Windage

[HO RacePro]

Windage -- it might not be a term you are familiar with...

It has to do with the power an electric motor loses stirring up the air. As the armature spins it acts like a centrifugal fan, and that consumes power. The classic 'can' motor typically has openings that allow air to escape, helping to cool the motor. But pumping out that air uses up some of the motor's power.

There is also viscous friction, as the armature poles spin past the magnets. The thin layer of air between them gets sheared, and that also costs power.

Now if you were to ask me what percentage of an electric motors power gets eaten up by windage I'd have to shrug, 'cause frankly I don't know. What I can tell you is it depends heavily on how fast the armature is spinning. A quick internet search revealed that windage losses increase approximately as the CUBE of the RPM's. Double the RPM's, eight times the loss!

Which still doesn't tell us how big a deal windage is. Is it small enough to ignore? Well, maybe. But that may be assuming too much. The very fact it has a name means at some point engineers thought it had a meaningful impact on motor performance.

Getting back to slotcar motors, would there be some advantage in reducing windage? Could closing off the vents in a can motor stop literally blowing power out of the motor? But would that cause the motor to overheat?

I'd say there might be a trade-off there. There are slotcars that operate at such extreme levels that any loss of cooling would quickly result in failure. But there are also many that aren't nearly so stressed.

I'm getting pumped to do some experiments with my 1/32nd scale slotcars. Could be interesting.

[Model Murdering]

Interesting question. Hmmmmmm.

At first glance? I'll venture that windage/airgap and cooling are inversely proportional and thus unassailably intertwined. As the numbers are ratcheted one way or the other from optimum performance, we begin to see issues that affect longevity. You'd be re-cycling heated air with a restricted flow, or cycling fresh ambient air in an open flow system?

I paid some dues in an electric motor shop. Based on a huge sample base, I can attest to a high mortality rate, where breathing has been restricted accidentally, or by poor design. Whatever the case, the final synopsis is always a shortened service life, and frequently premature death.

In industrial applications where real grunt is involved, we always see provision for ventilation in an electric motors. Either the shaft end or the bell sports a fan. Both ends of the casing are ventilated for through-flow. High speed DC double winds feature a large fan, and fully ventilated casings. Even the common low speed AC drives feature a primitive fan and fully ventilated casings. Certainly with low quality, and toy grade motors of the "disposable" variety, we may not see much in the way of cooling; but that hardly makes it right.

With slots, we can safely stipulate to the idea that: As the performance demand goes up, the amount of ventilation has to go with it. Notably, many high end unlimited slot chassis' all sport an open frame design using minimum airgap with a maximum exposure to free air. Also of note: As the DC mini motors go, the factory ventilation increases dramatically as the motor ratings increase. However! This is across the comm, and nowhere near the lam-stacks. Then again the high end gravity racers shred the cans down to just enough connective tissue to hold them together.

Much to digest, Ed.

I look forward to you shooting holes in conventional wisdom, and if'n ya dont; at least my brain cells were engaged by something of interest.

Bill

[HO RacePro]

Always good to hear from you Bill!

And I value your experience with industrial motors and unlimited slot motors. Motors generate heat, pretty much in proportion to their power, and that heat must be dispersed somehow or destruction is assured.

But I will cite my own experience with the industrial motors used in the food and pharma industries, designated TEFC -- Totally Enclosed Fan Cooled. Totally enclosed motors are almost universal in those industries because -- especially in the case of food -- you need to keep stuff from outside getting in and stuff inside from getting out. The fan only blows air over the cooing fins on the housing that encloses the motor itself. The armature and stator are carefully designed and rated to perform to spec without direct air cooling.

Can a slot motor survive without direct air cooling? Certainly. There are stacks of fully-enclosed DC permanent magnet motors in existence, and nothing would prevent them from being used in slotcars. They simply need to be designed and rated for enclosed service. And yes, that means they can't be as potent as a slot motor that features direct air cooling

And they may have a shorter service life, although that is a design option, not a requirement. If the internal temperature is kept within reason a long service life is achievable.

So my question is, could a standard 14K, 18K or 25K slot motor be raced without direct air cooling? Would they run faster and still have an acceptable service life?

Now where did I put my Magic 8-Ball???

[Model Murdering]

Oh swell Ed, today I'll be fussing seedlings in the cold frame today, pondering the application of constant and variable duty cycles. I'm having difficulty finding a workable "tube sock" (one size fits all) comparison between the two performance expectations. Ramping the duty cycle verses ratcheting the duty cycle presents entirely different demands in my pea brain.

The engineering is tailored to an extreme or opposite expectation. To my eye, the question is whether or not sufficient overlap between them exists for a practical application. As a simpleton, my gut sez no.

Banging apples and oranges together is fun. I have a kooky example that somewhat straddles this fence. Film at 11.

Bill

[HO RacePro]

Bill, let me throw another complication at you and see if it pops your circuit breaker...

Closing off the vent in a classic slotcar can motor offers the opportunity to improve the magnetic circuit that provides the motor's torque. If you bridge that vent with a magnetic material, such as carbon steel, the magnetic flux will have an easier path flowing across the magnets, which should give somewhat more magnetic flux passing through the magnets and the armature.

Nothing theoretical about the above statement. It's a fact, Jack! Question is, how significant is it? The motor will produce more torque, but it will also have reduced top speed. The entire performance curve of the motor will be shifted.

How much? Uhhhhhhhhhh...

My guess, a little bit. Enough to matter? Well in racing, yes, the little bits do matter. A lot. A hundredth of a second per lap can be cost you a win. A tenth of a second is a geologic age.

But it can only be proven on the track.

Watch this space...

[4424ever]

Many true statements have been made in these posts but in the end for home racing this is FAR too critical to concern yourself with

[Model Murdering]

LOL Dave ... practicalities? What on earth do they have to do with toy cars?

******

Ed,

After mulling it over, I think it'd be tough to measure air pressure, but I reckon a guy could simply subtract air, and measure any resulting change in draw, operating temp, and RPM. Seems like one could use a vacuum chamber, then run a cross section of chassis in it ... say Life Like M with a flux hood, a canned Mega G+ (minimal ventilation), and a free breathing open frame unit. H0 seems to provide all the weirdo combinations required ... giggle.

Using both heat and RPM wands one might see the result of paddling more or less air ... or not.

Bill

[Wobble]

And then maybe you need to factor altitude into it as well because slot cars are run at different altitudes and there's less air up there, isn't there?

[HO RacePro]

You might find it interesting that in some industrial applications sealed motors or generators are infused with hydrogen gas. This lightest-of-all molecular gasses is used, I believe, to reduce losses due to windage while improving heat transfer. Hydrogen molecules zing around like nobody's business.

To avoid Hindenburg-like explosions they need to exclude all oxygen. 'Course you could use helium if that scenario makes you nervous. Being a 'noble gas' helium doesn't burn. It'd work almost as well.

So here is another bunch of trade-offs. Yes, a vacuum would eliminate all windage, but would be terrible in terms of heat transfer. Hydrogen or helium will cost you some windeage, but be much better at heat transfer. Trouble is, those gases are really hard to keep contained. I remember learning that industrial leak detectors are calibrated by measuring the amount of hydrogen that leaks out of a glass capsule! Yes, hydrogen leaks at a detectable rate out of a bubble of solid glass!

Trade-offs. Engineering is all about trade-offs. I simultaneously laugh and groan when I hear a car commercial touting "uncompromising engineering". Engineering is ALL ABOUT compromise!