Yup, I’m back, and thinking about motors again. “Didn’t we already cover this?” I hear you asking (Where is that shrink?). Well, some of this, but Motors are a big subject, and as they are the only moving part in an EV (if you’re direct-driving, anyway) they’re kinda important.
So, what are our options?
- DC Permanent Magnet
- DC Series Wound
- DC SepEx (Separately Excited)
- AC Printed Armature
- AC
Great. That tells you not a lot at all, doesn’t it? Let’s try a little more description of each.
DC Permanent Magnet
In this motor, you have a wire-wound iron core with a series of permanent magnets attached to the outer rim, or a “Squirrel Cage” rim, with a core of permanent magnets. When you power the coils with a DC current in the right order, which is handled with brushes in the first case, and switches in the second, it creates a magnetic field that pulls the core around. The more power you apply, the more torque you get, and (potentially) the faster you can go. However, as you increase speed, the magnets begin to induce power back into the coils. This power (Called Back EMF, or Electro-Motive Force) must be overcome if you want to increase speed further, and there comes a point where the Back EMF is the same as the power coming from the battery pack, at which point further acceleration is impossible. It is theoretically possible to harness the Back EMF to slow the vehicle and supply power to the batteries (Called Regen, short for Regenerative Braking), but very few controllers do so, owing to the complexity required.
DC Series Wound
This is like the Permanent Magnet motor above, but rather than having the permanent magnets, their job is done with a second set of coils, which are connected in series with the first set. This makes for a more powerful, but potentially less efficient motor. Also, as the coils are all connected, Regen is not possible with this motor, as if the coils are energised, they will be trying to drive the motor.
DC SepEx
Here, the inner core and outer core can be powered separately. This means that if you want to slow down, you can apply a small current to the inner core, which will induce a current in the outer rim. That current can be drained to create drag on the core, and to create Regen. It also means at higher speeds, you can lessen the inner core power, which reduces Back EMF and allows more acceleration, at the cost of decreasing the available torque.
AC Permanent Magnet
This is very much like a DC Permanent Magnet motor, but rather than just having two points of contact for + and – terminals, these motors have connections for each set of coils. This means when you apply an AC current to them, not only do you attract the magnets toward your coil, but when the polarity switches you push them away again. If you time the waveform of the power to the rotation of the core, you get a very fast, and very powerful, motor system. As the power is flowing in both directions through the coils, it also means that back EMF is neutralised (Indeed, the back EMF is used, rather than having to be fought). An AC motor is also the only other kind that can be easily used for Regen.
AC Printed Armature
This is like the AC Permanent Magnet motor, but the coils are stamped out of flat sheets, and applied to a disc, rather than being wound wire on a stator. These are typically made small, and used in servo motors, though some companies have been trialling making these for vehicle-sized applications. Difficulties with making strong discs to mount the coils to, along with a seeming lack of interest from the industry, is making these almost impossible to find, and definitely impossible for an individual to buy.
In order of preference, These should be arranged:
- AC Printed Armature
- AC Permanent Magnet
- DC SepEx
- DC Permanent Magnet / DC Series Wound
But availability and controller complexity has made real-world implementations read more like this:
- DC Permanent Magnet
- DC Series Wound
- DC SepEx
- AC Permanent Magnet
And for most car-sized EVs, it’s PM motors only. SepEx are only available in sizes up to around motorcycle size, and there doesn’t seem to be any AC PM controller out there.
Oh, and all these (that are available) require some kind of transmission, as they don’t have the grunt to start a vehicle from 0km/h without some kind of torque multiplication (a gearbox).
If money was no object, I’d buy 4 AC Printed Armature motors (Custom made), a Citroen C6 “Glider” (That is, car without engine, transmission, differential, fuel tank etc), a massive load of Li-Poly batteries (200-500Ah minimum), plus a BMS to manage them all, and stack of meaty ultracapacitors to help with acceleration and regen braking. Which would be just about everything needed. Still, we can dream, right?
