Puffing packs - edf jet

[MattyB]

Posted by Richard Wills 2 on 13/08/2020 10:57:34:

If you're esc has active freewheel (important otherwise you will get excessive esc temps), you could use a 4s pack and limit the throttle to 75% to get the same power output but at lower current and therefore less stress on the packs

You could, but if he is using the stock ESC it's unlikely to have active freewheeling I would think (especially as this is a relatively old model). If it doesn't have that feature the ESC MOSFETS will be working quite a lot harder doing more switching - this adds additional heat into the equation which obviously isn't good.

Edited By MattyB on 13/08/2020 11:28:53

[Richard Wills 2]

I would have thought investing in an esc with the feature such as one of Hobbykings YEP range which are relatively inexpensive would be much more cost effective rather than constantly killing lipo's.

[PatMc]

Posted by MattyB on 13/08/2020 11:22:05:

Posted by Richard Wills 2 on 13/08/2020 10:57:34:

If you're esc has active freewheel (important otherwise you will get excessive esc temps), you could use a 4s pack and limit the throttle to 75% to get the same power output but at lower current and therefore less stress on the packs

You could, but if he is using the stock ESC it's unlikely to have active freewheeling I would think (especially as this is a relatively old model). If it doesn't have that feature the ESC MOSFETS will be working quite a lot harder doing more switching - this adds additional heat into the equation which obviously isn't good.

Edited By MattyB on 13/08/2020 11:28:53

Limiting the throttle to 75% doesn't necessarily mean the voltage is limited by that amount this would have to be measured either directly or indirectly.
But even if it was achieved the voltage between ESC & motor would be the same as with a 3s battery at full throttle, therefore the current & power would also be same as with a 3s.

[Richard Wills 2]

It will be the same power between the motor and esc that is the point. The voltage between the battery and esc will be higher and the current therefore lower, hence putting less stress on the battery which is the OP's issue. You are correct that a check with a watt meter to finalize the precise throttle limit would be prudent.

[PatMc]

Posted by Richard Wills 2 on 13/08/2020 11:39:21:

It will be the same power between the motor and esc that is the point. The voltage between the battery and esc will be higher and the current therefore lower, hence putting less stress on the battery which is the OP's issue. You are correct that a check with a watt meter to finalize the precise throttle limit would be prudent.

No, the current from the 4s battery will be exactly the same as it would from 3s.

[Richard Wills 2]

Pat, with all due respect you are incorrect. P=VA applies, for the same Power (W), if the volatage is higher from the 4s, then the current (A) has to be lower by the same factor.

[TonyS]

Gents,

Thank you all very much for the advice - what a wonderful forum this is!

It seems clear that I have a number of steps to take to get to the bottom of this - all of which I intend to put into place prior to the use of any of the new higher C packs I've purchased.

1. Put the set-up on a wattmeter. If I use one of the old packs (puffed) will I get an anomalous reading or will it be OK?
2. Re-position the battery - I realise that if I sit it on edge rather than flat there is space down each side to improve airflow.
3. Make the ventilation better in the battery bay
4. Invest in better batteries (done)
5. Maybe (depending upon the outcome of 1 above, limit the throttle
6. Invest in a better ESC with freewheeling (never knew these existed so a big thumbs up on that one)
7. If all of the above fails replace the fan/motor and esc with a set-up that I can tailor to my needs - the current stuff is the cheap generic stuff that came pre-installed

I think that covers everything. I'll let everyone know what the result is.

[Chris Walby]

Hi,

1. Old pack will give you a good idea of the power requirements, then compare to a new lipo!
2. No harm in that as long as the battery can't move about
3. Good idea, plastic spoons cut down to make scoops work well, just remember to let the hot air out
4. Can't beat a good lipo with edf's
5. NO...limit by thumb, normally you need 100% to get it off the deck or it will be a very fast ground squirrel.
6. Up to you, IMHO can't see the benefit
7. Can do, but there are some really nice well sorted EDF models out there that are cheaper than the individual components.

Chris

Edited By Chris Walby on 13/08/2020 12:35:21

[TonyS]

Thanks Chris,

I've never experienced the 'ground squirrel' yet and been flying EDF's for 12 years but there's always a first and it would make a great YouTube video (so much so I'm actually quite tempted!!)

If you know of any decent 'well sorted' EDF's I'd be interested. It's for a bit of fun and to hone my flying skills whilst I finish the Vulcan EDF build.

T

[PatMc]

Posted by Richard Wills 2 on 13/08/2020 11:46:34:

Pat, with all due respect you are incorrect. P=VA applies, for the same Power (W), if the volatage is higher from the 4s, then the current (A) has to be lower by the same factor.

Richard, the Wattmeter only reads Watts at WOT (wide open throttle). Because it's connected between battery & ESC it simply measures the current & voltage at that point then multiplies the two. It makes no allowance for the fact that the ESC can reduce the voltage applied to the motor.
It would need to be capable of 3 phase connection between ESC & motor to be able to measure the power being consumed by the motor. As it is it only measures the current passing through battery, ESC & motor ; and the battery terminal/ESC input voltage. It doesn't measure the ESC output to motor voltage.

For the motor to be turning the fan at the same speed with a 3s at WOT & 4s at 75% WOT the voltage applied to the motor would have to be the same.

[Bob Cotsford]

I thought the drive was PWM with full voltage pulses being applied and the current read being an RMS/average value after a bit of smoothing by the capacitors across the battery side?

[TonyS]

I wish I understood any of that......

[Frank Skilbeck]

Posted by PatMc on 13/08/2020 13:31:44:

Richard, the Wattmeter only reads Watts at WOT (wide open throttle). Because it's connected between battery & ESC it simply measures the current & voltage at that point then multiplies the two. It makes no allowance for the fact that the ESC can reduce the voltage applied to the motor.
It would need to be capable of 3 phase connection between ESC & motor to be able to measure the power being consumed by the motor. As it is it only measures the current passing through battery, ESC & motor ; and the battery terminal/ESC input voltage. It doesn't measure the ESC output to motor voltage.

For the motor to be turning the fan at the same speed with a 3s at WOT & 4s at 75% WOT the voltage applied to the motor would have to be the same.

But the Watt meter is reading the power being delivered by the battery at all throttle settings, so with more feed volts then for the same watts the battery will be supplying less amps. Remember these are not true 3 phase motors and the full battery voltage is supplied, just for shorter pulses as the throttle reduces, with the back emf determining when the coils are energised. In effect the watt meter installed between the battery and ESC will measure the power consumed by the ESC and motor combined, and any power consumed by the ESC, being turned into heat, will be fairly small compared to the motor.

[Richard Wills 2]

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.

[Vecchio Austriaco]

I suppose Tony is confused now with all the expertise. I suppose that he is using the standard equipment as it comes with this plane so it should work as thousands of the same model have been produced. He changed already to a higher C battery. I think next step is to shorten the flight time to see it the battery gets still hot. For my experience batteries get rather hot when you go to their capacity limit. If your transmitter has a timer set it to something short and see what happens after landing. If the instructions say 8 minutes flight time, set it to 4....

[TonyS]

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 2]

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.

[TonyS]

But isn’t a 3s pack supposed to be delivering 11 volts or so, so it’s stretched to deliver 12.2?

[Richard Wills 2]

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

[TonyS]

Thanks Ric. Useful to know.

[PatMc]

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 2]

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.

[Dickw]

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 Green]

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 Chaddock]

Dickw

Ho-Ray! At last.