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Power Systems Talk about motors, ESC speed controllers, gear drives, propellers, power system simulators and all power system related topics

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Old 04-13-2013, 08:41 AM   #1
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Default Strange motor

I recently got a large outrunner motor at a swap meet. It is new, never run or flown. No burned coils or oder from over heating.
The outside diameter is 49 mm
The width of the black magnetic ring is 17.5 mm
The shaft of the motor is 8 mm
The shaft of the prop adapter is 10 mm
The motor weighs 246 grams
The outside diameter of the stator poles is about 42 mm
The inside diameter of the magnets is about 43 mm
I have not taken the motor apart so the last two are as close as I can get.

I tached it at 14,450 rpm at 15.47 volts on my wattmeter, with no load, only white tape on each side, to get a tach reading. It was pulling 60 watts at full throttle.
That would work out to 934 kv if I am doing it properly.

That seems to be quite high for a motor of this size, I was expecting around 300 kv

I don't have any papers on the motor, no idea of a brand name, no marks on the motor at all.

Does anyone recognise this motor?
It seems to be made for a large prop, yet 14,450 is very high for 4s, most large motors use 6s - 8s or so. A higher cell count would make the rpm crazy high.

I have a project for a motor of this size so I hope someone can shine some light on this.


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Old 04-13-2013, 09:07 AM   #2
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My first guess is your tacho is off, those optical tachs can be a bit temperamental.
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Old 04-13-2013, 09:21 AM   #3
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2 white stripes for an optical tach? what type of optical tach? its possible for it to be a doubled RPM reading.

But... you may have an accurate reading also... KV is not completely dependent on motor diameter and length. Those are just a couple of factors in predicting KV.

Turns on the stator poles is a huge factor and you can wind a motor with 1 turn per pole to get very high KV. Lower number of stator poles will also increase KV. Then there's the magnet count... Fewer (wider) magnets = higher KV due to more angle per polarity shift of a stator pole.

AXi4120/14 stats:
(looks to be similar size)

No. of cells 12 - 16 NiCd
4 - 5 Li-Poly
RPM/V 660 RMP/V
Max. efficiency 85%
Max. efficiency current 20 - 40 A (>82%)
No load current / 10 V 2 A
Current capacity 55 A/60 s
Internal Resistance 41 mohm
Dimensions (diameter. x lenght) 49,8x55,5 mm
Shaft diameter 6 mm
Weight with cables 320 g

If wound at appx 8 or 9 turns per pole instead of 14 it might hit near 950KV (without pulling out a calculator)
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Old 04-13-2013, 02:28 PM   #4
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Nearest match I can find is the Fire-power 46 from Heads Up RC. Same size and near enough same weight once you add the cross mount and prop driver. The Firepower is 720kv, I'd expect yours to be in the 600 to 750 range. kv as low as 300 is usually found in larger motors.

Put a 14x7 prop on and a good 4s battery and watch the amps and watts on a wattmeter. The Firepower pulls about 750-800W with a 14x7 prop. If it really is 934kv then you will be much higher.
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Old 04-13-2013, 02:36 PM   #5
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Found it!

It is identical in spec to the Firepower 46, just different end covers: http://www.lightinthebox.com/C5045-B...-_p140744.html


It's actually a nice motor. I've got one of the Firepower's in my 51" AJ Slick with a Xoar 15x7 prop and it goes like a rocket. Pulls rather more amps than the official numbers recommend but copes just fine.
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Old 04-13-2013, 02:43 PM   #6
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Edit... seems that motor comes in different KV versions, and you can get one in 890kv, which is near enough what you measured: http://www.leaderhobby.com/product.asp?ID=9394001100034


So looks like you were near enough spot on with your kv
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Old 04-13-2013, 05:32 PM   #7
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Can't go by end bell color and expect to be an exact match...

But more good examples of the variations possible in a motor of the same physical size.

I don't recommend doing the prop test hoping to match full rated power on a motor when there is good reason to believe the motor you are testing is higher kv. Its a good way to overheat a motor and maybe even burn out an ESC.
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Old 04-13-2013, 07:16 PM   #8
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Originally Posted by fhhuber View Post
Can't go by end bell color and expect to be an exact match...

But more good examples of the variations possible in a motor of the same physical size.

I don't recommend doing the prop test hoping to match full rated power on a motor when there is good reason to believe the motor you are testing is higher kv. Its a good way to overheat a motor and maybe even burn out an ESC.
Agreed:

It's nearly mandatory to hook up one of those wattmeters to your battery/esc/motor while doing this sort of testing.

