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Old Jul 05, 2015, 04:09 PM
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Reduce voltage of 3 cell lipoly

I have an old speed 400 powered glider that I would like to convert to use the available 3 cell lipoly batts I used with my Radian - 1000 mah to 1300 mah.
I bought a brushed lipoly speed controller for it. On another glider the motor burnt out at this voltage. I used 8 cell (9.6 volts) nimh packs without a problem for the speed 400 motors.
Does anyone know of a voltage regulator that will bring the volts from 11.1 down to ~9.5 volts.

Please don't tell me to convert to a brushless. I have too many other projects to do this in the near future.
Please don't tell me to use 2 cell packs. I just bought a bunch of 3 cells and this glider has poor climb rate on 7.4 volts.
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Old Jul 05, 2015, 04:40 PM
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There are not many easily obtainable high current regulators that will take a 3S down to about 10 volts and carry 12-15A. I would suggest you search on the internet, (Amazon maybe) for such a regulator.
If I was faced with the same situation I would make one, but that is not in everyone's skill set.
A cheap and dirty way to do it would be with some high wattage low resistance power resistors, hanging out in the breeze for cooling purposes. As I said, this is a quick and dirty fix, and needs some experimentation to find the right values.

Iain
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Old Jul 05, 2015, 04:53 PM
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Quote:
Originally Posted by rev.iain View Post
There are not many easily obtainable high current regulators that will take a 3S down to about 10 volts and carry 12-15A. I would suggest you search on the internet, (Amazon maybe) for such a regulator.
If I was faced with the same situation I would make one, but that is not in everyone's skill set.
A cheap and dirty way to do it would be with some high wattage low resistance power resistors, hanging out in the breeze for cooling purposes. As I said, this is a quick and dirty fix, and needs some experimentation to find the right values.

Iain
Thanks Lain.
Electronics is Not my skill set. So far most regulators on Amazon are talking 2 or 3 amps. Will keep looking.
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Old Jul 05, 2015, 04:58 PM
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Prop size

Just thinking in another direction - voltage may not be what kills motors; it's more likely excessive current.
Could you reduce the current draw by fitting a smaller prop?
Cheers,
PeteM
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Old Jul 05, 2015, 05:53 PM
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The limiting factor for the motor is the current. 8 cell nimhs probably only delivered about 8v under load whereas 1000 - 1300mAH lipo would give about 11v. There would be a huge jump in current if using the same prop. As Petem says a smaller prop could be used to limit the current, problem might be finding a suitable prop.
Alternative solutions would be :
a) 2s lipo with a bigger prop to bring the current to the same level as 8 nimhs (this would reduce the potential max watts but the increased efficiency of the bigger prop would compensate)
b) 3s LiFe would give a smaller increase in the delivered voltage fairly close to a freshly charged 8 cell nimhs battery. This might be OK with the original prop size.
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Old Jul 05, 2015, 07:32 PM
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I was using a 6x3 folder on motor that burned out. Would a plastic zagi prop be better? The zagi is 5x5.
If I cut the zagi prop down to 4.5 x 5 would that work?

edit: Even I realize that if you multiply the diameter x pitch you would get a larger power requirement for a 5 x 5 or 4.5 x 5 prop.
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Old Jul 05, 2015, 08:27 PM
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If I limited my throttle to 75% using the computer transmitter, would that solve the problem?
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Old Jul 05, 2015, 09:12 PM
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Quote:
Originally Posted by John Gallagher View Post
If I limited my throttle to 75% using the computer transmitter, would that solve the problem?
Intuitively, you would think so, but the speed controller will still be delivering the pack voltage, around11.X volts, to the motor, just for shorter bursts. It would be nice if you could just average it out, but most motors don't know how to do that

ct
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Old Jul 05, 2015, 09:12 PM
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The problem with running any ESC at reduced throttle is that the excess voltage is dropped across the ESC, and with the subsequent current flow generates heat and so will overheat and possibly burn it out.

Without a suitable regulator, and utilizing the batteries and motor you have, it is almost a case of not being able to get there from here.

If I can find an old S400 motor and a suitable ESC I will run some experiments with dropping resistors as I mentioned above.

Iain
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Old Yesterday, 09:29 AM
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The ESC doesn't "drop" the voltage or heat up at low throttle, it switches the power on & off. The "off" proportion of each pulse of power increases as the throttle is closed.
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Old Yesterday, 10:34 AM
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I believe Brushed ESCs do in fact drop voltage, while brushless ESCs do switch the voltage but still are prone to overheat at partial throttle as the SCRs generate heat during the switch cycle.

Iain
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Old Yesterday, 02:00 PM
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Every brushed ESC I've had operated on a "mark-space" of full voltage as I described. None of them overheated when running at low throttle for prolonged periods.
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Old Yesterday, 04:18 PM
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Best answer in your situation is to either reduce the prop or add a gearbox so as to reduce the draw.

Either way investing in a wattmeter to verify the new current draw would be wise. There used to be a ton of motor current calculatros on the web when we brushless motors and controllers were stupid expensive all obsessed over this question.

Going up in pitch hurts- instead of the Zagi props look for props from some of the new small quads-- 5x3 or 6x2.
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Old Yesterday, 04:19 PM
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Quote:
Originally Posted by patmcc View Post
Every brushed ESC I've had operated on a "mark-space" of full voltage as I described. None of them overheated when running at low throttle for prolonged periods.
Overheating at part throttle was something people always talked about being a big problem back in the day. More electrical noise and potential Rf interfearance as well.

I'm not sure I ever experienced it, though, so you may be right.
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Old Yesterday, 10:26 PM
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Thanks to all.
Found a thread on wattflyer.com from 2006 that had at least two people that used a 3 cell lipoly on a Zagi with lower throttle setting and out in the air cooling without a problem.
Others gave much the same advice back then concerning the prop - go to a 5x3 prop.
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