EV Fundamentals: Inverter Control System
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Inverter Control System
05.02
| 00:00 | Now, that we understand that the inverter takes in DC voltage and converts it into AC 3 phase, we need to ask how the motor knows exactly how much voltage to target on each phase. |
| 00:09 | And this is determined by a process called field-oriented control. |
| 00:13 | Diving too deep into this is outside the scope of this course, but to cover it at a base level it's a process by, which the inverter creates a target magnetic field using a multi-step process. |
| 00:24 | The system calculates at up to 16,000 times per second how much current each phase of the system needs to achieve that target. |
| 00:32 | So, the magnetic field target is typically one , which puts the field 90 degrees away from the poles of the rotor magnets, creating the strongest magnetic attraction and thus the highest amount of torque for a given amount of electrical current passed through the motor. |
| 00:45 | To make more torque, a stronger magnetic field is required. |
| 00:49 | This is achieved by simply proportionally increasing the current. |
| 00:51 | So, double the current, double the torque. |
| 00:54 | Obviously, up to a point. |
| 00:55 | In theory, then an inverter can make more and more torque forever as long as we can increase the current, right? Well, unfortunately no, there are limits. |
| 01:03 | Heat and electrical resistance get in the way and when things are pushed too hard for too long, at some point we just reach the limit of the available battery voltage. |
| 01:11 | This is because to output more current, we need more voltage. |
| 01:15 | And the voltage needed also increases with RPM, because we must first match the back EMF voltage that the motor naturally produces. |
| 01:23 | As we talked about before, that increases proportionally with RPM. |
| 01:26 | So, while a large linear amount of torque is possible at low speeds, eventually at higher speeds the inverter just runs out of juice, aka battery voltage, which is kind of similar to a turbocharged combustion engine getting to the limit of its turbine flow. |
| 01:38 | The torque drops off and then the power remains relatively constant for a little bit before it falls off a cliff. |
| 01:44 | There are two main areas of an inverter's performance. |
| 01:46 | The constant torque region, which is limited either by the inverter's current output capacity or by the motor's rated torque limit. |
| 01:52 | Once the inverter runs out of voltage, it goes into what's called the constant power region using field weakening. |
| 01:57 | Don't worry too much about that for now, this is something we're going to look at in the next module. |
| 02:01 | Other than the input voltage limiting the amount of power, there are also current limitations of the power drivers inside the inverter and as a result, inverters will have hard-coded internal limits to prevent themselves from sudden death. |
| 02:12 | With that said, it's often the case that inverters aren't tuned to utilize all of their output potential, but unfortunately when it comes to OEM inverters, there's not currently any method of reflashing them in the way that you might be used to with internal combustion engines. |
| 02:26 | Although in some instances, there are replacement control units available, which effectively retain the high voltage electronics of the inverter, but change its brain to something that's easier to integrate with and somewhat adjustable. |
| 02:39 | It's also important to understand that there are two different sets of current and voltages when talking about the inverter. |
| 02:45 | The DC side has a direct current and voltage going into the inverter. |
| 02:50 | Those two are multiplied together and that's the input power. |
| 02:53 | The voltage into the inverter is fairly constant, that just changes with the voltage of the battery. |
| 02:58 | However, the current going into the inverter is very dynamic and that's tied closer to the total power the system is making. |
| 03:04 | So, at low speeds, even at full motor torque, the input current is not very high. |
| 03:09 | The voltages and currents coming out of the inverter on the other hand are AC, not DC, so they're a little bit different. |
| 03:16 | First of all, we talk about the currents and voltages as RMS, which stands for Root Mean Square and is the average of the current and voltage and makes it most comparable to DC currents. |
| 03:27 | The inverter's output current is tied linearly with the motor torque, as we've mentioned before, but the voltage is related to the back EMF of the motor, plus the additional voltage required to produce those currents above the base back EMF voltage in the motor. |
| 03:41 | The RMS voltage and current can be multiplied to give a very rough idea of the power, but due to AC currents and voltages not always being perfectly timed, that is, the peak AC current may not occur at the exact same time as the peak AC voltage, the math won't be totally accurate. |
| 03:57 | This is called power factor and is due to electrical inductance in the motor. |
| 04:01 | You don't need to be an electronics engineer to understand everything in depth, but it is important to understand the two separate sets of voltages and currents, as well as their relationships with each other. |
| 04:11 | So, just to reiterate what we've learned in this module, the motor knows exactly how much voltage and current to target on each phase thanks to field-oriented control, where the system calculates the required magnetic field position based on motor position and then calculates how much current each phase needs to achieve that magnetic field. |
| 04:28 | The stronger the magnetic field, the more torque can be made up to a point, and after that heat and resistance get in the way. |
| 04:35 | Both input voltage and current limitations of the power drivers are the limiting factors in the inverter's ability to create power, though inverters most often aren't calibrated to utilize all of their output potential. |
| 04:45 | And lastly, remember that the input current closely resembles power, where the output current closely matches motor torque. |
| 04:52 | Output voltage goes up with motor RPM in line with the back EMF motor torque constant that we discussed in the motor section. |
