[0:17]Hello all. Welcome to the course EV, Vehicle Dynamics and Electric Motor Drives. So we are continuing with the discussion on electric motor drives and in particular, we are going to see DC motor drives today. And here we have seen the overview of electric vehicle in the previous introduction. So of all the components which are available within electric vehicle, we are going to discuss this part which is within the blue boundary. And we would be avoiding the auxiliary part which is there installed on the vehicle. So essentially we are going to discuss parts which are related to only electric motor drive and we will proceed further with what are the basic requirements and how to deal with that.
[1:18]So going ahead, let us first understand what are the essential components if we talk about an electric motor drive. So, certainly, when we discuss about electric motor drive, we are discussing about a system where we have electric motor.
[1:45]Which is basically connected through mechanical assembly to a load. In our case, the load is electric vehicle and the motor could be anything of what we have discussed in the previous introduction. The motor drive also involves some kind of power source which is required to supply the electrical energy. In our case, when we are discussing electric vehicle, this is basically the battery bank which is installed on the vehicle. Typically, the voltage level which is given for a vehicle may go up to 400 volts. And nowadays, when we are going for high performance, high power vehicles, the voltage level is exceeding and reaching all the way up to 800 volts. Now, certainly, we cannot connect the battery bank to all the motors directly. We need some kind of interface and that is where our polytronic converter comes into picture.
[3:04]This polytronic converter takes power from this battery bank and supplies the required amount of voltage and current to the motor. The number of connections are dependent on which type of motor we are talking about. Now, the power converter as such may require information about when to operate the polytronic switches. So, basically, we need some kind of intelligence which comes in form of controller. Where we provide the gate pulses as per requirement to the converter. Now, the controller may certainly require certain type of reference what we need. It may ask about the speed reference or the torque reference which is required to be generated by the motor drive.
[4:07]Now, it may also require some kind of feedback from the output quantities. So, sensing unit is required for doing that job.
[4:22]This sensing unit may take feedback of current and voltage at the converter output side. It may also take the feedback of speed and or or torque and that feedback is basically given to the controller. Now, when we talk about these components, we have said that the options for motor could be any of the options what we have discussed in the introduction session. So, the motor could be PMSM, permanent magnet synchronous motor or induction motor or brushless DC motor or switched reluctance motor. Based on the motor, we are going to design our power converter, which would match the requirements of motor.
[5:28]Now, the sensing unit has basically two parts. One, which is taking feedback of the electrical quantities, so there could be some kind of hall effect sensors to sense the current.
[5:50]So, current and voltage sensors could be there. And the other part which basically involves the mechanical quantities, so speed and torque. So, we may have some kind of speed encoder or torque sensor. There are fairly accurate speed encoders and torque sensors and current and voltage sensors available in market and these are commercially used in many, many applications for power electronics. Now, if we talk about our controller, there are several options which could be there for a polytronic application. So, these controllers involve DSP based controller, which is commonly provided by Texas instruments and many other industries. There could be FPGA based. control board and etc. There are many other technologies which are being used. Now, when we are talking about these controllers, there are certain components which are essentially available on these boards. So, there may be some analog to digital channels for taking the feedback. There may be some general purpose input output pins for giving the PWM signal and other necessary digital outputs and taking the digital inputs. There may be certain kind of communication which is required for connecting with the other parts of the vehicle and connecting with the driver. This communication may involve can based communication or any other serial communication. We may have certain kind of digital to analog channels also for monitoring the current and voltage and any other quantity which is being processed on the controller. So, after these details, now we are ready to proceed further with the motor drive. Now, when we are saying that, this motor drive we are going to install on vehicle. Basically, we are looking for capability where the drive can be operated in either direction of speed.
[8:19]So, it should be able to move in both the directions, clockwise as well as anti-clockwise. And also, it should be able to generate torque in both the directions. So, when we are trying to accelerate, that time we may require to pump power into the motor. So, typically for a positive speed, we may require to provide positive torque. Now, when we are trying to reduce the speed, the option of dynamic braking should also be available with vehicle. So, our vehicle should be capable of providing negative torque, which is basically regeneration of the mechanical energy back into the electrical source. So, with this, when we are talking about the capabilities of drive, the torque and speed, which are required to be generated by drive, form a four quadrant system.
