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robert chamberlain10/09/2017 06:00:00
49 forum posts

Hello, I need a bit of input on my AXI 2418/20 motor. It is rated at 25 amps and when running it around 18 amps it gets really hot in a hurry. All I want to do with it is to launch my Gentle Lady to a few hundred feet and try to find some thermals. On the bench it will get to 170 F degrees easily in two minutes. At some point the magnets will fail due to the heat. ( do they recover?) I would probably need only a 30 second run to get to height so it probably would not be an operational problem but the rapid heat build up worries me. Someone suggested changing the timing which is 26 degrees out of the box. Don't know how however. Is this an ESC issue.? --Any suggestions on another motor to launch this thing? --best regards Bob Chamberlain

robert chamberlain10/09/2017 06:00:03
49 forum posts

Hello, I need a bit of input on my AXI 2418/20 motor. It is rated at 25 amps and when running it around 18 amps it gets really hot in a hurry. All I want to do with it is to launch my Gentle Lady to a few hundred feet and try to find some thermals. On the bench it will get to 170 F degrees easily in two minutes. At some point the magnets will fail due to the heat. ( do they recover?) I would probably need only a 30 second run to get to height so it probably would not be an operational problem but the rapid heat build up worries me. Someone suggested changing the timing which is 26 degrees out of the box. Don't know how however. Is this an ESC issue.? --Any suggestions on another motor to launch this thing? --best regards Bob Chamberlain

John Emms 110/09/2017 08:34:44
153 forum posts

Robert, I suspect that is a 2814/20. That motor has more than enough potential power for a Gentle Lady. To reduce the power if you need to, simply fit a smaller prop:

The motors are intended to be run unloaded, so they should not be run for a longer period that is necessary to check the static current drawn.

The heat build up in an outrunner is in the windings. Are the windings damaged? Can you smell burnt windings? It is possible to check the winding resistances, but very difficult to get accurate readings.

Core advice of mine is to run cheap controllers with cheap motors, and if spending money on quality (Axi are above average in quality), then spend money on good controllers to protect the motor from damage by the controller. Ideally you would be running Auto timing or "hard timing" with a Jeti, but otherwise, the setting for timing is done in the controller - check the controller instructions, or ask the supplier of your controller.

Also check for runout on the rotor. If the motor has been in an incident, it can throw the light cantilevered support for the shaft out of line, which will lower the motor efficiency considerably, and result in considerable heat build up. I know that most people mount outrunners using a radial mount set, but better support is provided by mounting on the front plate of the motor, so the loads are taken by the large front bearing in line with the substantial mounting plate.

Any magnet out of line will also reduce the motor efficiency considerably, so do rotate the rotor and check that all the magnets are in position - magnets should not be moving on an Axi produced during the last 12 years or more, so that includes all 2814/20s.

robert chamberlain11/09/2017 05:35:59
49 forum posts

John, thank you for all the good information. This is my first elec and the plan is to educate myself a little more. The ESC was given to me so I know little about it. With a 3S lipo it will not run very long (four min). I did all this testing a year ago and can not remember whether it was with an 1150 or 2200 MAH pack. Either way it got way too hot. How hot is considered too hot? Monday I'll head down to the hobby shop for a chat. I was given the broken GL a year ago and am only now finished making new parts (wing and tail). It is all a learning process but certainly a lot of fun.----Bob C

John Emms 111/09/2017 15:41:51
153 forum posts

Robert,

4 minutes is actually a long time with the throttle wide open on full load ie static running - and no less run time than I would expect. Only run the motor up to full revs static long enough to check that the current is safe.

The temperature builds up in the windings that you cannot feel, so if the rotor is hot, that is a lot of heat. To put this into perspective, if you are running at 180W at around 66% efficient, then 60W is building up in the windings as waste heat - think of the heat from a 60W light bulb running for 4 minutes. In flight, you will not be near 180W for most of the time, and you will have the cooling airflow as well as the prop wash. If there has been no damage so far, then you will be fine.

Do check the timing of your controller to make sure you are working near optimum for your outrunner motor.

