Why is AC motor used in the train engine

 Why is AC motor used in the train engine

 Why is AC motor used in the train engine? today topic is, why 3 phase AC motors are used, in electric train engine, now days? here, I have shown, technology improvement with time, in electric train engine, Olden days, DC motor were being used, afterwards, around year 2000 in India, 3 phase AC motor along with, GTO based control system came, and after some time, now days, 3 phase AC motor, and IGBT based control system are used, this AC motor, along with IGBT based control, are giving the advantage, olden days, this IGBT based control system, was not there. 1st advantage is, 3 phase AC motor, and IGBT based VFD/VVVF, technology provides, better speed control, and better acceleration control, in the engine, VFD means, variable frequency drive, when this control has, variable voltage also, so we call it, variable voltage variable frequency, now 2nd advantage, 3 phase AC motor, and IGBT based technology, provides unity power factor, and very less harmonics, now 3rd advantage, AC motors used in the train are, squirrel cage induction AC motor, this is easier to maintain, this kind of motor than DC motor, then there will not be, any mechanical contacts like brushes, now 4th advantage, AC motors are lighter than DC motor, for equivalent power, now 5th advantage, more effective regenerative braking, because of more effective braking, less wear and tear, of the braking system will be there, more energy saving will be there, and fast money pay back, payback is due to more saving, in the electricity bill, now I will explain, what is the payback period, pay back period, suppose money invested is Rs. 100, and after investing Rs. 100, we save Rs. 5 per month, in our electricity bill, so, to recover full Rs. 100, it will take 100/5 = 20 months, we will recover full Rs. 100 in 20 months, so this is called the pay back period, 6th advantage is, IGBT based control, with 3 phase AC induction motor, gives better performance, for a given weight and volume, 7th advantage is, AC motor in the train, with IGBT based control provides, better tractive effort, and higher adhesion level, I will try to explain these 2 points now, I have shown a train here, this is engine, this is rail, and these are wheels, when we have a CAR, there will a tyre over the wheel, and road will be rough, so, when we run the motor, or engine of the car, car will start moving, but in the train, there is no tyre over the wheel, so, it is not so easy, that we run the motor, and train start moving, I will explain you now, here, this is rail, this is wheel, gears, and motor, when motor is running, then 1st, power transfers to gears, then wheel, and a tractive force, is generate in the wheel, which try to rotate this wheel, it will not move, because for movement, rail is also required, then between wheel and rail, there will be friction, and there will be weight, like this, this together form adhesion, now this tractive force, and adhesion together, (tractive force is this), and adhesion is, in between wheel and rail, this and this together, make the train to move, here, more the utilization of friction, ( friction is between this and this), train motion will be better, in the case of the AC motor, along with IGBT based control system, utilization of the friction, is much better, here, I have given equations, and definition of adhesion variable, Adhesion variable is the ability, of the engine / locomotive, to convert the available friction, into the usable friction, at the wheel and rail interface, here, you can see 2 trains, running at the same speed, but here, wheel is moving faster than speed, so wheels are slipping here, this slip is due to, tractive effort more than adhesion level, here again, both the trains are moving, at the same speed, but 1 wheel of this train, is moving faster, so this wheel is slipping on the rail, this is due to, tractive effort more than adhesion limit, only for this wheel, in this train, both the trains are moving, with same speed, but one wheel of this train, is not moving properly, speed is less, this is sliding on the rail, this happens, if we apply the brake on the train, this is due to braking effort, is more than adhesion limit, we have to correct, the torque level here, this is tractive effort, and this is speed, this plot is tractive effort vs. speed, and this is adhesion level, this tractive effort, should be as high as possible, but should be less than adhesion limit, other wise wheel will start slipping, so, in the case of AC motor, along with IGBT based control, we can keep tractive effort, at high level, just below the adhesion limit, so utilization of tractive effort, is much more, this is slip control concept, between rail and wheel, this is time, this is torque, train speed is shown using red color, and speed is increasing with time, similarly, wheel wheel speed is shown in blue color, that also in increasing with time, wheel speed and train speed, should be equal, it is shown up to here, at this point, wheel speed, becomes more than train speed, in that case, wheel will start slipping on the rail, this difference is measured, using control system, and value of torque applied is reduced, until slip becomes zero, or speed of wheel, and train becomes equal, at that time, torque value is brought again, to normal condition, this concept of control, works better, with AC motor, and IGBT based control system, 8th advantage is, this one , I explained just now, better tractive effort, and higher adhesion level, by slip and slide control, 9th advantage is, better tractive effort, and higher adhesion level, by different power, to different axle or bogies, I will try to explain this now, during movement of the train, loading or weight distribution, is not uniform on all axles or wheels, so power or torque requirement, is different for different wheel, I have shown in this diagram, that 2 different control. and 2 different motor, are driving wheels, so depending on the requirements, we can adjust, different torque levels, for different wheels, such that wheel will not slip, this problems becomes more, when train is moving on a slope, or train is just starting, now, some big trains, will have more than 1 engine, similarly metro train, will have more than 1 engine, it is distributed in many bogies, suppose this is one engine, and this another engine, in such cases, torque and power requirement, will be different, for this engine, and this engine, then using control, we can set different torque value, here, and here, this means, power will be different, for different bogies or engine, this way, power and tractive force available, can be used more properly, in case of AC motor, along with IGBT based control system, today, we will close now,