Basic Brushless Questions - RC Groups
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Aug 31, 2003, 09:42 PM
Registered User
samhoff's Avatar

Basic Brushless Questions

Hello there,

I think these are pretty basic questions, but I’m having a hard time getting a straight answer from people I talk to. I'm in the middle of nowhere and have relied on this site heavily.

1. I have a brushless 1215/16 mini AC. Can I ruin this by running too many amps/too big a battery through it? (The controller is a Phoenix 25, so I know I can’t exceed that!). I assume I cannot.
2. Can I ruin it by running it “too long” or “too hard” or getting it “too hot”? Again, I assume the answer is no. If it were yes, why go brushless, is that right?
3. I am using a lithium 2100 2-cell block from radical rc. The voltage cutoff on the Phoenix 25 is currently at 5 V and it cuts out after about 5 minutes flight; for the remainder of the time I can get about 30 more flights of 30 seconds each. . . then the ESC kicks in again. But, on ground, it measures 7.5 V or so (the difference is between load and no-load voltages, I know). It is my understanding that I should never let a cell get below 3 V on a lithium (or 6V for a 2S) . . . but is that 3V WITH or WITHOUT a load? And how come it only gives me 5 minutes of good flight—I would think it would do better than that! (It’s on a Slowstick, with the above motor and a 6:1 gearbox, with a 13 X 10 prop).

Thanks to any who respond; I recognize the first two questions are much more basic than the third, but I’d appreciate info on any of the above.
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Sep 01, 2003, 02:36 AM
Registered User
You'll get better answers in the Power Systems forum so I'll move it there for you.

But some answers to start you off :
1. Yes you can ruin the motor by putting too much current through it
2. Yes you can ruin it by running it too hot for too long.
3. You can also ruin lithium batteries by pulling too much current out of them. That may be what you're doing.

Sep 01, 2003, 04:02 PM
Registered User
samhoff's Avatar
I thank Steve for the reply above, but I guess that's what I mean when I say I'm having a hard time getting answers. Does anyone have a more detailed explanation than "Yes, you can hurt your motor"? How many amps can I safely run? The insert says, "18 A/20 S" for Max. Loading. Does this mean it's 18 Amps max for 20 seconds?

Thanks in advance.
Sep 02, 2003, 09:34 AM
Tight is Right
Darren Hintze's Avatar
The MiniAC 1215/16 is rated (I think) at 16 amps continuous and 18 amps burst. Yes, it means you should not draw 18 amps for more than 20 seconds.

This motor, really, isn't very efficient above 15 amps (and it's max efficiency is around 10 amps). If you're running that high, another motor would probably be better.

By running a motor too hot you risk demagnetizing the core and/or melting the insulation on the winding wires. Either one destroys the motor.

Lithiums should not run over 8C (and 6C is better) so 2100's shouldn't be running over 16amps anyway. The quick ESC cuttoff is because lithiums will not hold voltage while over-drained (no batteries do). Once the load on them halts, they return to normal voltage (which is why you can disconnect them and still measure a nominal charge level).

Hope that helps!
Sep 02, 2003, 10:42 AM
registered user
DNA's Avatar

The 2100 cells you're using are good for 6 amps continuous. They are
the older Kokam cells and even at 6 amps they will lose about 1/2 of
their capacity. Your setup is likely drawing more than 6 amps, so the
kokam cells will drop in voltage even faster, as you've noticed, and you
will get short flight times. It is ok to discharge the cells to 2.5 volts/cell under
load. They recover to > 3 volts when unloaded.

With your 6:1 gearbox, you're not straining the MiniAC, so there's no
need to worry about drawing too many amps. You just need better cells
that handle more amps for the MiniAC brushless. I would recommend
a 2s2p pack of Etec 1200ma cells. They are capable of more than 14 amps
continuous and still deliver 90% of their capacity.

If you don't want to spend the $65 for the Etec lipos, your SlowStick
will easily handle an 8 cell pack of Kan 1050 nimhs for around $14.
Last edited by DNA; Sep 02, 2003 at 10:53 AM.
Sep 02, 2003, 11:30 AM
Registered User
samhoff's Avatar
(A little background on my situation: I started in powered electric RC 6 months ago and it seemed at that time that lithium was the way to go, so I got a lithium battery and charger. The fact is I don't even have a charger for NiMh batteries. I thought the lithiums would be more fun and cheaper in the long run. But my lithium battery has gone awry (one cell swollen like a balloon) and I am told there is no warranty at all on them. I had hoped it would last longer than the fifteen flights or so it's been on. So I am looking at buying another to the tune of $60 . . . or buying a NiMh charger and a battery for $70. Which way to go? I'm beginning to regret starting with lithiums for sure!)

With your 6:1 gearbox, you're not straining the MiniAC, so there's no
need to worry about drawing too many amps. You just need better cells
that handle more amps for the MiniAC brushless. I would recommend
a 2s2p pack of Etec 1200ma cells. They are capable of more than 14 amps
continuous and still deliver 90% of their capacity.

Does this mean that I can't hurt my miniAC as long as it's geared 6:1? Is that because the gearing will not allow it to go faster than what it can take? Or do I misunderstand?

Definitely what I need is about 14 amps continuous--I'm trying to use this thing for aerial photography and I want to be able to fly at, say, 70% of full power for a while to climb to photo-heights. Would these Etec 1200ma cells allow me to do it? Or should I abandon lithiums, sell my charger on e-bay and get started with NimH's?

Again, thanks to you veteran fliers. I live in the middle of Wyoming and have no one to help besides you guys. People here think I'm crazy to go electric instead of gas and I'm beginning to wonder if they're right.
Sep 02, 2003, 12:17 PM
registered user
DNA's Avatar

Yes, the Etec 1200 lipos will definitely improve your flight times and
give you more amps and capacity. Since a 2s2p pack has two cells
in parallel you effectively have a 2400ma pack. Since you're already
set up for lipos but not nimhs, you may want to stick with that chemistry.
I only suggested the nimhs because of their price. Lipos are fine if
you get a pack that can handle the amps you need. The Etecs will.

Low gear ratios like 2:1 will spin the prop faster but use a lot of amps.
High gear ratios like 6:1 will spin the prop slower but use less amps.
It's a juggling act to find the right prop and gear ratio combination to
give you the power you need, the correct amp draw for your batteries,
and the flight times you're looking for. You're already close with the
combo you're using with no need to worry about the motor. You just
need some better cells to handle the amps you're using and the flight
times you're expecting.
Last edited by DNA; Sep 02, 2003 at 04:25 PM.
Sep 10, 2003, 12:49 PM
Aerial Shutterbug
RMihara's Avatar
Hi Sam,

I'm not certain about the amp draw on the 13x10 but that same motor with a 5.33:1 g.b. and a 12x6 draws only 9A, a 12x8 pushes it up to 13A. So for your set-up I really think you're closer towards the 14A-16A mark with that big prop.

I would really recommend you go with the Thunder Power TP-2100, 3S1P pack. It's $79.95 at AeroMicro and while still expensive, I know you won't be dissapointed with the performance of your set-up using them.

In my Tantrum, I am pulling over 21A (peak) and nominally fly between 4A to 15A. Flight times have been between 20-30 minutes of very aggressive 3D but pulling back to gentle flight I've recorded a 45 minute flight, with about 250 mA left in the pack.


Note: Even though you will be able to sustain a vertical climbout to altitude, it's always a good idea to do it in stages, allowing the motor a short cool-down period before ascending upwards again.

But if you're going for 'speck-out' altitude like Steve and JMH then I'd recommend a good steady climb-out. And at 80% throttle, you will be able to achieve that continuously.