Capacitor For MM 4S

[Arct1k]

This looks like it will work on a MM on 4S as is 25v... $10 each though :whistle:

http://www.shopatron.com/product/part_number=5674/135.0.56.7400.0.0.0

Any thoughts or alternatives?

[BrianG]

Yeah, but that's a 2700uF cap, so it's bound to be a bit pricey. The ones on the MM are either 220uF or 330uF (can't remember which). You can probably find a better price online at an electrical distributer or something, you just gotta make sure it's a low-ESR type.

[Arct1k]

So brian - one should be enough as will be X4 cap with that installed... Correct...

[ICUBoy]

Hi guys
This one looks good and really cheap ($2.19 is about 2 USD)- do you need two per MM or just one?
http://www.jaycar.com.au/productView.asp?ID=RE6340&CATID=&keywords=low+esr&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

Thanks!

[phildogg]

where are you putting these?
phil

[Arct1k]

Across the power leads close to the MM

[bluonyx]

Across the power leads close to the MM

Kind of like this...

http://i160.photobucket.com/albums/t196/bluonyx/CIMG0353.jpg

[KaztheMinotaur]

What does adding a capacitor like that do?

Would this benefit my Hydra 120?

[bluonyx]

Like it says on the novak site, "dissipate noise and smooth voltage spikes, and will also improve the performance of these speed controls by reducing the operating temperature by 10-15 degrees Fahrenheit."

Noise, doesn't bother me because I'm spektrum

Spikes, OK?

Temperature, is a definite YES! It keeps it at least 10 degrees cooler. I never go above 90-93 on hard bashing day, but that's with a fan also. It used to be 105-110 range before the cap. I think it made a bug difference.

[KaztheMinotaur]

So this would work for any ESC?

Looks like it it just wired in parallel with the battery leads?

[Arct1k]

Should do - I've just bought three of them - 1 for each of my MM that run 4s.

I won't bother on the MM that runs 3S on my crt .5

[bluonyx]

Arct1k, I did the same thing. Anything 3S and under I wouldn't bother with, but I always end up stealing my MM from my other rides, so I installed it on all of them. The extra $10 of security per MM is worth it.

I hate being down, and just can't bear to pay $275 for an ESC when the $100 one will do just fine, especially when I only bash.

[sikeston34m]

I've ran the Novak 2700uf cap on 6S with no issues.

It doesn't get warm or anything.

It produces ALOT of zap also. It's kinda like having 8 of those smaller caps installed, only in a much smaller space. :yes:

[Arct1k]

Errk - Tad worried about the zap but if it helps the MM from self destructing game on!

Come on USPS!!!

[BrianG]

You can stop the spark by initially hooking up the ESC through a charging resistor. Once the caps are charged up, you can then hook the batt directly to the ESC. This would be ideal for any setup regardless of voltage or capacitance rating. That initial spark wears out the connections after a while...

[Arct1k]

Oh god I'm 17 again and thinking about capacitor charge and decay graphs!

RC circuits charge according to 1-e-t/RC

So if I want to reach 63% (1 time cycle) in 3secs on a 2700uf cap

3/2700uf = 1111 ohm resistor correct?

12 secs would get me to mostly charged...

[Arct1k]

....

[BrianG]

Since it takes 5 time constants to get to 100% (or really close). I would pick something like 10 seconds, divide that by 5, and then use the formula to get 740 ohms for your resistor. Anything between 680 and 1k would be good enough since any voltage value on the cap over 0v will help reduce that spark.

[entjoles]

well i have a ?

i installed the novak cap i had and also put some holes in the case, and i also run a fan, and my MM has 18 fets, and it runs on 5s (did a quick test and it had a little cogging , didnt change my settings from 4s - which has no cogging at current settings and gearing)

this is in my xt8 with a 9xl

....?....is it worth really running on 5s for the few less amps that this will draw or stay safe and run it on 4s and not blow it up.....

thanks

[Matthew_Armeni]

Just upping the voltage will not change the amount of amps the motor will draw, a 9XL draws the same whether on 4s or 5s (if I'm not mistaken, I'm pretty sure I'm not) it will just perform better on 5s.
Someone please correct me if I'm wrong about this:oops:

[sikeston34m]

Just upping the voltage will not change the amount of amps the motor will draw, a 9XL draws the same whether on 4s or 5s (if I'm not mistaken, I'm pretty sure I'm not) it will just perform better on 5s.
Someone please correct me if I'm wrong about this:oops:

I would like to take a stab at this one.

Watts=amps X Volts

Let's look at two setups both geared for 40mph top speed.

