Electromagnetic braking - How does it work?

[RegenStudent]

Hi all,

I am a student currently studying electromagnetic braking for my final year project.
I understand that generators generate more power at higher RPMs and less power at lower RPMs. As the wheel/generator spindle speed reaches 0 RPM standard motor-generators develop no power and no holding torque. As this is the case, how does the i3 brake to a complete standstill without using the friction brake? Also can the i3 be stationary on an incline without using the friction brake and if so, how?

Regards

 

[jadnashuanh]

It takes energy to produce the torque to drive the EV while the motor is driving the vehicle. If you stop putting energy into the motor, it has all of the features that can allow that motor to now become a generator, producing electricity rather than using it. Just like any generator...it takes energy to drive it. In this case, during regeneration, it is the store kinetic energy of the vehicle moving that drives the generator, thus slowing it down. While you can 'lock' the rotor of some electric motors, I'm not sure how the i3 'holds' the car on a hill. It could apply the friction brakes similar to you manually applying the parking brake until it senses you pressing the pedal to make it go. I find that mine will roll forward on a hill, but doesn't seem to roll backwards after stopping.

SOmebody may know the rest of the details.

 

[alohart]

While you're correct that the regen braking force decreases as the motor speed decreases, the i3, like all cars, has an additional braking force: rolling resistance. Even though BMW has worked hard to minimize the i3's rolling resistance, an i3 moving in neutral on a level road would stop due to its rolling resistance. The fact that other EV's won't stop using just their regen is likely due to the fact that most of them implement creep to mimic conventional automatic transmission ICE vehicles (i.e., they will move slowly without pressing the accelerator).

 

[FrancisJeffries]

I find that mine will roll forward on a hill, but doesn't seem to roll backwards after stopping.

I've read somewhere that it will stop you rolling back if you're in drive, and stop you rolling forward if you're in reverse. I've tried it and it's true.

Meanwhile, I'm a bit horrified at RegenStudent's request. He's studying electromagnetic braking for his final year project. His final year degree project?
If he's so ignorant about the basics of electric motors and generators (first year basics on electrical engineering degree courses here in the UK), he needs to talk to his project supervisor as a matter of urgency!

 

[WoodlandHills]

Sounds like someone is too lazy to do the work themselves and decided to cheat instead. Mommy and Daddy probably did his homework for him too......

 

[stumbledotcom]

Also can the i3 be stationary on an incline without using the friction brake and if so, how?

No, the i3 cannot hold its position on anything more than a minor incline without the driver applying the friction brakes. The previous references to it not rolling is due to a hill hold feature that delays releasing the brakes for approximately two seconds, preventing roll back in drive or roll forward in reverse. If you take longer to move your foot from the brake pedal to the accelerator though, the brakes will release and the vehicle will roll.

I drive over some of SF's steepest streets daily so I've become very familiar with the car's behavior on hills vs. flats. Contrary to statements I've read in reviews and road tests, the regenerative braking alone will not fully stop the i3 going down anything but minor hills. On a very steep street, the car maintains a steady 15mph or so even with the foot completely off the accelerator and the performance display showing maximum regeneration. For those familiar with the city, I'm referring to streets such as Divisadero to the Marina or California to the financial district.

 

[ultraturtle]

See page 65 of the US Owners manual, where it states that energy can only be recovered at speeds higher than approximately 12 mph (20 km/h).