Electric Vehicle Drivetrain "101"

[JeffKeryk]

While I own an EV and have solar panels on my home, I know next to nothing on how electricity and specifically motors work.
I have been wanting to approach @OVERKILL and our other in-house talent to enlighten me on this important subject.

I would ask that we start with the primary drivetrain components and their purpose. Maybe even approximate cost?
Further, we have seen examples of fires related to EVs and what we need to be aware of.
While I love our Tesla Model 3 and plan on buying a Model 3 Performance, I sure as heck do not want to burn down my home or injure anyone.

Thanks in advance to all. I hope this goes well as my curiosity surrounding our Tesla and other electric vehicles (Taycan baby!) is killing me!

 

[OVERKILL]

Well, as you've heard many times, EV drivetrains are extremely simple with few moving parts. That's not to be confused with total parts count, because the battery cell # is massive, as you know.

So, you have:
1. Power source. Typically a lithium-ion battery package, made up of thousands of cells.
2. Electric motor or motors. This can range from as few as one to as many as four in current designs.
3. Gearbox. What the electric motor is attached to in order to drive the CV shafts and provides the function of a differential
4. Transmission (Taycan), which isn't a common option, but offers multiple gear ratios. In the case of the Taycan, there are two. This may be integrated into the gearbox.
5. Drive controller. This handles PWM (pulse width modulation) to control motor speed, as well as regen
6. Charge controller. Interfaces with the various charge systems including regen, DC fast charge, and performing the inverter function for AC charging. *

*Inverter may be separate.

EV's are like more complicated radio controlled cars which have the same components, minus the regen, charge controller and transmission. The power from the battery is rapidly pulsed to the motor to provide motivation and the longer the pulses, the more power it produces. This controls both road speed and acceleration. If you put your foot to the floor, one would expect that you just get full power, but that's not the case. The PWM system rolls you into what is defined as full power; it's a bit of a "soft start" because 100% full torque instantly would be damaging, but it FEELS like 100% full torque with how rapidly it is applied, even though there's that buffer. 100% is also unlikely to be 100% motor power (direct), as there's likely yet another buffer there.

Your "gas" is most likely just like in a modern ICE car, a pair of redundant POTS (potentiometers) that are used to command a percentage of full power based on voltage. So, 0V could be "stop" and 5V could be WOT for example. This is like the trigger on a remote control car controller.

I have no idea on cost, that's likely going to vary significantly. I expect Tesla probably has their motor cost down pretty low at this point. BMW's brushed motor approach may also be more cost effective than others.

 

[wpod]

Sounds like how an electric forklift operates but it is more complicated than described. There must be safety control devices to prevent a wot runaway event. could you imagine turning on the power an having a component allowing uncontrolled wot?

 

[OVERKILL]

Sounds like how an electric forklift operates but it is more complicated than described. There must be safety control devices to prevent a wot runaway event. could you imagine turning on the power an having a component allowing uncontrolled wot?

Yes, I expect there are paired redundancies. I know there are in the pedal, that's what is supposed to prevent unintended acceleration events, both POTS have to agree or the system is disabled.

 

[Trav]

Yes, I expect there are paired redundancies. I know there are in the pedal, that's what is supposed to prevent unintended acceleration events, both POTS have to agree or the system is disabled.

Even modern gas vehicles with drive by have that, its a must.

 

[LazyDog]

If you really want to know how EV's nuts and bolts. Check this group at You Tube!

https://www.youtube.com/c/WeberAuto

 

[Cujet]

The electric motors in many EV's are AC motors, EDIT: with Tesla's DC perm mag motor in the model 3 (as I understand it, it still has a rotating field). Some are induction motors and some use permanent magnets. Both use a rotating magnetic field to "rotate" the rotor. In this example the windings are around the outside and the current goes to red, then blue then green. Yes, I know it shows it as "smooth" well it kind of is, as the current ramps up and down smoothly in each winding.

In simplistic terms, the rotor gets dragged around in an induction motor (yes, there is more to it, but that's a good way to visualize). In a perm mag motor, the magnets are attracted and repelled as each winding gets powa.

The really cool thing is that modern induction motors are very efficient at over 90-93%, (with extreme examples as high as 98%). Perm mag motors are generally a more efficient choice though at around 95-96% now.

