BMW iX electrically excited motors

[TheDeuce]

c2mike said:

What BMW are doing is good. They are not brushes - they are slip rings that don't break current and therefore don't spark.They just wear, but these rings don't handle full power, in fact they only do a few percent of full power, so this is good. Permanent magnet materials mean chinese engagement due to rare earth supply, hence with the geopolitical situation BMW is being wise.

Avoiding a loss of contact and subsequent arcing is an absolute requirement of any motor required to have a long and service free life. And I think that without atmospheric influence, because the unit is sealed, BMW should have no problem achieving that.

It's clever thinking imo. On the face of it it's easy to initially assume that in an EV efficiency and reliability are paramount.. But with a little further reflection, I can totally see that as a sealed unit their more basic motor design is simply good enough. It should be reliable enough and efficient enough easily. More than ever it makes sense if they're dodging the use of rare materials.

Perhaps this is the sort of thing we should expect to see more of as the seasoned car manufacturers begin to get serious about EV. It's not all about emerging technology, the wise application of less extreme technologies can have a worthwhile place too.

[amstrange1]

TheDeuce said:

I think it makes a lot of sense for the likes of BMW to instead chase more practical concerns such as materials cost saving, less rare materials that are potentially going to become supply problems too. Not least the fact that the motor unit costs less overall so is less of a cost down the line.

I think you're right on the supply stability point, but depending on magnet costs at a given time, I'm not convinced that the BMW wound rotor design overall is necessarily that much cheaper than a PMSM system. I'm sure the copper they've got in the rotor is a lot cheaper than magnets, but slip rings; brush assembly; additional seals and brush housing plus fixings will add a bit to the BOM cost and add manufacturing process time (plus tooling costs) too. I think it's more about supply chain stability, perhaps with some safety fringe benefits. I'm sure this year's German powertrain conferences will have a paper or two presented on it, telling us what they want us to believe the motivators were!

Here's what some of the key raw materials have done price-wise over the last 3 years (Nd = Neodymium, the key part in most PMSM magnets):

TheDeuce said:

It's clever thinking imo. On the face of it it's easy to initially assume that in an EV efficiency and reliability are paramount.. But with a little further reflection, I can totally see that as a sealed unit their more basic motor design is simply good enough. It should be reliable enough and efficient enough easily.

I think it's a bit hard on BMW to say this motor design is more basic, it's just a different e-machine topology which still needs careful electromagnetic design/optimisation. With wound rotor there's the DC field current to control in addition to your 3-phase AC, so a bit more I/O needed on the inverter side adding complexity there. This is probably offset by being able to remove some PMSM-specific HW and SW safety functions though!

On the reliability front, both BMW and the Renault Zoe wound rotor design clearly intend the brushes to be serviceable items as they sit in a separate sealed housing which is externally accessible. The BMW design looks to have 6 brushes (3 per slip ring) arranged equidistant around the slip ring vs the Zoe's 2. I assume that's a durability/robustness decision - anecdotally it seems it's not uncommon to need to swap the brushes on the Zoe setup due to noise, whereas the BMW setup is more balanced with (I assume) lower spring pressures and clearly some redundancy:



I think whilst the legislative drivecycles exist as they do, OEMs will continue to chase fractional in-cycle efficiency improvements just as they do with fuel economy and CO2 for ICE vehicles. Range helps them sell cars at the end of the day. Whilst it's clear that legislation has made ICE vehicles generally cleaner than they ever have been, I think as EVs become more prevalent over the coming decades the drivecycles and legislation needs to change to better reflect different real life scenarios - I reckon this would see more focus on light-weighting; reduction of vehicle size/frontal area and an increased obsession with aero. Unfortunately distilling everyone's real life use cases into a small handful of lab-controlled drivecycles is difficult, and any deviation from the current status quo costs the OEMs (and ultimately the customers) more money.

[GT9]

The BMW M cars and probably the more sportier variants are going to offer selectable 2WD and 4WD modes.

Making RWD/2WD available to the driver is part of their DNA that I think they will be very keen to rekindle or preserve.

Given that effective regenerative braking probably needs a motor on the front axle, driving in 2WD mode asks for a motor on that axle that doesn't consume energy during acceleration or cruise, but can apply a high degree of braking torque instantly when required.

How many BMW EVs will only have the rear axle driven?

If most BMW EVs have two axles driven, then the ability to completely de-energise/re-energise the front axle motor on demand is desirable.

[andytk]

Having looked at this, isn't it just an AC slip ring motor with the field coils excited by a variable frequency drive (aka vsd, variable speed drive / inverter drive). As per here: https://youtu.be/JPn5Ou-N0b0

If so, it's clever, as Tesla used a standard squirrel cage ac induction motor on their cars for efficiency (in dual motor mode).

This dispenses with the second synchronous ac permanent magnet motor (or brushless DC motor) as this can generate huge starting torque by itself, which is the squirrel cage ac induction motors weakness.

Also it means that BMW aren't hostage to fortune if China ban rare earth magnet export or are subject to sanctions if they invade Taiwan.

Andy