$6 EMAX CF2812/2822 Rewind Thread - RC Groups
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Jun 17, 2011, 12:21 AM
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KC Flyer's Avatar
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$6 EMAX CF2812/2822 Rewind Thread


This is an update after 3 years of experience and a thousand flights later - with the same rewound CF2812. I recently rewound the original motor using better technique and a heavier gauge of wire: 9Turns of 23g wire (vs. 9T/24g).

This is an excellent motor for a 1st rewind experience. It's also an excellent motor after it's been rewound!

A few specs up front to help decide your wind:

# Turns-Gauge___KV (Delta_Wye)_____Wire Length x3
7T-22/23g________2650__1530__________40"
8T-23g___________2318__1338__________46"
9T-23/24g________2060__1189__________50"
10T-24g__________1854__1070__________54"
11T-24g__________1685___973__________58"
12T-24/25g_______1545___892__________62"
13T-25g__________1426________________66"
14T-25g__________1324________________70"
15T-25g__________1236________________74"


Many on the ASSASSIN Thread are rewinding these same motors for a relatively easy hop up.

Here are a few links to help you get started if you are new to this motor and/or rewinding:

Rewinding Links
dLRK Rewind Thread #1
dLRK Rewind Thread #2

I did not use these links when rewinding my first motors . They should help you put in more copper (thicker wire) for a given number of turns . This should improve efficiency.

This motor winding thing, aka "man knitting" gets easier with practice. A few helpful hints to help things go easier:

After winding each strand (ie, 4 teeth), I burn off a small section of the insulation at each end of wire with a lighter or torch and scrape to shiny copper. I then use an ohm meter to check continuity end-to-end and not to bearing housing (don't want a broken wire or short circuit).

To make things easier, I use a needle nose pliers to pull end of wire tight, a hotel key card to compress wire closer to stator teeth, and a T-pin to gently ream out a hole for the last wind(s) to thread through (when using lots of copper). Be careful not to push pin through wire or damage insulation.

A curved awl helps remove old wire/glue from stator to be rewound. Be careful not to scratch green insulating paint or a short circuit might result.

A good magnifying glass is very helpful.


Motor & Accessory Links (Stick Mounts, Prop Savers, E Clips, etc.)
HeadsUpRC #1
HeadsUpRC #2
HeadsUpRC #3
PlaneInsaneRC
HobbyKing #1
HobbyKing #2

Magnet Wire, Bearings, Oil Links
MicroDan Magnet Wire
MicroDan Sealed Bearings
Scorpion Oil

These motors have 2 bearings:
Big is 8mm OD x 3mm ID x 4mm Thick
Small is 7mm OD x 3mm ID x 3mm Thick

Sealed Bearings ($6/set) need no oil or further maintenance . I'm moving toward them on my everyday flyers.

Unsealed bearings (stock bearings) should be oiled periodically. I clean them first by soaking/spinning in laquer thinner, let dry, then oil. Lots of black crud typically comes out when I clean them. >500 flights are possible with stock bearings if they are kept clean and oiled. You can also just replace them periodically as their cost is low or upgrade to sealed bearings if/when they go bad.


__________________________________________________ ________



In summary, this is a great motor to experiment with varying # of turns. It's lightweight (40g), durable and typically pulls over 300W with fresh batts. I've rewound several of these motors and none have failed me yet when using MicroDan wire. I've crashed them hard enough to bend shafts/bells, break C-clips, lose bearings etc. but am still using all of the rewound stators.

9 turns of MD 24 ga wire is a good starting point and the basis for my 2nd rewind (1st rewind used Radio Cr@p wire). High C batts (ie, 45-90) are a must for this combo using a 7x6e APC clone.

I rewound one for my flying buddy using 8 turns (24 Ga MD wire) for even more power with this prop (still no motor failures).

I've rewound others with 10 and 11 turns of 25 Ga MD Wire for use with 8x4 GWS props. I prefer the 10 turn motor over the 11 turn.

DON'T USE RADIO SHACK 26 GA WIRE AS IT WILL EVENTUALLY FAIL!

ORIGINAL THREAD:

The motor of choice for the Assassin EPP Flying Wing is a $6 EMAX CF2812 outrunner with a Kv of 1534 spinning a <$1 HK 7x6 prop with a 3s LiPo. See here for more details about this really cool and durable plane: ASSASSIN Thread

However, IMHO, the stock motor is a dog.

This motor is listed on HK's website as FC 28-12 or its sister FC 28-22 (1200 kv). Both are indentical once all the original wire is removed.

I first rewound the stock motor (per an instruction video in the above thread) using 26 ga Radio Shack wire. This was my first brushless motor in a plane and my first rewind. The stock motor has 12 stator teeth and 14 magnets wound with 12 turns of 3 strand wire terminated in a delta configuration.