As always, Elon and company do some really cool things with motors. The Plaid motors are designed to operate extremely well over a very wide range of RPM. Something that, until recently, was really not commonly available. It's not that the AC motor technology is new or that we could not do it back in year 1899, it's that Tesla has worked through the important shortcomings and produced a lightweight system with a very wide torque curve.

 

[thastinger]

The electric motors in nearly all EV's are AC motors. Some are induction motors and some use permanent magnets. Both use a rotating magnetic field to "rotate" the rotor. In this example the windings are around the outside and the current goes to red, then blue then green. Yes, I know it shows it as "smooth" well it kind of is, as the current ramps up and down smoothly in each winding.

In simplistic terms, the rotor gets dragged around in an induction motor (yes, there is more to it, but that's a good way to visualize). In a perm mag motor, the magnets are attracted and repelled as each winding gets powa.

The really cool thing is that modern induction motors are very efficient at over 90-93%, (with extreme examples as high as 98%). Perm mag motors are generally a more efficient choice though at around 95-96% now.

As always, Elon and company do some really cool things with motors. The Plaid motors are designed to operate extremely well over a very wide range of RPM. Something that, until recently, was really not commonly available. It's not that the AC motor technology is new or that we could not do it back in year 1899, it's that Tesla has worked through the important shortcomings and produced a lightweight system with a very wide torque curve.

The Model 3 uses DC motors

 

[Cujet]

The Model 3 uses DC motors

Thanks for the correction! I was unaware they did not reverse the current as they rotate the field.

 

[George Bynum]

The power from the battery is rapidly pulsed to the motor to provide motivation and the longer the pulses, the more power it produces. This controls both road speed and acceleration.

BMW's brushed motor approach may also be more cost effective than others.

In NO WAY detracting from your excellent answer, just expanding on it a little; The most common motors are 3 phase; the drives are almost certainly similar to the variable speed controls used in industry. Motor speed is MORE related to generated frequency than voltage. MOST LIKELY, it is the number and timing of full voltage (maybe 400-800VDC) that accomplishes frequency/speed control. There are hundreds or thousands of explanations of motor speed/torque control systems on the web.

I've not looked into BMW's system. Still with AC (no high current DC brushes), there are "squirrel cage", "permanent magnet", and "wound" field or armature. I doubt they use high current brushed connections, but I make dozens of mistakes every day.

 

[OVERKILL]

In NO WAY detracting from your excellent answer, just expanding on it a little; The most common motors are 3 phase; the drives are almost certainly similar to the variable speed controls used in industry. Motor speed is MORE related to generated frequency than voltage. MOST LIKELY, it is the number and timing of full voltage (maybe 400-800VDC) that accomplishes frequency/speed control. There are hundreds or thousands of explanations of motor speed/torque control systems on the web.

I've not looked into BMW's system. Still with AC (no high current DC brushes), there are "squirrel cage", "permanent magnet", and "wound" field or armature. I doubt they use high current brushed connections, but I make dozens of mistakes every day.

Some info on the BMW setup here:

BMW's Magnet-Less Motor: How Does It Work?

Here's why this old-school technology is back in BMW's newest high-performance EV.

www.motortrend.com

But the rare-earth materials that make permanent magnets are getting hard to find, so BMW is getting away from them by using electromagnets in both the stationary stator and the spinning rotor. It's relying on an old technology—brushed motors—to make this possible.

*snip*

This fifth-generation BMW motor has no magnets. It operates as a three-phase AC synchronous motor using brushes and a commutator to provide power to the rotor windings, meaning that AC brushed motors aren't just for third-world electric locomotives any more.

 

[Kiwi_ME]

EVs use nearly identical technology to servomotor systems used in industry; typically machine tools, robots and factory automation. DC brushed motors were common through the '80s then 3-phase "AC brushless" motors with rare-earth magnets took over. I worked in automation engineering during these decades and became very familiar with implementing these technologies into machinery.

One basic characteristic of these systems is that the motor runs in what's called "four-quadrant" control, meaning torque can be applied in either direction independent of the motor spinning in either direction. Part of that is what gives you regen in an EV and a reverse gear without actually shifting gears.

