[brian_]

Until this announcement from BMW, as far as I know the only modern production EVs using DC-excited wound rotors were the Renault Zoe and related models, with Continental motors.

The Interesting Engineering article, and others quoting it, erroneously say that the motor uses a commutator, and of course it does not - it has brushes and slip rings. Judging from this and other articles from Interesting Engineering, there is no one associated with the site with any significant technical knowledge: it is better to just read the releases from BMW.

What is new, and quite clever, is the holistic approach to making a drive unit that meets more than just the basic requirements of being efficient.

I don't think that's new at all, but it is good. After all, Renault considered the same factors and made the same decision - a decade ago - and has since switched to PM motors.

 

[brian_]

I did notice the use of commutator, which it obviously isn't. I guess their writer and editor aren't electric motor specialists.

That's more generous than I would be...

Mahle are claiming to be developing an induction coupled system to eliminate the brushes which is basically what truck alternators have done for decades

It's a little different from the truck alternators. The traditional system uses an auxiliary three-phase radial stator winding to induce current in an auxiliary three-phase radial rotor winding, which is rectified to power the rotor main rotor winding. The Mahle design uses high-frequency AC power to a stationary axial auxiliary winding, inducing current in a rotating auxiliary, which is rectified to power the rotor main rotor winding; this axial coupling approach relieves the rotor excitation system of any effect of changing rotational speed.

Any inductive coupling system for rotor excitation power is more complex than brushes and slip rings, but avoids any wear components (and the mechanical drag).

 

[brian_]

Tesla also, and also now have PM motors. I guess there is nothing new, just new reasons

Yes, except that Tesla went from induction asynchronous to PM synchronous, while Renault went from DC-excited synchronous to PM synchronous.

The simplest reason from a DIY perspective is that magnet motors are virtually impossible to disassemble safely without a very strong jig

True, but as long as the motor is complete with end housings (like a Leaf, for instance, but unlike a Tesla) there's no reason for the DIY conversion builder to disassemble the motor.

In his teardown video of the Bolt drive unit, Prof. Kelly of Weber State U. shows the GM special tool for rotor removal, which his shop bought (because they provide instruction in automotive technology - they don't just tear down stuff to see the cool parts inside). In one of the Tesla Plaid motor videos the rotor is only partially removed, because the person making it was clever enough to realize that just pulling it out would not go well and he didn't want to (or couldn't) buy or build appropriate tooling.

 

[brian_]

I bet they are switched reluctance.

No, they're not reluctance motors at all, and they're synchronous, not switched.

A reluctance motor has neither permanent magnets nor electromagnets (windings) in the rotor, only a stack of iron laminations which create preferential paths for magnetic flux (paths of different magnetic reluctance, which is analogous to electrical resistance). In a synchronous reluctance motor a typical multiphase (normally 3-phase) stator produces a rotating field and the rotor stays locked with that and rotates similar to other synchronous motors (up until the torque is to high and it breaks loose, which happens at much lower torque than a PM or wound motor of the same size). In a switched reluctance motor phases of the stator winding switch in sequence, and the rotor follows them around in jerking steps (but at least the stator phases can be crudely switched instead of sinusoidally varied).

This motor has a wound rotor producing a constant-polarity field just like permanent magnets do, but variable in intensity (by varying excitation current) and with copper instead of the materials needed to make a strong permanent magnet.

... switched reluctance. They haven't been possible until recently because of the switching and calcs needed for the micro.

Switched reluctance motors have been used (with clever switching logic) for many years. They're just not smooth and don't have enough advantages in small sizes (like an electric car size) to be worth using.

 

[brian_]

The ones I have seen are much simpler, just a stationary coil of wire inside the spinning claws, rather than a spinning coil of wire as part of the claws.

That's sounds like a normal alternator - with the cheap-to-build claw rotor configuration excited through brushes and slip rings - but with the axial coil stationary. The magnetic coupling of the stationary rotor winding with the movig claws around it would be terrible, but apparently Delco Remy offers that. The rotor is supported and driven from one end, and the winding is supported and held non-rotating within it from the other end.

The traditional configuration with separate excitation power transfer coils may only be used on larger machines.

 

[brian_]

I just realized that Nissan is going with this type of motor for the Ariya, perhaps due to the influence of their association with Renault. They are calling them "Externally Excited Synchronous Motors". I assume they're just using brushes and slip rings to get excitation current to the rotor, but I haven't seen any details on that aspect.

 

[brian_]

Wait what? Did I read that right? Did you say the armature has a fixed winding in it? So does the actual armature spin around this fix winding and the fixed winding is inducing a magnetic field onto the armature...so "sort of reluctance"?

Yes, I had not heard of it before this thread, but that's what 57Chevy described and that appears to be the design of the BorgWarner unit as shown in the YouTube video attached to their product web page. The magnetic coupling between the fixed winding and the rotor surrounding it is radially through the cylindrical area at the open end of the cup-shaped rotor, and axially through the other end. That's a magnetic flux path, with as low reluctance (the magnetic equivalence to resistance) as they could manage.

 

[brian_]

This is kind of interesting...I wonder if there is a weight savings? I'd assume the armature windings are wound onto laminates. I'm betting this motor has multiple internal windings so it can essentially make several magnet poles with them. They probably rotate with the actual armature.

Due to the extra rotor mass needed to provide sufficient magnetic coupling between the rotor and the excitation winding, I assume that this is heavier than a typical brush-and-slip ring design.

It appears to have a simple single rotor excitation winding. It's an automotive alternator, so excitation current is controlled to determine alternator output voltage, while the stator phases would feed a simple diode rectifier.