The video says to use 10 turns of Radio Shack 26 ga wire and this gives much better performance until, after ~70 flights, the wire finally gave out on me (shorted to stator).

I was flying in very strong winds way downwind above a forest of trees when the motor went quiet only a couple of minutes into flight. Luckily I was high enough to go into a dive and get back to field.

I had previously noticed that the green RS insulation had turned copper color in some places. I'm not sure what its design temp is but would not recommend RS wire for performance applications.

So I ordered some MicroDan 24 ga wire that is supposed to be good for up to 300 C (572 F).

The MicroDan rewind works great with 9 turns - maybe a little too good!

With fully charged batteries (12.6v), I got 14k rpm with the HK 7x6 prop. But also 33 amps out of my HK SS 25-30a ESC. After the batteries drained down a little, it was still getting 13,200 rpm at 10.6v.

Was really windy this morning but took it to the field anyway. Great unlimited vertical and really quick rolls. Motor was cool even after a spirited run. Flight time shorter and ESC/battery a little warmer than usual.

I did notice that the motor stuttered at 11,500 rpm with prop on but not at any other speed. Prop was balanced so I tried a different rotor (bell) and bearings but same problem. Then I changed the ESC timing from medium to high and the problem went away.

Here are a few performance numbers below with HK 7x6 prop (the Kv values were calculated by putting 2 pieces of electrical tape on the rotor with no prop, then WOT and taching and dividing by voltage):

Stock: 1480 Kv, 11,400 rpm @ 11.2v & 16a
RS 26ga/10T: 1780 Kv, 12,400 rpm @ 11.2v & 21a
MD 24ga/9T: 1975 Kv, 13,200 rpm @ 10.6v & 27a

Here are a couple pics. The first is the stator with burned out RS 26 ga wire. The second is the same stator with MD 24 ga wire. It was a little hard for a newbie to fit 9 turns on it. On the third tooth (out of 4) on each piece of wire, I used 8.5 turns which I later learned is equal to only 8 turns (only full turns count).

If you want to rewind this stator with 10 turns, I would use MD 25 ga wire to fit it all in and do 10.5 turns on the 3rd tooth. This should last much longer than 26 ga Radio Shack wire.

But I prefer the 24 ga 9T rewind. Time will tell whether or not the $9 ESC appreciates the hopped up motor (I have a 40a ESC on order).

This rewinding thing is fun!
Last edited by KC Flyer; Dec 06, 2013 at 07:53 PM.
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Jun 17, 2011, 07:09 AM
Jack
jackerbes's Avatar
Great report!

So when the green Rat Shack wire starts to turn black it is pretty well used up I guess?

I haven't messed with one of the Emax 28xx motors, it looks like they can be rewound without removing the stator from the bearing tube, is that the way you did it? If so, that makes them better rewind candidates then a lot of the backplate style motors.

If I understand the numbers right, you were pulling over 30A on 3S with that. That makes it a 40 gram motor that is good for about 7.5 Watts per gram! Was it holding up to longer full throttle runs or was that a burst?

Jack
Jun 17, 2011, 10:11 AM
Registered User
KC Flyer's Avatar
Jack -

The Radio Crap wire just seemed to turn gold after awhile - like the green insulation simply melted off. Never black though.

The motor never sagged nor got very hot in flight. I throttled back only to protect the ESC and batts (1300 3s Turnigy Nanotechs). I'm not worried about cooking the MD wire and the magnets seem to get plenty of cooling during flight with the Assassin. I think that the motor itself will hold up fine with extended WOT runs.

Here's an instructional video that shows a way to rewind the 2812. But the Rat Shack wire is low temp garbage and you should really go to the links in Post #1 to learn how to rewind. Anyway, in the end, the following video is fairly humorous but not too educational:

Motor Winding Tutorial (HobbyKing CF2812) (10 min 56 sec)


You just remove 2 - 3mm allen set screws to remove bearing tube/stator/rotor from red motor mount. Then remove a C clip to remove the rotor. Yes, the bearing tube and stator are one piece.
Last edited by KC Flyer; Mar 07, 2013 at 12:36 AM.
Jun 17, 2011, 11:53 AM
Registered User
flydiver's Avatar
The RS wire indeed does have crappy insulation. It flakes easily and seems to break down with pretty low temps. I've experienced both.

Next weakest link on the motor is the bearings. At that price point most of the similar motors have pretty low end bearings. Keep 'em oiled.

At that KV with the MD wind and a 7x6 prop you are making a lot of heat and not as much power as you think. The new wind is just shedding it more effectively. Try propping down and see if you actually get better performance and are easier on batteries.
Last edited by flydiver; Jun 17, 2011 at 12:01 PM.
Jun 21, 2011, 08:38 PM
A posse ad esse
Is the object to get the most thrust with the least watts. How do you know when you are generating heat instead of power?

