jimroad471

All you electronics guys can tell us the use of capacitors. I think capacitors act as regulators protecting motor from spikes and minimize glitching.

MacDuff

My Feedback: (2)

i dont think i solder on the capacitors on my motor correctly, when i drive about 35ft away, my motor starts glitching...

darthballs

are you refering to the large canister looking ones that go on the esc or the ones on the motor. the ones on the motor reduce rf noise reducing glitching. the ones on the esc smooth out the operation and aid in cooling.

Bhouse3000

yeah, the ones that mount onto esc, are mainly for touring, in which you do alot of breaking and hard accel. as when you do hard accel from a brake, the esc usually doesnt put out the 7.2 or 8.4v to the motor, as the voltage drops for a little bit, but with a capacitor direct to the battery wires/connectors(end that atatches to the esc) will keep a little charge evertime you let off the throddle, or brake, so when you come out of a corner, it give a little boost, so you have 7.2 or 8.4v directly to the motor, so there is no voltage drop, thuse causing a faster exit out of a turn, but like i said, its mainly for touring, as it isnt very noticable in offroad, or drifting
hope this helps

Access

Modern ESCs use PWM (pulse-wave modulation) which means that the ESC varies the duty-cycle of a constant-freqency waveform.
Adding to this is the fact that the motor coils have some inductance.
So capacitors, typically any number of 0.1uF ceramics, are necessary with brushed motors in order to minimize RF interference.

Otherwise, those larger can-type capacitors that people sometimes attach to ESCs, those can help 'prop up' the battery voltage (a little more punch) when the modulation duty cycle is increased rapidly. But a well-designed ESC will have the capacitors on the board itself already, hanging capacitors off wires is never going to help as much as soldering it directly to board itself.

jimroad471

Darth, I'm talking of the one wired to esc. Also, if cap is removed, are the two wires supposed to be shorted or removed?

darthballs

if you're taking the cap off then take the wires as well. the gtx in my opinion is very well designed and it has an external cap. there is simply not room inside as the controller is about the size of the cap.

Access

Not true, if it's a modern ESC and a good designer, there are tantalum surface-mount caps that come in just about any shape / size, or surface-mount electrolytics, for lowest ESR you are probably putting a 3-4 caps in parallel anyways. The ESR of a combination (ie. 3x1000uF) is going to be around .033 ohms, plus nominal board traces. Compare this to hanging a single larger capacitor (ie. 3300uF) off a wire, the ESR is going to be around .07 ohms for the capacitor, .1 ohms for the two wires, almost .2 ohms. Quite useless in that for every 10A, the cap is essentially losing 1.7 volts, compared to .3 volts in the former case; and in a modern 1/10th scale you are probably looking at 20-30A current draw when accelerating full from a stop. Sure it's better than nothing, it might get you an extra amp or two for a small fraction of a second, but it's nowhere near the advantage of putting the capacitor(s) on the board themselves.

jimroad471

I need to be clear. I think those are DIODES on the motor ( little buttons with long legs). The CAPACITOR is the big thing hangin' from esc and has a different funtion. I could be wrong. Anyway, I was asking Darth if capacitor is removed, do the two wires need to be connected for continuity or removed completely. We will have another discussion on motor diodes which are a must. Novak has a diode installation guide as well.

jimroad471

Access, GTB Esc's have little capacitors inside and on the board already? I've noticed Mamba has a series of caps outside and adjacent to the esc.

Access

No they're probably ceramic caps (mentioned in former post). If they're thin, and no larger in area than the nail on your pinky, that is. Diodes allow current to pass only one way, in years past they were used on some of the older forward-only ESCs. But generally not any more. A 50A shottky diode is going to be rather large, and most likely it would come in a TO-220 package or such. Small value ceramic caps are a given, practically required when using standard RF frequencies and brushed motors.

If the capacitor was to be removed, definately DO NOT connect the two wires, otherwise you'll almost certainly create a short. Caps work only when there is a voltage difference between the two leads, so you can be almost gauranteed if there's a cap between the two wires, under normal operating conditions there's going to be a voltage difference.

michelob78

I have one on a T4, I don't think it helped much at all. LRP recommends one if you read their manual only reason it's on there. Sooner or later I am probably gonna take it off.

Access

Yeah you might see a more noticeable effect by soldering the leads of the electrolytic capacitor directly to the ESC circuit board where the battery power is coming in (where the wires coming out of the deans connector meets the ESC) . But it looks like it could be difficult to place it then.

michelob78

It is soldered to the circuit board, there is black shrink tubing on the leads and they are bent to be as far away from each other as possible. The cap is the novak one with their decal removed and an LRP one in place.

jimroad471

Yes, that is very tight quarters to solder. So your'e telling me to keep the capacitor and remove the two wires. Yes?

