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Old Sep 20, 2009, 03:09 PM
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Powell River BC Canada
Joined Jul 2009
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Questions twin 480 set up

Well for starters anyways.

I have HET fans (Early one 6904?), 2w het motors, twin Jeti 70 Master-o-flights and evo batteries. 3700 3S1P and 4S1P. I need to recharge the 3S for meaningfull test. The 4S is pretty charged. 88 amps ish. on a quick power up. What worried me was with the whatt meter hooked up it sounded like one fan was ossilating or hunting in speed. Without the meter on it sounded better. I'm woundering if the Y-cable I made needs to be larger diameter. I think the one I made is 18 guage?? and about 4-5 inches. Without marking on the wire whats the quickest way to measure the guage. Or what diameter wire would you suggest I go out and buy?

A short burst yielded about 88 amps on the 4S. And 57 amps on the 3S.
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Old Sep 20, 2009, 06:53 PM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
12,377 Posts
18 ga is too small. 14ga minimum for those amps, 12ga better. What you were expriencing is a voltage drop. One ESC will slave the other causing erratic or the occilation you were experiencing. A 4s pack is too much for the 2W mtr. This is a 3s mtr system. About 450W per fan on 3s. Unless you have a real good pack, 3700 may not work. Consider 2000Mah per fan, or 4000 Mah 3s pack for both fans.
If you still have erratic operation, on the 3s pack, you may retard the motor timing in the ESC, to lower the amp draw.

Fuzz
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Old Sep 20, 2009, 08:20 PM
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Powell River BC Canada
Joined Jul 2009
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THanks Fuzz

I had hoped that was all that was going on. I know the 2W motor is supposed to be a 3S motor. But thought I would just refrain from WOT and have the extra volts there for an emergency. And maybe I'll go with the 3S pack anyways its lighter And I don't think I'll need WOT anyways to do some sedate scale like flying.

I'll go raid my friends parts box's for some cable.
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Old Sep 20, 2009, 11:44 PM
Big gov never Works
St. Martin's Avatar
Skunk Water, Rhode Island
Joined Jul 2002
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Quote:
Originally Posted by oilman2
THanks Fuzz

I had hoped that was all that was going on. I know the 2W motor is supposed to be a 3S motor. But thought I would just refrain from WOT and have the extra volts there for an emergency. .
Your welcome Brushless systems are not like brushed, in this sense. At partial thtl settings, the ESC/mtr still sees the full voltage. Voltage does not control the speed. It is the frequency of the switching. So..the by product is waste heat. Brushless ESC's always run hotter at partial.


Fuzz
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Old Sep 21, 2009, 01:29 AM
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Powell River BC Canada
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Hey Fuzz.

Oh I was not aware the motor saw the voltage constantly too. Hmmm thats a problem.
I never seem to get the kind of whatts others report.

Oh this fan is not a 6904 I guess. Its the three blade HET that wasn't produced for long.

My whatt meter reading was around 88 amps on 4s with two motors running. Can I divide that in half evenly for actual fan draw?

I'll charge up that 3S tomorrow and try running one fan to see what the whatts are. The label on the battery claims 92 amps constant. I would have thought I had enough battery. I know when Marcos did his tests he saw a lot of whatts on 4S. But that was on a Wemotec fan. I just looked back at a test I ran last year when the batteries were new. And I got 680 whatts peak right off the start on a short run with the 4S, HET fan. Still a lot more than what the 2W is ment for I guess. Well I`ll get that 3S charged up tomorrow. I guess 900 whatts in a 1800 gram (4lb) plane is enough for me anyways.
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Old Sep 21, 2009, 01:55 AM
Big gov never Works
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Skunk Water, Rhode Island
Joined Jul 2002
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Something doesn't add up here. 88A is about correct for a single HET 2W on 4s in a 70mm fan. I know I tried it w/minifan 505 @ 66mm. But for fans running that would be half each. Unless one is drawing more than the other. This is possible. I bought two 3W mtrs last week. One drew 37A, the other 30A, 425 and 525 watts respectively.

Run each fan seperately using the same ESC on 3s. Checking that they are the same values. Then the same test on one fan, switching controllers. All values should remain very close.

