EV Fundamentals: Purpose of an Inverter
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Purpose of an Inverter
05.09
| 00:00 | In this section of the course, we're going to be focusing on one of the most critical elements of an electric vehicle's drive system, the inverter. |
| 00:07 | Earlier we covered the basics of the inverter and now we're going to be delving much deeper in order to get a better understanding of exactly what these devices are, what they do, and how they work in conjunction with the rest of the vehicle's primary EV component tree. |
| 00:20 | To be clear though, like with the previous EV motors section of the course, the following modules contain concepts that will be a step up in technicality from what we've covered so far. |
| 00:29 | And while you might not need to know some of the finer details in order to successfully work on your own EV project, you're always going to be in a better position if you properly understand the hows, whats, and whys behind any technology that you're working with. |
| 00:42 | So, let's start at the beginning. |
| 00:44 | The inverter is at the center of the motor drive system. |
| 00:47 | It's a critical, extremely high power conversion electrical device that takes in DC voltage from the battery and converts it into the optimal alternating voltages on each phase of the three motor phases to get the most torque for a given amount of electricity. |
| 01:02 | High frequency, high power precision inverters are a relatively modern development. |
| 01:07 | A steady decrease in the price of advanced electronics is a major part as to the story of why EVs are dropping in cost, all while yielding ever more powerful outputs that rival and often surpass some of the fastest ICE vehicles ever sold. |
| 01:20 | So, what does the inverter do exactly? How does it function? What are its limitations? And what can be adjusted within an inverter to gain more power? Well, first let's ask what would happen if there was no inverter in an EV system. |
| 01:33 | This is already the case for most motors that are powered in buildings or industry. |
| 01:36 | They're simply switched on and run at full power at a fixed speed, which matches the frequency of their electrical grid. |
| 01:42 | This of course isn't going to be very effective for a vehicle where we need to be able to operate at all kinds of different speeds and torque levels. |
| 01:48 | So, enter the inverter, which is also sometimes called a motor controller for what's soon to be very obvious reasons. |
| 01:55 | By taking the raw DC input voltage from the battery and then modulating the output voltage on each phase of the inverter, we can cause the motor to consume as much or as little current as it wishes, within its own limits of course. |
| 02:07 | So, essentially more voltage will cause the motor to draw more current, resulting in more electrical power. |
| 02:13 | By precisely monitoring the position of the motor, the inverter adjusts the voltage on each of its output phases in such a way that the magnetic field generated by the inverter's currents are precisely positioned relative to the motor's rotor. |
| 02:28 | Now we get to the challenging bit. |
| 02:30 | To get optimal efficiency from the motor, the alignment of the magnetic field needs to be kept within a degree or two. |
| 02:36 | And it's kind of in the same way you can think of ignition timing on an internal combustion engine, how ignition timing is so critical for that engine performance. |
| 02:44 | To enable this precision control of the magnetic field inside the motor, the inverter drives solid state switches called MOSFETs or IGBTs at up to 16,000 times per second to create the desired voltages on each output phase. |
| 02:59 | We'll talk more about this in the next section, Inverter Control System, but in brief, beyond actually controlling the power outputs to the motor, the inverter generally also performs other tasks. |
| 03:11 | First of all, the inverter communicates with a primary vehicle control unit, though even some inverters do simple vehicle control internally themselves. |
| 03:20 | This vehicle control unit sends commands to the inverter, such as telling it to come online, engage a direction, and command a torque. |
| 03:27 | The inverter will then communicate back its status, the torque it's trying to command, as well as a number of other parameters, such as various temperatures it's monitoring, output currents, voltages, and other important channels and states. |
| 03:40 | Based on this information, the vehicle control unit can make critical control decisions. |
| 03:45 | So fundamentally, the vehicle control unit makes a request, and the inverter will either complete that request or explain why it can't. |
| 03:53 | Inverters are generally also programmed to never be able to do anything that would damage themselves, having their own internal current, voltage, and temperature limits. |
| 04:02 | In the next module, we're going to go into some more detail about the inverter control systems, but before that, let's run through a few things about what we've just learned. |
| 04:11 | The inverter is a device that takes in DC voltage from the battery and converts it into AC, free phase to be used by the motor. |
| 04:18 | The inverter can cause the motor to consume as much or as little current as it wishes, within the limits of its DC battery supply, of course. |
| 04:25 | So, more output voltage will cause the motor to draw more current, and that results in more power. |
| 04:31 | Voltage on the output phase is adjusted by the inverter so that the magnetic field generated by the inverter's currents matches the position of the rotor, and this works in a similar way to ignition timing on a combustion engine. |
| 04:43 | This kind of precision is achieved by the use of internal solid-state switches that ensure the desired voltages are provided on each output phase. |
| 04:52 | The inverter most often takes basic commands from the vehicle control unit, though some do offer simple functions internally. |
| 04:59 | The inverter then communicates all the information that it's constantly gathering back to the VCU for it to process and act accordingly on. |
