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Introduction Requirements of a traction system Control of DC motors Electronic Speed Control Methods for DC Motors Pulse Width Modulation Braking operation in DC motors Recent Trends in Electric Traction Conclusion References
Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion (traction). Traction motors are used in electrically powered rail vehicles such as electric multiple units and electric locomotives
High starting pulling effort in order to have rapid acceleration. Equipment capable of withstanding large temporary overloads for short periods. The locomotive should be self contained and able to run on any route. Capability of withstanding voltage fluctuations and temporary interruption of supply. Parallel running usually more than one motor (two or four motors ) should be possible.
D.C series motors. Single phase A.C series motors Three-phase Induction motors
D.C series motor develops high torque at low speeds & low torque at high speeds. This is the exactly the requirement of traction units. In case of dc series motor , up to the point of magnetic saturation, torque developed in proportional to the square of the current. Therefore, dc series motor requires comparatively less increased power input with the increase in load torque. Thus the series motor are capable of withstanding excessive loads. Speed of dc series motor can be controlled by various methods.
1. The construction cost of an ac series motor is muchmore easier than of a dc series motor.2. The starting torque of a.c single phase motor islower than that of dc series motor due to poor powerfactor at the start.3. The speed of an ac series motor may be controlledefficiently by taps on a transformer. Which is notpossible in case of a dc series motor.
It has simple & robust construction. Trouble free operation. Less maintenance. High voltage operation consequently requiring reduced amount of current. Automatic regeneration are the main advantages of 3- ph. Induction motor for traction. But due to their flat speed- torque characteristics, constant speed operation, developing low starting torque, drawing high starting current, complicated speed control systems they are not suitable for electric traction work.
Short form of SEParate EXcitement of traction motors where the armature and field coils of an electric motor are fed with independently controlled current. This has been made much more useful since the introduction of thyristor control where motor control can be much more precise. SEPEX control also allows a degree of automatic wheel slip control during acceleration
The purpose of a motor speed controller is to take a signal representing the required speed, and to drive a motor at that speed.• Motor speed can be controlled bycontrolling-- Armature voltage (Va)- Armature current (Ia)
It is obvious that the speed can be controlled by varying flux/pole, (Flux Control) resistance Ra of armature circuit (Rheostatic Control) applied voltage V (Voltage Control)
The above methods have some demerits A large amount of power is wasted in the controller resistance. Hence, efficiency is decreased. It needs expensive arrangement for dissipation of heat produced in the controller resistance. It gives speeds below the normal speed.
Compared to the electric and electromechanical systems of speed control, the electronic methods have higher accuracy greater reliability quick response higher efficiency
The main principle is control of power by varying the duty cycle. Here the conduction time to the load is controlled. Let for a time t1, the input voltage appears across the load ie ON state. for t2 time the voltage across the load is zero. The average voltage at output is given by Va = 1/T vodt = t1/T Vs = ft1 Vs = kVs where T is the total time period =t1+t2 k = t1/T is the duty cycle
• Pulse width modulation (PWM) is a method for binary signals generation, which has 2 signal periods (high and low).• The width (W) of each pulse varies between 0 and the period (T).• The duty cycle (D) of a signal is the ratio of pulse width to period. • D=(t1+t2) / T
• The motor is on for most of the time and only off a short while, so the speed is near maximum• The switch is on 50% and off 50%.• The motor will only rotate slowly.
Commonly two type braking system used in traction motors that are › Dynamic braking › Regenerative braking
The motors become generators and feed the resulting current into an on-board resistance. When the driver calls for brake, the power circuit connections to the motors are changed from their power configuration to a brake configuration and the resistors inserted into the motor circuit. As the motor generated energy is dispersed in the resistors and the train speed slows, the resistors are switched out in steps, just as they are during acceleration.
The motors become generators and feed the resulting current back into the supply system a train could use its motors to act as generators and that this would provide some braking effect if a suitable way could be found to dispose of the energy. Trains were designed therefore, which could return current, generated during braking, to the supply system for use by other trains.
PWM duty cycle control techniques enable greater efficiency of the DC motor . PWM switching control methods improve speed control and reduce the power losses in the system. The pulses reach the full supply voltage and will produce more torque in a motor by being able to overcome the internal motor resistances more easily.
The main Disadvantages of PWM circuits are the added complexity and the possibility of generating radio frequency interference . It can give speed below the full speed, not above. It cannot be used for fast controlling of speed.
1. Power is purchased from supply authorities who are responsible for the operation & maintenance of 132/110 kv transmission lines2. Supply authorities will give only voltage supply of 132/110 kv at the substation3. Modern microprocessor technology and the availability of efficient and compact power components have changed that picture. In 3-phase AC locos, the input (single-phase AC) from the OHE is rectified and then 3- phase AC is generated from it, whose voltage, phase, and frequency can be manipulated widely, without regard to the voltage, phase, frequency of the input power from the OHE
Multilevel converters: The main advantage of this kind of topology is that it cangenerate almost perfect current or voltage waveforms, becauseit is modulated by amplitude instead of pulse-width. That meansthat the pulsating torque generated by harmonics can beeliminated, and power losses into the machine due to harmoniccurrents can also be eliminated. Another advantage of this kindof drive is that the switching frequency and power rating of thesemiconductors is reduced considerably