#1 runs on 4S

#2 runs on 6S

Both cars accelerate at equal rates and in a race would be completelyl equal.

The idea that higher voltage setups draw less amps relates back to Watts = Amps X Volts

Since both cars are equal, both motors are putting out the same total watts of power.

#1 is drawing let's say 60 amps at top speed. 4S lipo will be about 14.8 volts loaded. 60amps X 14.8 volts = 888 Watts of motor power.

#2 being the equal to #1 that it is, will be drawing this kind of energy to produce the 888 Watts of power. 22.2 volts under a load on 6 S
888 Watts = 40 amps X 22.2 volts

What can we conclude from this information?

If you noticed, the word "equal" was used alot in that description. If you take #1 setup and just put it on 6S without making any other changes such as gearing, it's going produce more power and draw more amps.

If your goal is to lower the amp draw of a setup, you must change the gearing to lighten the load (resistence) on the motor so it will draw less amps.
This could even involve using a different motor of a lower kv to reach this goal.:yes:

[entjoles]

thats what i thought, the 9xl is a better 5s motor , but it runs good on 4s

i will have to get some smaller pinions and try it

[Matthew_Armeni]

Thanks! I never really thought about the re-gearing though it should have been ovbious, doh! Now, is there also a point at which increasing voltage and re-gearing will no longer be beneficial? I mean at some point the motor will be spinning too fast and cause overheating, right? Is this where finding the sweet spot of the motor comes in? Around 35K is what I've been reading on here.

But, the 9XL doesn't draw that many amps anyway, entjoles- what are you hoping to get by running 5s? I'm guessing more top end.

[sikeston34m]

Thanks! I never really thought about the re-gearing though it should have been ovbious, doh! Now, is there also a point at which increasing voltage and re-gearing will no longer be beneficial? I mean at some point the motor will be spinning too fast and cause overheating, right? Is this where finding the sweet spot of the motor comes in? Around 35K is what I've been reading on here.

But, the 9XL doesn't draw that many amps anyway, entjoles- what are you hoping to get by running 5s? I'm guessing more top end.

Yes, there is a point where just changing the gearing will no longer be beneficial.

Let's go back to the two setups I mentioned that were equal in performance with just a few things different to make them perform the same.

#1 on 4S, the motor spins at 35k but has the same gearing as #2. (2365 kv motor drawing 60 amps at 40mph)

#2 setup on 6S spins the motor at 35k. (1577kv motor drawing 40 amps at 40mph)

With the two setups equal in top speed, what are we hoping to get? Less amp draw usually means less heat build up in components. Heat build up is like an Energy leak. It's like running a battery powered electric heater. We spend that energy to make heat instead of motor power.

#2 setup will run longer on each charge and probably tend to accelerate faster than setup #1 because the lower the kv of a motor, the more torque it makes. :yes:

[phildogg]

I'm lost. could I add a cap and not worry too much about the zap? any Radio Shack carry a useful cap or not?
phil

[sikeston34m]

I'm lost. could I add a cap and not worry too much about the zap? any Radio Shack carry a useful cap or not?
phil

I kinda like the zap when I plug in those Lipo's. It tells me the power is going to be flowing like gang busters.

I don't know what all the big fuss is about on "wearing out connectors". I haven't worn one out yet, but if I do, they're pretty cheap ya know?

Out of all the capacitors that Radio Shack carries, NONE of them are low ESR or rated at 105C. They have the really cheap stuff.

[entjoles]

well i was thinking of the sweet spot, on 5s and using 3.7 v/p/c = right at 35k rpm , and also a speed of 38 mph with out gearing up on 4s(around 29k rpm)

i was also thinking of whether or not 5s will let the smoke out:lol:

[KaztheMinotaur]

What is low ESR and what will happen if the caps are not low ESR?

[JeffEmbracedDC]

Three questions.

1. Is 5600 uF completely unnecessary?:

2. Would you lose any performance (other than 2 seconds of runtime) choosing a 5600uF over a 2700uF cap?

3.Also, do we know if these Novak caps are Low ESR?

http://cgi.ebay.com/Novak-Power-Capacitor-4700-5675_W0QQitemZ320209522765QQihZ011QQcategoryZ44028 QQrdZ1QQssPageNameZWD1VQQtrksidZp1638.m118.l1247QQ cmdZViewItem

-Jeff

[JeffEmbracedDC]

What is low ESR and what will happen if the caps are not low ESR?

I have been told Low ESR basically means Low Resistance. Just a better performing capacitor. I was told a normal cap would work fine but a Low ESR would perform better and keep temps even cooler.

