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Old 01-12-2007, 12:34 PM   #1
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Default Nerdy Question On Exact Function Of Power Cap's?

OK, so I know that the power cap is supposed to help the efficiency of speed controllers, what I want to know is how?

This is nerdy I know but I work in the Computer/electronics industry but a couple of us were trying to understand exactly how it effects the voltages etc.

i.e. Does it at like a power reserve to simply add a fractional voltage when the motor causes the voltage on the battery pack to drop?

Or possibly helps to turn the inductive load of the motor into a more 'resistive' load to reduce strain on the speedo, a bit like Power Factor Correction on computer PSU's?

Anyone with good electronics knowledge able to answer?

Thanks
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Old 01-12-2007, 12:55 PM   #2
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I think we all want the true 100% fact on that. I'm under the impression it build's a surge to give more kick coming out of the turn's. And the way it keep's speed control's cooler is my wonder. If its compiling the negitive feedback to release a positive boost on throttle, once the boost is gone then I would be concerned that it would act as a regulater to the out put of the motor. Ok I'm crazy now, I can see a super tech coming and he'll probably drain us with a bunch of high tech intellect and I'll have 2 crossed eye's instead of 1.
My insinct would say bigger capacitor if the straight's are longer, And I would guess the bigger would'nt hurt in the short straight. It would probably be more efficiant to its worth. I sure hope they keep it leighman.
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Old 01-12-2007, 01:18 PM   #3
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The capacitor stores charge. The voltage of the charge will be no greater than the maximum charging voltage it was subjected to from the battery. It holds this charge in reserve and it flows back into the circuitry of the esc when voltage drops. So it acts like a buffer, to prevent transient drops in voltage. If voltage drops enough, the receiver, servo and esc will not function properly. Some say it actually adds punch to the motor. I'm skeptical. Can't the batteries that supply 30 or 35 amps down the straight do a better job of that than a 4400uf capacitor? I could understand that if the load was beyond the battery capability of miliseconds, but if you run at full throttle down the straight and the battery can handle that, what good is a few milliseconds of charge held by a capacitor?

It should not be subject to the regenerative power of the electric motor. The schottky diode serves that function, and acts like a short to keep that voltage from reaching the esc. That part of the circuit is driven by the esc which produces pulses of DC to control speed. You don't want a large cap across that part of the circuit. It would change part throttle from a square wave into semifiltered dc at a lower voltage.
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Old 01-12-2007, 01:23 PM   #4
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The power cap really doesn't have much to do with the motor. Most of the caps won't turn a motor over more than one revolution or so.

The reason for the power caps is to allow the FET's to turn on more quickly. This happens very quickly considering the drive frequency of the speed control so the cap also needs to be designed specifically for high frequency.

A FET is only efficient when it's completely "on" or "off" and anything in between isn't very efficient causing heat and stress on the FET's. Using a cap will allow the FET's to turn completely on more quickly/efficiently.
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Old 01-12-2007, 01:43 PM   #5
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The shottkey (sp?) diode is used to block the current generated by the motor while coasting from getting back to the esc and damaging it. I was under the impression that the cap stored that energy rather than blocking it. Then used later when amp draw was high like that first punch coming out of the corners.

I like Unregistered's answer better.
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Old 01-12-2007, 01:53 PM   #6
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How do you determine what size cap you need then? In the case of a brushless esc, I assume a 6 cell TC w/Novak 3.5 motor would require something different from a 4 cell 12th scale running the 13.5 motor?
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Old 01-12-2007, 05:11 PM   #7
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It all depends who you beleive.

Some would venture to say the power cap is there all for "ripple current".

When an ESC switches on and off, the power wants to keep going when the switch is off. This creates heat and slowly hurts performance. The power cap catches the ripple current when the mosfets are off, then gives it back when they switch on.

In turn, you get more voltage to the motor that would essentially be lost as heat.

