EV Fundamentals: Field Weakening
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Field Weakening
04.31
| 00:00 | So, far in this section we've covered the general operating procedure of an inverter, which is that after the inverter determines the magnetic field required through field-oriented control, it simply needs to adjust the total amount of current to proportionally increase the strength of the magnetic field and the resulting output torque. |
| 00:16 | But a totally different control system needs to be added on top when we start to run out of input voltage, and that's when the magic of field weakening starts to come into effect. |
| 00:25 | The concept of field weakening involves using the inverter to weaken the motor's back EMF-induced voltage, thereby reducing the voltage required to achieve a specific motor RPM. |
| 00:34 | The RPM point where this occurs is called the base speed, and this will vary based on the torque being produced, so the more torque we want, the earlier we'll run into base speed. |
| 00:44 | This is because it takes extra voltage to create the currents , which make up the output torque. |
| 00:48 | In this graphic we can see the voltage required to achieve a certain motor RPM with zero torque, and how much more voltage is required when a large amount of torque is added. |
| 00:57 | Essentially, the voltage required at the same motor RPM is much higher. |
| 01:01 | The more torque that's commanded, the more voltage that the inverter has to put out above and beyond the back EMF voltage, and thus, the earlier that base speed is hit. |
| 01:09 | Now, if field weakening wasn't possible, the torque would simply have to continue to be reduced until it finally got to nothing at the zero torque base speed. |
| 01:17 | Thankfully, field weakening is able to cancel out some of that back EMF by using the stator of the motor to create an opposing magnetic field. |
| 01:25 | This allows the motor to continue operating above its base speed RPM, and reduces the amount of torque loss that's experienced beyond base speed. |
| 01:33 | In a perfect system, field weakening allows the power to remain constant through the speed range, but in the real world, the system gets less efficient the higher the RPM, and the power drops off slightly. |
| 01:44 | Considering what we've already learned about how input voltage affects power output, we can understand that as the battery state of charge decreases, and the voltage of the battery goes down, the system will run out of voltage earlier, and output power will be lower. |
| 01:57 | This is why most EVs' performance will decrease as the charge level drops. |
| 02:01 | In addition to that, a colder, higher resistance battery will have a greater voltage drop. |
| 02:05 | We'll dig into this more in the next section of the course, which focuses on batteries, but for now, just understand that that means the inverter will run out of voltage earlier due to the increased voltage drop. |
| 02:16 | So, this takes us full circle into understanding how higher voltage allows us to make more power. |
| 02:22 | It may not be able to make more torque, as that's limited by the inverter and the motor, but the point where the torque drops off due to a lack of voltage will be reached later. |
| 02:31 | As we know that horsepower is simply torque multiplied by motor speed, the power will keep climbing linearly whenever the output torque is flat. |
| 02:38 | Additionally, a higher voltage system is able to utilize a motor with a higher torque constant, so more torque could be produced with the same inverter current simply by using a more concentrated winding in the motor. |
| 02:49 | In this example, we can see three configurations. |
| 02:52 | The first is the initial low voltage system with a low torque constant motor. |
| 02:57 | Then, by doubling the input voltage, the second motor is able to carry the torque to twice the RPM, where its higher base speed results in the power being doubled, assuming the motor could spin that fast, of course. |
| 03:07 | In the third configuration, we imagine a rewound motor with double the torque constant to get the RPM back in line with the starting configuration. |
| 03:15 | Here, we now have double the torque of the first configuration and the same power output as the second configuration. |
| 03:20 | The ideal situation is always going to be having the most voltage and RPM you can get away with, because that's going to result in the lightest motor, inverter, and drivetrain. |
| 03:29 | So, that's a little bit of an explanation of what field weakening is and the purpose it serves. |
| 03:34 | Now, that you have an understanding of the different control modes an inverter operates in, in the next module, we'll discuss how an inverter must be calibrated for the specific motor it's being paired with. Let's quickly round up a few key takeaways from this module first. |
| 03:47 | Field weakening involves using the inverter to weaken the motor's magnetic field, which reduces the back EMF effect and lets the motor achieve a higher RPM. |
| 03:55 | The point of this occurring is called the base speed. |
| 03:58 | Field weakening is able to cancel out the back EMF by using the state of the motor to create an opposing magnetic field and this allows the motor to be able to output a nearly constant power level above base speed. |
| 04:09 | The system does get less efficient as RPM gets higher, which means the power output will drop off a little. |
| 04:15 | As the battery state of charge decreases and the voltage of the battery goes down, the system will run out of voltage earlier, resulting in less power output. |
| 04:23 | This also applies to batteries that are colder and therefore have higher resistance and more voltage drop. |
