Both motor/generators in the Prius are very similar in design. They are synchronous, AC, permanent magnet motors, which are very flexible and efficient. They do require complicated control electronics. The rotor (the part that spins with the shaft) contains powerful permanent magnets. If you put these magnets on your refrigerator, you'd need tools to get them off again! There are no coils of wire in the rotor and no electrical connections to it. This increases reliability over DC motors which have windings in the rotors. All windings are in the stator (the part of the motor/generator that stays still and is fixed to the frame of the car). The control electronics passes an alternating current through these windings to turn the rotor. This current must be "synchronous" with the rotor's movement. This means that the current must pass first in one direction and then in the other at the precise time that each magnet embedded in the rotor passes the winding. A position sensor on the shaft tells the control electronics where the rotor is and how fast it is spinning.
The animation at right shows a hypothetical synchronous, AC, permanent magnet motor. I used to think that the Prius motor/generators were like this, but someone has begun taking a wrecked drive train apart and it is now clear they are somewhat different. Still, this animation took me a while to create and I'm not about to delete it! It does illustrate the general principle well enough.
It may take a little study to figure this animation out, so let me direct your attention to several things in sequence. First stare at the pole of the top-most (orange) winding. Observe that it pulses alternately red and blue, indicating that the current in the winding generates alternately a north and south magnetic pole here. This is, of course, due to the current in the winding passing first in one direction and then the other. Now look at all the other orange windings. Their poles pulse in the same pattern as the top one and at the same time. The orange winding are all connected together and carry the same current. Next, look at the green windings. Their poles pulse in the same way, but not at the same time. This is also true of the magenta windings. To see how the different windings generate magnetic poles moving in a circular manner, wait for a red (or blue, if you prefer) pulse at one pole and then move your eyes to the next counter clockwise winding and wait for your color there. It should take about a second. With a little practice, you will be able to follow a pulse around the stator. Now for the really tricky part. As you follow a red pulse around the stator poles, glance at the rotor and note the positions of the magnetic poles there. You will see a red rotor pole being "pushed" around in front of the red pulse you're following by magnetic repulsion. You will also see a blue rotor pole being "pulled" by magnetic attraction behind the red pulse you're following. That's it. That's how it works. The control electronics senses the rotor position and keeps the stator poles magnetized so as to both push and pull the rotor around.
If you watch a rotor pole pass by a stator pole, you will see that this is the time at which the winding current changes direction and the stator pole briefly has no magnetization. It changes from having opposite magnetization to the approaching pole, pulling it closer, to having the same magnetization, thus pushing it away. If you watch for the time at which a winding reaches peak current and the stator pole reaches its most intense magnetization, you will see that a rotor "magnet" is then exactly adjacent to it. The like pole of this magnet is toward the counter clockwise direction, and is pushed away. The unlike pole is toward the clockwise direction, and is pulled onward. This is why the motor, as I have drawn it, runs counter-clockwise. To run the same motor clockwise, it would only be necessary to change the timing of the current pulses so that at the peaks of stator magnetization the like pole of the adjacent stator magnet is in the clockwise direction and the unlike pole is in the counter clockwise direction.
It is my belief that the timing of current in the windings relative to the rotor position will always be as I have shown for operation as a motor. That is, current will peak in a winding when exactly between rotor poles. If this did not happen, the motor would operate less efficiently. If the computers require less torque from the motor, they tell the control electronics to pass less current though the windings. The rotor poles are never allowed to "catch up" to the rotating magnetic field when motor operation is required.
Watch this space! So far, we know that the stator of MG2 has 48 "teeth", that is 48 metal protrusions towards the stator. Each winding passes around several teeth, in the "slots" between them. The windings for each of the three phases are overlapped. They probably produce 8 magnetic poles which rotate more smoothly than in my animation (which only has six). The rotor probably has eight poles too, with eight permanent magnets embedded in it.
Although motor/generators they make are not quite the same as the Prius ones, eCycle, Incorporated are more forthcoming than Toyota in their technical information. The information they publish about the motor / generator used in their hybrid motorcycle is worth a look. It is a synchronous, AC, permanent magnet motor, like the Prius, and eCycle add "brushless" to the description. It is also described as "12-pole". The implication is that the stator and rotor both have 12 poles. The rotor is described as having "12 neodymium iron boron (NdFeB) magnets mounted on its circumference". The stator appears (from the photorgraph) to have 12 overlapping windings on 36 armature poles. This is where I think it differs from the Prius MGs. I have shown the Prius MGs with nine stator armature poles and non-overlapping windings. Even so, in my animation you can see that the currents in the windings cause only six magnetic poles to rotate around the stator. The number of effective magnetic poles can be different from the number of physical poles around which the wire is wound.
One-stage vs Two-stage power conversion: explanation
Example of motor / generator and control electronics system efficiency.