Fuzz
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Old Sep 21, 2009, 07:23 AM
Effumall
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HUH??? 88 amps with a single 2W is instant smoke.

The 2W should run on 3S only, and no more than 50 amps. Therefore 88 amps IS appropriate for two 2Ws.

Quote:
Originally Posted by St. Martin
Something doesn't add up here. 88A is about correct for a single HET 2W on 4s in a 70mm fan. I know I tried it w/minifan 505 @ 66mm. But for fans running that would be half each. Unless one is drawing more than the other. This is possible. I bought two 3W mtrs last week. One drew 37A, the other 30A, 425 and 525 watts respectively.

Run each fan seperately using the same ESC on 3s. Checking that they are the same values. Then the same test on one fan, switching controllers. All values should remain very close.

Fuzz
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Old Sep 21, 2009, 10:10 AM
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Powell River BC Canada
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I don't think Fuzz actualy said it would actualy run at 88 amps.

Wow 7 amps differance on the same motor type.

Maybe my controler timing is not on high.....
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Old Sep 21, 2009, 03:15 PM
Big gov never Works
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Skunk Water, Rhode Island
Joined Jul 2002
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Quote:
Originally Posted by kelberts
HUH??? 88 amps with a single 2W is instant smoke.

The 2W should run on 3S only, and no more than 50 amps. Therefore 88 amps IS appropriate for two 2Ws.
I agree...read post #2 please. I was drawing 80+A in a 505! But only to try and see. No smoke tho..but would have come eventually. And 88A would be approx correct for two 70mm fans on 3s, using the 2W.

Fuzz
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Old Sep 21, 2009, 07:59 PM
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Powell River BC Canada
Joined Jul 2009
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Ya I think these 3700's are a little stretched for this job.

Fresh off the charger the 3S poacks got me 410 whatts 44 amps. And it was winding down basicaly like a normal count backwards speed.

When I get the new y cables made I'll try some more tests.

My 88 amp reading was with the 4S pack!! Between the choker effect of the light y cable and voltage sag I guess. New Y cable might bring that up.

At the moment I'm thinking... One 3700 4S pack running the two motors, taking voltage sag into effect. Maybe a slower timg rate. Maybe I can get away with it?? I was hoping to take this plane with me next weekend on a trip. Not really any time left to get new batteries... Or maybe this plane will have to wait till next holiday.
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Old Sep 22, 2009, 04:57 PM
Big gov never Works
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Skunk Water, Rhode Island
Joined Jul 2002
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I would not use the 4s. Go with what the mtr system requires. Your going to need a 4000Mah 3s minimum, for decent power and flight duration. Also, do a stic run, to see how much time you have, before one ESC slaves the other. Keep this in mind during flight. Or, you will end up with a jet turning into the weaker side.

My prefered twin set-up, is the HET 3W. With a 4s pack it draws only 60A on two fans. Smaller packs can be used. And only 50W(25W each mtr) differance. But at the lighter weight, you actually have a higher watts per pound ratio. Not to mention greater efficiency with the higher V/lower amp system= less wasted heat.

Don't force yourself to use less than what is required for dependable flight. Wait!

Fuzz
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Old Sep 23, 2009, 03:38 AM
Julian T
Fleet, UK
Joined Jun 2005
531 Posts
I have experience of only one twin - an RBC Raptor. I am using Mega 16/15/2 motors in 6904 fans on 3s 3850 - 5000 capacity packs with completely separate ESC and battery for each motor. This works really well but the Raptor has plenty of space for the batteries. I have seen problems running 2 motor/escs off one battery where one side starts 'hunting', presumably because the escs, although identical, have slightly different low voltage cut offs.
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Old Sep 23, 2009, 10:26 AM
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Powell River BC Canada
Joined Jul 2009
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Hey thanks for the advice guys. I am putting thicker wires on some battery packs that I originaly installed in this plane and it flew well enough on them. 4x1550 3S packs. Two per motor. The cables that came on the batteries are very thin 20 or 18 gauge. I guess a cost savings measure. I'll see tonight what difference 14 gauge makes. Maybe I can fly on these if the batteries are still any good.
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