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It's more than that.

A great write-up by Tesla (2007) [1]

  In an ideal brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable. When maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum – so that inverter and motor currents are maintained at their lowest possible values. This minimizes the I² R (current² resistance) losses and thereby optimizes efficiency. Likewise, when torque levels are low, the B field should be reduced such that eddy and hysteresis losses due to B are also reduced. Ideally, B should be adjusted such that the sum of the eddy, hysteresis, and I² losses is minimized. Unfortunately, there is no easy way of changing B with permanent magnets.

Essentially, this new tech combines the best of Permanent Magnet Brushless DC and Induction motors. It allows you to change the magnetic field B without contact.

The losses in the shaft coils are low when B is low so the gains in efficiency for an "eco mode" would be excellent as you modulate B for low torque and you would still retain the ability to crank up the power in a "sport mode" at the cost of efficiency.

[1]https://www.tesla.com/blog/induction-versus-dc-brushless-mot...

Exactly. The B field can be cranked up for short a duration to maximize torque. The rotor could also have much lower inertia.

I sketched out this same topology, with inductively coupled coils for the rotor's magnetic field. If you don't need the fields to reverse (they wouldn't with a magnetic field). Then you could still have small PMs in the rotor with a coil to provide boost field for additional torque during acceleration.

> Unfortunately, there is no easy way of changing B with permanent magnets.

There’s the old semi-permanent magnet trick, although this only allows a couple of discrete field strengths, not continuous tuning. Basically, a neodymium (or other hard) magnet and alnico (or other soft) magnet are put in parallel, surrounded by a pulse winding. A pulse can flip the magnetization of the alnico, but not neodymium, so you get field strengths of either neo + alnico or neo - alnico. (This is most often used sized for similar strengths to turn a “permanent” magnet on or off, but can be used in non-equal strengths as desired.)

Brushes and slip rings seem like they would be more efficient, but they are adding friction and sparking that isn’t part of this, and as they wear become less efficient. BLDC showed that induction wins through one conversion, but this is making 2.

Simple napkin math: If you compare BLDC to Brushed efficiency. Best BLDC is 90% efficiency and best brushed is 80%. Even if the inductive rotor loses as much energy as the complete BLDC motor, it would still perform on par with a brushed motor.

Well, BLDC doesn’t really use induction at all. It just makes a magnetic field that pushes on magnets.

I don’t understand why this is being downvoted? Is this a terminology thing? I design my own brushless motor controllers. A brushless motor operates by generating a magnetic field in fixed coils and using that magnetic field to directly act on magnets.

Wikipedia defines magnetic induction thusly: “Electromagnetic or magnetic induction is the production of an electromotive force (emf) across an electrical conductor in a changing magnetic field.”

So basically using a changing magnetic field to create voltage in a conductor. While it is true that induction occurs in a brushless motor as a secondary effect (the rotating rotor induces back EMF on the stator coils), the primary mechanism of movement is a magnetic field physically acting on permanent magnets - no induction involved.

This is distinct from an AC induction motor, about which Wikipedia says: “An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding.”

Basically in an induction motor, induction is a required effect for the operation of the motor, rather than a side effect as in a brushless motor.

So while induction occurs in a brushless motor, if induction could be magically disabled the motor would still work. This is not true for an induction motor.

Indeed, BLDC doesn't use EM induction to excite the rotor, instead it uses directly macroscopic magnetic force due to current in stator acting on the permanent magnets in the rotor.

The EM induction is present in the winding of the stator, and to some extent has to be present, to keep the current there low enough, otherwise we would get locked stator situation, and the winding would burn out.

That’s true, each individual coil is an inductor, and without that inductance you would need a more complex drive circuit.

To be fair, is practically on one of the first lines of the press release: "...In contrast to the magnet-free concepts of so-called separately excited synchronous motors (SESM) already available today..."

That's the normal/traditional advantage of "brushed" motors vs synchronous motors, but the brushed motors have the massive disadvantage of having a mechanical contact (the brushes) that wear off, need to be sealed, etc. The advantage here is that it behaves like a brushed motor, but without the brushes!