[9:18]For this course, particularly, I would be considering torque on the Y axis and speed on the X axis.
[9:32]So, ideally, the drive may be required to operate on the complete torque speed plane.
[9:44]However, when we are talking about practical drive, there are restrictions on the mechanical and electrical parts. So, the mechanical assembly may suggest that we are having a restriction of speed up to certain level. So, it can be marked as omega max on the axis. And similarly, when we are going for negative direction of rotation, it can be marked as -omega max.
[10:10]Similar to electrical, similar to mechanical assembly, the electrical components also impose restrictions on the developed torque by the motor, which are basically related to the current carrying capability of motor. So, we may have certain value of torque, which is called as rated torque for the motor. We can call it tau max. And it is equally available for the negative direction also.
[11:11]However, most of the drives, they have restrictions on up to what point the tau max can be applied. Up to what omega. That omega can be called as the rated speed of the motor.
[11:36]Beyond this point, due to several reasons, the torque must be reduced. And in most of the cases, it follow the inverse proportionality with the omega. So, the modified torque speed area or the plane looks like this polygon.
[12:10]So, what we are trying to say that the drive should be capable of operating anywhere within this polygon. And when we say that, we are having different modes of operation for this drive. For example, we are having different quadrants, say quadrant one, where our torque and speed both are positive. So, essentially, we are having positive power output, the P mechanical is positive, which means we are converting electrical energy into mechanical energy. This, when we are doing with positive omega, this is called forward motoring. Now, similarly, if we talk about the third quadrant, what we find that again, though our tau and omega both are negative, but the product has become positive. And because we are going in the negative direction of omega, this is called reverse motoring. Now, considering when tau and omega have opposite polarities. So, if we take the fourth quadrant first, we have positive omega, however, negative tau. So, here we are having negative value of mechanical power output, which means we are talking about some kind of regeneration in the system, mechanical energy being converted to electrical energy. And this with positive speed can be called as forward breaking. Now, the second quadrant, again, when we have tau and omega of opposite polarity, we are having PM less than zero. And so, we can call this as reverse breaking. So, drive should be capable of operating in all four modes and over the entire area of this marked boundary. Now, when we talk about power, the reason where our torque can be taken to maximum value is called as constant torque zone. In this reason, the power basically increases linearly and reaches up to the maximum value, P max. Beyond that, the power graph becomes constant. And we have the rest of the zone can be called as constant power zone, which is applicable for the other part of the graph as well. Now, the other quantities, voltage and flux, we will see specific to the motor how they are behaving and where the values can be kept constant and where it must change. So, moving further, now, when we say that the drive has to generate so much of torque and speed at a given moment, we also need to understand how to do that. How to supply that kind of voltage or current that drive is basically able to operate with such capabilities? So, with this, let us move into the basics of DC motor.
[15:43]Since it's an EV course and we are having the discussion of electric motor drive, we are not going to discuss too many details about DC motor. However, some of the necessary details which could connect with our drive system, we would be discussing over here. So, when we talk about DC motor, there are two terms basically. First thing is the term motor. This essentially says that we are talking about an electromagnetic system. In fact, electromechanical system, where we are trying to convert electrical energy into mechanical energy.
[16:41]We also have capability to regenerate. So, which is why, whenever we are converting the electrical energy into mechanical energy, this is called as motoring mode. And whenever we are doing the opposite, conversion of mechanical energy to electrical energy, this is called generation or generating mode. So, here the drive is regenerating. The other term which suggests about the supply, the nature of the supply what we are going to feed to the motor is basically DC. So, when we say DC motor, we are essentially going to provide DC supply, a DC voltage or DC current to the motor. Now, let us understand some basic principles on which this motor works. How the torque is generated and how it can be used for generating a uniform torque.
