Getting the power to the road

Having thoroughly discussed wheelmotors on here, I think it’s time to change subject completely and talk about wheelmotors.

Yes, you did read that right, and no I haven’t lost it.

Several home-built EVs, especially ones done for-cheap (For instance, Forkenswift), they’re done with DC motors taken from old forklift trucks. Usually the “drive” motor is used, and all is well. But flicking through the Princess Auto catalog a little while ago (What else do you do at 2:30am when you can’t sleep?), I came across a few interesting things, not least were some small, but reasonably spec’d Hydraulic motors. (I can’t link to them, their website is stupid – If products have the same name, you can only see one of them!) A little more digging, and I find a set of things called “Inline Axial-Piston” motors, which have great characteristics (Torque up to 83 ft-lbs (each!), top speeds of 3-4000 RPM). So my thinking is that, perhaps, one could take the “Pump” motor from a forklift, hook up some high-pressure hoses, and have the basis for a very space-efficient vehicle. With no need for drive shafts at all, it gives all kinds of space for batteries, and if you were custom-designing your vehicle, you could have a flat floor with the batteries inside (Using thin cells like LiFePO4 laid on their sides, for example), a small “Engine” compartment with the motor, controller and hydraulic splitter, and just some high-pressure hoses going from there to the wheels. Driving the motors in series/parallel would give reliable performance with behaviour like a limited-slip differential, and the difference in displacement between the pump and motors will act as the gearbox. And the units themselves are reasonably small and light, which completely throws away the whole “messes up the handling” problem.

But I’m no hydraulics expert (Heck, I wouldn’t know where to begin, to be honest), so ideally what I’d really like is for some hydraulics guru to take a look at this and tell me if I’m nuts for considering it. I know hydraulic drive is used in some low-speed vehicles (dump trucks, forklifts, that kind of thing). So why not high(er)-speed situations like this? We don’t need all 3000 RPM (heck, with 15” wheels and average tyres, you reach 120KPH (75MPH) at 1185RPM! 1500RPM would suit perfectly (give a little leeway for the racers out there :) )), but we do need the torque and power. Ideally, this system should also drive backwards. Not only for reversing the vehicle, but to be able to drive the motor ‘round, giving the option of regenerative braking too.

So, someone who has an idea what to do with these pipes and things, drop me a line and let me know if this’ll work or not.

Motors: DC? PM? AC? SepEx?

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:

1. AC Printed Armature
2. AC Permanent Magnet
3. DC SepEx
4. DC Permanent Magnet / DC Series Wound

But availability and controller complexity has made real-world implementations read more like this:

1. DC Permanent Magnet
2. DC Series Wound
3. DC SepEx
4. 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?

Motors: High Speed, Low Torque, or Low Speed, High Torque?

Yes, we’re going back to the question of drive motors.

For most EV conversions, it seems, the trend is to go with high-voltage battery packs, spinning the motor at high (5-7000 RPM), and running that high-speed, low torque drive through the standard transmission and differential (To get a geared-down wheel speed of 2000RPM). This seems to be because the only way to control motor throughput is to control the voltage, with higher voltages making faster cars. It’s my understanding that a motor will take as much current as it “wants”, which makes low-speed, high torque motors very difficult to get (at least, off-the-shelf).

Personally, I believe that simplifying the drivetrain as much as possible is definately the best way to go. My real preference would be for a pair (or two pair) of wheelmotors. This isn’t feasable (As explained in a previous post), unfortunately. So my next choice would be to mount a pair of motors inbound, driving the wheels from their driveshafts (So, replacing the differential with a pair of motors, running in parallel). This is a problem, however, as driving these motors at the typical EV’ers voltage would give a ridiculous top speed (with 15” rims and fairly standard tires, with a motor speed of 5000RPM, you’d get a top speed of 1032KPH(!!!)) with little torque.

Now, I could be wrong with all my assumptions here (Which is probably quite likely). It could be that if you run a motor with such high torque loads on it, it takes all the current it needs to drive the wheels. It does mean that I need a very high current controller (5,000 amp? Higher?) which doesn’t exist yet, and I’d need very high-draw capable batteries (Or several paralleled strings of batteries).

How would other people deal with this problem, apart from using the transmission and differential and including all the weight, friction and power loss included therein? Should I just bite the bullet and leave the tranny in, rather than having to worry about all this high-current nonsense?

Thoughts, comments, and more are always welcome!

Motors, Transmission, Wheelmotors Oh My!

In my mind, there seems to be 2 ways to convert an EV.

1. Take out the ICE, petrol tank, exhaust system, hook an electric motor up to the drive shaft on the (manual) transmission and stuff it with batteries
2. Take out the ICE, petrol tank, transmission, differential, drive shafts, brakes¹, petrol tank, and fit a wheelmotor to each wheel.

Option 1 is obviously simpler, but introduces plenty of losses with transmission and differential gearing.

Option 2 is more complex, but rids the car of a lot more weight, and gives no gearing loss, along with simple 4-wheel drive. However, it does change (increase) the “Unsprung weight” of the car (A bad thing). Oh, and (at the moment) there is the not-so-small issue that there are no wheelmotors available on the market at the moment.

There are a few companies advertising wheelmotors, and they all have glowing stories (and video, in some cases) of their wheelmotors in action. But none are available. Let’s list the ones I’ve found:

PML Flightlink’s Hi-Pa Drive.

I’ve been watching this one for a while. They used to have a lot of technical details on this system on their site, including nice PDFs. But now the company is in administration, and their Hi-Pa Drive link goes to a very pretty site that is sadly lacking in technical information. They also used to state that they were not going to sell to individuals. While that piece of text is gone from their site, important details are still missing, and it’s not looking good for prospective EV converters.

E-Traction’s TheWheel.

(You’ll have to find it on E-Traction’s frames-infested site. It’s under “The-Wheel direct drives” –> “SM450”)

This is apparently up and running on a few busses in the Netherlands, and while they have pictures up on their site for a car-sized unit (The SM450/1), it’s not actually been made in any working form yet. They, too, will not sell to individuals.

TM4’s “Electric Corner Module”

(I kid you not – It’s meant for using to supplement existing ICE drive systems, but can (as they admit) be used as a totally electric drivetrain.)

This Canadian company (Quebecois, of course) shows these wonderful systems. The key phrase here, however, is that “Our experienced design team will work closely with you to meet your specific needs for an AWD drivetrain, using electric wheel motors or any other configuration, whether centrally mounted or not.”

In other words, call us, Mr. motor industry executive, and we will quote you a nice, exorbitant price for a complete system, including assembly-line tooling and the like. Small-batch buyers (Like individuals and small businesses), you’re not welcome.

---

And that’s it. There are no other makers of automobile-grade wheelmotors, anywhere. Why is this? Is it that these three companies, between them, hold the patents to effectively choke anyone else out of making wheelmotors? Is there no other use for a small, efficient, and agile motor like these? You would think, with EVs, PHEVs, Hydrogen Fuel Cell’d EVs and the like, there would be more demand for such a groundbreaking technology.

1. Yes, brakes. With wheelmotors, you can Regen down to 0MPH, and they have a small mechanical brake for parking.