EV Fundamentals: Motor Types
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Motor Types
04.10
| 00:00 | The electric motor has been around for a long time now, and while plenty of different types have come and gone, currently the most popular, especially in the automotive world, are variations of the permanent magnet motor. |
| 00:10 | These motors are chosen primarily due to their high efficiency, and they're also more suitable for long duration, high load power than alternatives like induction motors, which we'll still be taking a look at later in this module. |
| 00:22 | The downside of permanent magnet motors though, is that they use rare earth magnets, which are resource intensive and expensive. |
| 00:28 | So, the basic operating principle of a permanent magnet motor is actually very simple. |
| 00:33 | A magnet, well a series of magnets, are attracted to an opposing magnetic field created by current flowing through the stator. |
| 00:41 | This creates torque. |
| 00:42 | The most basic type of permanent magnet motor have magnets mounted on the rotor surface, and are called SPM motors, short for surface permanent magnet. |
| 00:50 | The field produced by the stator in these SPM motors doesn't interact with anything other than the magnet of the rotor to produce a torque. |
| 00:58 | This means that relatively large magnets are needed, and field weakening must be relied on entirely above base speed for these motors. |
| 01:05 | Variations of the permanent magnet motor are able to interact with other parts of the rotor to create a torque, rather than relying only on the magnet. |
| 01:12 | This is possible using steel or other ferrous materials inside the rotor, which is able to react with the magnetic field created by the stator. |
| 01:19 | These motors are referred to as IPM, or more specifically IPMSM, interior permanent magnet synchronous motors, and are the motor type most EVs employ today. |
| 01:29 | They combine the high torque and efficiency of working with a permanent magnet with the basic principle that magnetic fields want to align themselves. |
| 01:37 | Okay, so let's explain how this works in a bit more detail. |
| 01:40 | Firstly, the rotor of an IPM motor is created from a series of specifically shaped steel laminations. |
| 01:45 | Slots are cut in these laminations in a way that allow the magnetic fields passing through the rotor to create a torque called a reluctance torque. |
| 01:53 | And as the rotor tries to align itself with the stator's magnetic field, this torque is added to the torque from the magnets. |
| 02:01 | The combination of these two torques gives us the total output torque of the motor, resulting in a combination that does better at high speeds than a normal permanent magnet motor. |
| 02:09 | As we touched on earlier, an alternative to the permanent magnet style of motor is the induction motor. |
| 02:14 | Though this doesn't see much use in the automotive space anymore, as they aren't as efficient and aren't as well suited to applications that require constant rapid acceleration or sustained high speeds. |
| 02:25 | Induction motors use the same stator design we just looked at, but rather than having an internal permanent magnet rotor, they use a totally different rotor design that looks like a swirl cage. |
| 02:35 | The magnetic field created by the stator induces a current on metal bars inside the rotor. |
| 02:39 | The current flowing through these metal bars then creates their own magnetic field, and these two fields interact with each other, resulting in motor torque. |
| 02:47 | The differences between the rotational speed of the stator's magnetic field and the rotational speed of the rotor is called slip. |
| 02:55 | This slip, or difference in speed between the two, is what induces current into the rotor, and that in turn is what creates a torque. |
| 03:02 | So, higher currents in the stator create a stronger magnetic field, and thus more torque, but also higher slip creates more torque. |
| 03:08 | The interesting thing about induction motor is that theoretically it has no torque limit. |
| 03:13 | The more current that you put into the stator, the more torque will be produced. |
| 03:16 | But practically speaking, there are limitations due to saturation of the magnetic materials. |
| 03:21 | The downside is that these motors aren't very efficient due to additional losses created in the rotor circulating currents. |
| 03:27 | One of the main issues is that the light load efficiency of an induction motor is poorer than a PM motor, and in an electric vehicle, light load efficiency is obviously quite important. |
| 03:37 | Induction motors excel at having very large constant power regions and have a high peak torque capability. |
| 03:43 | To sum up, while you can find both surface permanent magnet and induction motors used in some vehicles, by far the most common type of electric motor you'll find in the modern automotive space are IPM motors. |
| 03:54 | These combine the torque created by the magnet and reluctance torque created by the rotor trying to align itself with the stator's magnetic field to produce high levels of power very efficiently. |
| 04:03 | On the downside, they are more expensive and resource intensive to produce. |