[Arct1k]

The novak caps are low esr i believe but the 5600 is low voltage - you need to 2700 HV novak cap... This is the best price i found

http://cgi.ebay.com/Novak-Power-Capacitor-Harness-2700mF-5674_W0QQitemZ320210595585QQihZ011QQcategoryZ44028 QQrdZ1QQssPageNameZWD1VQQtrksidZp1638.m118.l1247QQ cmdZViewItem

[JeffEmbracedDC]

Yeah that's the one I was lookin at...Just ordered it! ... Also ordered a pair of novak 25mm fans, CCBEC and castle link...

-Jeff

[BrianG]

Three questions.

1. Is 5600 uF completely unnecessary?:

2. Would you lose any performance (other than 2 seconds of runtime) choosing a 5600uF over a 2700uF cap?

3.Also, do we know if these Novak caps are Low ESR?

http://cgi.ebay.com/Novak-Power-Capacitor-4700-5675_W0QQitemZ320209522765QQihZ011QQcategoryZ44028 QQrdZ1QQssPageNameZWD1VQQtrksidZp1638.m118.l1247QQ cmdZViewItem

-Jeff

1. Higher capacitance is usually better for this type of application. However, the heat that comes from these is usually dissipated better if spread out over several caps. Using several smaller caps vs one larger one also helps transient response. And, finding a 5600uF cap rated for 35v would be physically large.

2. You would not lose ANY runtime with caps. Think of caps as short-lived batteries. On the contrary, you may actually gain a very little but I don't want to get into that

Or, the famous water/electricity analogy:

Think of a battery like a water faucet and a large tank of water as a capacitor. Also, think of water pressure as voltage and water flow as current. Resistance would be restrictions in the pipe and whatever the water powering (water wheel maybe).

A faucet never "empties", but it's output flow is limited by pipe size and whatever source feeding it (river/stream/city water works/etc). If you try to pull too much flow then the output pressure will drop.

A tank (cap) needs to be filled, but can supply large amounts of water for a short time. Empty it too much/too fast and it wont be doing anything really.

3: Usually, the higher rated temps (105* +) tell you if it's low-ESR.

[JeffEmbracedDC]

Thanks, Brian! Great everything. I don't know what made me think you'd LOSE runtime.. I just wasn't thinking for a second...

Thanks, guys

[suicideneil]

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=P10286-ND

I believe this is the same item...

[GriffinRU]

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=P10286-ND

I believe this is the same item...

Finally good man found good partnumber!!!

Hey, guys big caps dangling on the wires that is bad thing.

The only true place for capacitor is as close to Fet's as possible. All the rest is to compensate for long wires and poor connectors, do not even want to mention bad batteries.

And we are here all about performance, quality and looks of our trucks and systems, so lets not look like dummies and do it right.

- in general each capacitor has its resonance curve and bank of different caps will work better for the circuit then one big one. That is a little bit more advanced but can be researched in spare time in the library or at school :)

- And I am sorry but 9XL will draw more Amps at 6S then at 4S...
And yes gearing is important, because it would set the load for the motor, but Voltage will drive dynamics i.e. acceleration. Not to forget it is also depends on throttle, because of PWM, right... :)

[suicideneil]

I have my uses....

As for the 9XL drawing more amps at a higher voltage, is this simply because the the motor is greedy by design? Given an unlimited supply of amps to draw from the batteries and an esc powerful enough to supply them, will it pull more amps at a higher voltage because it needs/wants them at higher speed (doing more work)... or something like that? The mathematics on this area is a tas bit beyond me to be honest.

[BrianG]

A motor has a certain resistance. Current is determined by voltage divided by resistance. So, you increase voltage, current will increase as well.

However, motors are NOT simply resistors. They do have a baseline resistance value, but also inductive reactance, which depends on motor speed and load (and frequency). Resistance and reactance creates impedance. Increase the speed increases the impedance. Increase the load decreases the impedance.

So, you can actually keep the same current draw when going to higher voltage IF you also reduce the mechanical load by reducing gearing. Motor is doing the same amount of work, but with different voltage/current requirements.

[suicideneil]

Ah, that makes alot of sense when its explained like that.

Increasing the load decreases the impedance, which increases current draw as a result = hot motor correct? A bit like working out increases adrenaline flow (reduced impedance), but you also get rather hot and sweaty (increased amp draw).

Decreasing the load (gearing down) increases impedance which restricts current flow and amp draw, so you run cooler for longer basically.

A higher voltage will make the motors life harder since when its spinning faster, its workload is increased (and it has limitations to how much work it can do regardless), which also decreases impedance = more heat.

