The actual alternative is induction motors, which are just a bit less efficient than PMSM and otherwise basically the same. Except that the frequency fed to them isn't exactly proportional to speed.
They've been used to great success since we had the needed power electronics to drive the electric trains of Europe.
Yeah so the relationship between speed, power, frequency, size (both in the direction of primary flux excitation and in the direction orthogonal to both that and the movement), and torque at nominal values of current density (for a given conductor losses are proportional to the square or this value and to the total mass of that conductor in the machine; that's independent of any of the other scaling parameters; note this is absolute power not percentage) and peak flux limitations (core saturation, permanent magnet demagnetization), are sadly not trivial if you express them in a way that is even just _valid_ for the modern days where we can support electrical frequencies up to around a megahertz at scales up to around 100 kW, and even harder when you remember that core material has severe frequency dependence of it's limits.
E.g. for example for a given electrical frequency and decent radial flux synchronous machine, power density is quite static and torque density can actually be dialed quite freely from 2-pole machine (turboset in gas turbine running on the grid at 3600 rpm (or 3000 rpm outside NA and some Pacific Islands) to 40(+) (example deployed at Hoover dam, 180 rpm).
At those higher pole counts, the center of the rotor is no longer electromagnetically active, because the magnetic field lines keep to a narrow ring only about as thick as each pole is wide.
Unfortunately it's mechanically not that trivial to handle a cylindrical shell with a small air gap (this needs to be significantly smaller (about at least 10x) than the pole width) when using substantial torque and speed.
Circumferential velocity is practically limited by hoop strength of whatever the outer region of the rotor is made of, even if it's all very nicely balanced, because eventually the magnetic armature flux source (wires or magnets) will fly out.
Higher electrical frequencies limit the field winding core's magnetic permeability (magnetic field/force strength amplification relative to vacuum, for same electrical current) which hurts efficiency by dropping the useful mechanical power component of field voltage while the voltage resulting from the current (that needs to happen to cause the magnetic field in the direction of movement that causes the mechanical force) due to wiring resistance stays. (I think the permeability gives the ratio between voltage and current for otherwise identical mechanical load conditions and winding shape?)
Thinner wires have less fill factor because the insulation has to stay the same thickness as per-winding voltage stays, but magnetically inactive terminations are less wasteful (for losses and mass) when a decent number of effective turns (>>1, think >10~50 for most of the benefits) are used.
Note while the armature necessarily has an even number of poles in it's construction (north/south), the field is not forced to that.
Indeed, the iirc most smooth torque (under practical mechanical feasibility limitations and without undue sacrifice of efficiency) results from having a prime number (of field windings, in WYE-style connection) exactly one off from the armature pole count.
Note that for low losses all these torque-smoothing techniques _require_ only a single electrically directly driven winding in each slot (per mechanical field pole) and with that only GCD(field_slots, (armature_poles / 2)) windings get to share an electrical half-bridge (one single wire going to a single voltage-output terminal on the electronics board; note mainstream BLDCs have 3 of these, classic fridge compressors have 2, and modern stepper motors (e.g. 3D printer) have 4).
Any time you have multiple windings driven by different electrical source voltages you're wasting heat in the winding because the lowest-loss would require all conductor in the slot to to perfectly evenly share current.
There's just one problem with that: you need a nearby slot with exactly opposite phase to even possibly use more than a single (half) turn of "winding" in the slot.
If the voltage is still enough to not loose too much in the connections, you can use transistors developed for efficiently powering modern computer chips from comfortable voltages like 12V, but even then a "winding" has to be much longer than an armature pole to mitigate the losses of spreading the return current sideways to where a slot carries the current in the reverse direction.
Once the voltage at the transistor is over around 10V the benefits of more precise control of the field magnetization to the armature position (and how the shapes distort the field lines from anything that would look like a sine wave) could be useful.
In theory that'd also provide direct access to electronically control the air gap (well, net force normal to the air gap "surface") which _could_ be an alternative to mechanical bearings for very thin-shell constructions.
See maglev trains for a pretty practical application of using an electric motor to also levitate the "rotor" in a place where a mechanical bearing ("train wheels + bogies") performs poorly.
Actually large data centers at least if done in a vaguely alirack style architecture, can do this with a decent fraction of their nominal power for very little hardware cost, as reactive power and real power add up via Pythagoras (`apparent=sqrt(real^2 + reactive^2)`) to the apparent power (rms voltage times rms current, which is what the 60Hz electronics and 60Hz transformers care about).
The first 10-ish % are nearly free.
And alirack style datacenters have large 3-phase converters between the grid and some 240 (nowadays often 350) V DC bus, with the battery banks directly (with just fuses and sometimes a little bit of balancing/nudging power (think 10% of battery power rating)) on the bus, and then the servers also directly consuming from that bus.
The large converters on the battery bus thus allow synthetically smoothing load transients to the grid using the batteries to smooth that power draw.
This has just minor additional wear on the batteries and a small power efficiency impact from hitting through the batteries, both of which are easily paid by anything market-rate of providing that grid service.
Because they already need the power electronics and batteries anyways, unlike a utility battery farm that at best can argue day/night load shifting of solar production as the reason for the electronics and batteries to exist.
In that same spirit it's also effective to put batteries on the DC bus (between MPPT and inverters) of large solar farms, because they need the electronics anyways and it's actually reducing the required inverter&transformer capacity of the solar farm by peak-shaving.
10+ MW voltage-source converters that can't do up to around 80% of their nominal capacity as mostly-reactive apparent power with stabilizing synthetic inertia scaled as desired/specified are a mostly software issue, stemming from lack of regulatory pressure incentivizing the engineering complexity of that.
Though if you want to do a smoothing action on real power flux you'll have to colocate battery capacity with the converter.
Which to be clear is fairly cheap to do as long as you get compensated for the substantial frequency stabilization capacity this represents. I'm talking like 15~120 minutes at converter nominal AC power of battery capacity.
The first 10~20% of reactive power are almost free from the converter electronics, btw....
Ehhh if it's just for looking and you don't have anything lidar just go for splats they're way better behaved, mostly because they don't need to understand a concept of "surface" they just understand "splat with spherical harmonics of view-dependant color".
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