Motor in a can??

[Konrad]

Motor in a can, as these are marketed to the glider community I'm posting this power thread under "Gliders".

I can’t for the life of me understand why anyone would want to use these! These are just motors supplied with their own mounting cage. The design as I see just adds weight and if not properly installed will allow the motor windings to heat up sooner!

They are claimed to fit well in the nose of gliders. I’m sorry I don’t see this. The cage mount actually means that the diameter of the motor is larger than the rotating drum of the motor. This means the ID of the nose needs to be larger to allow the cage to meet the front fuselage mount. So for any given diameter of the rotating can the space normally given to wire clearance is now taken up with the cage mount. So that’s one hit against the marketing hype!

Lead placement. If one really wants the motor leads to exit the rear of the motor assembly it is just a mater of placing the firewall (mounting bulkhead) further down the fuselage nose, and back plate mount using a cross mount and “standard” outrunner .

To properly mount an outrunner in a glider one should use the nose ring (firewall) followed by a rear bearing support (See attached). This eliminated the problem that the cantilevered motor presents with resonance while spinning large props. It also offers a means to keep the motor leads away from the rotating can. And it allows for the largest outrunner motor for any given nose space.

Yes, the motor in a can can offer the advantage of glueing blocks to the rear of the can to brace it against the fuselage. But I’ve never seen this done in practice.

Now we need to address the thermal path. Outrunners by design are very poor at cooling the motor windings. This is why they generally are less powerful than real inrunners! Adding a cage mount only makes this problem worse! Luckily we can do a bit to help the motor in a can survive. Most cage mounts have air vents at the front and rear. We need to direct what air that enters the fuselage down inside the cage mount NOT around the cage mount.

This actually is easy to do. Just add a sealing bulk head ahead of the rear vents. What you are trying to do seal the outer part of the cage mount against the inside of the fuselage nose. This will result in any air that enters the front of the fuselage through the turbo spinner or side inlets is forced down the inside of the cage mount. This way the rotating can is cooled directly by the cooling air. Also with the rise in air pressure while in the air, the motor air gap might allow some airflow by the motor windings.

Now if one thinks ahead a bit one can use the sealing bulkhead as an aft support for the motor.

If there is interest I could draw up a schematic showing how to properly mount, the poor design of, a motor in a can?

All the best,
Konrad

P.S.
I'd like to see Top Models reintroduce the outrunner rear mount (Hyperion style) for their line of outrunners.

 

[Johnnashville]

Still trying to figure out, why put a motor in a glider?

 

[Konrad]

Still trying to figure out, why put a motor in a glider?

LOL,
Because real (original) soaring machines (Hawks, Eagles, Vultures) don't leave their launch systems on the ground! And besides laying out the winch just isn't practical. The winch was the best solution in the 60's but technology has left it way behind! Heck, I was flying high powered electrics (properly set up gliders) way back around 1986.

If flying gas bags, electrics are even lighter than ships with wings stressed to handle the winch.

Electric, the only way to fly - to paraphrase Mr. Magoo


All the best,
Konrad

 

[Hank GB Z]

Are we sure they aren’t trying to sell us “can motors”?

I have a NIB CG Electra with the old 550 Manuchi motor maybe what is old is new again?

Hank

 

[Konrad]

Ha ha,

As much as I think the outrunner in a can is an evolutionary dead end and maybe even a step backwards. They "CAN" be a bit more efficient than the brushed Mabuchi can motor. Ok just a bit better but still better.

That Electra with a real 480 sized outrunner might be fun for a gas bag of a sailplane.

All the best,
Konrad

 

[Hank GB Z]

Konrad,

Yeah they're are a couple of updated Electras out there. Big things is to fix the nose and get rid of the plastic cowl and plastic hatch on the bottom. I like those mounts you show in the pictures for setting up outrunners in the nose of a glider. Are they not avaible any more?

Hank

 

[Konrad]

I don't know about the availability of the rear bearing support from Hyperion. I've only seen them for the 28 mm diameter outrunner.