Reminds me of the Hvmaxx- high rpms + weedy magnet = low impedance, hence why it always runs hot in my Gmaxx. Gearing down helps, but then the speed suffers....

Thankyou Brian, I think I learned some very valuable info there.

[BrianG]

Yeah, that sounds about right. Interesting analogy too. :smile:

I like to equate HV-Maxx systems to a 4-cylinder engine. To get the same power as a larger BL motor or V8 engine, you gotta rev 'em real high.

[pipeous]

Thanks for the advice Brian. I met up with a local guy that does testing and such for Castle and he hooked me up with 4 X 470uF low ers caps today, 25v. now I just have to work out where to fit them into the chassis. I am running the buggy tomorrow so we'll see how it goes.

[GriffinRU]

9XL - business - hopeless :(

[BrianG]

Whaddya mean Griffin??

[GriffinRU]

Whaddya mean Griffin??

Brian, help me out here...

Get a iron nail, wrap couple turns of wire around it. And run couple test, like:
Apply 1V to leads, check current
Apply 2V - . check current
Get a magnet and move it around nails end and check for current change at 1V and 2V.

After that draw some lines with DC motor.

[suicideneil]

Im lost...

What are you trying to get at exactly? This all rather beyond me.

[BrianG]

I guess I still don't know what you are disagreeing with, but I'll assume you disagree with a previous statement I made, so I'll try to explain:

Current is going to increase when you increase the voltage, but that's because there is the same "load" on the electromagnetic nail (zero in this case).

Please correct me if I am wrong here; but in a motor, loading (among other things) plays a big role.

So, take an actual motor of whatever wind.

Apply 1v, measure current. Apply 2v, measure current. Current will increase by about a factor of 2 (probably not exactly double because rpms are higher and is inducing more opposing voltage in the adjacent coils). Is this correct?

Now, apply 1v to it and add some physical resistance to the shaft. Measure the current. Now, apply 2v, but remove the physical resistance. Measure the current. The current will not double from 1v to 2v because the load was reduced even though the voltage doubled.

So, if all other factors being equal (gearing, motor wind, etc), when going with higher voltage, current will increase. But if you gear down (on the same motor), which reduces the mechanical load, current will be less even though you increased the voltage.

[GriffinRU]

Im lost...

What are you trying to get at exactly? This all rather beyond me.

That is exactly what I am trying to say, to make it simple current will go always UP with voltage in our simplified world of electronics.

When you use your throttle control, you are changing average voltage applied to the motor. So if you start from 4S by throttle you apply, using PWM, voltage from near 0 volts to max of 14.8V, which corresponds to RPM change from 0 to Kv*Volts.
And in 5S case you do the same from 0V up to 18.5V...

[GriffinRU]

Brian do not change my nail to the motor, right away, you might think that you can jump to the conclusion so quick, but we are not here yet.

In simple motor example applying DC voltage to phase will blow it or overload. Voltage and Current in the motor are out of phase, but load defines that. But we are not here yet.

So, how much change in current would be when you move magnet around nail tip, on the same scale as going from 1V to 2V?


My main problem here is that there is a believe that BL motor will regulate power and that is what I am not agreeing with.

Second, it would be impossible to come to conclusion here if we not agree on system parameters, Like motor is fixed (9XL) and load is fixed. Otherwise we not just changing voltage applied to the motor, right?

[BrianG]

OK, I see now. Yeah, current is limited by the DC resistance of the coils which is really low generating really high potential currents. When the motor starts moving, back-EMF becomes the major factor in limiting current.

Maybe I'm not explaining it right, but I don't think a motor will regulate power either. All I was trying to say was that current is not dictated solely by the voltage because there ARE other factors.

[GriffinRU]

Now we can easy explain operation voltage for given motor, right? Meaning you will never apply 10S to 6XL motor :)

From here each motor has:
- load limit, not related to voltage. Motor need to spin.
- Maximum operational voltage, when power regulation can be managed by gearing change, within safety envelope of maximum load. RPM limit.

[BrianG]

I agree with both those points exactly...

[GriffinRU]

Ok, then from here 9XL will draw more amps from battery with higher voltage.
All the rest is proper setup!

[BrianG]

Yes, unless you reduce the gearing for lighter load...

[GriffinRU]

Yes, unless you reduce the gearing for lighter load...

Just to make sure...

After you done with gearing, measure your current draw at 5S and then switch to 4S, current draw should drop, right?

In simple world you can treat a motor as a light bulb:
- low voltage low light/poor efficiency/long life
- Normal voltage good light/good efficiency/expected life span
- high voltage sligtly more light than at norm/poor efficiency/short life
- too high voltage brief light(flash) -> broken

[BrianG]

After you done with gearing, measure your current draw at 5S and then switch to 4S, current draw should drop, right?