Out of interest, what could make such a design better than a traditional inductive motor? Such motors already suffer inductive losses, and do not need slip rings or brushes, and presumably do not an additional set of windings to transfer power to excite the stator?

Just could the two sets of coils be optimized for their own purposes?

Inductive coupling in an ASM is at the drive frequency (hundreds of Hz), inductive coupling in an inductive electrically excited SM can work at an arbitrary frequency, e.g. 100 kHz. You can also see in the ZF release how small the coils for transferring the excitation current are compared to the main windings, they are the small rings on the left side.

>what could make such a design better than a traditional inductive motor

Traditional induction motors, compared to these with wound rotors, are heavier and bigger in size for the same power output, and have less efficiency especially at starting/low-speeds.

that is only true if you don't have a VFD. At that point you can basically get whatever you want out of them until they melt or blow out a bearing

So then it comes down to cost? If this is cheaper than a motor and vfd, then it's still a win?

Source: the one time I restored a massive JT Towsley jointer and had to get a vfd for the giant motor I had to put on it. In other words, I have no idea what I'm really talking about.

as others commented, this is not new tech. It's the result of the PR department needing to make a press release.

Oh I was just going to comment that those looked like some sort of brushes and thus that would negate other advantages, but after reading your comment and carefully reading the notice yeah that is definitely great! Induction coils instead of brushes for the rotor current def seems like a key advantage/development.

The animation seems to show that the rotor has a DC (ie. constant) field. That is easy to do with slip-rings and a DC current source, but with any induction mechanism, there must be a rectification stage on the rotor. They don't show any of that.

I'm wondering if performance could be improved by controlling the polarity of the rotor coil fields, to push against the stator-induced ones. Since some semiconductor rectifiers are necessary in the stator, it doesn't sound much more complex to control the polarity. Part of the control logic could be handled before the inductive power transfer mechanism, by modulating the frequency, though it gets complex as you probably want to tune the resonant frequencies of the sender and receiver.

Wind turbines work that way. Early wind turbines had to mechanically rotate at a fixed speed to match the grid. With modern ones, the field is sometimes electrically rotated to adjust for rotor RPM. This can be done quickly as the wind changes. The range of control is not large, though. This is a quick adjustment made electronically until the blade pitch can slowly be changed by motors to re-tune the wind turbine for the current wind speed. Look up "doubly fed induction generator".

Interesting to know, thank you. They seem to mostly use this to adjust the frequency of the induced current in the stator, as they control the rotation speed of a "virtual rotor" (rotation of the rotor + rotation of the magnetic field around the rotor).

That's indeed similar, but in the wind turbine use-case, the stator is passive. A direct application could be to tune the torque/speed response, or improve motor weight by reducing the number of coils on one of the elements.

Hm. Do they have diodes in the rotor?

It's common for large, power station sized, generators to have that. The field windings are on the rotor, and the heavy power comes out from the stationary armature windings. To power the field windings, there's a smaller generator as part of the machine, but it's the other way round - stationary field, rotating armature producing power. The output of the smaller generator is rectified by diodes on the rotor to provide DC power for the main rotating field. No commutators or slip rings. No wear, no arcing.

Did someone just scale this down for motors?

Brushless synchronous induction motors have also existed for a long time.

Yes, but these ones with externally excited wound rotors have the advantage of more torque and efficiency at compact packages but have had the disadvantage of brushes that would wear out, create carbon dust and electrical noise.

So it's the `I2` from the `I2SM`-name which is the key part, referring to the induction of current into the outer coils, and then the magnetic induction to create the forces.

There will be large losses due to induction method.

"large" is meaningless unless you can compare it to the friction cost of the brushes they replace.

Except you're competing with brushless motors using magnets, not other induction motors.

> The good part is there's no more brushes to wear out and no dry seal needed so the motor can now be oil cooled from the inside as well, but to me it raises the big question about the power losses incurred by the energy that's transferred to the rotor via induction versus the previous direct contact via brushes and slip rings.

Slip ringed motors can be perfectly oil cooler as well. And you will need oil seals and drains in under any circumstances you use oil cooling.