umm okay, The brushes will last at least 300K miles. After that they can be replaced without having to replace the whole motor. This was the info from another website.

 

My understanding is that the synchronous motor has a higher power density and better efficiency than asynchronous motors used by other manufacturers. The synchronous motor is also able to free wheel, which makes coasting more efficient. Other motor types have drag from the permeant magnets always being on as compared to the separately excited motor the i4 uses.

 

Go to German, in an i4M50, put the ACC on 180 kmh (111 miles/h), when traffic allows press the speed pedal and wait a few seconds, until max speed of 225 kmh (140 miles/h) is reached. Then you will understand the reason for this motor technology.

 

I like those avatars. A coupe and a Cobra being driven in anger. Here's my old flame...

 

Each motor type has a different torque vs. RPM curve. This will affect the responsiveness and performance at different speeds. This was a factor in BMW's decision to choose this motor type.

Nissan has made the same decision with the new Ariya, I believe. It uses "externally excited synchronous motors", which appear to be the same design.

If you want to geek out on this motor design, here are some good articles.

• Fairly short: Vitesco presents externally excited synchronous electric motor for future EV drive systems
• Much longer: A closer look at brushed AC motors in EVs - Charged EVs

 

Here are some additional infos:
Motor maintenance | BMW i4 Forum (i4talk.com)

 

...considered by some to be "old tech".

Nope. And don't let any fanbois throw that shade at you, they're just flaunting their lack of motor knowledge. Motors use brushes for varying reasons, with varying performance and wear traits.

The "brushed motors" in kid's toys, vacuum cleaners, and car starters have commutators to crudely-but-effectively switch their magnetic fields, and are indeed noisy, dirty, and old-school (but also cheap, rugged, and stalwart for the last 150 years). Early EVs in the hobbyist era (pre EV-1) used commutator motors.

The brushes in BMW's motor ride on slip rings that are smooth and low wear. They're there to carry small currents to windings on the rotor that generate the motor's fixed magnetic field -- a job done by magnets in permanent magnet motors. Similar to a permanent magnet "brushless motor", the big currents that switch the magnetic fields that make the BMW motor rotate are controlled by big transistors, controlled by fast CPUs and realtime software. So in the BMW motor, traction power does not go through the brushes -- they are a lightly loaded component and should have a long life.

The BMW wound-rotor synchronous motor is a little (percent or two) less efficient than a permanent-magnet synchronous motor (what most EVs use) under heavy load, since the rotor windings need a bit of power. But over the whole operating map -- and in particular over the whole drive cycle of a passenger car which is mostly part-load operation with no small amount of coasting -- the wound-rotor motor catches up, because the rotor current can be reduced at part load and switched off when coasting. When the rotor's not magnetized, there are no eddy-current and hysteresis losses, things that cause non negligible drag in a spinning PM motor... thus, the very competitive efficiency (km per kW-hr) posted by BMW compared to Audi and VW. And the rotor windings are plain ole copper and iron, not neodymium and cobalt permanent magnets... which leaves more money for the suspension design, and of course fewer toxic holes in the ground.

 

I'm still learning this new technology, but I understand our cars use a brushed AC synchronous motors, which are considered by some to be "old tech". I believe most other EVs and hybrids use brushless induction motors.

I believe the upside of our motors is that they don't use permanent magnets, which are made from scarce materials. The downside is that the brushes are subject to wear.

(This is a whole new vocabulary to me. Compression ratios, compressor maps, valve timing... I got all that, but not this stuff. Might as well be speaking Mandarin to me.)

Discuss!

Ni hao!