Yes.

[rschoi_75]

dang ... after reading this thread, I feel like the biggest electronics noob of all time. Just wanted to say thanks for this informative discussion. I learn something new every time I'm on here.

[Maciolus]

Will Panasonic FC 3300uf 25V be also suitable? I am using 4S2P A123 cells.

[GriffinRU]

Will Panasonic FC 3300uf 25V be also suitable? I am using 4S2P A123 cells.

Yes,
18X25/2750mA/0.020Ohms - looks alright for 4S A123

[Maciolus]

Yes,
18X25/2750mA/0.020Ohms - looks alright for 4S A123

Thanks!

[Bernie Wolfard]

Hmm, there have been a couple of things discussed in this thread I feel I need to comment on. I will also try to provide some information about the use of capacitors, the original reason for this thread.

First, the motor’s Kv affect on current.
Kv means RPM per volt. It has nothing to do with kilovolt or other electrical measurement. This is a constant all motors have. Part of Kv is determined by the number of turns of copper on the motor but turns affect different motor types differently so are not a constant and hence not really useful in determining a motors performance. When a motor spins up it becomes either a generator (brushed) or alternator (three phase brushless). As the motor goes faster, the amount of voltage it generates goes up. When the voltage of the motor matches the voltage of the battery pack it cannot go any faster, hence it has reached its Kv.
Given two similar motors with different Kvs and the same voltage, the higher Kv motor will pull more current (AMPS) than one with lower Kv. This is simply because the higher Kv is trying to get a higher RPM and it takes more current to go faster. There are other things that will cause the higher Kv motor pull more current such a lower copper to magnet ratio, less backplane, perhaps wider airgap etc. but you get the point. To get the efficiency advantages of getting watts from volts instead of AMPS you need to lower the Kv of the motor as your volts go up or you wind up pulling a lot more current as your voltage goes up.

Second: Capacitors, why?
Sole purpose of adding Caps to a controller is to control ripple current. Ripple current is a side affect of pulling pulsed current from a DC source where nothing in the system is electrically perfect. In this case this means the components of the system, particularly the battery, have electrical resistance*. A speed controller works by switching full throttle current on and off really fast to the motor (in our case about 11,000 times per second). This is called pulse width modulation or PWM. The motor averages these pulses out. If 50% of each pulse if off, 50% on the motor sees 50% power. This means the ESC is also pulling pulsed current at the same rate from the battery. In a ideal world these pulses would form a square sine wave where the on part of the pulse went straight up to full throttle current, straight over to the cut off point, straight downs to off current, straight over to on then repeat over and over, then rinse. Unfortunately, as some of you may have found out by now, we don’t live in a ideal world, electrical or otherwise. Because of the batteries internal resistance each pulse it puts out starts a little late, slopes up, overshoots, flattens out, shuts off a little late and tapers down and undershoots. All of this lateness, sloping and overshooting is called ripple current. The caps simply help smooth this out. If the batteries are inadequate, there is high resistance wire or plugs between the pack and the ESC or a high resistance plug the caps will quickly become drained trying to mitigate ripple current. If this happens the caps overheat. Worst case they blow up. As they are overheating they allow more ripple current to reach the FETS which in turn heat up. All of this is bad and can lead to catastrophic failure of the ESC. However, it is important to remember that capacitors cannot make up of inadequate batteries or poor wiring, they provide no increase in power and if they are the wrong ones make matters worst by increasing resistance. A properly designed ESC with good batteries does not need more capacitance. With inadequate batteries no amount of capacitance will help. To ensure long life in an electric power system just make sure you use the best (lowest internal resistance) batteries available with large enough capacity. Always ensure are least 20% more battery discharge capability than you think you will need and everything is OK an power will actually go up as more amperage equals more power, more voltage equals more motor RPM.

Bernie

* A batteries internal resistance is the one and only number that determines how fast the cell can discharge. The lower the resistance, the quicker the cell can discharge. Wouldn’t it be nice if battery companies published this statistic so we could easily judge which batteries are the best?

-----------------------------------------------

For the more technically minded here is a responce from our chief engineer on why ESCS have caps.

You might notice that every brushless controller on the market has an input capacitor that goes across the battery leads - the plus of the capacitor goes to positive battery and the negative of the capacitor across the negative of the battery. The reason for this capacitor is to smooth the ripple current from the battery, so that the battery sees a smoother current demand.

