EV Fundamentals: Limitations & Cooling
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Limitations & Cooling
04.35
| 00:00 | In this module, we're going to look at the role that heat plays in an electric motor and how its effects can be best mitigated with different types of cooling systems. |
| 00:09 | Intuitively, we tend to think that heat goes up in proportion with power. |
| 00:12 | You double the power, for example, you get double the heat. |
| 00:15 | But that isn't actually the case. |
| 00:17 | As we touched on earlier in this section, the heat produced in a motor increases with the square of current, meaning that the heat increases exponentially with current. |
| 00:26 | What this means is that the amount of power a motor can withstand really depends on the amount of time that you're demanding that power. |
| 00:33 | Even the manufacturer's published motor limits are likely on the conservative side if you just wanted to push a motor for a second or two. |
| 00:40 | So, now that we understand how motor builds heat, what actually happens when that heat gets high enough to become a problem? Well, there are two major problem areas at play here. |
| 00:49 | First is the stator, where the windings exceed an acceptable temperature and get to the point where the insulation on them can break down or fail. |
| 00:57 | Obviously, if that happens, and the windings short on each other, the motor is done for. |
| 01:01 | Additionally, the hotter the stator windings get, the higher the resistance of that stator becomes. |
| 01:06 | This then causes the motor to heat up even faster, and unless the current is quickly reduced, the motor will be destroyed. |
| 01:12 | Inverters do of course monitor stator temperature and have what's known as D-rate curves, which are designed to reduce the power gradually as the motor gets close to this maximum allowed stator temperature. |
| 01:23 | Secondly, excessive heat also affects the rotor, or the central part of the motor. |
| 01:27 | This is more complicated, but in permanent magnet motors, the rotor heats up due to eddy currents caused by the changing magnetic field. |
| 01:35 | Rotor heat tends to increase with motor speed, and to a lesser extentalso from motor torque. |
| 01:40 | To mitigate this internal core heating caused by eddy currents, the rotor is generally made up of thinly sliced laminations , which are electrically insulated from one another, rather than one large block of ferrous material. |
| 01:53 | Regardless, when the core gets too hot, there's a risk of demagnetizing the permanent magnets. |
| 01:58 | Initially, the magnetism will be reduced with elevated temperature, but beyond a certain point, permanent damage to the magnets will occur. |
| 02:06 | In induction motors, this problem also exists and is in fact much worse, as the currents developed inside the rotor are much higher. |
| 02:12 | As you rightly assume, with all this in mind, the design of a motor's cooling system and its ability to manage heat is critical in achieving any type of high performance. |
| 02:22 | There are three main cooling methods we see in EVs. |
| 02:25 | Air, water, and oil, or combination. |
| 02:28 | A fluid cooled motor can be much smaller and lighter than a motor that relies solely on conductive air cooling, simply because the manufacturer is able to use smaller and lighter materials due to not needing a large surface area with, which to shed heat. |
| 02:41 | Some fluid cooled motors have cooling only in the stator, and this limits the endurance of those motors, as the heat inside the rotor can't be addressed. |
| 02:49 | Generally, if a motor uses water only, it's most likely only cooling the stator. |
| 02:54 | If the motor has oil, or both oil and water cooling on the other hand, it's most likely cooling many parts of the motor, which is going to give us much greater endurance. |
| 03:02 | Having a direct path to both the stator and the gap between the stator and the rotor allows for cooling of the entire system, and these systems are able to get very hot, over 150 degrees Celsius. |
| 03:13 | So, even having a small amount of fluid in contact with these components can really remove a lot of heat. |
| 03:18 | Depending on the use case of the vehicle, the type of the cooling system that's inside the motor can make or break the success of said vehicle. |
| 03:26 | It's common to see most drive units of modern EVs now employing oil cooling inside the motor, and this is possibly the most significant reason why the high power endurance of motors has improved so rapidly in the last few years. |
| 03:38 | Getting the heat out of the stator and rotor is key to maintaining high power. |
| 03:42 | That now covers the basics of cooling EV motors. |
| 03:44 | So, to reiterate, here are the main points. |
| 03:46 | Heat increases exponentially with current, meaning that the amount of power a motor can withstand really depends on the amount of time you're demanding that power. |
| 03:54 | The heat is also impacted by the battery voltage and the inverter's pulse with modulated frequency. |
| 03:59 | Excessive heat is a danger to both the stator, where the insulation on the windings can break down and cause a short, as well as the rotor, which can see a permanent demagnetization of the magnets. |
| 04:10 | Both scenarios are terminal for the motor, so we want to do our best in avoiding it through efficient cooling and temperature monitoring systems. |
| 04:18 | The best motors will use both oil and water to keep an acceptable internal temperature and prevent the inverter from having to reduce power. |
| 04:27 | Of course, if these systems don't control the motor properly, complete motor failure can occur. |
