Keefer

My Feedback: (2)

I've been doing some studying on ratings, getting ready to put up an E-Skylark 56 vintage as a twin. I'm using MotoCalc, which seems very useful, but it's not answering one question:
I wish to run 3S sets (maybe 3S2P), and found several motor/prop combinations to get the flight characteristics I'm looking for. I'm trying to approximate a pair of .15 nitro engines, so maybe 9x5 props at 12K rpm. So I get some Kv values between 1100 and 1300.
The problem is that MotoCalc's motor database doens't necessarily list motors that are easily found by the local hobby shop or those big on-line distributors.

So Isays to self: "Self, all ya gotta do is find motors in that Kv range." But I'm finding that many of the motors I find can't handle a 9x5 prop without overloading, or I'm finding motors that weigh upwards of 12 oz.
So what combination of manufacturers specs can lead me to understand what their current draw will be under the load of the prop Iwant?
Do the no-load current or winding resistance play into that?

I also see several posts where guys say "just change the voltage/cells to get the RPM you want". When you plug in another cell into MotoCalc, it sometimes screams "Oh, you idiot, you're gonna start a fire!" - or something like that - the motor will overheat, or the batteries can't handle the current.

Happy New Year!

Hauling-A

My Feedback: (7)

Keefer, If you haven't tried this site, give them a try. They give quite a bit of information about their motors. These are decent motors at a reasonable price. http://www.innov8tivedesigns.com/

Jim

Keefer

My Feedback: (2)

Thanks, Hauling-A -
They do publish a lot of info on their motors.

So my question now is, how does MotoCalc calculate actual motor current?

DaveFlynn

As I understand it, motor calculators can't calculate the current unless they have a value for L and a table for Kt. All four values R, L, Kv and Kt are needed to make a motor calculator work. R, L and Kv are easy to measure Kt is not, it varies with RPM. The problem comes from running a Delta wound motor on DC instead of 3 phase 4 quadrant AC where the Kt would be the inverse of the Kv.

Hauling-A

My Feedback: (7)

Keefer, I don't use motoCalc so I'm not familiar with it. The three Calculator's that I do use arn't exact. There is a variance between all of them when using the same parameter's ie; motor, battery, esc, and prop.

I use a "Watts up" Watt meter to determine what amp draw, watts in, and battery pack voltage under load that my motor & prop combo is putting out. For on board flight data you would have to use a data logger to know exactally what your motor and prop combo is doing.

If my motor prop combo isn't working then I either use a smaller prop or rethink my motor selection. I may need to go to a lower kv rated motor of the same size so that I can run a larger prop with more pitch for a faster mph. If the size of the plane permits the weight increase I also may have to increase the voltage of my battery pack.

The type and size of plane that you are going to use will more or less determine what motor, esc, servos, battery, and prop that you will be able to use and there are many, many option's available.

I would say your best bet, if you already don't own one is to buy a watt meter, it will make selecting motor and prop combo a lot easier. All you have to do is burn up one motor or battery or esc and you've most likely exceeded the cost of a watt meter.

I hope that this can be of some use to you.

Jim

jooNorway

It sounds odd to say you want to run a specific propeller at a specific RPM and still saying this should be equal to a .15 motor...
Just from one of my setups; if I spin a 9x6 (Graupner CAM) at 11700RPM the load is 42A, and use 420 Watt. I think this is a bit more than a 0.15 [X(]
You have to do an other aproach to this...

Keefer

My Feedback: (2)

jooNorway - you're right - I'm up towards approximating a 0.19.

Keefer

My Feedback: (2)

Jim - IT is useful - thanks. What I'm trying to avoid, though is having to use a watt meter on a prop/motor combo and then realizing too late that the combo is wrong. I'm trying to understand the motor specs better (in the design phase) so I only have to drop $300 once.
Happy New Year!

svorkoetter

I suspect a .15 nitro isn't going to turn a 9x5 at 12k rpm, but ignoring that, I'm guessing that you arrived at Kv values of 1100 to 1300 based on an estimated 10V battery voltage under load. The problem is that the Kv value is what the motor will do, per volt, with no load. So if you run a 1200 rpm/V motor at 10V with no load, it will spin 12,000 rpm (a bit less actually, because of Io and Ra losses). Thus, a 1200 rpm/V motor will never reach 12,000 rpm on 10V with a prop on it.

Yes, hugely. The basic factors that affect the current draw with a particular prop are Kv, Io (no-load current), Ra (resistance), and of course battery voltage. For an idea of how these affect current draw, see my article, [link=http://www.stefanv.com/rcstuff/qf200204.html]Motor Math[/link].

That just means you haven't selected an appropriate motor or battery.

Brushless motors don't run on DC. They run on 3 phase AC generated by the brushless ESC. It's exactly the same as what the mechanical commutator does in a brushed motor. For calculation purposes, you just need Kv, Io, and Ra just as with a brushed motor.

By the way, we've recently added some new motor and battery data to MotoCalc, so I suggest you do an "Update All" to get the latest data.

Keefer

My Feedback: (2)

Nicely done, svorkoetter! Now that's some information Ican digest!

DaveFlynn

Name ONE hobby ESC that puts out AC. I have 9 and they all put out DC.

Keefer

My Feedback: (2)

Um, well, any ESC for a brushless motor. No brushes, no commutator, no turny-turny without something to cause the field to move relative to the rotor.

DaveFlynn

Rotating DC is not AC. Look on a scope, one wire is not powered. I have industrial ESC's that drive all three wires all the time and use hall sensors for commutation.