"TRUE AC MOTORS" versus brushless motors

Hello. At one time, I read Link claim that true AC motors don't have a "torque constant", they just provide a constant torque line all across the RPM range. That fascinated me.

But, now that I've learned a bit, are these so-called "AC motors" really just the same thing as brushless motors except with a sinusoidal drive instead of trapezoidal drive? It appears that if you drive RC motors with insane voltages, they'll have a super high no-load speed which result in their destruction from spinning way too fast. So the solution is to electronically limit the RC motor's RPM. Also, it's going to have insane current if it's not limited, so what to do? Phase limiting. If you limit the phase current and the "normal current" would exceed that current limit within the electrically limited RPM range, you'd basically have the same motor current within that RPM range. Well, if you do that, the torque is going to be the same all over the range. Despite that, though, they still have a "torque constant". Is this how AC motors actually are, and they really do have a torque constant and it just doesn't seem like it in practice due to motor RPM limiting and phase current limiting?

Now I've heard of AC motors that are driven by the sinusoidal frequency and that frequency determines their no-load speed... are these typical electric car motors?

I think there is some relevant info to your questions here:
http://www.diyelectriccar.com/forums/showthread.php/pure-horse-power-dc-vs-ac-14110.html
but I don't recall what page it's on; been a while since I read thru it.

swbluto said:

Now I've heard of AC motors that are driven by the sinusoidal frequency and that frequency determines their no-load speed... are these typical electric car motors?

Click to expand...

Google: "VFD motor".

If they are perm magnet, there is no difference between AC brushless and DC brushless.

I think you must be thinking of induction motors, which have no perm magnets, and operate in a continous state of phase-slip to generate a torque through induction to thw rotor, because its easier to spin with the field than create a boat load of eddy currents (heating).

not sur what 'true ac' means but to me a BLAC as opposed to BLDC i tend to think of the switched reluctance motor.
http://www.allaboutcircuits.com/vol_2/chpt_13/4.html


a decade ago when i first got into ebikes i was absolutely certain vsrm would have taken over by now.
for the the manufacturer i thought the appeal of no commutator, brushes or magnets lends itself to lo material cost/high profit margin.
for the end-user what's not to like about no heavy magnets & performance that punts ass of a hub or r/c motor alike.
yet another example of just how far behind the state-of-the-art our ebikes have to be content with using last century old china-tech.

oh well, at least there's the possibility of finding an old vacuum cleaner in a dumpster somewhere in japan.

Dyson claims that the motor is the fastest ever developed for domestic appliances - up to three times faster than conventional vacuum cleaner motors - but it is much smaller and weighs around 1kg, compared to the typical 1.3kg. It also generates 400 Air-Watts - a measure of the "suck" in vacuum cleaners. This is about a third more than conventional motors.

The 100,000 rpm motor spins more than five times faster than a Formula One engine, and has a similar power-to-weight ratio.

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http://www.drives.co.uk/fullstory.asp?id=1109
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=11265

Well, there is(was) one electric motorbike with a VRM: http://www.electricmotorbike.org/index.php?page=lectra [scroll down for more on the motor].

I don't think it's got anything to do with old China tech. If some of the problems with VRMs had been overcome, they would be more common generally.

isn't it galling to see a vacuum cleaner or a blender with a motor that's more advanced than what we're allowed to have?

I guess it makes more sense for those applications because you can use the high speed without needing to gear back down again. Just imagine the torque ripple in a VR hub motor

I assume swbluto's original question was about AC vs DC permanent magnet machines, not induction.

Forwarded from my buddy, Paul Rako at EDN (article written by Margery Connor)
http://www.edn.com/index.asp?layout=blog&blog_id=1470000147&blog_post_id=1700012370

This quote pretty much sums it up, but doesn't really answer the question in my (admittedly somewhat dull) mind.

"One thing everyone agrees on is that the drive signals for the two motor types are different: A PMSM is driven sinusoidally, while a BLDC is driven trapezoidally making the PMSM much quieter, both electrically and mechanically. Plus, it has virtually no torque ripple."

Ohhhhh K... So if we dismember a Hobby City 100 dollar coffee can outrunner and make a skewed stator and drive it with a sine control, I guess it's now a PMSM, or BLAC, or PMAC?

swbluto said:

Hello. At one time, I read Link claim that true AC motors don't have a "torque constant", they just provide a constant torque line all across the RPM range. That fascinated me.

Click to expand...

If you're talking about a motor which has separate powered field windings, instead of permanent magnets, you can vary the field strength, so it won't have a fixed torque constant. Is that what you mean?

Toorbough ULL-Zeveigh said:

isn't it galling to see a vacuum cleaner or a blender with a motor that's more advanced than what we're allowed to have?

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They work well in applications with minimal starting torque needs, and situations where having a large amount of airflow is included as part of the design/function to begin with (like a vacuum cleaner).

They would be a poor choice for an EV application, and the required gang of planetary gearboxes would be 3x the size and weight of the motor itself. If you look at it from a system perspective, you would end up with something heavier, more complex, and much lower efficiency than just an RC motor.

Don't be jealous of the vacuum or blender