At partial throttle, the controller is turning the motor on and off at a high rate (for our controllers it is typically 13khz.) During the "on"
cycle, there is a fairly high current demand on the batteries. During the "off" cycle, the motor current is recirculated through the controller, and the battery has a near zero current demand. The capacitor recharges during the controller "off" cycle, and discharges during the controller "on" cycle.

The apparent ESR of the battery is reduced, and some of the strain on the battery is transferred to the capacitor.

In most cases this works well -- the battery efficiency is increased because it sees a lower peak current demand. Also the strain on other components (MOSFETs, back-emf detection circuits etc.) is reduced because of a reduction in the ripple voltage on the battery rail.
Without the capacitor, some setups would see ripple voltages exceeding 50% of the battery voltage. So you have to think of a battery as you would any other device - - there is parasitic resistance (ESR) and inductance (ESL.) Batteries are pretty good DC sources when the load is constant, but when the load makes large step changes, their output voltage also makes large step changes.

If, for example, I tried to run a motor which would draw 10 amps on a "perfect" DC supply, on a battery that had .1 ohm of resistance and an output voltage of 10V at no load, we would see a ripple voltage of just under 1V, with a loaded voltage of just above 9V, and an open voltage of 10V. Placing a capacitor across the battery would average the current demand on the battery, and smooth the output to a constant 9.5V.

Ok, at this point I can assume you understand the role of the capacitor. The reason electrolytic capacitors are used is because they are inrush current surge resistant (tantalums are not -- so they are dangerous to use in this type of application) they have a high Q so they tend to suppress ripple well at lower frequencies, and they have good bulk capacitance at moderate to high voltages (where ceramics do not.) As you pointed out, lower ESR would be desirable, especially in marginal systems where the batteries have very large voltage ripple (high ESR.) Tantalums cannot be used because the inrush currents would destroy them (they burn up) so electrolytics are used instead. With electrolytics, ESR and bulk go hand in hand -- so we selected a capacitor with a low enough ESR for the majority of applications, that won't be too large or too expensive. However, in some marginal systems, the capacitor is forced to do a lot of work, and the ESR of the capacitor creates heat. With an electrolytic, there is a failure mode where the temperature of the capacitor exceeds the boiling point of the electrolyte, causing a catastrophic failure of the capacitor. Usually, this will only happen on systems where the current demands of the motor are significantly higher than the capability of the battery to supply current.

I hope this helps clear it up!

[GriffinRU]

Hmm, there have been a couple of things discussed in this thread I feel I need to comment on. I will also try to provide some information about the use of capacitors, the original reason for this thread.

First, the motor’s Kv affect on current.
Kv means RPM per volt. It has nothing to do with kilovolt or other electrical measurement. This is a constant all motors have. Part of Kv is determined by the number of turns of copper on the motor but turns affect different motor types differently so are not a constant and hence not really useful in determining a motors performance. When a motor spins up it becomes either a generator (brushed) or alternator (three phase brushless). As the motor goes faster, the amount of voltage it generates goes up. When the voltage of the motor matches the voltage of the battery pack it cannot go any faster, hence it has reached its Kv.
Given two similar motors with different Kvs and the same voltage, the higher Kv motor will pull more current (AMPS) than one with lower Kv. This is simply because the higher Kv is trying to get a higher RPM and it takes more current to go faster. There are other things that will cause the higher Kv motor pull more current such a lower copper to magnet ratio, less backplane, perhaps wider airgap etc. but you get the point. To get the efficiency advantages of getting watts from volts instead of AMPS you need to lower the Kv of the motor as your volts go up or you wind up pulling a lot more current as your voltage goes up.

Second: Capacitors, why?
Sole purpose of adding Caps to a controller is to control ripple current. Ripple current is a side affect of pulling pulsed current from a DC source where nothing in the system is electrically perfect. In this case this means the components of the system, particularly the battery, have electrical resistance*. A speed controller works by switching full throttle current on and off really fast to the motor (in our case about 11,000 times per second). This is called pulse width modulation or PWM. The motor averages these pulses out. If 50% of each pulse if off, 50% on the motor sees 50% power. This means the ESC is also pulling pulsed current at the same rate from the battery. In a ideal world these pulses would form a square sine wave where the on part of the pulse went straight up to full throttle current, straight over to the cut off point, straight downs to off current, straight over to on then repeat over and over, then rinse. Unfortunately, as some of you may have found out by now, we don’t live in a ideal world, electrical or otherwise. Because of the batteries internal resistance each pulse it puts out starts a little late, slopes up, overshoots, flattens out, shuts off a little late and tapers down and undershoots. All of this lateness, sloping and overshooting is called ripple current. The caps simply help smooth this out. If the batteries are inadequate, there is high resistance wire or plugs between the pack and the ESC or a high resistance plug the caps will quickly become drained trying to mitigate ripple current. If this happens the caps overheat. Worst case they blow up. As they are overheating they allow more ripple current to reach the FETS which in turn heat up. All of this is bad and can lead to catastrophic failure of the ESC. However, it is important to remember that capacitors cannot make up of inadequate batteries or poor wiring, they provide no increase in power and if they are the wrong ones make matters worst by increasing resistance. A properly designed ESC with good batteries does not need more capacitance. With inadequate batteries no amount of capacitance will help. To ensure long life in an electric power system just make sure you use the best (lowest internal resistance) batteries available with large enough capacity. Always ensure are least 20% more battery discharge capability than you think you will need and everything is OK an power will actually go up as more amperage equals more power, more voltage equals more motor RPM.