Get them while you can!
Remember that you will need a bit of a stub shaft coming out the rear of the motor to receive the bearing.

Optional motor support plate for GS30 Glider series motors

Give additional protection and support to your GS series motor with this Spare/Replacement Motor support plate. Reduce chance of motor failure and protect motor from battery contact during rough landings.

hyperion-world.com

 

[Wayne]

To answer your question - The outrunner in a can concept works well and is pretty darn popular for a number of reasons.

Cost - They tend to cost less than an inrunner, but more than an outrunner.

They are easy to mount and fit well. - Yes, they are when compared to another outrunner and the spinning of the outrunner and wire and battery clearance issues is eliminated with the CAN design. The motor is also easy to remove and service the plane when needed. Yes, it adds a bit of width to the simple outrunner, but less so then the normal outrunner will actually need to clear its own wires, and now a battery can actually be pushed right up against the motor if needed. (I think this is the main reason they are popular.)

Cooling - Seems to be just fine. I will say that I have had more problems cooling inrunners then I have with outrunners. I find that most outrunners have a lot of cooling options these days and this simply has not been a problem if the motor is at all within specs with the prop and battery.

The mounting of an outrunner with a rear mounted firewall in a glider is very rare due to the mess of the install and the weakness of having such a long arm between the motor mount and the prop flange. A bearing mount is going to be much harder to install well and is super rare in the real world. If you like, this can motor is pretty much exactly that.. It is an outrunner with a rear bearing mount.

I will run an outrunner when I can. They cost less, weigh less and work great. If I kill one I do not cry too much.

I rarely run an inrunner due to the cost and weight of the motors. But if I am going for all out power or all out efficiency or packaging is a critical concern, then I will use them. Sometimes they are the right option for the job.

There is nothing wrong with a canned outrunner.

I will add that I think some of the best power to weight ratio motors are outrunners, especially some of the modern quad motors. If we could get some of the motor companies to make us some glider outrunners built like the little quad motors we would have some sick options out there! (I have tried but they do seem to interested..)

 

[Konrad]

Thank you Wayne for your perspective.

But I think we are confounding two different types of motor construction. In this thread I’m trying to find ANY advantage to the outrunner in a can concept. I’m not looking for a comparison between the outrunner and the real inrunner (Motor with the windings against the case and the rotating magnets inside the case).

To your points Power to weight is highly dependent on the ability to shed heat. The out runner is at a great disadvantage here as the winding are inside and shielded from most of the cooling air by the rotating can. With an inrunner the winding are against the case which can be in direct contact with the cooling airflow.

Then there is the issue of thermal efficiency (input energy verses output power). Again the outrunner is at a disadvantage here with all the electrical losses in the stator, compared to the electrical losses in the inrunner.

These two issue make the outrunner less than ideal when looking at power to weight and efficiency, in aviation these properties are paramount! I’d like to point you to the motors used on the B787 for the cabin air compressors. These motors are 3 phase DC inrunners that generate about 110 hp with 100% duty cycle Yet they are about the size of two of your fists.

The only strong point for the outrunner is mechanical. In that for each rotation of the shaft it has many many more power impulses from the higher pole count. This means that at any given RPM the outrunner has a smoother higher torque curve. This comes at a price in the thermal limits and energy efficiency.

The inrunner can make up for this with the use of a gearbox. Unfortunately for the inrunner if cost is a factor, the price of a gearbox is a lot more than the cost of a few feet of copper winding wire.

So in applications where one needs the highest energy density and high thermal efficiency the inrunner rules. I point to the FAI F5J class of glider. All the top end power systems are geared inrunners.

The problem with the inrunner in a quad copter is that the cost multiplier of needing to have 4 gearboxes.

Now in the next post I’ll try to show just how horrible the outrunner in a can really is!

All the best,
Konrad

 

[Konrad]

Here I’ll try to show what I see as the gross short comings of the outrunner in a can motor. Basically it comes down to weight and cooling. Yes, size can be an issue but that can be addressed by using a longer stator motor.