Access

You'd have to re-orient the ESC then, shorten the leads, don't know if it's really worth it. For what it costs the sphere is / had better be a well-designed ESC so maybe it has enough capacitance built-in. But the general recommendation in app notes and things like that is 5mm, half a centimeter or less for trace lengths, if it's longer than that it's probably not doing much good at all. And it adds weight / raises the CG, a little bit.

jimroad471

Access,
I emailed Novak with my questions. You've answered the ESC power cap function correctly and clearly as Novak would. I am told now that caps inside the motor can be removed. I really thought any diodes or caps inside the motor are there to eliminate rf interference created by high rpm metal and electronics. I truly don't know electronics worth beans but I solder pretty good (lol)
Jim

michelob78

I wouldn't go removing any capacitors on the motor, stick with the manufacturers suggestions on the motor.

hands without shadows

That cap is huge!! But dont like all modern escs have caps inside of them, offroad and onroad alike? Anyways, ditch the bullet connectors man, there draggin you down. And the things on the motor are caps, not diodes, acces had to teach me too!!!

michelob78

The bullet connectors are gone this pic is several months old.........

zaphod42

My Feedback: (3)

These days most new ESCs don't have large power caps inside for a good reason. Whether they are in a surface mount package or not it still takes about the same volume of package to get the same capacitance. Tantalum capacitors aren't very good for high ripple current applications (they get pretty hot when you demand high current pulses out of them) and even surface mount electrolytics have their drawbacks. The main concern is that they too get hot and they transfer that heat to the circuit board they are on. Using an external capacitor gets that heat out of the ESC package. Even if you don't notice a difference in the performance of the ESC with the addition of an external capacitor you have still dropped the instantaneous peak current demanded from the ESC at each PWM (Pulse Width Modulation) turn on by more than ten times which will make the ESC run cooler and extend it's life.

Access suggested earlier in this post that an external capacitor had far higher resistances and was therefore ineffective but there were some inaccurate assumptions used in the rationale. Sorry to be picky but I wanted to get the facts out here.

First it was suggested that you could use 3 1000 uF surface mount caps in parallel in the ESC. The highest SMT aluminum electrolytic capacitor value that is available 'off the shelf' is 470uF for a 16V rating and they are slightly more than 3/8" in diameter and almost 1/2" tall so you have increased the size of the ESC by about 1.5" surface area. Also the best ripple current rating you can get in an SMT package is about 670 mA and they cost about $0.30 each if you buy them by the reel. By comparison you can get an external 1500uF capacitor (1/2" diameter X 1" long) with a 2.2A ripple current rating for $0.33 each in the same purchasing volume. Also, the ESR (Effective Series Resistance) of the 470u SMT caps is 0.15 ohms (3 in parallel is therefore 0.05 ohms) where the external 1500u is at 0.03 ohms for about 1/3 of the cost .

The resistance of the wire used in the example was 0.1 ohms for the wires. Assuming the wires are a reasonable length (I will use 2" leads as a reference (so 4" overall wire length to and from the cap)) the actual resistance even if you use 18 gauge wires is actually 0.0021 ohms. This is straight from the AWG tables (18 AWG wire has 6.385 ohms of resistance per 1000 feet). (To get the 0.1 ohms suggested for a 4" wire run you'd have to use 35 AWG wire.) This makes the resistance of the path 0.0321 ohms counting the wires and the capacitor. At a 10 amp draw this means a voltage loss of 0.321 volts - better than the 0.5 volt loss for the SMT caps with the added plus of a higher ripple current rating. Traces on the PC board are not going to be enough of an improvement from 2.1 milliOhms of the wire to offset the difference in ESR and with the combination of radiated and conducted heat inside the ESC from the capacitors it makes it a bad tradeoff.

It was also suggested that soldering the capacitor leads directly to the ESC could help. Since the capacitor leads actually have a smaller cross-sectional area than the wire you would likely use to hook them up you will drop more voltage across the leads than across the wires (assuming your solder joints are good) so it's actually better to use the wires. In either case the actual lead or wire resistance is less than 10% of the loop loss and is in effect swamped out by the internal resistance of the capacitor. Given 18 AWG wires the actual voltage drop across the wires is only 0.021 volts compared to 0.3 volts dropped across the cap.

