Features of Traction Drives:
Various Features of Traction Drives are:
1. Large torque is required during start and acceleration in order to accelerate the heavy mass.
2. The motor is subjected to torque overloads during acceleration and when negotiating up gradients.
3. Because of economic reasons single phase supply is used in ac traction. Indian Railways employ 25 kV, 50 Hz single-phase supply.
4. The locomotive ratings can be 6000 HP and higher. Again because of economic reasons the substation ratings are not very high. Consequently, the traction supply is weak and locomotive supply voltage varies in wide limits. In 25 kV ac traction, the supply voltage may vary from 19 to 27.5 kV on continuous basis. For a short duration (around 10 sec) the voltage may vary from 17.5 to 29 kV.
5. The supply has sharp voltage fluctuations, including discontinuity when the locomotive crosses from one supply section to another.
6. Since, the supply is weak in nature, the reactive power has very adverse effect. Therefore, power factor should not be allowed to be lower than 0.8 and it should never be allowed to be leading to avoid over voltages.
7. The harmonics injected into the source, both in ac and dc traction, can cause maloperation of signals and interference in telephone lines.
8. Dynamic braking is widely used. Mechanical brakes are also provided for use when the train is stationary.
9. Regenerative braking is used when the energy saved is large enough to justify the additional cost of the drive and transmission lines and is possible only when the system is able to absorb the energy generated. Mostly, system is not able to absorb all the energy that is generated. Therefore, dynamic braking is combined with regenerative. The energy which cannot be absorbed is dissipated by dynamic braking. Such a combination of regenerative braking and dynamic braking is known as composite braking.
10. When regenerative braking is used, the distribution system is subjected to higher voltages during regeneration. The distribution system and the drive equipment must be designed to account for this.
11. Wheel slip should be avoided. The coefficient of adhesion, which is a measure of the tendency for wheel slip, depends on the following factors:
- Coefficient of friction between wheels and the rail.
- Nature of motor speed-torque characteristics—a characteristic with low speed regulation is preferred.
- Series-parallel connections of motors.
- Smoothness with which the torque can be controlled.
- Speed of response of the drive.
12. In a locomotive more than one motors are fed from a converter. The load sharing between motors is more uniform when the motors have a large speed regulation.
13. Suburban (including underground) trains have more than one motor coach. Electrical interconnections between motor coaches are provided so that the drive for all motor coaches can be controlled automatically from the master controller in the motor coach in front. In main line trains also sometimes more than one locomotive are required to drive a train. In locomotives also, arrangement for inter-connections may be provided so that both the locomotives can be controlled from the master controller of any one locomotive.