First let's look at a properly mounted outrunner. I’d like you to notice that the rotor motion actually can help draw cooling air though the motor to cool the windings. Some brands of outrunner will place a fan on the back of the rotor to aid with drawing the much needed cooling air through the motor.(See lower part of top drawing) But remember any power taken off the rotor to drive the cooling fan is power NOT available to drive the prop. In my drawing I also have added a rear bearing support to add stability to the mounting system (Non- cantilevered mounting). Not that it is needed but this bulkhead can be used to direct the motor leads away from the rotating rotor of an outrunner. Also notice that the down thrust is adding clearance to the motor leads at the down thrust moves the rotor away from the fuselage.

Now for the engineering abomination and marketing deception that is the outrunner/inrunner motor. (See lower drawing) First the term inrunner should never be associated with this kind of construction! No part of the winding are against the case for cooling. And the stator still has high electrical losses!

The first thing you will notice is that the actual motor is just like the standard outrunner. That is because it is a standard outrunner! But that it has been reversed only for the benefit of allowing the motor leads to come out the back. To do this and still allow the motor to be mounted on the front mounting ring of most gliders, this motor comes with its own mounting cage (can). This mounting can comes at a very high cost in weight and worse than that huge cooling penalties! I hope the weight is obvious in that the can has to be strong enough to handle all the torque that the outrunner can produce. Unfortunately it doesn't add any stiffness to the rotor assembly. It is just added weight! Not a problem you say as most gliders need nose weight. OK fine!

But now comes the killer with the design. The cooling fan action of the rotor* is actually fighting the natural airflow from the forward motion of the sailplane. (Top part of the second drawing).This means that any air that does get inside the motor from the front will be pushed (or stalled) and get pushed back out the inlet holes (slots). To make things worse what little air was drawn in from the rear of the motor by the cooling fan/rotor) will tend to be kept close to the outside case by the inlet air entering the fuselage. Now this preheated air will get sucked back into the motor by the cooling fan. And in a short time the motor will experience thermal breakdown. (This is why this type of motor is only sold to gliders drivers as it is hoped that the motor runs will be short)!

Now all is not lost should you find yourself the proud owner of an outrunner in a can. The installation of a blocking bulkhead will force any air that enters the motor past the fan and down the motor air gap (lower part of bottom drawing). It will also offer some rear support to the motor to help counter the gyroscopic precession forces from the large prop. I like to use some fuel tubing as my grommet to seal the outside can to the 360° bulkhead.

All the best,
Konrad

 

[Konrad]

Wayne,
If this isn't a typo, "(I have tried but they do seem to interested..)" I fear you meant "don't".

But if you have an OEM that is willing to make a glider motor, I’d like to see a motor with a highly tapered nose. Much like we had with the Hyperian GS series of motor. The smaller diameter of the nose adresses any issue with getting the motor leads around the motor. It also adds some distance between the bearing supports to help stiffen the shaft/rotor. And unlike the motor in a can allows the rotor to properly draw cooling air through the motor.

In the mean time these "L" series fit the bill rather well.

Electric Glider Motors

alofthobbies.com

All the best,
Konrad

P.S.
I see that Axi is still marketing such a motor with the sailplane long. These have that tapered front housing. Don’t confuse these with the motor in a can Axi Cyclone. Other than the price structure the sailplane long looks to be a fine outrunner for any glider.

 

[Konrad]

...
The mounting of an outrunner with a rear mounted firewall in a glider is very rare due to the mess of the install and the weakness of having such a long arm between the motor mount and the prop flange. A bearing mount is going to be much harder to install well and is super rare in the real world. If you like, this can motor is pretty much exactly that.. It is an outrunner with a rear bearing mount.
...

What, am I living in a fantasy!
In the real world the aft bearing mount is simple to install as the motor self aligns the mount as one waits for the bulkhead to be glued in. Size the bulkhead a bit undersize and let the glue take up the 1mm or 2mm of clearance.

