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Puffing packs - edf jet

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TonyS13/08/2020 18:51:26
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Vecchio,

quite right, all very confusing. Seems as though it's not as black and white as I'd thought however...just to add fuel to the debate which is the lifeblood of any community.

First test on the Wattmeter shows 450Watts and 37 Amps drawn at WOT. The last pack I wrecked was an Overlander Extreme 1700 30C. Upon landing the charge was 12% (ouch - never usually go that low but in my defence it is a small capacity battery so the flight time went by very quickly...). As always it recovered, in this case to show 21% when it had cooled off a bit.

Richard Wills 213/08/2020 18:58:08
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250 forum posts
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37a at 450 watts gives an under load voltage of 12.2 ish so the pack is not really being overworked at that. I think lack of cooling and possibly taking too much of the capacity from the battery may actually be your main issues.

TonyS13/08/2020 19:12:06
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1186 forum posts
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But isn’t a 3s pack supposed to be delivering 11 volts or so, so it’s stretched to deliver 12.2?

Richard Wills 213/08/2020 19:20:31
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250 forum posts
7 photos

11.1 is the nominal voltage which is pretty much empty. 12.6 is the fully charged voltage, so a 0.4v drop under load is pretty good. They are being worked pretty hard at WOT but If it was being overworked the voltage drop under load would be much worse.

Edited By Richard Wills 2 on 13/08/2020 19:23:07

Edited By Richard Wills 2 on 13/08/2020 19:24:14

TonyS13/08/2020 22:18:44
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Thanks Ric. Useful to know.

PatMc13/08/2020 23:15:40
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Posted by Richard Wills 2 on 13/08/2020 17:15:49:

Pat it is only the voltage and current at the battery to esc we are interested in here. A 4s at a 75% transmitter limited throttle will give the same watts as a 3s at 100% on the same motor giving the same rpm at the fan, if watts are the same, the voltage is higher and the current lower for the same watts. You need to limit in your transmitter as if you use 100% on the 4s, you will blow everything up immediately. The higher the voltage the lower the current and the cooler and less stressed everything is. This is the reason I run everything on at least 6s these days, which you could also do with the edf and run at 50% throttle limit with active freewheel and get the same power on half the current which is what I would do if the plane was mine.

Richard, it's the voltage that the ESC is feeding to the motor that determines the RPM.
We know that under no load RPM = KV x Volts and with a load the RPM is governed by this formula.

If a wattmeter is connected at closed throttle it will display the full battery voltage. Progressively opening the throttle the voltage will drop, the current and Watt readings will rise. But despite this drop in voltage the RPM will be increasing. This is because the voltage at the output end of the ESC is rising and causing the RPM to increase.
The ESC output voltage is not being measured by the Wattmeter but it is the "Volts" in the formula Watts = Volts x Amps being usd by the Wattmeter.
At WOT the ESC input & output voltage are virtually identical at which point the "Watts" reading becomes meaningful.


Richard Wills 214/08/2020 00:33:00
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250 forum posts
7 photos

Richard, it's the voltage that the ESC is feeding to the motor that determines the RPM.
We know that under no load RPM = KV x Volts and with a load the RPM is governed by this formula.

If a wattmeter is connected at closed throttle it will display the full battery voltage. Progressively opening the throttle the voltage will drop, the current and Watt readings will rise. But despite this drop in voltage the RPM will be increasing. This is because the voltage at the output end of the ESC is rising and causing the RPM to increase.
The ESC output voltage is not being measured by the Wattmeter but it is the "Volts" in the formula Watts = Volts x Amps being usd by the Wattmeter.
At WOT the ESC input & output voltage are virtually identical at which point the "Watts" reading becomes meaningful.

Respectfully I do not think you quite understand how a brushless motor and speed controller system operates.

Dickw14/08/2020 11:00:24
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752 forum posts
101 photos

Just to add something that may pull these technical arguments together:-

With the PWM "pulsed" supply from the ESC to the motor there is no "commonly understood" voltage. The pulses will be at battery volts, but the pulses could be so short they have no noticeable effect on moving the motor (arming tones are an off topic example).

The effective voltage we neeed to consider is surely the rms value measured/calculated over at least one complete on/off cycle. That way any rms reading voltmeter across the ESC output will show a steadily increasing (effective) voltage as the throttle is opened (changing on/off ratio) even though we know the instantaneous voltage at any given time is actually either battery volts or OFF.

(for the purists - yes, the volts/pulses will be modified by circuit inductance and capacitance).

Dick

Phil Green14/08/2020 11:14:44
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1639 forum posts
344 photos

If its just one or two puffed cells in a larger pack, its quite easy with care to remove the bad cell and reassemble the pack, shuffling the balance wires. Obviously you end up with a smaller cellcount pack but it avoids wasting the remaining perfectly good cells. This isnt just about a Yorkshireman's pocket - there is only so much lithium on the planet.

Simon Chaddock14/08/2020 11:15:03
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5776 forum posts
3055 photos

Dickw

Ho-Ray! At last.

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