Hope that helps,

John

Frank Skilbeck11/09/2017 16:02:47
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3716 forum posts
88 photos

Do make sure that air can get into and out of the motor though, there are spinners with holes in the front and some with vents in to allow air into the motor (providing you cut holes in the bulkhead to line up with holes in the motor) and then some vents further back to let it out, if you have no air flow over the motor then it could get quite warm. Just think how hot your 18w soldering iron gets.

robert chamberlain11/09/2017 21:26:17
49 forum posts

Hi Frank, you are right on about air flow but in this case the motor is sitting completely exposed bolted to the firewall. I think there is something else going on here.

robert chamberlain11/09/2017 21:34:39
49 forum posts

John, I understand what you are saying and thanks for input. Timing is the next to take a look at if only to understand how it is done. Another hobby shop just went out of business,--sign of the times

Piers Bowlan11/09/2017 22:07:09
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873 forum posts
18 photos

What size prop are you using Bob? I also wondered what prop size was recommended with this motor on a 3s LiPo but I cannot find this motor on the net. Another thought I had was, what are you measuring the current with and how reliable is your tester? You say the motor is pulling 18A, can you verify that this is correct and not more? John suggested trying a quality ESC, you can always use it in another model if it doesn't make any difference. AXI make excellent motors, 'motor problems' can sometimes turn out to be ESC problems in my experience. Good luck finding a solution.

robert chamberlain11/09/2017 22:22:02
49 forum posts

Hi Piers, I tested the set up with one of those Amp/Volt/Watt meters units. Recommended prop is an 11/5 although I have an11/6 on it. I bought it through Hobby Lobby here in the U.S. Note: now called Hobby Express . The next step is the ESC . Maybe motor timing? Motor is rated for 3 or 4 Lipos and 25 Amps

Simon Chaddock11/09/2017 23:39:05
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4942 forum posts
2558 photos

robert

I can't find the exact specification for 2814/20 but I note the max rating for all these AXI motors is for 60 seconds only

Even at 18A for 4 minutes static I would expect it to get pretty hot.

Is the recommended 11x5 prop perhaps when using a 2s LiPo?

robert chamberlain12/09/2017 03:21:52
49 forum posts

Simon, thanks for the good info. I can't find that 60 sec note anywhere on the paper that came with the motor. Is that true for all 2814/20 motors and if so, what good are they except to quick glider launches?! According to the Hobby Express catalog it is good for 3 and 4 Lipos. Sorry I spent $100 on this. At what temp do magnets not become magnets anymore? Thanks again--Regards, Bob C

robert chamberlain12/09/2017 03:51:54
49 forum posts

Yep, there it is on their web page.----depending where you read it. either 20/30/ 60 sec run time! No wonder it got hot in 3 min! ----Bob

Piers Bowlan12/09/2017 08:12:11
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873 forum posts
18 photos

Robert, at 18A your motor is developing almost 200W (before losses) and that is about 150W per pound, if your Gentle Lady weighs 25oz (Usa a 1300mAh LiPo to keep the weight down). Your gentle lady will not be so 'gentle' with 200W up front and will more than likely climb vertically to 500ft in about 15sec. Irrespective of what is recommended you could try a slightly smaller prop and still find the performance more than adequate without it getting so hot.

As for 'what good are they except for quick glider launches?' use a smaller prop to keeps the amps under control and you can use them for anything and they will give long and reliable service. (see here) If you need more power use a bigger motor not just a bigger prop!

John Emms 112/09/2017 11:51:15
153 forum posts

Sorry, you are Bob,

This is the manufacturer page for the motor Bob:

**LINK**

ALL model electric motors have a maximum rating for a short period because the mass is low, and will only absorb a limited amount of heat. AXi are rated for 30S (normally 60s for the fan cooled), and almost all of the Chinese ratings would be for far less time (that is a whole different story!). Massive industrial motors can have a continuous rating because of their high mass, and large cooling surface area.

I was suggesting that perhaps you may be better with a smaller prop, 1, because the recommended props are a maximum, 2, the recommended prop may be too much with a very low resistance battery (low resistance = higher voltage - higher amps = higher power through the motor), and 3, I doubt you need all of that power for this model.

And exactly what Piers says.

Hope that helps further,

John

John Emms 112/09/2017 12:00:42
153 forum posts

Neodymium magnets vary depending on the grade, but look at them starting to degrade at around 130 degrees C. The more the magnets degrade, the lower the motor efficiency, and it then follows that there will be an even greater build up of heat.

Also, as the copper heats up, the resistance increases, and again, that leads greater heat build up (and damage) in the windings.

The answer is to keep all of the power system as cool as is reasonably possible during use.

This all starts to make electric power seem quite dire, but I have been having great success for many number of years.

PatMc12/09/2017 23:12:07
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3346 forum posts
451 photos

I'm afraid I disagree with some of the advice given.