Quote:
Originally Posted by flydiver
At that KV with the MD wind and a 7x6 prop you are making a lot of heat and not as much power as you think. The new wind is just shedding it more effectively. Try propping down and see if you actually get better performance and are easier on batteries.
Jun 21, 2011, 08:56 PM
Jack
jackerbes's Avatar
Quote:
Originally Posted by daddyrabbit1954
Is the object to get the most thrust with the least watts. How do you know when you are generating heat instead of power?
Sometimes the seat of the pants impression will tell you, like you're flying at full throttle and back off two, three maybe even four clicks on the throttle, do the same maneuvers again, and their is little or no difference in performance.

When that happens, if you have a data logger attached and flying too, you'll see a small drop in the RPM and a big drop in the Watts.

Like start flying big open loops at constant throttle and back off one more click each time you start another loop. Some people just can't get past the habitual, balls to the wall, I'm flying at full throttle thing, others can.

It can get pretty subtle sometimes.

Jack
Jun 21, 2011, 10:35 PM
Registered User
flydiver's Avatar
To investigate that most effectively you need a thrust stand, wattmeter, tach, and temp gauge. Then you can start testing stuff somewhat properly.

Without that you can start with seeing if WOT is maintaining at least 75% of unloaded RPM. If it is or better > good. If not you are bogging the motor.
Rapid temp rise in a short time frame on the bench indicates too much prop. It can be more difficult to test this in the field. I do a 1 minute WOT run in close proximity, bring it down rapidly and test the motor as soon as I can get to it. Too hot to touch is too hot.

Or, just try a smaller prop and see if you get same/better/worse performance and go from there.
Jun 21, 2011, 10:53 PM
A posse ad esse
I have this motor that I would like to try as my first rewind:

http://www.headsuprc.com/servlet/the...lectric/Detail

To stay in a 840~940kv range, what guage wire and how many wraps will be required? I have not removed the existing wire yet, but it appears to have quite a few wraps.

Quote:
Originally Posted by flydiver
To investigate that most effectively you need a thrust stand, wattmeter, tach, and temp gauge. Then you can start testing stuff somewhat properly.

Without that you can start with seeing if WOT is maintaining at least 75% of unloaded RPM. If it is or better > good. If not you are bogging the motor.
Rapid temp rise in a short time frame on the bench indicates too much prop. It can be more difficult to test this in the field. I do a 1 minute WOT run in close proximity, bring it down rapidly and test the motor as soon as I can get to it. Too hot to touch is too hot.

Or, just try a smaller prop and see if you get same/better/worse performance and go from there.
Jun 22, 2011, 12:38 PM
Jack
jackerbes's Avatar
Quote:
Originally Posted by daddyrabbit1954
I have this motor that I would like to try as my first rewind:

http://www.headsuprc.com/servlet/the...lectric/Detail

To stay in a 840~940kv range, what guage wire and how many wraps will be required? I have not removed the existing wire yet, but it appears to have quite a few wraps.
That will have 9 turns made with multiple strands, I think 5 to 7 strands or something like that. All of the 2410 motors have the same stator (12 arms) and they vary the turn counts, numbers of magnets, and the termination (Delta or Wye) to get the various models.

So once you get the windings off the stator they are all the same. I have not rewound a 2410-09 but all the other 2410's I have had were 12 arm stators and with 14 magnets. If your's is that way too, the attached screen shot of the output for Turn Calculator 5 will show you the turn counts for the various winds and terminations.

One of the best winds I've used on this motor so far is the 11 turn LRK "Y" wind I described in this thread:

https://www.rcgroups.com/forums/show....php?t=1382108

I did a 9 turn wind in that thread first, then did an 11 turn on a second motor and that is the one that turned out best for me. The Kv was too high on the 9 turn wind (1423) and produced a just so-so 2S motor. But the 11 turn wind is a great 3S motor with a 1310 Kv. That wind doubled the power output of the 2410 motor for me with a HD 8060 prop.

If you want a lower Kv you can just add more turns, looks like 15 or 16 turn would be about right for what you want for a Kv.

Jack
Last edited by jackerbes; Jun 22, 2011 at 04:21 PM.
Jun 22, 2011, 01:10 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by daddyrabbit1954
... To stay in a 840~940kv range, what guage wire and how many wraps will be required? ...
Keep in mind that winding wire thickness has no effect on Kv. However, it does effect efficiency, max.power and voltage drop under load.