The only issue I've had is in finding or adding the needed stub shaft protruding from the rear of the rotor. This usually isn't a problem as one can use the replacement shaft from the next longer stator motor. (Cut the excess length with a Dremel cut off wheel. I've even been known to turn down the rotor boss to expose this stub shaft. All but one of my six "30mm" outrunner equipped gliders are using the rear bearing support. Yes, I use outrunners in just about all my sport gliders!

The only one that isn't using the rear bearing support is my Samsara. This is because with that long nose it is difficult to install the upper mounting screws. Also Hacker grinds the pinion gear into the shaft for that geared A30M-18 motor, so replacing the shaft isn't practical. Yes, I'm using a geared outrunner!

All the best,
Konrad

 

[Hank GB Z]

Wayne,


All this talk about motors in cans got me curious so I went and looked at them on your website. They look like a well made product. I am curious about the hollow shaft. I can't imagine it helping with cooling even if using a turbo spinner, do you think Top Model is working on a variable pitch prop system for these high performance gliders?


Hank

 

[Konrad]

I recall the variable pitch prop being tried way back in the late 90's in FAI F5B. Don't know if the complication of keeping up with the unloading motor was an issue (should be easy to do now with telemetry ESCs) or if the issue was durability at those power levels. Today in F5J this shouldn't be a problem.

 

[Konrad]

I should have said that many ESC have the ability to sense current and output this data. A simple Arduino feedback circuit* driving a pitch servo might be neat. I can see this being an "advantage" in the last part of the 30 second F5J power run where we add energy to the plane in the form of speed and then convert this to height by coasting.

This is assuming that the airframe can handle the added speed.

* Or for an old fart like me an Op-Amp circuit!

All the best,
Konrad

 

[Wayne]

Konrad - I know this is getting under your skin because you replied 4 times to my post. I'm not an expert, I'm not the engineer, but I like a good debate.

Your drawings do not show the correct airflow. In fact a good motor will have almost no gap between the stator and the magnet. The main airflow on an outrunner is via the space between the stators and coils, and heat also carries over to the magnets and those are bonded directly to the steel flux ring (outer housing) so yes, they cool via the outside of the case too.

Another cooling aid for an outrunner is the larger diameter, oftentimes the motor is in the prop blast. Not on a typical glider instal.

IMHO, and I MAY be WRONG!! I find an inrunner more critical of the correct setup to avoid catastrophic heat. But then again, maybe that is just what we all do after spending $$$ for that sexy inrunner, we run it with the suggested prop and find it lacking.. Ah, gee, I can put a little more prop on this motor.. Ah gee, why did it melt my magnets? I personally have only managed to melt one motor, it was an outrunner, and it was the first commercially wound outrunner I had ever bought. I didn't know what I was doing and put way too much prop on it. Prior to that I had wound my own brushless motors or ran brushed 400s.. What a pain!

I think it would be interesting to do some testing and see how comparable a good outrunner is compared to a good inrunner. Maybe in the 300 watt range..? I'll put up a $17 DYS motor against whatever inrunner you can find to match it. LOL I can tell you who will win in the weight to power ratio and in the price to power ratio.

I am not championing one motor over the other, just trying to correct what I consider to be some big errors. The reality is that no one actually selects their motor based on efficiency for this hobby. We simply cannot afford the weight of a good efficient setup. I think we will all happily give up a tiny bit in efficiency to save a pound of weight. Correct me if I am wrong, but the efficiency difference between an inrunner and an outrunner comes down to something like 1 or 2%?? Assuming you can find an inrunner tuned to your specific needs, lets face it there are probably 100 times more offerings with outrunners then inrunners. Or, you can wind you own if needed. I sure don't want to wind another motor in my life, and certainly no desire to wind an inrunner!!

 

[Konrad]

Wayne, I think I only replied to specific parts of your post. And yes I think it is a gross disservice to call a "motor in the can" any kind of inrunner.