[17:51]Now, the principle of operation for DC motor is whenever there is a conductor which is carrying some current and it is placed in uniform magnetic field. Because of Lorentz force, so say this is carrying current I and the field intensity is B, the Lorentz force suggests that the conductor is going to experience a force, which is equal to this. And the direction can be decided by right hand thumb rule.
[18:42]So, if we see that, in this particular picture, what we obtain that this conductor is going to experience a force in this direction. Now, if we place the counter conductor also on the same plane, which means perpendicular to the writing board.
[19:07]And we are having current basically going into the plane for this conductor. Say we name the first conductor as small A and the return conductor as small B, what we find that the force experienced by these two conductors are basically opposite in direction. The magnitude is same.
[19:26]So, if we place this assembly on some kind of mechanical arrangement which is basically hinged at the center, and we pass the current I through it, this is going to experience a certain amount of torque in this direction.
[19:57]Now, if we do not disturb this system beyond this point and allow it to rotate, this is basically going to flip its position. And the conductor A is going to move down and conductor B is going to move up.
[20:22]Now, because of this arrangement, A is going to experience force same as what it was experiencing earlier, the direction is same.
[20:36]For B also, the direction is same and because of the assembly, now the torque direction has flipped. So, which means, if we allow it to move in this orientation, this is going to have an oscillatory motion, which is nothing but going back and forth about the same point. Now, if you want to operate it like a motor, we need to have uniform torque generated by this system. And so, we can generate a speed which is unidirectional. So, this is where certain assembly that is required to be involved in this is called commutator and brush. So, let us understand first through a picture. So, basically what this assembly is doing is that we have arrangement of copper connections with the conductor A and B, for example. And this is mounted on the shaft. So, it can rotate along with shaft and we have the conductors connected on this permanently. Now, when this assembly is rotating on account of torque generated by the motor, this can these conductors move along with this commutator. Now, this this commutator is basically connecting with the supply which is kept outside. For example, battery is kept outside, that is connected through another unit called as brush. This is basically a carbon brush.
[22:28]So, what we find that, as it rotates from its position, the conductor A is going to flip its polarity and it is going to get connected to negative polarity. And conductor B is going to move to positive polarity. So, in effect, what would happen is that compared to the previous case where we had the torque getting flipped.
[23:30]Now, this maintains the direction of torque in the same manner. And so, we can have the direction of rotation as a uniform number. It is not going to alternate with the connection. So, that is what is achieved through this commutator and brush assembly. So, there are certain important components which are involved with the DC motor, we can see it in the next picture. So, as suggested, we have the assembly of commutator and brush, which is shown over here, the brushes and the commutator is installed on the rotor itself. Now, there are two types of coils which are present on this machine. One, which is marked as field coil, which is basically present on the stator. The other one, which is mounted on the rotor, that is called armature coil. The purpose of field coil is to basically maintain the flux in the system. What we have seen as north and south, this could be one potential north and south made by the field coil. So, this works like an electromagnet. Now, the armature coil is basically the component which is basically responsible for generating torque in the machine by having current carrying conductor within the North South field. Just a very, very basic layout of DC motor is shown over here. The modern machines may have many more poles and a more complex structure. There are certain more non-idealities which are followed to avoid some of the complications in the system. Now, if we go for the equivalent circuit of DC motor, we can model this through electrical quantities.
[25:54]So, the armature part can be represented like a cylinder with brushes shown over there.
[26:07]It is having certain amount of resistance and certain amount of inductance involved with the coils.
[26:24]And the field winding can be shown as a coil which is mounted on the stator with certain amount of resistance and inductance. So, supply of VF is given to the field winding and current IF is flowing into that. At the armature terminal, supply VA is provided and current IA is flowing. Now, because there are conductors, there are coils which are moving in a magnetic field, it tends to generate certain amount of back EMF EB on the armature terminals.
[27:03]So, if we keep this arrangement of armature and field in this manner, which means, if we have separate supply for both the windings, we call this arrangement as separately excited DC motor.
[28:21]In another possibility, the field winding can be connected in series with the armature.
[46:34]So, either load is connected to the source or it is shorted to zero voltage. So, when we say that, of course, this cannot be done with actual resistance and we need to go for something else. We can have some kind of switching network.
