Electric assistance needed!

I have a Multiplex MiniMag which I've been flying for some time with the following setup:
HiMark 2815 brushless inrunner
7x5 or 8x6 APC E props 3 cell 2250 LiPo
Originally I had a Castle Creations 25A ESC running with the 7x5 prop. The ESC used to cut out until I realised mounting it directly to the foam fuselage (a great insulator) was not a good idea. Since letting it 'float' within the vented fuselage all has been well.
I changed the ESC to a 40A Turnigy unit as the original 25A was pretty much at it's rated limit.
A friends E-meter showed I was drawing upto 60A+ with the 7x5 prop. Changing to the 8x6 dropped that to 50A.
Question 1: Why would a larger prop spinning at roughly the same RPM draw less current? It doesn't make sense.
Question 2: Why would a 40A ESC deliver more than 50A (static) without getting hot or cutting out? It doesn't make sense.
Question 3: Why would a 25A ESC run perfectly if there is a 50A draw?
Anyhow, multiple flights later the motor quits. Inspecting the motor and testing the ESC shows the motor is burnt out.
Question 4: Why would the motor burn out? Yes, I was doing some full throttle runs whilst towing a small glider (Alula under 400grams) which it handles with ease.
Question 5: The HiMark motors are very good and fit the plane perfectly. Should I fit another and be a little cautious on throttle application and duration?
Question 6: Any other suggestions as to why I'm drawing so many Amps and how I can keep that within the ESC's rating? Is it a matter of bigger or smaller props?
All constructive comments and suggestions welcome.
Regards
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On 07/06/2010 03:32 AM, The Raven wrote:

No, it doesn't! Meters don't always read correctly, though. ESC's chop the voltage going to the motor, so the motor current tends to be very spiky. That makes measuring the average current difficult -- any electrical engineer knows that a meter must be built with care to measure correctly under such conditions.
So it could just be the meter. In particular, if the meter was measuring the instantaneous current to the motor (see my answer to question 3, below).
Also, one 8x6 isn't necessarily like every other 8x6 in the world. I very much doubt this is the case, but if you switched a 7x5 with really fat blades for an 8x6 with skinny blades the power could drop. I really, really doubt that this is the case, and certainly isn't if you chose a prop that was generally similar, just bigger in every way.
Do you happen to know if it went slower with the 8x6?
I think that if you've found out anything the least bit startling with your meter vs. prop test, it's that the meter isn't reading what you think it is.

A well-engineered piece of equipment will always take more than the label says. I would expect some heating if the current measurement is accurate, however.

A really good ESC is going to limit its current by throttling the motor back if the current draw is excessive. I absolutely positively don't know if the CC ESCs do (I've only been involved in designing industrial motor controls, which _most certainly_ do). If it does limit the current it'll be what the ESC can stand. If it's limiting the _average_ current, then the instantaneous _peak_ current would go up to 50A easily.
Note also that a bigger prop may raise the instantaneous current without changing the average current.

_If_ the CC ESC was limiting the current to 25A and the Turnigy limits to 40 (or doesn't limit at all), then that would explain it. Or if you switched to the 8x6 prop based on an erroneous current reading.

Yes. Better yet, find a prop that doesn't cause an over current condition at full throttle and use that, or fit a motor with a lower Kv to slow it down. Or use a 2-cell pack. Motors don't have a heck of a lot of thermal mass, so it wouldn't take long to fry the windings -- I would expect that just a couple of seconds at a serious overload would do it.
_Lots_ of airflow to and through the motor will help -- hide that thing behind a spinner without any sort of a scoop to route air through the motor is just asking for trouble.

Smaller prop, lower pitch prop, lower battery voltage, lower Kv on the motor. All of theses will reduce climb performance.
Or find a stouter motor and rip into the fuselage to make it fit, and get a rocket-like climb.
Or just buy a big pile of motors, and do stunts as soon as you see a smoke trail.
--

Tim Wescott
Wescott Design Services
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I realise not all E-meters are the same but I doubt they'd be out that much, noting the 25A ESC didn't complain at all.

WOT static run, not an instanteous peak.

True, but all props were the same brand and type.

