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Amperage Question

Old 12-01-2017, 04:25 PM
  #16  
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Originally Posted by Skiddins View Post
Are you sure about that?
Yes.
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Old 12-01-2017, 05:20 PM
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The last time I did a data capture on a 100X48 carpet track with a 17.5, max amp spike was over 100a and average amp draw for the run was 22a.

Art
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Old 12-01-2017, 05:28 PM
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Originally Posted by Skiddins View Post
Are you sure about that?
That's over 1.1KW at 8V

21.5's tend to be rated at a couple of hundred Watts, the 25-30A (whilst accelerating etc) sounds far more reasonable.

I can wheelspin a TC off the line with a 13.5
My F1 (21.5) just pulls away without any drama even if the throttle is buried (Taking 1200mAh out during a run equates to an average of 14A)
Stall currents tend to be very high but only last for a moment. The total amount of energy used during that current spike isn't very high.
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Old 12-07-2017, 11:10 AM
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Originally Posted by howardcano View Post
The latest ROAR-legal 21.5T motors will draw over 140 amperes peak from a standing start if the battery can handle it.
That may happen for a few milliseconds from a dead stop but a 175 gram motor would not last long, or be at all efficient, if that spike was for any length of time. If you run a modern BL motor at any more then like 2-3 watts per gram all you are really doing is making heat.

It is interesting looking at that Reedy chart. I personally do not buy the efficiency numbers (extremely optimistic) but note how once you start getting into the 400 watt range the efficiency starts dropping off.....Hmmm, maybe there is something to that 2-3 watts per gram.

I fly some moderately high performance (by today's standards) gliders with 1000+ watts on 3S and all those motors are in the 300+ gram range, after a 10 second run they are producing some serious heat. Modern high performance gliders are now in the 2-3Kw range but are making that power using higher cell counts. What I have found after flying e-powered planes for the last 25 years is that higher voltage and lower amps is more gentle on the electrical components but with that said there is only so far you can go because a watt is a watt.

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Old 12-07-2017, 01:19 PM
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Originally Posted by Skiddins View Post
Are you sure about that?
That's over 1.1KW at 8V

21.5's tend to be rated at a couple of hundred Watts, the 25-30A (whilst accelerating etc) sounds far more reasonable.

I can wheelspin a TC off the line with a 13.5
My F1 (21.5) just pulls away without any drama even if the throttle is buried (Taking 1200mAh out during a run equates to an average of 14A)
This is where things get tricky. Instantaneous power can be some really freaky numbers. But as rotor speed starts to climb, the inductance of the coils in the motor start to produce a whole lot of back EMF. Reducing the effective voltage at the coils, and in the end, limiting motor speed.

This isn't so bad, as the case, windings, and core that the wires are wound on all have quite a bit of mass, and can take some really freaky high currents for a second, or a few seconds at a time.

The stronger the powerplant, the less time you're going to spend with it at full throttle. So you can have even higher surge currents.

Back in the day, when I was running 540 brushed motors, I could tell if I had the current limiting on my ESC set to less than 100A. On a 1500mah battery. With a 27 turn single. That means peak currents were over 100amps... Not for long mind you, but they were up there.
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Old 03-20-2018, 10:04 AM
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Testing no load current draw is virtually worthless info. I can tell you if your pulling 1.3 amps no load, full battery, full throttle you have room to raise the timing of the motor.
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Old 03-20-2018, 10:51 AM
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Interesting just how much current these little cars can actually draw!! I remember MANY years ago racing for Novak--and he was first developing speed controls! We were ASTOUNDED to find--- to protect the early esc's--he had a 140 amp fuse in the line. We were popping those fuse's from a dead start in 1/12 scale cars on sub-c nicads!!! OVER 140 amps being pulsed from a dead stop!!! It finally stopped when we went up to 160amp fuses!!
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Old 03-20-2018, 10:57 AM
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Originally Posted by Tim Neja View Post
Interesting just how much current these little cars can actually draw!! I remember MANY years ago racing for Novak--and he was first developing speed controls! We were ASTOUNDED to find--- to protect the early esc's--he had a 140 amp fuse in the line. We were popping those fuse's from a dead start in 1/12 scale cars on sub-c nicads!!! OVER 140 amps being pulsed from a dead stop!!! It finally stopped when we went up to 160amp fuses!!
Actually, nickel based battery chemistry can supply higher instantaneous load than lithiums, not to mention the current surge as brushes hop on comms, interesting stuff
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