vmsguy

OK...

I'm trying really hard to do this whole "electric motor setup" thing the right way, and I have to agree with those that say it's frustrating.

So I'm trying to find a calculator. I looked here, and found a link to one on-line one. So I went there. And tried to plug in my info. It didn't have the batteries, or the ESC, or the motor I have. And I DON'T have some fly-by-night manufactured hardware.

I say, "OK, this is free, you get what you pay for. I'll plug in my info." I look up the specs of the motor at Tower, but what's listed as specs for the motor aren't listed as calculator input specs. I say "OK, the online calc says 'this' but on tower it says 'that'. Maybe I can come up with a translation matrix."

So I find a motor on tower that the calculator DOES have, and the specs don't match. RPM/v doesn't match.

What good are calculators if there isn't a consitent way of listing specs? What good are calculators if they're not updated, or update-able, with new hardware specs.

So.. the question I have here...

Please, name me a calculator that:
1. I can rely on to have accurate specs.
2. That I can update, or download updates for.
3. That I can input custom (new) hardware specs in some common-sense way.


Like I said, I'm trying to do this, and have been reading on this forum since spring trying to unlock these mysteries. I'm not stupid, and I'm not lazy. But I am frustrated....

exeter_acres

My Feedback: (2)

Motocalc might do....


but post what you are trying to do and someone here might be able to help

vmsguy

I didn't want to get into specifics, because I didn't want a "you should try this motor, or that battery" discussion.

But I will if I have to.

I wanted to see how my current motor/ESC/battery/prop combo would perform

So I found one link to "Pcal" (I think that's what it's called.) I go out there, and look for my motor. An AXI 2826/12. It's not listed. OK, I say to myself. I'll plug in info. I went out to Tower and looked up my motor. There's a bunch of specs. But few of the specs listed at Tower are called the same name as those listed on the online site. OK.. I say to myself again, maybe the sites are speaking different languages, (like english versus french) maybe they're saying the same thing, just differently. What does this site have listed. I find one, a AXI 2820/10 (I think, can't remember) So I go to Tower, and look up this motor. The online site has RPM/v listed as 1100, yet Tower has it at 1200. My ESC (JETI 40 Amp) wasn't listed. My batteries (Thunder Power 4200 3s2p) weren't listed.


What I want to do, I guess, is be sure the information I'm getting is accurate and up-to-date.

I want to come up with a configuation for a given plane and be sure it'll work. I don't want to blow up my ESC or motor because it can't handle the current going to it, etc.

I want to be able start with a plane, doesn't mater make or model. I know the finished weight will be "this much". I want to fly this plane utilizing "this many" watts per pound. Due to the design of the plane, I can not have a prop larger than "this long." I like "blank" brand motors, and I think "this" model motor might work. Now I have no particular allegence to ESCs, what ESCs will power that motor, what batteries will I need to drive a given prop.


In short, I want something that works, intuitive to use, and accurate. I'm not made of money and I don't want to show up at the field only to see my plane barely crawl across the runway because it doesn't have enough power to get out of its own way. Or see the motor suddenly die because the ESC overheated and the plane crash. Or have the plane literally go up in flames because the batteries or ESC overheated. Or land after a couple flights only to find my multi-hundred dollar battery investment is now trash because they were overtaxed trying to drive the motor.

Is this too much to ask for????

ozrcboy

Hi vmsguy,

OK - I think we all have a clear understanding of your frustration. []

I don't use a simulator program when trying to calculate these things. Instead I take an educated guess (based on the motor I have bought, and what other motors of the same kv, or rpmv, say) and then check the static amp draw on my actual test.

So, the things you need to know about your motor are the kv (or rpmv it is sometimes referred to - revs per minute per volt) and it's maximum continous and burst amps.

When you look at your system (any motor + any esc + any battery) you need to determine the current limit for the system - this will simply be the lowest of the currents that each of the devices can handle. For example if the motor can handle 20A, the esc can handle 25A and the battery is a 20C 1500mAh (So 20 x 1500mA = 30A) then your motor is the limitation on the system (20A). If the motor could handle 35A then the limitation would be your esc (25A), and if the esc could handle 40A it would be your battery and so on.

Picking the prop is how you control the number of amps your system draws. Now some rules of thumb - as you increase prop diameter the current drawn increases on the square of the diameter. As you increase the prop pitch the current increases linearly. So let's say you started with a 10x5 and it drew 20A. An 11x5 would draw approximately 24A (10x10=100, 11x11=121 ratio is 1.2) and with a 12x5 you would draw 29A, with 13x5 33.8 etc.

Now looking at your particular setup - a 2pack in parrallel configuration is going to give you something like 30+A per pack, so 60+A when connected in parrallel. So far your ESC is the lowest current device. I'm guessing your motor will probably have a limit around the 30A mark, but you need to check this.