7x5E APC and 8x6E APC. PCPhill is suggesting these may be too big, and is probably right. However, I couldn't get anyone at a hobby shop or the flying field suggesting I go smaller. They all said bigger equalled less Amps - which can only be true if the RPMs drop. I'm no e-flight guru but I know enough engineering to understand that increasing load (at any revs) will also up the Amp draw (at those revs).

We missed that on the e-meter with the 7x5E. All I can say is the 8x6E was pulling 11000RPM static.

What's the efficiency ratings of ESCs like? Are some 'horrible' and others reasonable? Perhaps the Turnigy ESC is highly inefficient compared to the Castle Creations.

Neither ESC got hot (once proper airflow was established).

Perhaps the Castle Creations do limit max current draw and the Turnigy doesn't.

I'm starting to believe that.

If I could get a HobbyCity motor with the same mounting screw spacing (16mm) then I'd be buying a few of those.
Regards
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On 07/08/2010 03:17 AM, The Raven wrote:

Obviously you haven't designed products with a sales guy hovering over you, trying to push things into production when you know they're still crap.

Instantaneous = instantaneous. An ESC pulses the current to the motor (that's why it makes all those chirpy-whiny sounds), and the motor current takes microseconds or milliseconds to respond to the motor voltage. Meters read over seconds; to see what's happening with the winding current you need an oscilliscope, and you need to look at things in the microsecond or millisecond range.
If the meter reads the peak current instead of the average, or if the meter samples the instantaneous current and pretends that it's average, you'll get all sorts of confusing results.

Then that's not it.

You're right, they're confused. There's more than one way to drive a motor, but the way that ESCs drive brushless motors makes them 'want' to turn at a constant speed, and as they slow down they'll draw more current to produce more torque. The two easy limits to this are to imagine the motor with no prop at all, and imagine the motor with the shaft glued to the case. With no prop the motor will only need to pull enough current to overcome mechanical friction and electrical losses. With the shaft glued to the case the motor can't turn at all, and the current is limited only by the winding resistance and the ESC.

Based on my modest experience designing switching amplifiers, and my extensive experience hanging out with designers of switching amplifiers, you should expect efficiencies ranging from 80% to 98%. Based on price tags and Castle's reputation I would expect they're above 95%. I don't know Turnigy, but I doubt they beat out Castle.
When a switching amplifier (which is the "business end" of an ESC) is inefficient, all the power lost in the amplifier turns into heat -- so a less efficient ESC is a hot ESC.

So there was some heating, which you controlled with proper airflow.

Certainly that's more likely than the other way around.

--

Tim Wescott
Wescott Design Services
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Depending on which KV version you bought, the motor is mildly to severely over propped. I'm surprised the ESCs survived, but it was only a matter of time until they died. You definitely need smaller props, the tables I saw showed anything between 5x4 and 8x4 depending on the KV.
Do you know the KV rating?
2815 -1400 2815- 2000 etc?
PCPhill

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Hi Guys,
I haven't had a good chance to read your replies yet and will try to get back with some more info within 24 hours.
The motor is 2000KV and the 7x5 prop is the recommended size (at least according to some other research).
Regards.

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The recommended prop for that one is 5x4 - 7x4 so the 8x6 was definitely too much. The motor probably overheated. You might want to consider a small outrunner and mount it to the outside of the removable front plate of the Minimag. That's how I flew mine with no problems. I used a cheap ($20) brushless & ESC from Grayson Hobby and flew it for a year. http://www.graysonhobby.com/catalog/disposable-foamie-brushless-combo-sport-p-940.html It's currently living as a floatplane in TX. You might want to go slightly bigger on the motor for aerotowing...
PCPhill

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I was told 7x5 and saw similar mentioned on another site but you are probably correct on sizing. Yes, the motor definitely cooked one of the windings, I can see the charring looking in the front of the motor.

I chose the Himark because of it's near bolt in fitment and don't really want to try pulling the beast apart to fit an outrunner.

http://www.graysonhobby.com/catalog/disposable-foamie-brushless-combo-sport-p-940.html
Here's a video of our aerotow

http://vimeo.com/12887656
. It's only 21seconds but you can see how well it goes (or went...).
I looked at HobbyCity and they have "similar but not quite the same" motors in the 2835 sizings. All have too high a KV rating.
I may switch back to the CC 25A ESC and buy another Himark with more rationally sized props.
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