EV Fundamentals: Motor Concepts
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Motor Concepts
08.45
| 00:00 | Now, that you have a basic understanding of the electric motor's design and function, it's time to get a better feel for what they look like inside, how they produce their power, and where the limitations lie. |
| 00:09 | We don't need to get too deep into the weeds on this subject, but it is important we have an idea of what's going on when a motor is operating. |
| 00:16 | Let's start with the stator. |
| 00:17 | Stators have windings, which pass through slots, creating the poles. |
| 00:21 | The stator iron brings the magnetic field created by the windings as close as possible to the rotor, so the magnetic field created by the current flowing through these windings can act on the rotor as directly as possible. |
| 00:32 | There can be a different number of poles in the stator, but they're always designed to match the number of poles in the rotor. |
| 00:38 | So, to repeat, motors have a certain number of pole pairs, and the number of pole pairs a motor has acts like an electromagnetic gearbox. |
| 00:46 | They multiply the torque produced, but require the current to be at a higher frequency. |
| 00:51 | A stator will generally have some form of liquid cooling, either oil or coolant jackets. |
| 00:56 | The majority of heat produced in the stator is typically in the windings, however, some is also produced in the stator iron itself. |
| 01:04 | The heat produced in the stator increases with the square of current, so you can understand that a little bit more current equals a lot more heat in these small windings. |
| 01:13 | On top of that, the hotter the windings get, the higher their resistance becomes, furthering the heat created. |
| 01:19 | Needless to say, a proper cooling system for the stator is critical to long endurance at high power. |
| 01:26 | Next, up is the rotor. |
| 01:27 | The design of the rotor varies significantly based on motor type, and we'll discuss that in the next module, but in all cases, rotors will have a position sensor, either a resolver or an encoder, so that the inverter is able to know the precise position of the rotor relative to the stator at any moment. |
| 01:44 | Now, before we go further in explaining the limitations of making more power with motors, we need to talk briefly about back EMF. |
| 01:51 | This concept is often misunderstood, but in simple terms, back EMF is an expression of the amount of voltage that is produced in the motor for a given motor speed. |
| 02:02 | For motors that use permanent magnets, the back EMF, or electromagnetic force, is a constant value, often expressed in volts per 1000 RPM. |
| 02:10 | This voltage is induced in the stator by the spinning rotor. |
| 02:14 | We won't get deep into the physics of it here, but just know that this voltage exists and grows proportionally with motor RPM. |
| 02:21 | So, the amount of voltage versus motor speed is called the back EMF constant, and it's determined by properties of the motor, like the number of stator turns and the size and strength of the permanent magnets, as well as the number of poles. |
| 02:34 | The back EMF effectively puts a speed limit on the motor, because once the back EMF voltage, plus the additional voltage necessary to push current into the motor, matches that of the inverter's maximum output voltage, which is limited by the battery voltage, the motor RPM can't be increased any higher. |
| 02:51 | This is called base speed. |
| 02:53 | Luckily, thanks to a magic trick played by the inverter called field weakening, we're able to reduce the effect of back EMF and then spin the motor above base speed, but at the cost of reduced torque. |
| 03:04 | And so this is why motor dynographs look like a straight line of torque, right up to it abruptly drops off. |
| 03:09 | This straight line is called the constant torque section, where the torque is limited by either how much current the inverter can put out, or by how much current the motor can safely handle without suffering permanent damage. |
| 03:20 | This, by the way, is where we can sometimes push the limit on motor output and find more performance. |
| 03:25 | The drop-off in torque occurs as the motor is entering the constant power region, and the inverter is doing that field weakening magic. |
| 03:32 | And don't worry, we'll be covering that in detail in the inverter section of the course. |
| 03:37 | Depending on motor and inverter design, the power in this region is fairly flat, but it does decrease as RPM increases. |
| 03:44 | Next, we need to consider the balance between torque and RPM. |
| 03:49 | Motors are available in a number of different configurations. |
| 03:52 | Even motors, which are physically the exact same size and model are available with different winding configurations that allow the user to select the best motor for their use case. |
| 04:01 | You can think of this like selecting a camshaft for a gasoline engine. |
| 04:05 | It's possible to have a very high torque, but run of torque and power at low RPM, or you can select a winding with lower torque that extends out to a higher RPM and generally produces more power. |
| 04:16 | As power is simply torque multiplied by RPM, the desire is always to keep the torque up as RPM is increasing. |