These motors are DUMB! They will happily turn over a far to large propeller, until something starts sending out smoke signals.

FYI, a very crude measurement of motor capability is around 100 watts per ounce of motor weight, assuming the proper number of LiPo cells are used.

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Old 04-13-2013, 08:14 PM   #9
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Kyle......

"FYI, a very crude measurement of motor capability is around 100 watts per ounce of motor weight, assuming the proper number of LiPo cells are used"

Not meant to be critical....maybe I lack complete understanding of this "watt per POUND" rule....just went though this on another thread.......

Is what the "crude" rule meant to imply, without getting all scientific, is that, as a general rule of thumb based on total weight of the plane (battery, motor, craft) for every ounce of plane weight, based on the type of flying one intends to do, a motor that provides at least (using the 100 watt per pound) 6.25 watts per ounce will be sufficient...?....or am I confused since you've suggested a (motor weight not plane weight) comparison........I'm lost on this now, just when I thought I had a handle on the "crude" rule methodology......lol

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Old 04-13-2013, 08:42 PM   #10
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Originally Posted by fhhuber View Post
I don't recommend doing the prop test hoping to match full rated power on a motor when there is good reason to believe the motor you are testing is higher kv. Its a good way to overheat a motor and maybe even burn out an ESC.
Well yeah... that's why i said to do the test with a wattmeter hooked up


Seems 'case closed' to me.. It's externally identical in every detail and dimension to the Leader Hobby C5045-890KV and the kv as tested is within 5% of the stated 890kv for that motor, which given the variability in kv measurement is as spot on as you could hope for.
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Old 04-13-2013, 08:50 PM   #11
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Originally Posted by pizzano View Post
Kyle......

"FYI, a very crude measurement of motor capability is around 100 watts per ounce of motor weight, assuming the proper number of LiPo cells are used"

Not meant to be critical....maybe I lack complete understanding of this "watt per POUND" rule....just went though this on another thread.......

Is what the "crude" rule meant to imply, without getting all scientific, is that, as a general rule of thumb based on total weight of the plane (battery, motor, craft) for every ounce of plane weight, based on the type of flying one intends to do, a motor that provides at least (using the 100 watt per pound) 6.25 watts per ounce will be sufficient...?....or am I confused since you've suggested a (motor weight not plane weight) comparison........I'm lost on this now, just when I thought I had a handle on the "crude" rule methodology......lol
No problem.

These motors will put out a given amount of power without burning up. What limits how much power the motor can put out is, for the most part, heating up the motor. The largest part of the cause of "Heating up" is the current flowing through the motors windings.

Now, these motors are rated with a bunch of stuff, mainly their maximum watts rating, maximum voltage rating, maximum current rating. And, the watts is directly equal to the voltage times the current. Its that simple.

So, if you have a motor running on say, 4 LiPo cells, and pulling perhaps 31 Amps, here is how the power is calculated. Each LiPo cell can put out about 3.5 Volts under load. So, four of them would be 3.5 times 4 or 14 Volts DC. To get watts, multiply that 14 Volts times the 31 Amps, and you get 434 watts. Here is where you've got to be a little careful with some of those El-Cheapo motors on the market. We indicated for the most part, 100 watts per ounce of motor weight is fairly safe. So, that 434 watts requires a motor that weighs about 4.34 ounces. (434 watts/100 watts per ounce) If your motor actually weighs 2.07 ounces, that would be 434/2.07 or 209 watts per ounce. A motor running at 209 watts per ounce would likely fry its windings, unless it is a very high quality, high RPM motor.

Now, it takes 746 watts to make one horsepower. So, that 434 watts divided by 746 equals 0.58 horses!

Now, for the model weight, versus watts. This is apples and oranges. Totally different subject. A fairly good rating of a model would be to have a power system that puts out 100 watts per pound of model weight. Yeah, the model will fly at 50 watts per ounce, but it might not have enough stuff to take off from a grass field. And, to me, it would be under powered. Running a model at 100 watts per pound of model is getting into reasonable performance, in the same ball park as the same model under glow engine power.

Upping that to 150 watts per pound of model weight is getting close to a high powered model, likely outperforming that same model with a glow engine up front. And, 200 watts per pound of model weight, that model will easily fly straight up, and gain speed in doing so.

Take some numbers.

