What is VFD & Why VFD is used in the train

  

What is VFD & Why VFD is used in the train.

  Today topic is, What is VFD & Why VFD is used in the train. VFD means varaible frequency drive. In VFD system, speed of motor is controlled by changing frequency. Now here, we have one 3 phase AC motor. This is 3 phase supply. Generally it will be induction motor. Speed of 3 phase AC motor, depends opon the frequency of AC supply. Sp speed of motor can be controlled, by changing the frequency of AC supply. Now synchronous speed of AC motor, is given by formula, Where f is the frequency of the supply. And P is the number of poles. suppose  f is 50 Hz, and P is 2. Then sysnchronous speed will be, Revolution per minute. This is synchronous speed. Actual speed will be little less, About 2 to 5 % less than this. VFD system. Here I have shown the VFD system. This consist of 4 parts. 1st is rectifier. 2nd part is DC link. 3rd part is inverter. 4th part is 3 phase AC motor. Now days, in train engine, we use 3 phase AC motor. Earlier days, DC motor were used. By now days, in latest technology, 3 phase AC motor are used. Input will be 1 phase AC. Rectifier function is, to convert single phase AC in to DC. Rectifier is made using diode also, using IGBT also. Latest technology is IGBT based. Because of IGBT based control, we can get unity power factor here. This is called the DC link. One capacitor is there. Voltage across this capacitor, may be 650 volt DC, may be 750 volt DC or may be 1800 volt. It may be anything depending upon the system. Inverter converts DC in to 3 phase AC. This also made using IGBT nowdays. Now to control the motor speed, frequency of this AC supply need to be controlled. That also is done by this inverter. This inverter does 2 jobs atleast, One is, to make 3 phase AC supply, 2nd function is to control the frequency. But in traction system, there may be so many controls. Simplest one is to control the frquency here, to control the motor speed. This is inverter. It coverts DC into 3 phase AC supply. You may be aware of 3 phase rectifier, which converts 3 phase AC into DC. But inverter is reverse type. This convert DC into AC. There are 6 switches. These are IGBT. They will be becoming ON & OFF in sequence. If this becomes ON, we get +ve voltage here. I have shown here in green color. If this becomes ON, we get negative voltage here. So these switches will be becoming ON & OFF. I have shown only 1, 2, 3, 4, 5, 6 times becoming ON & OFF. Actually these will be becoming ON & OFF many times. When you filter this green color, you get red color, which is AC. Here IGBT is ON for longer duration. So we get more voltage. Here ON period is less. So we get less voltage. So this is AC. I have shown only one phase. There will be 3 phases like this. And this is the time period, for this red color AC. Frequency will be equal to 1/T. If you want motor speed more, so we want frequency more, So we have to reduce T. So this will have to be reduced. So this inverter does 2 functions mainly. One is to generate 3 phase AC, And 2nd is to control the frequency. VFD controls. The types of controls for VFD, can be basically devided in 2 parts. One is scalar control or V/f control. 2nd is vector control. In scalar control, we keep V/f constant. V/f decides the flux. Flux we try to keep constant for better performance. Suppose we want motor speed double. We will increase the frequency to double. Then voltage also, we will make double. Suppose we want motor speed half. Then f will become half. Then V also will become half. But still speed will become half. This scalar type VFD control, are low performance type. They are simple control. And price also is low. In market, generally for small size motor, 3 phase motor, we get VFD control, those are scalar type. Because price is low. 2nd is vector control. These types are used in train engine nowdays. They have high performance. They have very omplex control. Price is also very high. And good steady & transient response. When motor speed is changing. We call it transient condition. When motor speed is steady, We call steady response. In both conditions, this vector control system will give good respose. Now benefits of VFD. First is, performance is very high. Then energy saving. VFD system consumes less electricity. In case of train & railways, electricity expenditure are very high. They want to reduce it. To reduce the electricity expenditure, They use vector type VFD. Then regenerative breaking is possible here. What is regenerative breaking ? When train is moving, It stores lot of energy because of very high mass of the train. So when we apply the break, that whole energy or part of the energy, is converted back to main power. That is called the regeneration or regenerative breaking. Means, train energy is converted, into electrical energy back to AC system. Then very good transient response. Transient means, just now I told, When speed is changing. When motor is starting, train is about to start from station, That will be transient response. So VFD control provides very good transient response. So we should feel less jerk. Very good steady respose. Steady means, constant speed. When train is running at constant speed, Then also respose of the system is very good. That, when we use vector type control. That is very complex system. High power factor. If we use IGBT based recrifier in VFD system, Then unity power factor, or about unity power factor can be achieved. Soft starting, when we start the train, that time we should start slowly, not with jerk. That is the meaning of soft starting. This is required in train, when train starts. It should not move suddenly. It should move slowly. Then torque control. We will not go in detail about torque. But as an example, when we start any vehicle, say train. Then lot of torque is required just to start. That is why, we use 1st gear during staring of the car. Because 1st gear gives more torque. So torque control is possible in VFD. That to, in vector type VFD system One more thing, I will tell about transient. That input supply will be changing. When input supply changes, that is also called transient condition. During that time, VFD system has very good transient response. Today we will close now.