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Question about ESC, Over amping and part throttle.

Old 12-14-2008, 05:57 PM
  #1  
Figure.N9ne
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Default Question about ESC, Over amping and part throttle.

this is just to satisfy some curiosity i have.

lets say this is a setup

plane makes 50 amps full throttle
flies fine at under half throttle, full throttle is never used
40 amp esc

i've read that the esc still see's the 50 amps momentarily since it pulses the throttle to maintain the lower throttle settings. but isnt the 50 amp full throttle draw a combination of prop size and rpm. wouldnt the motor have to be at full rpm for the prop to have the resistance that causes the 50 amp draw? so the motor at half throttle isnt seeing full rpm so the resistance is lower, why would it still see 50 amps in pulses?
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Old 12-14-2008, 07:46 PM
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slipstick
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No. Resistance has almost nothing to do with it I'm afraid....motors do not act like resistors.

The point is that part throttle consists of pulses of full voltage/current but switched rapidly on and off. When you are reading 25A current from the battery (half throttle) the ESC is actually switching 50A but it's only ON for only half the time. The AVERAGE current is 25A but the output devices in the ESC are still having to switch 50A.

That's simply the way an ESC works.

Steve
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Old 12-14-2008, 07:49 PM
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Originally Posted by slipstick View Post
No. Resistance has almost nothing to do with it I'm afraid....motors do not act like resistors.

The point is that part throttle consists of pulses of full voltage/current but switched rapidly on and off. When you are reading 25A current from the battery (half throttle) the ESC is actually switching 50A but it's only ON for only half the time. The AVERAGE current is 25A but the output devices in the ESC are still having to switch 50A.

That's simply the way an ESC works.

Steve
if resistance doesnt have much to do with it then why does switching 1 inch of prop vary the amp draw by 10,15, 20 amps in some cases?
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Old 12-14-2008, 07:55 PM
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phupper
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I've never seen my ammeter spike to full current on startup. On a 25A motor the the needle sweeps smoothly from 0 to 25. A meter like mine may not detect the short pulses you're speaking of, need a scope for that. But I think the motor's amp rating is based on constant current, and in that case you'd be ok at half throttle.

Someone will chime in with a definite answer for you.
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Old 12-14-2008, 08:20 PM
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Dr Kiwi
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Originally Posted by Yaniel View Post
if resistance doesnt have much to do with it then why does switching 1 inch of prop vary the amp draw by 10,15, 20 amps in some cases?
That's because of increased load on the motor... essentially nothing to do with the ESC, which simply acts as a conduit to let through whatever current the motor demands.
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Old 12-14-2008, 11:06 PM
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Figure.N9ne
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so wouldnt the motor at part throttle have less load and in turn be demanding less current from the esc? i'm just trying to get a grasp on this concept, thanks.
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Old 12-14-2008, 11:57 PM
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slipstick
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Originally Posted by Yaniel View Post
so wouldnt the motor at part throttle have less load and in turn be demanding less current from the esc? i'm just trying to get a grasp on this concept, thanks.
I'm having trouble understanding what you can't grasp. It's pretty simple.

When the load is less the AVERAGE current is lower. But even though the AVERAGE current is lower the PEAK current is still the same. The average is lower simply because the peak current is on for a shorter proportion of the time. The ESC switches usually several thousand times a second. To get 25A average you have 50A on for half the time. To get 10A you have 50A on for one fifth of the time. But the ESC is always switching the full 50A and that instantaneous current can damage it.

Steve
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Old 12-15-2008, 12:08 AM
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Figure.N9ne
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Originally Posted by slipstick View Post
I'm having trouble understanding what you can't grasp. It's pretty simple.

When the load is less the AVERAGE current is lower. But even though the AVERAGE current is lower the PEAK current is still the same. The average is lower simply because the peak current is on for a shorter proportion of the time. The ESC switches usually several thousand times a second. To get 25A average you have 50A on for half the time. To get 10A you have 50A on for one fifth of the time. But the ESC is always switching the full 50A and that instantaneous current can damage it.

Steve
i think you may have misunderstood me earlier when i talked about resistance. i didnt mean electrical resistance, but the actual resistance caused by the drag on the prop spinning through the air. what i dont understand is, why does a smaller prop draw less current than a larger prop with the same pitch? i'd assume its because its harder to turn the prop through through the air, and as the rpm get higher would it become harder to turn the prop? what i keep thinking is that a larger prop at lower rpm (by using less throttle) would need less current than the same prop at higher rpm because there would be less drag and less effort needed to achieve the rpm. why would it pulse the full throttle amperage if the drag from the higher rpm isnt there?
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Old 12-15-2008, 10:36 AM
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slipstick
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Once more with feeling .

The power affected by prop size is AVERAGE power (electric motors work that way). Switched current is always PEAK current for variable proportion of the time (because that's how ESCs work).

I give up...anyone else want to try ?

Steve
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Old 12-15-2008, 10:48 AM
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I think I get it. I'll give it a try.

You need a 50A ESC.

How was that?
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Old 12-15-2008, 11:16 AM
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phupper, that was pretty straightforward, elegantly simple and direct to the point!
Well done!
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Old 12-15-2008, 03:29 PM
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Originally Posted by slipstick View Post
Once more with feeling .

The power affected by prop size is AVERAGE power (electric motors work that way). Switched current is always PEAK current for variable proportion of the time (because that's how ESCs work).

I give up...anyone else want to try ?

Steve
well thanks for trying, i'll figure it out on my own.
Originally Posted by phupper View Post
I think I get it. I'll give it a try.

You need a 50A ESC.

How was that?
gee thanks.. i didnt know that ... the point of this thread was so i could figure out WHY, i prefer to know how things work, not just do things blindly.
Originally Posted by Dorsal View Post
phupper, that was pretty straightforward, elegantly simple and direct to the point!
Well done!
at least someone thinks so.
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Old 12-15-2008, 03:38 PM
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Originally Posted by Yaniel View Post
i think you may have misunderstood me earlier when i talked about resistance. i didnt mean electrical resistance, but the actual resistance caused by the drag on the prop spinning through the air. what i dont understand is, why does a smaller prop draw less current than a larger prop with the same pitch? i'd assume its because its harder to turn the prop through through the air, and as the rpm get higher would it become harder to turn the prop? what i keep thinking is that a larger prop at lower rpm (by using less throttle) would need less current than the same prop at higher rpm because there would be less drag and less effort needed to achieve the rpm. why would it pulse the full throttle amperage if the drag from the higher rpm isnt there?
When you spin the prop at a lower RPM by using less throttle, you are still using the same BURST or PEAK current, but it is being pulsed fewer times per second than when you are running a higher RPM with more throttle. The ESC pulses those 50A bursts many times per second. The more bursts, the faster the motor will turn the prop.

A smaller prop draws less current because the motor isn't "working as hard" as it would with a larger prop. The motor at any given setting will try to spin at a given rpm; hence the kV rating. Of course, some of the RPMs are lost to inefficiency. But the motor is always trying to spin at that given RPM. With a larger prop, it takes more amps to power the motor to spin at that same given RPM.

Hopefully this helps a little.
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Old 12-20-2008, 07:44 PM
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Originally Posted by Dr Kiwi View Post
That's because of increased load on the motor... essentially nothing to do with the ESC, which simply acts as a conduit to let through whatever current the motor demands.
You know Doc I actually had electronics profs that didn't visualize the water ananogy. I don't know why it isn't taught more.

Last edited by Bill G; 12-21-2008 at 07:48 AM.
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