Lets say we have a model that weighs 5.5 pounds. At 100 watts per pound, we would need about 550 watts. That 5.5 pounds also includes the motor, ESC and Battery weight.

And for the size of the motor, at 100 watts per ounce of motor weight, that would require a motor that weighs in at 550 Watts/100 watts per ounce of motor weight, or a motor that weighs in at about 5.5 ounces or perhaps a little bit more.

The battery would also weigh somewhere around 5 ounces, more or less. So out of that 5.5 pounds, or 88 ounces for the total model weight, you would have about 5.5 ounces of motor, an ounce or two for the ESC and 5 ounces for the battery pack. So taking that 88 ounces, minus 5.5, minus one, minus 5 or about about 76 ounces for the model, including the radio system.

Hope this helps.

One of those computer programs, such as www.motocalc.com does all of this stuff automatically, and even predicts how the model will fly with the model wingspan, area, weight, motor, ESC and battery pack. You can learn more in an hour with this motocalc program than you can after buying $$$$ worth of stuff and trying it out. Motocalc is free for 30 days, then $39, and IMHO worth every penny.

If after entering in all the stuff motocalc requires, and motocalc indicates it will fly well, you are quite safe in buying the materials, putting it together, and flying it.

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Old 04-13-2013, 08:50 PM   #12
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Originally Posted by pizzano View Post
Not meant to be critical....maybe I lack complete understanding of this "watt per POUND" rule....just went though this on another thread.......

He wasn't talking about the 'watts per lb' rule. that is a rule for estimating how much power you need to fly a plane of a certain weight in pounds.

The 'rule' kyle was refering to is '100W per oz' which is a conservative rule of thumb for how many watts a motor of a certain weight in ounces is capable of delivering safely... Nothing at all related to the 'watts per lb' rule.

Anyway, they are both only very rough rule of thumb guidelines.
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Old 04-13-2013, 09:06 PM   #13
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Thanks Kyle and JetPlane........sorry.....I get it now..!....I was comparing apples to oranges....a rookie mistake since I seldom get that technical with motor/power system compatibility...still feeling my way around with smaller craft and simple plane weight to power system relationships.

I've pasted the example Kyle gave to my last conversation (somewhat related to this issue)....and saved it to my personal tech folder.......Thanks much!. I'm sure it will come in handy when I graduate to the more complicated aspects of this hobby.

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Old 04-14-2013, 05:12 AM   #14
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Thank guys I think you have figured it out.
Without the mount and prop adapter my motor weighs 5g less than these, that could be a slight difference in lead length.
When I checked rpm, I used a non pulsating led light to get a pure light source, since my hobby room has fluorescent lighting, and a known good optical tach.
The Leader hobby motor, I think is the one I have, they claim 1185W with 12/6 - 13/10 props. I will do some testing with a prop when I can.

I use an infrared optical thermometer, I can sense the temp of the windings from the back of the motor.

This all sounds good, I want to use this motor, if it tests ok, for a Berkley Custom Privateer, 114" ws. The motor is in a pod above the wing, so I can't swing a 14-15" prop.

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Old 04-14-2013, 06:33 AM   #15
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if you are limited in diameter but have the power to swing more prop... more blades is the answer.

(one of the main reasons for the 4-blade on a full scale P-51 D.. the landing gear are too short for a 3-blade that can deliver full engine power)

I'm not familiar enough with the old Privateer to predict your results well... but usually you end up with having LOTS more power than the design expected using a modern brushless power system (or even a modern glow engine) and you still come out lighter than the kit designer expected when updating equipment in the old Berkeley kits.
Caution about that...
You can sometimes have the power to pull the wings right off in level flight.
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Old 04-14-2013, 08:32 AM   #16
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I understand what you mean, I have a power 32 clone, that I am absolutely sure would have enough power to fly the Privateer with no trouble. but it has another home.

The Privateer is really just a big glider with a watertight fuselage, I have over 50 years in modeling so I can spot a weak point before the plane is finished. I am using carbon fibre in some places I don't feel good about. The design was created for a Fox 59 long shaft engine, it was never a real strong motor, about like a modern sport 40. I didn't want to be forced into a 3 or 4 blade prop for a few reasons cost being one. I wanted a large outrunner that would turn a 12" prop fast enough to equal around a 50-60 size motor. I think I fell into a good one, if not O well, I sure didn't pay much for it.

I may even insert a temp prob from the back, against the windings of the stator, while I do my testing. I have the equipment to do so.

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