It's a glider it shouldn't have a long slow climb. A short fast climb to thermal hunting altitude is what's required.

If the motor runs for 30 secs at 18A without getting unduly hot that should be fine for the equivalent of a good tow or bungee launch without the same stress on the wings. When it's RTF I think the model will weigh over the 25oz quoted but even at that weight 200W power will give 128W/lb. That power/weight ratio will get the model to around 200mtrs (650ft) in 30 secs which would be good for ALES comps or reduce the power burst to 20 - 25 secs for around 450 - 500ft.

Using a 3s 1300 - 1500mAH lipo should be good for 5 or 6 launches without recharging - that's around 45 - 60 minutes flying time without any thermal help.

John Emms 113/09/2017 15:40:55
153 forum posts

I am going to qualify, and perhaps partially disagree with, Patmac.

Most of my aerobatic models are running at little more than 100W measured/lb, and most would agree that my models are "adequately powered". I always worked power systems out for customers wanting thermal and vintage power systems at around 75W/lb, and under certain circumstances, I would be happy to go lower (qualifying the advice that the climb rate would be at the lower end, but nowhere near marginal). A rudder/elevator thermal soarer will not want to be overpowered, to maintain adequate control. I also agree that there are thermal comps where a high climb rate is required, and that, of course, will require high power levels.

Why do I say W measured/lb? The Watts measured on a meter (actually consumed) for that power system in that weight of model.

I suspect that many saying that to be adequate, a power system must be over 200W/lb are using the labels on Chinese motors for there calculations. A "400W motor" will have that written on the label, but in normal use may normally used at 250W when actually measured with sensible props for that size and type of motor.

Hope that helps, and doesn't muddy the water too much.

John

PatMc13/09/2017 18:50:55
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3346 forum posts
451 photos

John, I'm suggesting that Robert uses what he has, it won't overpower his model if he uses it for climbs or simply throttles back. The power available is no more than the average bungee launch [AKA "hi start"] used with many GL's & similar pure gliders, it's more controlable & won't stress the model during the launch phase.
OTOH an underpowered thermal model (or any type for that matter) can be a real liability. If it's climbing slowly it's actualy taking up a lot of time to clear the airspace over the launch area. Also in this situation the controls are usually fairly sluggish making it difficult to take any action to avoid any other flyers if it becomes necessary. Another downside is that many glider pilots feeling under pressure to increase the climb rate try to do so by pulling some up elevator, which in fact has the opposite effect or worse causes a stall.

I don't fly in any competitions but for my own satisfaction I limit my glider launches to 30 secs power/200mtrs alt then thermal hunt from there.
Personaly I don't believe in the power figures often quoted as being necessary for various types of models. My thermal floaters have the highest power to weight ratios of my models at between 100W/lb & 135W/lb.

I'm not sure why you pick on Chinese motors as giving unrealistic power figures I've found them to be no less accurate or reliable than any others in their claims but a lot better value in terms of performance/£.

Incidentaly there is no such thing as an "X watts motor" unless the applied voltage is also quoted, as I'm sure you're aware. And of course neither is there such a thing as a voltage limit for motors.

 

 

Edited By PatMc on 13/09/2017 18:52:25

Piers Bowlan16/09/2017 19:04:57
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873 forum posts
18 photos
Posted by PatMc on 12/09/2017 23:12:07:

I'm afraid I disagree with some of the advice given.

It's a glider it shouldn't have a long slow climb. A short fast climb to thermal hunting altitude is what's required.

If the motor runs for 30 secs at 18A without getting unduly hot that should be fine for the equivalent of a good tow or bungee launch without the same stress on the wings. When it's RTF I think the model will weigh over the 25oz quoted but even at that weight 200W power will give 128W/lb. That power/weight ratio will get the model to around 200mtrs (650ft) in 30 secs which would be good for ALES comps or reduce the power burst to 20 - 25 secs for around 450 - 500ft.

Using a 3s 1300 - 1500mAH lipo should be good for 5 or 6 launches without recharging - that's around 45 - 60 minutes flying time without any thermal help.

I don't think I was advocating a 'long slow climb', non of my gliders do that. The motor is rated at a max current of 25A for 20s but from the specification, max efficiency is between 12A and 17A with this motor. Beyond this the motor will start to turn the extra Amps into heat rather than power but not a problem if the motor run is short. Propping down slightly might not make a noticeable reduction in performance.

The 25oz weight I quoted came from the The Tower Hobbies website but it was probably fake news!

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