Helis & Planes Compared

[BTCat]

Motor specs say: "Max. efficiency current : 10~15A" That doesn't mean a lot to me. When I static tested it, it didn't get into the 20 amp range until over 75% throttle, if I remember right.

Now I am really curious about Scar's information. Kent?

[Solitaire]

While we wait on the engineer...

I have to think a 15A motor drawing 30A is definitely over-propped. No news there. I'm thinking a prop that will get the motor in it's efficiency range of 10-15A is the way to go. I also have to think a static test isn't a fair test. Wouldn't the motor draw a little less while in flight rather than holding it down?

Inquiring minds want to know.

[BTCat]

I believe a static test is the worst of the worst. It should draw the most amps.

Finally found a 9x7.5 today. I'll try it out. That was 19k's first recommendation. I think it will be better all around.

[akent]

Motors are rated for certain currents/power. Overpowering them can fry them!!

I know that first hand. I took a 1500kv HC motor and tied that to a 9:5 prop and within 20 seconds had smoke, even though it was within the power limits.

The wires they use on a lot of these Chinese motors are really thin.

Copper might need to be our next big investment item.
Kent

[akent]

Mar 13, 2010 10:56:32 GMT -6 scarface26 said:

You better get a bigger amp esc soon, BT. Read NoFlyZone's blog on ESC's at the RC Group Ultimate SC thread. If I read it correctly, you will be pulling 30.2 amps at any throttle setting. It just shows on a watt meter, the average amps for any throttle setting. The pulses of 30.2 amps are just further apart at lower throttle settings.

www.rcgroups.com/forums/showthread.php?t=865319#post9873236

This is true. The pulse current will be quite high on any brushless motor, although some of it is smoothed out from the motor's high inductance.
However, it is the average current that fries ESCs. The FETs (Field Effect Transistor) in them are rated for high peak currents with much lower average or sustained currents. What fries them is thermal runaway, where high current increases the temperature, which increases the resistance, which increases the temperature, and on and on.
For instance, a common 20 amp continuous rated FET (the FDS8812) can handle pulsed currents of 80 amps. So we don't generally have to worry as much about pulsed currents.

All of these brushless motors have a very small resistance through the windings.
===
In your battery example, the light bulb will draw current based on it's resistance. If the bulb has say 50 ohms of resistance, then at 12 volts, the current will be I = E/R (I = current, E = voltage, and R = resistance), then 12/50 = 0.24 amps, or 240 milliamps. This bulb would draw P = I*E (P = power in watts) .24 * 12 or 2.88 watts.
A starter motor has VERY low resistance and works from direct DC. These have on the order of a few milliohms of resistance in the coils with large wires, so they draw a LOT of current.

If you were to apply a high current DC into a brushless motor, it would instantly fry the motor. These are a completely different animal than a brushed motor like the starter motor. The brushed motor is internally mechanically commutated, while a brushless motor is commutated via a computer in the ESC.

I hope this helps.
Kent

[BTCat]

Believe it or not, I can actually understand some of that Good info as always, Kent. You wrote: The system will also draw more power with more load, which should be obvious. I am really curious about the link Scar posted on page 11. Is it accurate to assume if a motor is drawing 30 amps at full load and shows say 10 amps at 50% load this is just a mean average and the actual amp draw is 30 amps pulsed further apart? If so, this not only adjusts my view of the ESC, it also makes me wonder about my fundemental understanding that amperage is determined by the load ie a car battery with 600 amp potential will deliver milliamps to a light bulb and 500 amps to a starter. I am familiar with "pulse width modulation" in other applications, but this is messing with my head

[Solitaire]

So, what's the verdict on the 9x7.5 prop? Do the trick or do you still have a strain on the motor?

[BTCat]

Mar 16, 2010 23:17:11 GMT -6 Solitaire said:

So, what's the verdict on the 9x7.5 prop? Do the trick or do you still have a strain on the motor?

That's the prop I broke. I'll have to try again later.