Vriendelijke groeten Ron
diy motor tips Drive Calculator
diy motor group Cumulus MFC
Jun 22, 2011, 01:19 PM
A posse ad esse
Obviously, there is more reading and research I need to do. From what I understand now, I should use the maximum guage that will still allow the number of turns I need to achieve the kv value, in order to maintain efficiency.

Quote:
Originally Posted by Ron van Sommeren
Keep in mind that winding wire thickness has no effect on Kv. However, it does effect efficiency, max.power and voltage drop under load.


Vriendelijke groeten Ron
diy motor tips Drive Calculator
diy motor group Cumulus MFC
Jun 22, 2011, 01:51 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by daddyrabbit1954
... I should use the maximum guage that will still allow the number of turns I need to achieve the kv value, in order to maintain efficiency.
Correct

Efficiency governs power
Higher efficiency does not only mean that the motor makes better use of the batteries' power, it also means the motor is able to handle a higher power input before hitting its maximum temperature mark i.e. a the power/weight ratio will be higher.

An example
Say the motor has an efficiency of 70% and it can handle 50Watt input. That means it can get rid off 30%*50=15Watt excess heat. Now, by cramming in thicker wire (and/or using better stator-iron, segmented magnets), efficiency increases to say 75% (I'm a bit optimistic here). The motor's ability to loose those 15Watts has not changed (by radiation, convection and conduction). This means the motor now can handle 60Watt before it hits the 15Watt (25%*60Watt) losses mark. An efficiency increase of 5% gives an increase in the power to weight ratio of 20% (from 50Watt to 60Watt). That's why efficiency plays such an important role, in any motor design: efficiency governs maximum power. The motors weight may have increased a bit due to more copper.
A rather extreme example, Just for calculation's sake/fun: going from 80% to 90% efficiency would increase power by a factor two.


Vriendelijke groeten Ron
diy motor tips Drive Calculator
diy motor group Cumulus MFC
Jun 22, 2011, 05:01 PM
Jack
jackerbes's Avatar
Quote:
Originally Posted by daddyrabbit1954
Obviously, there is more reading and research I need to do. From what I understand now, I should use the maximum guage that will still allow the number of turns I need to achieve the kv value, in order to maintain efficiency.
The help you can get here from guys really make the learning curve pretty painless. That thread above was one of my first rewinds and because I posted the details on it I got of lot of feedback and help during and afterwards. The second rewind I detailed in port #19 of that thread, is the wind that really gave me the motor I was looking for.

One of the limitations you'll always run into in rewinding motors is that the turns wound on two adjacent stator arms must share and fit in the bottom of the "V" shaped space between the arms. So when tried the LRK, wind which only uses every other arm, I was able to both use a larger size wire and fit the desired turns in without it interfering with the arm next to it.

In that rewind the motor's current carrying capacity was increased to 150% or more of the original ratings and the power was doubled.

A lot of the basics and terms you'll start hearing and reading are explained here:

http://www.bavaria-direct.co.za/models/motor_info.htm

Here is a kit motor assembly instruction manual that will give you a good idea of the step by step process for rewinding using the DLRK and ABC winds. Yeah, I know those terms are meaningless to you but they won't be for long once you get started.

http://www.micronradiocontrol.co.uk/...o/KH-257-2.pdf

Jack
Jun 22, 2011, 05:44 PM
A posse ad esse
Outstanding - thank you very much, Jack

Daddyrabbit
Jun 22, 2011, 07:14 PM
tree scavenger

still scratchin head....


Quote:
Originally Posted by Ron van Sommeren
An example
Say the motor has an efficiency of 70% and it can handle 50Watt input. That means it can get rid off 30%*50=15Watt excess heat. Now, by cramming in thicker wire (and/or using better stator-iron, segmented magnets), efficiency increases to say 75% (I'm a bit optimistic here). The motor's ability to loose those 15Watts has not changed (by radiation, convection and conduction). This means the motor now can handle 60Watt before it hits the 15Watt (25%*60Watt) losses mark. An efficiency increase of 5% gives an increase in the power to weight ratio of 20% (from 50Watt to 60Watt). That's why efficiency plays such an important role, in any motor design: efficiency governs maximum power. The motors weight may have increased a bit due to more copper.
A rather extreme example, Just for calculation's sake/fun: going from 80% to 90% efficiency would increase power by a factor two.
could someone please help me get this?
every time i see this (for several yrs now, mind you), i think im getting it,, but then when i run thru the numbers, i get all mixed up, what am i missing in the math??

i just cant see where the extra 10 watts comes from?
how did we get from 50(watts) to 60 (watts)??
%5 of 50 watts is 2.5 ?? how did we get to 10??

KCFlyer i sincerely apologize if this is too offtopic, but its Your thread, you know where the buttons are, i wont be offended the least sir,,, or maybe (hopefully?) you'll be interested as well.
thanks
ss


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