There are "lots" of ways to look at the side effects or how it feels or what happens. I love the internet because you can read ever version twisted one way or other, and in some respects "everyone is right" a little.
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Old 01-12-2007, 05:51 PM   #8
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Ok now I have two cross'ed eye's and my tongue is sticking out.
I thought the center photo statice was charging the rotory osalater, and telling the thingy majig to go forward. And the watcha macall it would stay cool. Gee.
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Old 01-12-2007, 07:03 PM   #9
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Quote:
Originally Posted by Charlie
It all depends who you beleive.

Some would venture to say the power cap is there all for "ripple current".

When an ESC switches on and off, the power wants to keep going when the switch is off. This creates heat and slowly hurts performance. The power cap catches the ripple current when the mosfets are off, then gives it back when they switch on.

In turn, you get more voltage to the motor that would essentially be lost as heat.

There are "lots" of ways to look at the side effects or how it feels or what happens. I love the internet because you can read ever version twisted one way or other, and in some respects "everyone is right" a little.

I spoke with charlie and bob about this subject shortly before I departed the race hotel for the Novak race.

Looking at power supply designs for other applications, I noticed that the function of a cap in an esc is much like an LCR filter. Bob and charlie confirmed this (as charlie posted above). Speed controls output PWM which and the cap makes this occilation less dramatic, and as charlie mentioned can cause the high end of the pwm cycle to actually be at a higher voltage. To see a more dramatic version of this effect look at a cockroft walton charge pump which uses an AC or pwm source with a network of caps and diodes. Basically the pulses stack up in the caps increasing the output voltage.

The whole thing about the cap unloading out of corners isn't really why they make the esc punchier, that would barely crank the motor over for half a rotation.
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Old 01-13-2007, 06:10 AM   #10
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when you apply full gas throltle (or something) a motor is like a short ciquit ... your pack (battery) has a internal resistance and so the voltage drop would be high if there is a bigger current because u = i x r so if i goes up u drop will be higher .. a cap has a virtual resistance of zero so the voltage will be kept higher .. and the oscillation are not seen at the battery side .. for instance hook an oscilloscoop up on the pack and apply throtle .. you won`t see anny pwm cycles or what so ever ....


an lcr filtewr is a completly different as just an c

there`s no resonance freq. in there ..... or at least not just because you`ll ad a cap ..

the bigger is better but it will also be slower again so virtual resistance will be higher ... that`s why some prefer more smaller versions but than the effect to weight ratio is going bad .........
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Old 01-13-2007, 06:49 AM   #11
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I am not too sure about the scale of things, but there are very few circuits you can build that does not have any "intrinsic" inductance , resistance and capacitance. As electrons run in the wires, there will be a magnetic field , which will cause inductance (whether this inductance has any significance i am not sure), the material of the wire will cause resistance, and since we are putting a cap in all of this, it will have capacitance.

I agree with a couple of things said on this thread:
1. the capacitor can release charge quickly, making the PWM oscillation to the motor less tramatic.Possibly allowing a "smoother feel" on the throttle, and adding fractions of voltage to the motor, which is in most cases, insignificant.

2. The capacitor allows the FETs in the ESC to turn on and off faster, so the FETs generate less heat and as a result reliefs strain to the ESC.

I am wondering more about how the cap allows the stacking of PWM waves to create higher output voltage though!
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Old 01-13-2007, 07:14 AM   #12
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So the stayvis davis and the do wah ditty really do keep the Speedo cool.
Now that I can drive standing on my head crossing my eye's and sticking out my tongue. I think I'll leave my capacitor on just for look's.
And please dont ask skiddins how to solder we'll never get out of here.
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Old 01-13-2007, 10:31 AM   #13
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YES THER`S INDUCTANCE BUT ALSo (damn caps) stray capacitance ... but then again res. freq. will far away from everything we reach ...


voltage doublers are commonly none but not allowed ....
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