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Old Dec 05, 2003, 07:06 PM
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galloping gimp's Avatar
Ohio, USA
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I've had some success with my CD-ROM motor, but I need help analyzing the results. I have no tach, so I can only report amps and volts at this time.

26 turns, #26 (~4mm) wire
stock ring magnet (have not yet installed N45 magnets)
controller: Jeti 08-3P
battery: 370 mAH NiMH, 7 cell

The motor started and ran the first time I applied power, which was nice to see.

I immediately installed the only prop I have, a GWS 9x7, which I know is too much prop.

With the 9x7, the motor throttles up smoothly. As I reach about 85% of full throttle, the motor stops. As I advance the throttle, battery voltage decreases from 8.9V (no load) to well below 6V under load, at which point the motor stops. Current is about 4.5A when the motor shuts down.

After about 10 minutes of intermittent running, the controller is barely warm. The motor stator is very hot -- too hot to touch.

What does it all mean?

I think the motor is stopping because the battery voltage has dropped below 5.3V, which is the cut-off for the Jeti controller. Does this mean that the battery pack is simply incapable of sustaining such a large current draw?

Does the current (4.5A at 85% throttle) sound about right for this motor and prop?

How will installing 5x5x1 N45 magnets change things? Will current draw increase?

What about the heating of the motor?

All advice and comments are very welcome.

Thanks - Jeff
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Old Dec 06, 2003, 12:10 AM
Why not Delta?
rysium's Avatar
Sacramento, CA
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Quote:
Originally posted by galloping gimp
I immediately installed the only prop I have, a GWS 9x7, which I know is too much prop.
You mean direct drive????
I just tested my motor on GWS 300 C (5.33:1) with 8x4.3. Full throttle it gets 3.8A and static thrust about 6.5 oz. On 10x4.7 it gets up to about 5.5A with static thrust about 8 oz. So my guess is 9x4.7 would be the perfect solution.

9x7 DD on that motor is way to much. Anything over 5A is to much. My motor runs good and warm (not hot) up to 5A, but I would keep it about 4.5A max - longer life for motor and the battery.

Quote:
I think the motor is stopping because the battery voltage has dropped below 5.3V, which is the cut-off for the Jeti controller. Does this mean that the battery pack is simply incapable of sustaining such a large current draw?
On my 350 NiMh I get good 3.5A (I found it's very happy at 2.5A. 5A may just kill the battery. Unless your battery is rated 20C, but I didn't see NiMh 3500 for 20C. If you pull 5A from this battery, you may need the new battery soon.

RysiuM
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Old Dec 06, 2003, 12:55 AM
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Ohio, USA
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RysiuM -

Thanks for your advice and comments. This is uncharted territory for me. Until now, my experience with R/C motors has been limited to flying the IPS-A in various planes.

Yes, it's direct drive, and yes, I know I am abusing the battery packs. And yes, I know that a 9x7 is too much prop. I'm a bad boy.

However, these are the only batteries and props I have at the moment, and I wanted to see if the motor worked at all. Now that I know it works, I'll be getting some better props.

I only briefly pulled more then 4.5A from the battery packs, so hopefully I have not cooked any of them. The packs never got more than mildly warm.

I measured 3.5 ounces thrust with a freshly-charged 7-cell pack and the 9x7. I could not use 100% throttle because above about 80%, the voltage would instantly drop below the controller's cut-off.

Basketcase is flying a similar motor in his Tiger Moth and is using a GWS 8x4.3 and 2 li-polys with good results. I think I will order a selection of props to see what works best.

- Jeff
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Old Dec 06, 2003, 05:59 AM
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USA, FL, Orlando
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I've been reading this with great enthusiasm. But I'm wondering, does anyone know why you use different gauge wires and number of turns. I mean, does 20 Turns produce more voltage or thrust than 16 Turns. What about the gauge wire? What does different gauge wire do? Thanks.
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Old Dec 06, 2003, 08:25 AM
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mmormota's Avatar
Budapest
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Quote:
Originally posted by taekwondo
I've been reading this with great enthusiasm. But I'm wondering, does anyone know why you use different gauge wires and number of turns. I mean, does 20 Turns produce more voltage or thrust than 16 Turns. What about the gauge wire? What does different gauge wire do? Thanks.
The Kv value, ie the rpm/V depends on the number of turns. The more the turns, the lower the Kv.
On a given turn number, the thicker wire is the better, thicker wire has lower resistance, lower resistance means lower copper loos, higher efficiency.
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Old Dec 06, 2003, 08:30 AM
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Vacaville, Calif
Joined Jul 2003
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Grinding N45s?

I installed my new 5x5x1 45s in the cap, now it is a tad too big to fit over the stator.

Can I take the Dremel and reduce the bulk and/or if I do will I damage the magnets or create other problems?

