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Old Jan 31, 2013, 10:16 AM
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Gimbaled
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morgan hill california
Joined May 2004
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Quote:
Originally Posted by jackerbes View Post
I don't think you are asking the right question. The wire size is the last thing you will find out and the number of turns has to give you a motor turning at the needed or optimal speed.

I'll answer that here because the details and considerations apply to both the gimbal motor and the flight motors. But the considerations for the gimbal motors is quite different and also a relatively new area of work as far as rewinding knowledge.

The motors, as you buy them, are wound with multiple parallel strands of finer wire. The parallel strands (I call each of the three groups a "bundle" of strands) are effectively the same as a single larger strand as far as current capacity.

Using fine parallel strands allows the strand count in a bundle to be easily changed so as to adjust for how many turns will physically fit on a stator arm. And it also allows the winding shops in China to wind different motors without having to stock an large assortment of wire sizes. The fine strand bundles also will bend around the arms easier and fit closer to the arm than a single larger strand in some cases.

On average quality motors the most commonly used wire in the bundles measures about 0.25mm or so and we generally consider it to be equivalent to 31 AWG wire which is 0.227mm uncoated. And this table is often used for estimating wire sizing and surface areas: http://en.wikipedia.org/wiki/American_wire_gauge - sizes and specs there are for typical uncoated magnet winding wire.

One example is the DAT-750 motor. As you buy those they have 18 turns with 8 strands of 0.25mm wire in the bundle. Each strand has a surface area of about 0.0404 mm2 so 8 strands have an area of of 0.0404 x 8 or 0.3232 mm2. So (looking at the mm2 numbers in the table) a single strand of 22 AWG wire with an area of 0.326 mm2 will give you a little more current capacity than the original 8 strand bundle.

The Kv is determined by the turn count and termination, the current capacity is determined by the surface area. Copper is King for current flow! So for turning props we want the most amount of copper we can get in the motor.

The Kv that results from each collection of parts (stator, magnets, flux ring, bearings, etc.) and a given turn count is unique to that particular assembly. But a production run of motors made from the same parts will only vary the Kv for each motor by a small amount, a few percent or so. Similar quality motors with similar Kv's will have similar turn counts but we really need to measure the Kv for a given motor to get more accurate numbers. The results can vary widely from the advertised specs in some cases.

It is the miscounted and misplaced turns, crossing turns, and even broken strands in a bundle that all contribute to the variations in the Kv. And they cause disruptions in the quality and harmony of the magnetic fields as the motor runs. And we can rewind more carefully to improve things. This is where my technical knowledge runs out, I am a re-winder, not really a technical expert on motors.

For driving propellers the maximum RPM desired and battery voltage determine the Kv needed for a given prop. Battery voltage x Kv = no load RPM. No load RPM / battery voltage (taken at same instant in time) = "raw" Kv. A "raw" Kv is accurate enough for our purposes, it is within a few percent of what the motor lab will determine it to be.

Motors will run at about 75% of their no load RPM when running at their continuous current rating (input power). So we can estimate the needed Kv from the max RPM number and the battery voltage.

One quick example: Say a raw Kv with a fully charged and rested 3S pack is found to be 12,000 RPM at full throttle with the pack at 12V under load (no prop=small load, little voltage drop). That tells us the raw Kv is about 12,000 / 12V or 1000. We then find the prop that will let the motor run continuously at full throttle without overheating and measure the RPM and voltage at full throttle again. The 75% rule of thumb says that will be about 9,000 RPM.

If the prop in the final test above is smaller than you wanted to use, you will need a motor that can handle a higher input power to spin a bigger prop. That would be a motor that weighs more or that is of higher quality.

Another rule of thumb has proven reliable is that the input power on average quality outrunner motors, running at their continuous rating, will be about 3 Watts for each gram of the motor weight (bare motor with connectors, no mounts or accessories). So if the motor in the example above weighed 80 grams I would expect it to have an input power of about 240 Watts. And on a 3S pack at 11V under load (3.7V per cell, about as low as you should go) I would expect the motor to be drawing about 22A (240W / 11V = 21.8A).

As the quality of motors improves the 3 Watts per gram rule of thumb can go up and our rewinds can raise that value. And buying better motors can raise it too.

I think JussiH is right, that the 0.16mm wire is too small for this application. And I'm not even sure that a DAT-750 would be a good motor for the gimbals because it is the open back CD-ROM type. I suspect that the back plate type motors as he has will be a better choice for their size.

But if the question is still "..using .16mm wire, what would be the suggested number of turns for a DT750 motor?..." the answer would be that it is the number of turns that gives you the Kv you need. And if the stator arms were not well filled at that number of turns, you would be giving up some of the motor's input power potential.

I further suspect that motors with more stator arms and more magnets will be the motors that are best and have the lowest cogging effect for use on the gimbals. I think it will be that way because of the low Kv's (400 or so?? or even less??). I suspect that with lower Kvs and slower (compared to turning props) rotational rates, having more stator arms and more magnets will make for having more cogging steps and smaller increments of movement from one cog to the next best. That will be where the smoothest movement is found I think.

So maybe the new pancake style motors with 20, or 24, or even 28 arms will be the best motor in the long run. And maybe people will even want to increase the magnet counts on the pancake motors to get more steps and even smoother motion?

Jussih, do you know what the Kv is on the motors you are using now? And just out of curiosity, is your controller applying the magnetic braking when the movement is stopped?

And if you want to share the below listed bits of info, it will good for this project. I can give this thread a quick lesson on using a turn calculator spreadsheets to help decide the turn counts they need to have to get the motor rewound.

- Model of motor you are using
- Kv specification of original winding (or a raw Kv if it was measured)
- Turns used in original winding
- Termination method on original winding

If the original winding was dLRK, I can use that as an input on a spreadsheet (like the screenshot seen here for the DT750) and it will give the turn count predictions for doing the dLRK and other winds and terminations on the motor you used.

If you can measure the raw Kv of one of your rewound motors it will also give you a chance to confirm that the speadsheet's turn count predictions are valid and give these folks the Kv they need for their gimbal motor rewinds.

You mention needing a resistance between 5 and 15 Ohms on a phase, that is interesting and good to know. But it is the raw Kv that is most needed to predict the turn counts.

Wow! This is getting awful lengthy but I guess it all goes for the benefit of the project?

Jack
Great post Jack, thank you for taking the time.
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