Bernie

* A batteries internal resistance is the one and only number that determines how fast the cell can discharge. The lower the resistance, the quicker the cell can discharge. Wouldn’t it be nice if battery companies published this statistic so we could easily judge which batteries are the best?

-----------------------------------------------

For the more technically minded here is a responce from our chief engineer on why ESCS have caps.

You might notice that every brushless controller on the market has an input capacitor that goes across the battery leads - the plus of the capacitor goes to positive battery and the negative of the capacitor across the negative of the battery. The reason for this capacitor is to smooth the ripple current from the battery, so that the battery sees a smoother current demand.

At partial throttle, the controller is turning the motor on and off at a high rate (for our controllers it is typically 13khz.) During the "on"
cycle, there is a fairly high current demand on the batteries. During the "off" cycle, the motor current is recirculated through the controller, and the battery has a near zero current demand. The capacitor recharges during the controller "off" cycle, and discharges during the controller "on" cycle.

The apparent ESR of the battery is reduced, and some of the strain on the battery is transferred to the capacitor.

In most cases this works well -- the battery efficiency is increased because it sees a lower peak current demand. Also the strain on other components (MOSFETs, back-emf detection circuits etc.) is reduced because of a reduction in the ripple voltage on the battery rail.
Without the capacitor, some setups would see ripple voltages exceeding 50% of the battery voltage. So you have to think of a battery as you would any other device - - there is parasitic resistance (ESR) and inductance (ESL.) Batteries are pretty good DC sources when the load is constant, but when the load makes large step changes, their output voltage also makes large step changes.

If, for example, I tried to run a motor which would draw 10 amps on a "perfect" DC supply, on a battery that had .1 ohm of resistance and an output voltage of 10V at no load, we would see a ripple voltage of just under 1V, with a loaded voltage of just above 9V, and an open voltage of 10V. Placing a capacitor across the battery would average the current demand on the battery, and smooth the output to a constant 9.5V.

Ok, at this point I can assume you understand the role of the capacitor. The reason electrolytic capacitors are used is because they are inrush current surge resistant (tantalums are not -- so they are dangerous to use in this type of application) they have a high Q so they tend to suppress ripple well at lower frequencies, and they have good bulk capacitance at moderate to high voltages (where ceramics do not.) As you pointed out, lower ESR would be desirable, especially in marginal systems where the batteries have very large voltage ripple (high ESR.) Tantalums cannot be used because the inrush currents would destroy them (they burn up) so electrolytics are used instead. With electrolytics, ESR and bulk go hand in hand -- so we selected a capacitor with a low enough ESR for the majority of applications, that won't be too large or too expensive. However, in some marginal systems, the capacitor is forced to do a lot of work, and the ESR of the capacitor creates heat. With an electrolytic, there is a failure mode where the temperature of the capacitor exceeds the boiling point of the electrolyte, causing a catastrophic failure of the capacitor. Usually, this will only happen on systems where the current demands of the motor are significantly higher than the capability of the battery to supply current.

I hope this helps clear it up!


Very well done!

But, your chief engineer simplified capacitor role way technical inclined hobbyists :) Hopefully he knows that and tantalums caps can do the job but cost allot and size/power /temp/voltage wise are not good for high current/voltage ESC's.

But good cover for bad batteries and poor connectors. And most important upon discharge batteries internal resistance goes up as well, so cap helps here as well even when you have top notch batteries and connectors.

[Arct1k]

When we say poor connections / resistance - is it measurable at no load with a multimeter?

PS lets buy castle pizza

[Bernie Wolfard]

Arct1k,

You cannot measure the amount of resistance with a standard multimeter it is too low. Even though it is low, it is significant because resistance goes up with the square of amperage.