That is also sort of a mistaken impression. When the voltage on the ESC output is above the voltage in the capacitor the capacitor is being charged. When the voltage drops below the level in the capacitor the current flows out of the capacitor in an effort to oppose the change in voltage. The ESR specification is, then, really a rating of the impedance of the current source the capacitor represents and therefore the maximum current that it can source. Using the 1500u capacitor above as an example again since it has an ESR of 0.03 ohms then using Ohm's Law (Current = Voltage / Resistance) and using a standard 6 cell battery voltage (7.2V) as an example it can source 240 amps on an instantaneous basis if it drives into a 0 ohm load (in other words a dead short). Now you can see why a capacitor can get hot when it is filtering a PWM output. That is why you see a ripple current rating - no capacitor is going to survive a 240 amp ripple current so the cap I used as an example is only really good for a ripple curent equal to about 1% of it's total capacity on an average basis before it overheats and blows up and it's one of the better ones.

Access

Here is the link.
http://rocky.digikey.com/scripts/Pro...100ADA102MJA0G
They have surface mounts up to 6800uF, they may not be the best for this particular application.
another one by panasonic with 800mA rated ripple current.
http://rocky.digikey.com/scripts/Pro...M=EEV-TG1A102Q
And the only reason to use SMT is assembly and sometimes the shapes SMT parts come in can be easier to fit inside of an enclosure. They could use a non-SMT one too if it makes sense.

As for the rest of the post, zaphod do you have a lot of formal schooling in this type of electronics? I'm only basing my conclusions on app notes by the manufacturers and what I've learned from other people with my job, I'm not an electrical engineer myself. It's been 10yrs+ since I took any kind of formal analog or power electronics class. So you're probably right, or at least more up-to-date in your knowledge than I am. It's just that all the app notes I have read say 'keep the board traces wide (as wide as possible) and if at all possible, mount the cap within 5mm of the ...'

zaphod42

My Feedback: (3)

Access

I didn't set out to 'dis' you - I hadn't seen these particular parts before but I stand by my statements. By the way, I do have a formal education in electronics - I am a junior engineer for a company that makes high horsepower brushless motors and controllers (up to 150 horsepower). I am currently laying out a PCB for a high energy DC-DC converter (more than 50A at over 300 volts) so I know quite a bit about trace widths. It is true that you want to keep trace widths as high as possible (less voltage drop means less heat buildup in the board). Normal design procedures call for decoupling capacitors (usually 0.1 or 0.01 ufD) as close as possible to the device using the current but it is common practice to put the big filter capacitors off in the corner of the design and away from any signal electronics to keep down EMI noise problems.

The caps in these links are quite pricey (three times the cost of the ones I mentioned earlier) but the real problem lies with their intended usage. These caps (and the SMT ones I referred to) have their ripple current rated at 120 hz - they are intended for filtering a full wave bridge for a 60 hz line transformer. The ripple current capacity falls off rapidly as you rise in frequency because the transitions in voltage are much faster so the current either sinked or sourced is far higher in a shorter period of time. Add to that the fact that a PWM application is in effect a square wave output (a straight switch on / switch off relationship which causes huge transitional currents compared to the relatively low slope of a sine wave) so therefore the ripple current spec falls off very quickly and the heat buildup goes up exponentially as frequency rises. The 1500 uF caps I referred to are rated at 100khZ and are intended and designed for switching applications and can take the abuse that PWM filtering dishes out.

The other problem with SMT caps is that you have all of that weight suspended on two small surface mount pads. Given the impacts that an RC car endures you have a good chance of ripping the pads off the board with an impact rendering the ESC useless (if you don't short it internally and burn the battery down with it). There is also a large amount of both radiated and conducted electrical noise created by the caps and having them in close proximity to your control circuitry is asking for trouble aside from adding another heat source inside of a tiny little sealed box.

Access

No don't worry about that, it's more important to get the right information out there, that's what these forums are for. For board layouts I've only done a few high-frequency RF switches and a single-chip DC-DC convertor (5V, 4A), and certainly you seem to know more about things like ripple currents than I do. Is your board going into an electric vehicle or some kind of heavy machinery?

zaphod42

My Feedback: (3)

The particular board I am working on is more for power grid level conversion of energy. It can be used for either converting high voltage AC (pre transformer power line stuff) to mid level DC (200 - 450 VDC at 50 or so amps) or for converting high energy DC (say from a wind farm or solar panel array) to AC that can be fed onto the grid for distribution. (I didn't design this particular system - that was done by one of my colleagues who was with NREL (the National Renewable Energy Laboratory) before they downsized - I am just doing the layout.)

We also do electric vehicles - my other current main project is an electric pickup (380V Lithium Ion battery pack with a 120 hp motor - it should be wheels on the ground in the next couple of weeks). We have also done electric bus systems for a couple of cities so they can have non-polluting vehicles for walking mall shuttles and such. It's actually kind of a cool place to work