Niels … this is not only a fantastic simulation .. but also a great explanation of the rationale for the development of each element of the hybrid drive. I never really understood why the planetary gearset was really required. Also I like how you have shown that an electric motor can serve as a clutch in an ideal world. Having watched this video I can see other applications such as diesel electric trains and ships where a planet gear could be employed to improve efficiency. Also do you know the efficiency of power transfer from ICE -> wheels .. versus ICE -> MG1 -> Inverter -> MG2 -> Wheels ? Well done.
Can someone please explain how the 'generator clutch' that replaces the millstone clutch works? The author describes it between 4:00 and 4:40 in the video. Are the coils that are connected in the video to the drive shaft supposed to be wrapped around another permanent magnet (different from the permanent magnet on the rotor connected to the engine's output shaft)? Are magnetic clutches that are fully contactless really feasible? I understand he's using it as a pedagogical device, but it just blew my mind because I'd never even considered that magnets without contact could be used as a clutch to transfer rotation.
Several of his pedagogical devices in the middle of the video are physically unbuildable, until he has added enough of the parts of the actual solution to make it actually work. His "generator clutch" shows up at a part of the video where the power-split planetary gearset hasn't been added yet. It just has a rotor that rotates and 'stator' coils that ... also rotate. Easy to do in an animation where they don't have to be mounted to anything or have any wires connected. After he mentions that later at 7:00, he adds in the planetary gearset, and now it's a thing you could build. Once you build it the real way, the MG1 stator really is stationary, and you probably wouldn't use the word 'clutch' as a description of MG1 anymore. In old-school automatic transmissions, there were things called clutches (which would use friction between two rotating parts, to transmit rotation) and other things called bands (which would use friction between the stationary case and a rotating part, to stop that rotating part and force some other part of a planetary gear train to rotate instead). That's closer to what's happening in MG1 in real life: the electromagnetic forces resist the rotation of MG1, forcing rotational power to pass between the engine and the output. Where the 'band' analogy breaks down is you never want a friction band to be partially applied. When the band is fully released and the part is spinning freely, that's fine. When the band is fully applied and the part is stopped, that's fine. Any intermediate stage where the part is making frictional heat against the band is only happening while the band is applying or releasing, which is a stage to be passed through as quickly as possible. When you replace a friction band with an MG, now you don't have to avoid those 'partial' stages anymore. They don't make damaging, wasted frictional heat; they just make electricity, which you can use. So in the Prius transmission that's what's actually happening most of the time. The MG1 rotor is spinning, but not spinning freely: there are electromagnetic forces restraining its rotation (and so making power flow mechanically through the gears), and the side-effect of electrical power out of or into MG1's coils also serves as a separate, electrical, power path. When you prevent any current flowing in MG1's coils, you allow MG1 to freewheel (like a band fully released) and you have the neutral function. The case where you hold MG1 to zero RPM (like a band fully applied) is possible, but doesn't really happen a lot. A three-phase motor is usually driven by sending changing currents into the three phase windings so the resulting magnetic field has an orientation that changes (and the permanent-magnet rotor follows it around). To just hold the rotor still, the ECU has to know the rotor's position—that's what the 'resolver' is for—and send, effectively, DC into the stator coils, proportioned among the three so the resulting stationary magnetic field is holding the rotor in place. Doing that isn't quite as efficient as holding something stopped with a friction band. The friction band wastes energy when it's partially applied and there's motion and friction and heat, but there's really no energy being lost when the part is really held at zero RPM. Holding the permanent-magnet rotor stopped using a steady winding current costs you some power. The winding current must be strong enough to resist whatever torque is on the rotor, and that current produces some waste heat according to the DC resistance of the windings. That may sound alarming if you are used to hearing the terms "inrush" or "locked-rotor" in connection with industrial motors. When you slam full voltage into a motor whose rotor isn't turning yet, you get a very large current and heat that would burn the motor right out if the rotor didn't come up to speed quickly. That's not what's going on with a Prius MG. To hold the rotor stationary only requires 'enough' current, and the inverter is able to supply just 'enough' voltage to do that. It isn't slamming full voltage into a stopped motor. We talk of these as being high-voltage motors (300-ish volts in gen 1, 500 in gen 2, 650 in gen 3, I don't know the later numbers), but those are the voltages needed at speed (you have to overcome the "back EMF" that the spinning motor generates back atcha). In the zero RPM case, with no back-EMF at all, you can get substantial torque out of them with more like eight to sixteen volts. At that rate you are producing some waste heat, but measured more in the hundreds of watts, not thousands. That stopped-rotor situation was considered in more detail in ➡this post⬅. Returning to the original question, if you really had a way to build the free-floaty-no-wires-attached stator from 4:00 in the video, you could indeed make it work like a clutch, again by using steady winding currents to hold the rotor and stator in a fixed position relative to each other. You wouldn't need extra permanent magnets or anything. But you would have the energy loss of the steady DC winding current just described, compared to a real clutch, which doesn't lose any energy when fully engaged.
