There are certainly quality issues with these motors--but sometimes it is hard to make the correlation with price!

Electric motors are pretty simple compared for example with a good "stunt run" glow engine. In the latter there is a lot of "black magic" and most of the time you get what you pay for.

But in electric motors, there is a just a simple theory about how they operate, so even a crappy one will work as the theory says--just may have crappy parameters.

But as far as I can see, here are some of the issues with electric.

1) Quality of materials---good strong heat resistant magnets, heat resistant wire insulation, good bearings, thin low loss laminations for the stator (0.2mm seems to be about the best.
2) Quality of stator design---you don't want to have your iron saturate
3) Construction quality--consistent winding (same number of winds/tooth), magnet gluing, good solder connections at the winding termination
4) QC--does it work when you get it. And is it easy to get it replaced if not.

Item 2 is probably not a big determinant-- since most knock-off companies will be copying any good design after a short time I bet.
Item 3 is probably better with the better (higher cost) manufacturers--but there are issues there to (like magnets coming loose)
Item 1 is probably the largest differentiator in cost, although except for bearings perhaps, we aren't really stressing our equipment in control line (at least I'm not).

I think the issue with the European motors is the exchange rate with the $. It is brutal. So even if companies like AXI have improved manufacturing efficiencies, the cost doesn't fall as fast as the Euro has been rising.

Like I have said, I have typically gone middle of the road--not the highest, not the lowest. Up to now I have been satisfied. Could I go cheaper and still be happy? Probably, although I might have issues with my batteries if the cheaper design is less efficient (because my batteries are not over-sized with their capacity).

My 2 (US $) cents!

Unless good motors are a lot lighter or use less electricity for the same power as mediocre motors, it might be a reliability issue.  A modiocre stunt engine is mediocre every flight.  A mediocre motor may run fine, but have a high failure probability.  This might be OK for most folks, but a guy like Jim Aron who takes two years to build a front-row stunter would pay a little more to reduce the probability of having all the magnets fly off during an overhead eight.  Even if the failure mode isn't that dramatic, if you spend a thousand dollars going to a contest, you don't want to risk losing a flight to a bad motor.  

One problem is that I have heard of magnets being tossed with about every brand---even some of the best ones. To some extent some of these cases may have been exacerbated by overheating problems (especially when I read them in RC forums), but I am not sure.

I had one case in which my own hand-wound and glued motor began losing a magnet (fortunately I discovered it by noticing the magnet was protruding a bit from the shell). I had used a recommended product --a rubberized Loctite adhesive, but evidently didn't use enough (or something  ). With my last build, I used JB-Weld and I think I used a pretty good technique. We will see!

But lately I have been using production built motors.

I suppose if I had a $1k plane, even with a top of the line motor, I would consider running it in to make sure any poor gluing would show itself as early as possible.  Of course I don't run $1k planes, so I won't worry that much!

The other thing to consider is that when we talk about "high cost", these motors aren't really that expensive in the big scheme of things. To me the major expense is still the batteries.

Dennis--re the cooling fans.

My feeling is that if you need cooling fans, you are overstressing the motor. If you are doing that, then it is probably running past its best efficiency point and converting too much of your battery energy into a space heater!

In this case I'd argue you should go up a size larger in the motor. Saving weight in a motor, and then having to carry a larger battery isn't a good tradeoff IMHO!

How hot do you think your setup is getting?

I haven't flown the Vector in a while--result of a takeoff "incident". When I was flying it, I don't really recall thinking that the motor really felt that warm at all. With the Nobler, the fiberglass cowl doesn't make motor access easy--without removing the prop. However I "think" I touched it after a flight some time back and again didn't think it felt hot. If it were warm, I think I would have noticed it.

Fortunately with the Scorpions, the magnets are rated to a pretty high temperature (200^oC), as is the wire insulation. However if you are getting that hot in a stunt application, I think the motor is just too small.

I do have a temperature probe for my datalogger, but have never plugged it in.