Got my second CDromer to fire up..no rewind.. with a CC 10.
No Tach yet nor way of affixing the prop but alas.. ingenuity will soon strike (translates to a trip to the online RC shops LOL)

Anway, the grinding of the magnets is the big question of the day for me..
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Old Dec 06, 2003, 12:02 PM
Why not Delta?
rysium's Avatar
Sacramento, CA
Joined Jun 2003
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Quote:
Originally posted by galloping gimp
RysiuM -

Basketcase is flying a similar motor in his Tiger Moth and is using a GWS 8x4.3 and 2 li-polys with good results.
I think you can get rid of gearbox when use more turns on the motor.
My stator must be very small compared to others. Using 26G I couldn't fit more than 16T in 4 layers (5,5,4,2). When I used 2 x 30G I was able to fit 18 turns in 5 layers (4,4,4,3,3).

How do you fit 26T on 26G I need another CD-ROM
However without the prop on WOT my motor sounds like turbine and takes 1.5A.

RysiuM
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Old Dec 06, 2003, 01:31 PM
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USA, FL, Orlando
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"The Kv value, ie the rpm/V depends on the number of turns. The more the turns, the lower the Kv.
On a given turn number, the thicker wire is the better, thicker wire has lower resistance, lower resistance means lower copper loos, higher efficiency."
Thanks MMoromota: So less turns and thicker wire is best, ie more rpms per volt?
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Old Dec 06, 2003, 01:44 PM
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Ohio, USA
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RysiuM -

Here's my stator. It is about 24mm in diameter. I wound four layers of #26 (~ 4mm) wire: 7, 7, 6, and 6 turns per layer. I used #26 magnet wire from Radio Shack.

I was able to keep the windings fairly neat and parallel, although you can see a couple of loose turns in the photo.


Trprhook -

I have seen a number of warnings that you should NOT grind the magnets. If you have clearance problems, the knowledgeable guys recommend grinding the stator instead.

The problem is heat. If the magnets get hot, they can begin to lose their magnetic properties. There are several reports of magnets drastically weakened by grinding.


Question for all: When replacing the stock ring magnet with more powerful N45 magnets, what are the effects on RPM, torque, and current drain, assuming batteries and prop remain the same?

- Jeff
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Old Dec 06, 2003, 01:56 PM
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Quote:

Question for all: When replacing the stock ring magnet with more powerful N45 magnets, what are the effects on RPM, torque, and current drain, assuming batteries and prop remain the same? [/B]
Kv (rpm/V) will be lower. Torque, efficiency, maximum effective power will be higher.
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Old Dec 06, 2003, 02:20 PM
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Tokyo, Japan
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Re: Grinding N45s?

Quote:
Originally posted by Trprhook

Anway, the grinding of the magnets is the big question of the day for me..
http://www.rcgroups.com/forums/showt...5&pagenumber=6

I use 5x5x0.9t N45.

Takao
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Old Dec 06, 2003, 10:29 PM
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Sacramento, CA
Joined Jun 2003
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Tiny with CD-ROM HP x 52

Today I finally got my measurments.

So on the C drive, 2 cells of 1500LiPo I got 6020 rpm, with GWS 8x4.3. giving me a little more than 7oz thrust (the plane with this battery weights 7 oz and on WOT it's going up a little). Funny thing, but this RPM on that prop gives the same result on my prop calculator.
The battery gives around 8V and motor takes 3.8A.

Now the time for calculations:
Prop calculator gives me the prop power 22.7W
Motor takes 8*3.8 = 30.4W,
so the efficiency is 22.7/30.4 = 75%.
That means the motor gives out 30.4-22.7 = 7.7W in the form of heat.

I think is quite not bad. I didn't include the efficeincy of ESC and the current for receiver because it's very marginal.

How rpm/V of the motor is measured? can I somehow measure it for my motor? Under load of GWS 8x4.3 the motor is turning 5.33 * 6020 rpm = 32086 rpm, but it depends on the load. without load it sounds like turbine, but I didn't measure the rpm. It's just very high.

RysiuM
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Old Dec 06, 2003, 11:53 PM
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Thanks to everyone who contributed valueable information in reply to my questions.

Special Thanks to "galloping gimp" for such an illustrated answer !! The information you provided me has been of great help in understanding the complete picture.


Thanks
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Old Dec 07, 2003, 12:11 AM
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galloping gimp's Avatar
Ohio, USA
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taekwondo -

Quote:
So less turns and thicker wire is best, ie more rpms per volt?
Not necessarily "best." It depends on what you are trying to accomplish. In some applications, you want more torque, which will let you turn a larger prop. In other applications, you want to maximize RPMs.

As I am learning, one of the neat things about re-winding your own motor is that you can tailor the motor to the application. There is a lot to learn and many forum threads to be read. It will take you days, but it is worth reading them all from beginning to end.

- Jeff
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Old Dec 07, 2003, 12:21 AM
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AirBorne_AZ -

I still have a whole lot to learn, and I would never claim that my methods are "correct" in any sense of the word. If you read the many threads on this topic, you'll see other construction methods.

Because of all the brass in my motor, it is heavier than many CD-ROM conversions. But brass is a material that was readily available and easy to work with, so I used it. Assuming my first motor proves flight-worthy, I fully intend to construct some others, this time with lighter materials.

- Jeff
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