So - buy yourself a wattsup meter - this thing plugs inline between your battery and esc and will give you the readings on current and voltage when you actually spin the plane up for a static test. Put a prop on the plane and with the wattsup meter in place bring the throttle up (with the plane tethered!) and watch the amp draw. Provide you don't exceed the maximum amps of the system bring it to full power, read the amps of, and figure out whether you are close enough, need a bigger, or smaller prop (and remember you can increase pitch to increase amps as well).

Calculating your input watts - once you know the current you draw with the right prop, multiply this by your volts (so 11v for a 3s setup) and there is your input watts. In terms of whether your setup can ever deliver the necessary power that's easy to figure out too - find your lowest current device, and multiply that by your volts, and that gives you the most your system can ever safely produce, provided you can find a prop that draws exactly that much. Now - the one caveat on this is whether or not you can fit a large enough prop to the aircraft (due to structural issues as you mentioned).

So, that's the theory - now - your setup - at 1200rpmv I would probably start with a 10x5 to 10x7 prop. I would expect that to pull around 20-25A. However, many motor makers give a list of recommended prop sizes with their documentation - use my suggestion only if such a list doesn't exist with yours. If you want more than 20-25A (you have a 40A esc after all) then maybe start with an 11x5-11x7 (somewhere in the order of 24A-30A. You can also look at increasing pitch size to draw more amps out of your motor.

The important thing is - if there is no documentation from the motor manufacturer on recommended setups then I think you should measure the amp draw to be sure. A wattsup meter is a once off purchase that you will find invaluable. Also, if you happen to have access to one, a clamp meter (that can measure large current) is also good, although it can't measure volts under load.

I wrote a primer on electric flight here - you have probably read these same things already, but you might find it useful:

http://www.oz********.com/2007/01/el...ic-flight.html

Cheers,
oz.

Time Pilot

When I input information into those calculators, I'm not too concerned about the ESC or battery. I normally pick an ESC about the size I'd use or larger and the largest rated battery listed. I'm not sure, but I'm guessing that the ESC doesn't make as much difference as all those other parts. The battery can limit the power output of the motor system. If the battery isn't up to the task of providing the current, you'll see lower power output numbers.

My concern was to know the maximums so I could ensure I wasn't overtaxing the ESC, battery or motor.

If the motor isn't listed, you pick one that's close to the one you want to examine by looking at weight and KV. When I was starting out, I'd be looking as specs from maxxprod, aircraftworld, and various others. Usually, you can find something similar.

There is a relationship between the weigh & kv of one motor and that of another of the same weight and kv. You can expect them to be similar in performance. Not exactly, as some motors are just better quality and more efficient.

For the most part, Tower has a great site, but I have noticed occasionally specs that don't quite make sense. Lots of information to input! One question though: Axi has gold and silver lines and the specs are different, if I recall, between two similarly numbered motors.

This is the online calculator I use:
http://brantuas.com/ezcalc/dma1.asp

There are many users here who would be happy to input some numbers into motocalc for you.
http://www.motocalc.com/

In the end, the calculators are only a guide. They aren't 100% accurate so the best thing to do is use them as such and get a meter to know for sure what's going on in your plane.

And get a battery rated for max amps of your system. One of the myths that I had to unlearn was that you can cut amp draw by using a lower throttle setting. ESC's work by either being on or off for a different percentage of the time. For example, the circuit may only be 'on' for 10% of the time at low throttle, but while on, it's drawing the max current during that time for that particular system.

Keep at it. It's a bit frustrating at first, but it's all good.

ron_van_sommeren

English and German, Windows, Mac and Linux.
Download, help, suggestions, announcements:
www.yahoogroups.com/group/d-calc

ozrcboy

Time Pilot, I can see your reasoning, but why does my clamp metre show these higher amps at lower throttle positions - is it just a matter of the current being averaged over the sample time?

And does it matter? Surely if an ESC is rated to 20A they mean 20A for 100% of the time. Or is it a cell chemistry thing?

Glacier Girl

My Feedback: (4)

As T/P was saying. A Brushless esc is nothing more then an on/off switch to be simple.

Unlike a water faucet where flow is controlled/ regulated by opening and closing a valve.

The esc just turns on and off, sorta like this. Low speed might cycle like this, on/off/off/off/ on.
Higher speed might be like this, on/off/on/off.

AND everytime it turns on, it pulls the full amps required by your set up. If it's pulling 40 amps at WOT, it's still pulling 40 amps at low throttle. The esc just regulates the time the motor can draw power to determine speed.

And yes most gages average amp draw and they can be fooled and in turn fool you into thinking the amp draw drops as throttle position drops.

Lots of magic smoke has appeared because of this one.


And best of luck, you'll find there is no 100% perfect answer when dealing with electrics.
No two of anything will be exactly alike.

Time Pilot

And it's not only the ESC's that you have to worry about, but the batteries too. You need to keep max amps under the C rating, at least.

Some of the experts suggest, should you want to maximize your battery life, that you run your batteries at 80% of their C rating. So a 10C 2000mAh lipo pack should only be run at 16 amps rather than their rated 20A.