[Technical question] How does a MY go in reverse?

[boulder.dude]

Been wondering for awhile… to drive in reverse, does MY have a planetary gear like ICE cars or can the motors run backwards?

 

[QtownTaxiGreg]

I think I read somewhere that if it wasn't for the electronics it would go just as fast in reverse so I am picking polarity swap. Interested to find out also.

 

[jjrandorin]

Maybe @Tronguy knows?

 

[smogne41]

The motor is just driven in the opposite direction by the inverter. Electric motors can be driven either way. It is the same as reversing the direction on your electric drill, etc. There is just the motor, a fixed reduction gear and your differential.

 

[Tronguy]

Maybe @Tronguy knows?

Yep. I know this one.

No shaft-reversing gears required.

I'm not a motor specialist, but I am a half-decent general electrical engineer. So, I know a bit less about the induction motors used in Teslas than the PMG (Permanent Magnet) motors.

So, say that you've got a permanent magnet rotor. In Teslas, it's not just a single north and south poles, but pairs of the blame things. Let's say three pairs. These are on the rotor; i.e., the thing that spins.

Let's say you've got a North pole on the rotor at some position. Using the windings in the Stator, one creates a South Pole on the stator near a North Pole on the rotor and, just like Kindergarten, the North pole moves towards the South Pole. At this point, with handy-dandy switching transistors, we move the North Pole further around, the South Pole chases it, and Away We Go, Forwards.

(And the same thing is happening on the matching South Pole on the rotor, except that we're simultaneously creating a North Pole on the stator; and, if we got, say, a triple N-S set of poles on the rotor, we create a triple moving S-N poles on the stator, so everything whips around. Lots and Lots of switching transistors.)

Now: Note that I said that we create a North or South pole on the stator in the position that causes the rotor to move in a particular direction. Where and how big those created North and South poles on the stator are created is Completely Up To the Software Driving All Those Switching Transistors. So, instead of creating those stator North and South poles in positions that make the car move forward, we can just as easily make those Stator South and North poles appear on the other side of those Rotor North and South poles and, when the rotor turns, move the Stator North and South poles away and: Ta-Da! The car moves in reverse!

Now, I dunno about the car moving in reverse as fast as it moves forward. First off, I've been messing with autos for 'way too long, and one of the tidbits I've picked up over the years say that the kinds of gears that they use to reduce the speed of the motor to the speed of the driven axles tend to have a preferred direction of rotation. If you've ever heard a manual transmission in reverse, it sounds 'way different - and it precisely because of the way the individual gears are shaped that you hear those funky noises.

Second: As you all may have noticed, cars' wheel alignments are designed to have camber, caster, and toe; and it's the caster that's important here. Just like with a bicycle, a wheel moving in the forward direction has a bit of caster on it so, if one takes one's hands off the steering wheel, the car tends to go straight. If one takes one's hands off the steering wheel when the car is in reverse, the opposite tends to happen: The steering wheel will try and go max left or max right, just like a bicycle driven by a non-circus entertainer would do. This makes driving in reverse Really Dangerous and the provenance of car stunt men and their cars, highly tuned for the purpose.

So, given that the electric motors are completely drive-by-wire (no throttle cable involved), it doesn't take much thought to realize that the software guys may have very well limited the maximum reverse speed, in the interests of drivers not killing themselves by rolling the car.

If things weren't complicated enough, I've got this for ya: If one takes the shaft of an electric motor, spins the sucker, and puts ones fingers on the stator windings, one will get a Nasty Shock. That's because a moving magnetic field (the rotor magnets spinning about) intersecting wires in the stator will cause currents to flow in the stator windings, complete with voltage to spare. No surprise here: Every electric motor is also, built in, a generator.

As I said, this is no surprise. And if you were wondering how this gets used: This is precisely what regenerative braking is. Put yourself in the car at the top of a hill and get it moving: The spinning rotors will generate AC waveforms on the stator windings: Our handy switching transistors work Just Fine as rectifiers; they convert the AC waveforms on the stator windings to DC, and use them to charge up the battery. Doing all this creates drag on the rotor, and that's regen braking. The amount of energy extracted doing all this can be near-zero (by never turning the switching transistors on) or something More Rugged by turning them on for more and more time in a cycle, and that's our Tesla Software CS majors doing their bit.