I'm with Alan on this one: if the motor is too hot and you have decent cooling air ingress/egress then you are running too small (read that light) of a motor. It's not that hard, really.

Dennis' observation that under-stressed motors are somewhat more efficient is true, and they are because of my favorite reason to declare one motor better than another ... low winding resistance. This improves both efficiency (assuming both have similar Kv's) and it also means that in the very short term, if the ESC governor adds 0.1 volts to help go up, then more instantaneous current flows to help fight all that gravity and induced drag.

My rule for better (aside from build quality) if 2 motors have similar weight and Kv, then the one with the lower resistance wins. Many motor manufacturers don't or won't specify it! If the Kv's are different, then compare the resistances by using my fudge factor of (Kv-a / Kv-b)^1.8. The exponent should be 2, but the reality of wire sizes and packing them in results in a great fit to 1.8.

later,
Dean

How hot do you think your setup is getting?

Hottest I ever MEASURED was around 130F.  However I had another that was clearly running hot despite best efforts to fix.  Never had the temp sensor handy - but after flight you could tell there was a lot of heat there.  The motor definitely was NOT overloaded: Reimfire 35-36 with a 10x5 APC at around 10k.  Same motor had flown often and well with an 11x5.5 on another bird...

The hot motor I mentioned was a smallish Turnigy 28-36 swinging a  10x5 APC at 10,900.  FLown just one flight - it got it done but was obviously laboring.  Post flight also showed battery usage at over 2400mah, on a bird that uses maybe 2000-2100 using a larger motor.  My conclusion was that the higher current draw was a byproduct of the motor being overloaded.  Had a similar experience when I replaced the 35-36 in the MB with a 35-42.  Data size is small but the 6mm longer motor uses around 100mah less.  I would expect larger motors would be diminishing returns tho...

So Alan & Dean - (forgive me if I am mis-representing what you said) are you saying that motor cooling is (almost) a non-concern as long as the motor is comfortably large enough for the task at hand?

   

>>>Less one thinks that you can keep supersizing the motor, eventually you will have to worry about magnetic losses (coming from having to magnetize and demagnetize all that iron every commutation cycle), so there is an optimum size.<<<

Exactly, we have iron loses and copper loses. Best efficiency depends on voltage and so if you use large voltage, you can come to best efficiency current which is higher than max allowed current for that motor, so it is not everytime true that bigger is better ... as you say "there is an optimum size"

Hmm ... I see that my table is somehow corrupted, but you can see some examples that bigger is sometimes worse .. for example those two mvvs motors. It will change if you will load them by larger prop, that larger motor will win.

It makes sense that larger wire diameters have the lower resistance. It's not just a matter of cross sectional area, however, because AC tends to flow around outer surfaces (skin effect). However, skin effects may be reduced or even negligible at lower frequencies. As frequency increases, effective resistance of wire increases, and the skin effects are due to eddy currents in the conductor. There are specially wound wires and even tubular conductors designed to reduce these effects. Not sure how they work for motor windings..

And, classically, almost all common conductors exhibit more resistance when they are heated. There are some materials that do not do this, but I can't remember what they are. The material in some thermistors (some kind of sintered alloy semiconductor?) has a negative temperature coefficient.

Wondering aloud again, someday when you hear of extremely efficient electric motors using sputter constructed semiconductors instead of metal wires, remember my fickle pondering about it. Maybe it will be the result of some kind of stacked "printed circuit" motor.. They are making stacked structures in integrated circuits nowadays. Perhaps nano technology can someday be applied to build a very, very good motor.

The other thing which comes to mind is using a solid state cooling device, like a Peltier thermal device, to provide cooling of windings. (However, such devices are not particularly efficient, I suspect it might be something less even than a zero sum game for such an application.)

It would seem the solution would be superconductors; we could top off the liquid He in the old dewar before each flight!

L.

"You moon the wrong person at an office party and suddenly you're not 'professional' any more." -Jeff Foxworthy