Perhaps silly electronics question

All of them, working as a committee.

With a 10A ESC you should get 9A no problem.

That depends on how the ratings relate to reality. One hopes they're conservative, but nominal ratings are never "real" ratings -- sometimes they're more, sometimes they're less. I would expect that for modelers they're pretty close, however.

It depends on whether the 10A rating on the ESC is a current limit rating or a maximum don't-exceed-or-it'll-melt rating. I believe that nearly all ESCs have current limits, but I don't know how accurate they are (has anybody tested?).

Not without knowing the load (i.e. the propeller). The current draw of a motor is a function of the motor's speed constant, it's shaft speed and its armature resistance -- if the speed constant is stated correctly the current should be zero right at the calculated shaft speed, going up to V / Ra at zero shaft speed. This is why you want a current-limited ESC: you prang the aircraft, stall the motor & forget to turn off the throttle and that current will go sky high without limiting.

I measure the current on the ground at full throttle; if it's too much I put on a less aggressive prop, a different gearbox or I drop a cell. It'll unload in flight so you can let it go right up to the maximum rated current for the most wimpy component in the system (unless you're flying a 'copter, in which case you should probably give it a safety rating -- I'd guess 20%).

| My question is: what part of this circuit actually determines the | maximum current draw?

All of it. And you forgot two other important variables -- the gearbox and the prop. And there's more ...

| If I push the throttle up so that the motor WANTS 9A | will the battery simply refuse to supply it (i.e., drop | its voltage, etc.)?

The more current you draw out of the battery, the lower the voltage leaving the battery and the more heat generated in the battery. To be more precise, if you double the current, the heat generated in the battery will approximately quadruple. (heat = I^2 * R, where R is the internal resistance of the battery.)

| Or will it overheat as it discharges, damaging itself and eventually | melting my plane and my wallet?

It could.

Note that when they say it'll do 10C, usually that's only for a short period of time (though some of the newest can do 15C for a short period.) If you try to sustain 10C for 6 minutes, the batteries will lot last long.

| Surely the ESC won't prevent a 9A draw -- it's meant to allow 10A. | But what if the draw wanted to be 11A --- would the ESC prevent that, | or would it melt?

Melt. The ESC doesn't prevent anything. Mostly that rating is about being able to get rid of the heat generated, and you could possibly exceed the rating if the cooling is adequate, and it could melt (let the magic smoke out) at lower currents if the cooling is inadequate.

| And (last but not least) is there a way to tell how much current a | particular motor will draw at max throttle for a given voltage?

Yes.

There's a lot of variables (even more than we've mentioned here), but they're all very well understood and can be calculated with relatively high accuracy by programs like this.

Generally, it is the load. If the motor wants 9A and the ESC is capable of passing that much, then the battery will try to deliver it. If your battery is rated 10C, that only means that it can safely deliver that amount without major harm. It is probably capable of delivering a lot more but at a cost of battery life and voltage. Draw too much and you may get flames!

That depends on the design of the battery. Most will deliver a lot more than the rated current but will overheat and likely be damaged.

See above.

The ESC would probably overheat. Some have thermal protection that would shut down the motor part. Others could be damaged.

That would depend on the load the motor will see. Which prop, gear ratios and such. Motocalc will get very close to actual numbers.

The motor.

The voltage will drop.

It can. Try to always get a battery that will supply the current you need.

It won't prevent it. Only 1 amp over probably won't hurt it , but it may.

I use an Astro Flight 'Whatt meter'. It tells you the amperage , voltage and watts. Great little 'gadget'.

Ken Day

| >My question is: what part of this circuit actually determines the | >maximum current draw? | | The motor.

Oh, it's a lot more than that. The prop and gearbox is almost as important as the motor, for example, and if the battery isn't adequately sized, it can be the limiting factor.

| >If I push the throttle up so that the motor WANTS 9A | >will the battery simply refuse to supply it (i.e., drop | >its voltage, etc.)? | | The voltage will drop.

... and the battery may still provide more current than it's rated for.

Really, there's little preventing your entire setup from catching fire in flight, beyond 1) a fuse, if you add one and 2) careful selection of appropriate equipment.

Simply putting on a bigger prop can turn a properly flying plane into a plane that's ready to turn into a flying fireball with just a few minute's warning.

| >Or will it overheat as it discharges, damaging itself and eventually | >melting my plane and my wallet? | | It can. Try to always get a battery that will supply the current you | need.

Try to get a battery that can supply more than the current that you need. If you keep your batteries right at the maximum rated current all the time, they'll not last long.

| >And (last but not least) is there a way to tell how much | >current a particular motor will draw at max throttle for a given | >voltage? | | I use an Astro Flight 'Whatt meter'. It tells you the amperage , | voltage and watts. Great little 'gadget'.

They're nice, but you need to keep in mind that you're measuring the draw with the plane at rest. In flight, the prop will speed up (unless you're hovering) and the current and power draw will decrease.

(Fortunately, this means that if you have a battery rated at 10C, and your Whattmeter reads a 10C draw on the ground, you may be OK, as in the air it may be an 7C draw. The exact numbers will vary, of course.)

Motocalc is really the ideal answer to this sort of question, as it's accurate and it costs nothing to test other configurations.

The short answer is that the motor, the ESC and the battery are all capable of passing more current than is good for them.

Which one catches fire first is a difficult question.

How much the setup WILL draw is mostly dependent on how loaded up the motor is.