So, to recap: By playing silly buggers with the switching transistors on the stator windings, we can:

1. Drive the car forwards
2. Drive the car in reverse
3. Brake while the car is going forwards
4. Brake while the car is in reverse
5. And the Null value: Do nothing, and let the car glide. Effectively in Neutral.

And all of the above without switching any gears around, anywhere.

In case you were wondering: There's reasons that Tesla says that the electric motors/reduction gears/differentials in the car are million-mile items: They're hellaciously simpler than any ICE transmission and have ridiculously fewer moving parts.

I mean.. take an automatic transmission on an ICE. The ICE has a limited range of valid RPMs where it generates reasonable power, and that forces multiple gear ratios (and gears), along with synchronizers (wear items that force gears to match each others' rotational speed when shifting gears), splines shifting back and forth, that horrible, inefficient mess of a torque converter that takes crankshaft RPM and puts it into the main drive shaft of the automatic transmission, and complicated gears, control assemblies, electronic solenoids that force the transmission fluid around to switch gears with.. Ugh. Just ugh.

Finally: For quite some time, I owned a Gen 3 Prius. Priuses have this whacko planetary gear set with, effectively, three shafts coupled together: A shaft from the ICE (only turns one way), a shaft of sorts on one motor-generator, and a third shaft that goes into a second motor generator; that third shaft is directly coupled into the differential and front wheels of the car. Interestingly, while the Prius ICE has a clutch of sorts, it never actually gets disengaged: It's there in case there's a spike in torque, and allows the ICE to decouple from the planetary gear set for short periods of time without jerking the heck out of the passengers and hardware.

Having said all that, the full-time hybrid transmission does items 1-5 above, all without actually shifting gears a la non-hybrid ICE. There turns out to be a degree of freedom in the planetary gear set so the car can be getting energy out of the ICE via the first motor-generator, and then using that energy to drive the car in reverse. Unlike a regular ICE or a Prius, the effect of the two motor-generators and the fact that motive energy can be going through both the shaft between the planetary set and the second motor-generator as well as in parallel with ICE energy being converted to electric by the first motor generator and then back to mechanical torque by the second motor generator, makes the "transmission" of a Prius act like a continuously variable transmission with a battery boost. That battery boost is important. With an regular ICE, one can make a car driveable, but the ICE becomes horriblly inefficient. If one puts in an efficient ICE, the car gets great gas mileage over a limited RPM range, but it's get-up and go and driveability will have gotten up and went.

So, Toyota put in a non-Otto Cycle ICE: An Atkinson Cycle engine. (One of the characteristics: Both the intake and exhaust valves open at the same time...). Wildly efficient, horrible driving: But then Toyota uses the motor generators in the transmission to smooth out the Atkinson spikes in torqe and to provide good get-up-and-go while the Atkinson plays catch-up. And, with the continously variable transmission part of all this, keeps the Atkinson motor in its efficient zone. And that's why Priuses get 50 mpg.

But, take another step back: Yes, the transmission in a Prius is wildly simpler than a normal automatic transmission or even a manual: There's no torque converter or shifters. But that whole planetary gear set is still 'way more complicated that what's in a Tesla.. and one still has the thousands of moving parts of an ICE. And as efficient as that Atkinson engine is, it's still not as efficient as a BEV.

Teslas win, hands-down.

 

[chillaban]

Yeah I used to write EV motor firmware for defense/aerospace. The easiest answer is that motors are rotationally symmetric and go forward or backward just the same. From a software standpoint, basically you command a negative speed in the speed loop, or a negative torque in the torque loop or negative voltage in the space vector modulation layer.

Most EV motors can run at the same performance forward or backward, but in practice, software limits are put in place to reduce the acceleration and top speed in reverse. This is mostly for obvious safety reasons, but it's also uncomfortable for occupants. When you accelerate briskly, you get pushed back in a nice seat cushion. When you reverse abruptly, it's just an asymmetrical seatbelt holding you in place. Even worse when you don't have a seat belt on, jabbing the pedal in reverse can throw you forward which in turn causes you to involuntarily hit the pedal harder.

 

[drb3333]

Even in some ICE situations, also.

My Ford Escape Hybrid introduced in 2004 had no reverse gear for the gasoline nor electric engine; they simply commanded the electric backwards for reverse function. Not new but rather conventional technology.

 

[boulder.dude]

Very helpful, thank you!