Another characteristic of these systems is that the motor has a resolver to provide rotation positional feedback to the servo controller such that the motor can be precisely positioned and held stationary for a period of time. By differentiating the resolver output (calculate the rate of change) we get the motor speed. By differentiating that again we get the motor acceleration. The resolver provides directional information as well.

In an EV with a 3-phase motor (all of them now) the servo amplifier is called the "inverter" because it can provide 3-phase power from DC. But it does much more than that. With integrated computer control of that inverter combined with the motor resolver signal it can drive the motor to move any load within the motor's power envelope precisely as commanded.

Taking into account the demands of the driver and/or other control inputs in an EV it can make the motor run in various modes like constant torque, constant speed, constant acceleration, or follow any sort of predefined torque, speed, or positional curve.

So when you're driving and pressing the accelerator it creates a demand for torque. The controller works out what current to apply to the motor to satisfy that demand. If you set the cruise control then a demand for speed is created so the controller ramps up the motor at a moderate rate until the speed demand is satisfied, including on downhills. If you have "one-pedal" driving then when the pedal is relaxed the controller follows a predefined deceleration curve that will bring the motor to a halt. Once a low speed is reached or when holding position a small amount of power is being consumed, so some EVs automatically use the hydraulic brakes at speeds under a walking pace.

The other major parts of an EV are the on-board battery charger, the relay and network comms interface for high-power DC charging, and the DC-DC converter that runs the 12V electrical system and keeps the 12V battery charged.

An EV traction battery may be nominally configured as 400 or 800 volts and is fully electrically isolated from the vehicle's chassis ground. That isolation is monitored constantly while driving and whenever the car is woken up to charge the traction battery or the 12V battery. A 12V battery is needed for "booting" the system and also to carry out tests on the traction battery, but not disturb it by loading it. Typically the battery enclosure contains a large power contactor and the charge management system (BMS). When the car is "off" there is no high voltage power present outside of the battery casing.

Weber Auto channel (John Kelly) on YouTube does very detailed teardowns and specifically his Bolt videos cover all hardware required for any EV.

 

[PandaBear]

If you put your foot to the floor, one would expect that you just get full power, but that's not the case. The PWM system rolls you into what is defined as full power; it's a bit of a "soft start" because 100% full torque instantly would be damaging, but it FEELS like 100% full torque with how rapidly it is applied, even though there's that buffer. 100% is also unlikely to be 100% motor power (direct), as there's likely yet another buffer there.

Your "gas" is most likely just like in a modern ICE car, a pair of redundant POTS (potentiometers) that are used to command a percentage of full power based on voltage. So, 0V could be "stop" and 5V could be WOT for example. This is like the trigger on a remote control car controller.

Regarding to PWM, basically it is a way to rapidly switch on and off something very fast, so that when it is powering a capacitor or inductor (electric motor is an inductor / coil), there's enough capacitance or inductance to feel like it is smooth. It is a much more efficient / simple way to control something with variable load / voltage / current that is analog with only a digital on/off switch. The key is, it must be done fast enough that you do not feel it, and you do not hear it. Usually it has to be at least more than 20kHz and often more than 2x that. Your dog may hear it but you should not, if the system is designed correctly.

The reason EV's 200 lb/ft torque feel way more instant than a gas vehicle that is also advertised as 200 lb/ft torque, is that electric motor's torque came at almost 0 rpm while gas vehicle's 200 lb/ft torque is only 200 at the peak of the power band, and probably only 1/4 of that at 1500 rpm, even a turbo diesel would probably need at least 1500 rpm to reach peak torque, which is why locomotive now use diesel generator to power the electric motor to move the train.

 

[denwood]

..and don't forget the 8:1 final drive ratio with no clutch slip, or torque converter loss. These factors along with the electric motor properties contribute to that instant low end grunt "feel" you get from pretty much any EV. You'll find that the intitial squirt to 30mph on the street, (even on our lowly 2018 LEAF) will leave a lot of much faster cars behind in urban traffic.

What you can't do in an EV is store up kinetic energy (as in spinning up the engine/flywheel while stopped), to in turn drop that energy into forward acceleration during a launch from zero.