New motor inventions from Finland
The last few months have been awesome for motors here in Finland. Indeed, we’ve been treated with not one but two novel inventions on this field. In a way, both are aimed at improving the speed-torque characteristics of the motor.
Here’s a brief rundown.
Motory geary thingy
The first one is from Lappeenranta University of Technology, from a spin-off company called Egear. They’ve developed a new kind of traction motor, intended especially for heavy duty machinery.
Their product is based on a rather smart idea. Basically, these folks first took a fairly typical oil-cooled high-performance permanent magnet machine. Then, they put a two-stage epicyclic gear inside the rotor.
This is brilliant for a couple of reasons.
Reason number 1
First of all, simply having a gearbox is nice. Electric motors do have nice torque characteristics already. They can produce their maximum torque all the way from zero speed to their rated speed. And below that, their power stays constant. Quite nice compared to the spikey curves of typical gasoline engines.
However, even that may not be enough. Sometimes, even higher torques are needed at low speeds. Think of un-stucking a loader that’s gotten stuck in the mud. That places certain requirements on how the motor has to be designed.
And once that loader is moving again, it probably wants to go quite fast, to make up for the time lost. Now this, on the other hand, places wholly different requirements on the rotor.
For that purpose, having a gearbox is nice. By shifting gears, the motor can stay much closer to its optimal speed range.
And having two gears is usually plenty enough for that. Remember the wide torque range I mentioned just moments ago? That’s why. Even Tesla is content with two gears, and their products go quite a bit faster than typical heavy machinery.
Reason number 2
Yeah, having a gearbox with an electric motor is nothing novel itself. Been there, done that. Well, at least somebody has.
Having it inside the rotor, on the other hand…now we’re talking. That’s an awesome idea for permanent magnet motors especially.
You see, traction motors typically need to create relatively high torques no matter what. They are not high-speed machines in any case. And with permanent magnet motors, an easy way to increase the torque is to increase the number of poles. (For non-electromechanists: the N-pole of a permanent magnet is one pole, and its S-pole is another. Together they form a pole-pair.)
More poles means one pole-pair is shorter, i.e. occupies a smaller section of the rotor circumference. And if the air-gap flux density stays about the same, a shorter pole-pair means smaller flux per pole. And that can be carried by a thinner rotor back-iron (called a yoke).
Another trick to increase to torque is to increase the rotor radius. This works simply because of the increased moment arm. Basic mechanics, at least mostly.
So, we combine the larger rotor radius with reduced yoke thickness. Of course, we end up with a large amount of unnecessary space inside the rotor.
And putting a gearbox into that space makes perfect sense.
Epicyclic gears are a perfect match for t hat kind of an application. They consist of a sun gear in the middle, a ring gear on the outer perimeter, and planet gears fixed to a planet carrier in between. For more information, I recommend you check out the Wikipedia article.
Epicyclic gears offer a designer a plethora of choices. Basically, the sun, ring, and planet carrier each can function as the input or output of power. Correspondingly, the range of possible gear ratios is quite wide indeed.
In this case, I think one configuration makes clearly more sense than the others. Since the gearbox is inside the rotor, the ring gear is probably fixed to it. So that would be where the power goes in. The sun gear will probably then function as the output; possibly with compound planet gears to increase the ratio.
Electric gearboxes – what?!
The second invention comes from a company called Quantum electric. With their new modular motor, they claim to be able to increase the range of an electric vehicle by up to 50 %.
Based on the promotional video on their website, their motor is an axial flux one. The rotor consists of a holder disk (magnetically passive, I’d guess), and permanent magnets placed neatly in its holes. Most likely, they are alternately magnetized in the axial direction.
The active part of the stator then consists of U-shaped coils. Each spans roughly two magnets, so of course they will be subject to an alternating flux as the rotor rotates.
The company claims the motor is modular and easy to manufacture. That indeed seems plausible.
Especially the rotor assembly seems nice. You see, in many machines the magnet flux passes through the rotor iron. This can make assembly difficult, as the magnets start repulsing each other. However, in this case the rotor disk can be magnetically passive, like stainless steel, or even plastic. Thus, the repulsive forces should be minor.
The stator coils are probably also easy to make, but I’m not certain about how easy they are to fix in place.
Another claim is even more interesting. Apparently, the voltage and the number of poles can be changed on the go. Accordingly, the motor can be used at 24 V, 48 V, 72 V, and 96 V supply voltage.
If I had to guess, this is probably done by switching the connections between the U-shaped stator coils. They are in parallel whenever a low back-emf is required, and in series when the opposite is true. Most likely, this is done with simple power electronic switches.
This is probably what they mean by having an electric gear, something they repeatedly mention on their site.
Whether or not this actually changes the number of poles is somewhat less clear-cut. A large portion of the flux will anyways be excited by the permanent magnets. And changing its distribution, its number of poles, is much much easier said than done.
But things are more flexible on the stator side. With this kind of concentric winding, with solenoidal coils wound around teeth, the number of poles for the winding is not so easily defined. This is because the same winding can be used with different rotors, each with different number of poles. So you could say that the number of poles in the stator winding is changed.
But I digress.
This voltage-changing thingy is behind the supposedly-increased vehicle range. With today’s power electronics, in motor operation voltage doesn’t matter that much. As long as the preferred voltage of the motor is below the DC link voltage, the converter can always step it down easily enough.
However, this does not work so easily in the opposite direction. For instance, when braking at low speeds, the motor might only be producing like 20 Volts of back-emf. If the battery voltage is for instance 48 Volts, harvesting the braking energy for charging them gets somewhat difficult.
In this case, this Quantum Electric motor probably switches more of its stator coils to series, increasing the voltage.
There you have it. Two motor inventions.
Both from Finland, both in a way aimed at increasing the speed range of an electric drive.
One does so by having an integrated gearbox. Ideal for heavy-duty applications, where colossal torques are required. There’s simply now way to produce them with the motor alone.
The other instead opts for changing the winding configuration of the motor any time needed. I may not like the motor topology itself, but this switching approach seems like it could be applied to other kinds of motors too. Hope they can patent it in time.
Check out EMDtool - Electric Motor Design toolbox for Matlab.
Need help with electric motor design or design software? Let's get in touch - satisfaction guaranteed!