EV Fundamentals: Electric Basics
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Electric Basics
05.29
| 00:00 | Before getting any further into this course, it's important that we get a handle on some fundamental electrical concepts that are going to calibrate your brain to the possibilities and limitations of this new type of powertrain. |
| 00:10 | The first basic concept we need to understand is exactly what electrical power is. |
| 00:15 | So, put simply, the power you get in watts is just the current, defined as the rate at, which electrical charge flows, multiplied by the voltage, the force that drives that current. |
| 00:24 | So, to get more power, you either need more voltage or more current. |
| 00:27 | And just for clarity's sake so we're all on the same page, current is measured in amps and voltage in volts. |
| 00:32 | In EV terms, we usually speak in kilowatts, which is a thousand watts, and since power is voltage times current, we can think of one kilowatt coming from 100 volts at 10 amps, with one kilowatt being equivalent to 1.34 horsepower. So, we can easily convert between the two if we want to get a clearer understanding of a system's performance. |
| 00:50 | So, for example, a 500 kilowatt EV has 670 horsepower. |
| 00:53 | And similarly, energy is also measured in kilowatts over a function of time, kilowatt hours. |
| 00:58 | This is simply just a way of expressing how much power can be discharged from an energy source. |
| 01:03 | So, for example, an energy source containing one kilowatt hour is able to discharge one kilowatt of power for one hour. |
| 01:10 | A 75 kilowatt hour energy source can discharge at 75 kilowatts of power for one hour, or one kilowatt for 75 hours. |
| 01:18 | Where it all starts to get a little bit more confusing is that there are two separate voltages and currents. |
| 01:23 | This is because the voltage coming out of the battery isn't the same as the voltage going into the motor, and neither therefore is the current. |
| 01:31 | The voltage produced at the motor changes with the motor speed, and the inverter needs to produce a voltage in excess of that to provide positive torque. |
| 01:40 | We'll get more into this in the EV motor section of the course, but for now it's important to keep in mind as we move forward that there's two voltages and two currents. |
| 01:48 | Next, let's talk about understanding heat produced, which is a big factor in all electrical systems, but it's especially important when it comes to high power electric vehicles. |
| 01:57 | The same formula we just used, organized in a different way, tells us that the heat produced is the current squared multiplied by the circuit resistance, which is the circuit's opposition to current flow, and that's measured in ohms. |
| 02:08 | So, simply put, this means that the heat produced from a little bit more current isn't proportional, but it's exponential, meaning that it increases at an accelerated rate when compared to the change in current. |
| 02:18 | For example, if you double the current, you quadruple the heat, and that's why a little bit more motor or battery current makes a lot more heat. |
| 02:26 | So, following on that logic, this is the reason why higher voltages allow more power for a given weight. |
| 02:32 | So, for example, to make the same power using a 100 volt battery versus a 1000 volt battery, we would need to carry 10 times the current, which would be 100 times more heat produced in the circuit, and that's assuming the cables and all the components were the same size, but that just gives you a picture to how much larger things need to be when you're working with large amounts of current. |
| 02:51 | So, when you go up in voltage, that means you can use smaller conductors to carry the reduced current, as well as lighter inverters and motors, achieving the same power output but with less weight. |
| 02:59 | This is the same reason the electricity and power lines near your house run at much higher voltage than the voltage inside your house. |
| 03:07 | Unlike with internal combustion engines, more power in a properly calibrated EV system doesn't mean more damage or a likelihood of failure, when properly controlled. |
| 03:14 | It simply means the system will overheat faster and then need to reduce its power output to prevent overheating or damaging itself. |
| 03:21 | EVs have a number of different temperature monitoring systems in each component, and these components are also hard-coded to not be allowed to just self-destruct or melt themselves. |
| 03:29 | So, this all means that a compromise is required in terms of discharge power and runtime. |
| 03:34 | Lower power will allow for a significantly longer runtime, thanks to the concept that heat produced is not just proportional to current, as we just discussed. |
| 03:42 | Okay, so the last electrical concept we need to get our heads around is that similar to gasoline, high voltage can be a very calm beast until you piss it off. |
| 03:51 | Speaking from personal experience, a 400 volt dead short through a 22 gauge wire sounds like a bomb going off when that wire blows. |
| 03:58 | And because it's so silent and peaceful at rest, high voltage can be very easy to underestimate and it's easy to become complacent around. |
| 04:05 | Working with these systems needs to be taken very seriously. |
| 04:08 | It only takes one moment of carelessness to end up in the hospital or much worse. |
| 04:12 | So, please pay high voltage the respect it deserves and you won't end up as someone else's cautionary tale. |
| 04:18 | All right, let's wrap up the main points covered in this module before moving on. |
| 04:22 | Input power is measured in watts, and that's simply the voltage multiplied by the current. |
| 04:28 | That means making more power requires either more voltage or more current. |
| 04:31 | When discussing EV applications, since EVs make hundreds of thousands of watts of power, we generally talk in kilowatts. |
| 04:38 | A unit of energy is expressed as a kilowatt hour, which is simply the energy to discharge at a power of one kilowatt for one hour. |
| 04:45 | And it's important to not get kilowatt and kilowatt hours mixed up. |
| 04:49 | Okay, one is power and one is energy. |
| 04:51 | Confusingly, there are actually two separate voltages and currents to consider, because what leaves the battery isn't the same as what goes into the motor. |
| 04:59 | We learned that heat plays a big role in an EV driveline and is generated at an exponential rate in relation to current, meaning that a little more current can equal a lot more heat produced. |
| 05:10 | Also, unlike combustion engines, more heat doesn't necessarily spell disaster in an EV system, because it will protect itself. |
| 05:16 | Lastly, it's crucial that we have a healthy respect for the high voltage systems involved in electric vehicles. |
| 05:23 | It only takes one slip up to find yourself in serious trouble. |