With that out of the way I think you meant to say that most Chinese outrunners have such a low copper fill ratio that there is significant cooling between the windings (not the stator). And yes my 2D drawing does not show this cooling path. As you know there are 2 primary losses in an electric motor, Iron and Copper. Again as you know we want to stuff as much copper into the stator for any given wind number, to minimize the copper losses. This means that if there are air gaps between the winding that the motor will be very inefficient at high power. This is a big hit against most outrunners.

The air gap I show between the stator and the magnets is a design feature used to fine tune the Kv* or Kt* value for the motor given any particular winding. (larger air gap results in a higher KV and a smaller air gap results in a higher Kt).

So yes I was being too generous in showing that this air gap is a primary cooling feature. The main thermal path for both the inrunner and outrunner is through the outer case. Now as we know the heat comes primarily from the resistance in the winding as the current flows through the windings. With the inrunner there is a direct thermal path from the windings to the case to allow the airflow to carry away the heat. With an outrunner that air gap I show actually acts as a thermal break (insulator) allowing only thermal convection to carry away heat from the inner windings across the air gap to the outer case. With the motor in a can we now have two air gaps that the heat must cross to get to the cooling air! You can see this in the lower half of the motor in a can drawing.

I too wound a lot of my own motor. In fact I still have a Torcman outrunner kit from the late 90’s.

What is nice about Fr Sky is that they have a real nice thermal coupler for their telemetry. I actually used this in coming up with my pressure cowl for my EF-1 racers.

Not sure what issue you may have had with properly cooling a high powered inrunner. But by design the inrunner has more cooling area than an outrunner simply because the winding are close the the maximum circumference of the motor allowing more cooling area that is in direct contact with the case and through the case directly to the windings.

And this is my point a few percentage points in the overall thermal efficiency means that at the high efficiency values of an electrical motor there is a huge impact. For example if one motor is 95% efficient and the other is 93% efficient it means that the thermal load of the 93% efficient motor is 1.4 times higher than the 95% efficient motor for any given input power. So to carry away that heat means the 93% motor need a lot more thermal mass or a lot more cooling air drag, nether of these things is desirable in an aircraft.

I like Hank GBZ observation about Topmodel's hollow shaft and what that might open up.

Now I’d like to be shown that I’m in error as this would mean than I’m learning a few new things. Unfortunately with the inrunner verses the outrunner for any given power to weight the inrunner wins!

Now is the inrunner a good match for the torque requirements of any given prop? That is another topic and why we often use gearboxes.


* Kv is rpm per volt, Kt is torque per amp

All the best,
Konrad

 

[Konrad]

Now the sharp eyed ones will be saying; Hey Konrad that motor in a can looks a lot like your pressure cowl! And you would be correct! We can use the can (cage mount) as the baffle tube.

We take a little bit of high velocity air from the forward motion of the sailplane. We then slow it down by expanding it in the plenum in front of the rotor. (And if you remember your high school physics* this ups the pressure). We then allow this high pressure air to flow down the gap between the rotor and can towards the low pressure air outlet.

The key to this installation is that one needs to add that 360° baffle. Without that baffle all that high pressure air will be deflected by the fan/rotor to the outside of the can offering very little in the way of cooling benefits.

*They still teach Physics in high school?

AMA & STEM

So I was reading my EAA (Experimental Aviation Association) magazine last night and I see they have a little article about the EAA teaming up with the AMA to help with STEM programs at schools. I think this is great, this is something we like to help out with at Aloft Hobbies where we supply a...

forum.alofthobbies.com

All the best,
Konrad

 

[Konrad]

Ok, since I started kicking this can down the road, I stoped off at Aloft Hobbies to see the new "motor in a can" from Top Models.

TopModel

alofthobbies.com

If questioned I'll deny I ever said this, but the Top Model motors actually look like they are a nice quality piece of work.?
(I still think they are an improper engineering solution to an almost insignificant problem, keeping the wires away from the rotating rotor).

All the best,
Konrad

 

[Konrad]

It looks like this small forum is getting noticed! Thanks Tic.

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