the tracks. The positive of the wire goes via resistor banks to the positive point on the motor. The negative terminal of the motor is connected to the locomotive’s wheel assembly which passes the current to the earth via tracks i.e the rails acts as a return conductor(negative terminal). Current flows only when the circuit is closed and there should be a potential difference and ground is neutral so it closes the circuit.