| 04:23 | However, having enough torque to not need gears and to have the desired torque and acceleration up to base speed is also an important consideration. |
| 04:32 | So, just as the case with an internal combustion engine, a compromise must be made with electric motors. |
| 04:36 | The way the stator is wound affects this. |
| 04:39 | A motor with many turns of wire per stator pull will offer more torque per amp of input current, but it will have a matching higher back EMF constant. |
| 04:47 | That means that more voltage is required to extend the motor's torque and power versus a motor with fewer turns. |
| 04:54 | A stator with fewer turns will instead have more wire in parallel or larger wire, making the motor resistance lower. |
| 05:00 | That's convenient. because it'll take more current to produce the same torque. |
| 05:04 | However, generally speaking, a motor with too few windings will be less efficient as it must operate at much higher currents. |
| 05:10 | The concept that's most important to understand and remember here is that output torque is multiplied by speed. |
| 05:17 | Selecting a very high torque motor without enough input voltage will almost always produce less power than the same input voltage with a lower torque motor that was operated at a higher speed. |
| 05:27 | Motor RPM is our friend. |
| 05:29 | The size of an electric motor will typically determine how much torque it can produce. |
| 05:33 | The larger the motor, the higher the torque, but the speed of the motor determines how much power it can produce. |
| 05:38 | And that's why modern EVs have motors that spin upwards of 16,000 RPM. |
| 05:42 | As a general rule of thumb, we want all the voltage and RPM that we can get because these are basically weight-free ways to get more power, assuming that the gear reduction can handle that speed. |
| 05:52 | Whenever there's a desire to find more power without increasing the voltage or speed of the motor, we're essentially forced to increase the motor size. |
| 05:58 | An example of this would be a double motor configuration. |
| 06:01 | When two motors are stacked together, twice the current is supplied at roughly the same voltage. |
| 06:06 | Now, the voltage is actually less because there's more power results in more sag from the battery. |
| 06:12 | But for the sake of this example, a double motor will give about double the torque and power produced. |
| 06:17 | For most motors though, this solution would require separate inverters, one for each motor. |
| 06:22 | However, if the input voltage can be doubled, the power can be doubled as well, assuming the maximum motor RPM isn't exceeded, without the added mass of the second motor. |
| 06:31 | The final drive ratio could then just be changed to trade off the torque for speed. |
| 06:35 | So, now you have an idea of how to theoretically get more power from motors. |
| 06:39 | Assuming controlling them isn't an issue, being able to raise the voltage almost always opens up the possibility to make more power above base speed. |
| 06:47 | And if the inverter isn't already pushing the motor to its maximum below base speed, there's an opportunity to get more power there by increasing the current from the inverter. With all that covered, let's take another look at the key takeaways from this module. |
| 07:01 | The stator is cooled, either by oil or coolant jackets, and has windings , which pass through slots, creating the poles. |
| 07:07 | The current flowing through these windings acts on the rotor, and the design of, which varies based on motor type. |
| 07:13 | It's helpful to think of motor torque as being created by motor current and motor speed increasing with motor voltage. |
| 07:20 | A very important concept to understand is that we don't just command the speed of the motor. |
| 07:25 | When it's connected to a heavy vehicle, really we're just commanding a torque, and that torque will accelerate the entire vehicle's mass, assuming the tires are not slipping, to higher speeds, which therefore require higher voltages. |
| 07:36 | With that said, all rotors will have some kind of position sensor, so that the inverter is able to know the precise position of the rotor relative to the stator. |
| 07:45 | Back EMF is voltage that's produced in the motor in opposition to the motor speed, and grows proportionally with RPM. |
| 07:51 | This effectively puts a speed limit on the motor, because once the back EMF voltage matches the inverter's maximum output voltage, the RPM can't be increased any higher without field weakening. |
| 08:03 | The balance between torque and RPM is a defining factor in an electric motor, and different motors are designed in different ways to lean either more towards torque or power. |
| 08:13 | Thinking of it like a combustion engine, it's like the difference between a towing cam that makes a bulk of power and torque down low, and an aggressive high power cam that makes all its power the top of the rev range. |
| 08:24 | So, just as the case of an internal combustion engine, a compromise needs to be made depending on the application. |
| 08:30 | Lastly, remember that output power is torque multiplied by speed, so a high torque motor without enough input voltage will produce less power than the same input voltage with a lower torque that's able to really run out to a high speed. |