To measure very low resistance values you put a set amount of amperage, say 10 amps, accost the part being measured with a lab quality power supply then measure the voltage accost the part with a voltmeter accurate to at least ten thousands of a volt. Then use Ohm law to calculate the resistance, ohms = volts / current. I don't know how to measure internal resistance or a battery but obviously this method will not work with them.

Artur, I think Patrick, Castle Creations owner and chief engineer explained why tantalums are not appropriate for these applications, one of these being they can explode from the amount of inrush current we get in these applications. Patrick is a systems engineer who has a long resume including being lead engineer at Garmin, Allied Signal and others.

Bernie

[BrianG]

Low resistances can also be measured with a conductance meter (reciprocal of resistance). However, measuring low resistances like that is hard because the meter leads and connections can affect the measurement appreciably. For this reason, measuring voltage drop is easier. Why would you need a lab quality PS though? As long as you have an accurate and calibrated voltmeter (with the proper resolution), you could measure the voltage output of any PS and the V drop on a small resistance (shunt maybe).

Also, IIRC, the internal resistance of batteries is not linear. Instead of supplying the resistance value of cells, manufacturers should supply the resistance curve.

[GriffinRU]

Arct1k,

You cannot measure the amount of resistance with a standard multimeter it is too low. Even though it is low, it is significant because resistance goes up with the square of amperage.

To measure very low resistance values you put a set amount of amperage, say 10 amps, accost the part being measured with a lab quality power supply then measure the voltage accost the part with a voltmeter accurate to at least ten thousands of a volt. Then use Ohm law to calculate the resistance, ohms = volts / current. I don't know how to measure internal resistance or a battery but obviously this method will not work with them.

Artur, I think Patrick, Castle Creations owner and chief engineer explained why tantalums are not appropriate for these applications, one of these being they can explode from the amount of inrush current we get in these applications. Patrick is a systems engineer who has a long resume including being lead engineer at Garmin, Allied Signal and others.

Bernie

:)

You can do the same measurements with batteries as with caps.

BrignG - You need to have something as precision reference.

P.S. Resistance also would be temp and current dependant not to mention vibration...

[Arct1k]

I guess thats a no for me and my $20 multimeter then :)

[BrianG]

lol, probably not.

@Artur; As long as you measure the output voltage of the PS, know the value of the precision shunt/resistor, and measure the v drop of the shunt, why would you need a good PS? Wouldn't the various measurements account for any PS deficiencies?

[GriffinRU]

lol, probably not.

@Artur; As long as you measure the output voltage of the PS, know the value of the precision shunt/resistor, and measure the v drop of the shunt, why would you need a good PS? Wouldn't the various measurements account for any PS deficiencies?

You will be better with precision PS then shunt. You can improve your measurements with Kelvin type resistors but it is much easier with PS.

[pipeous]

I just wanted to update. I finally ran my RC8 today with the Neu 1512 2.5D. The caps worked awesome. I checked it a few times and ran for quite a while at a BMX track. Not even close to getting warm on 4s lipo. motor or esc. I'll never waste time with fans again. In fact, I am going to add caps to my Sidewinder now to cool the B44 down a bit.

[Cain]

Saw pipeous other post, he is running 4 of the 440 caps. So with my simple logic, would that mean its basically similar to running 1760 cap? If so, Then running the 1 novak 2700 cap could lead to even better cooling seen on a mamba max ESC?

Thoughts on this? Thinking of picking up 1 novak cap, maybe 2, and ditching the fan.

[BrianG]

Yes, if running caps in parallel, you just add the values up. So, 4 x 440uF caps (what an odd value) is the same as 1 x 1760uF cap.

However, there is a point where you get smaller gains. Sure, you can add 20 caps and it would be cool, but would be a waste. Approximately doubling whatever is on the ESC would make the most difference and still be easy to fit on/near the ESC. Adding more certainly won't hurt (although it does make the initial hook-up spark bigger), but you just don't get the same amount of gain for the effort and cost. If your setup NEEDs the extra caps just to function and not overheat, maybe it's time for a different ESC choice.

Also, since these caps heat up, it's better to use several smaller ones in parallel because there is more surface area to help the air cool them. Plus, transient response is typically faster.

[Cain]

Thanks for the response.

The main thing I am looking for is the ability to eliminate the fan currently used with the Mamba max on 4S. I am curious based on pipeous experience, using 1 Novak HV cap will basically give the same or even more benefit that he is seeing.

I think I may give it a shot.

[pipeous]

I do notice the caps get warm, about the same as everything else. The MM works good on the 8th scale with 4s. it's smooth. I can crawl along with no cogging, burn laps and it is just such a consistent feel. I could come arund a corner onto a table top and keep the throttle on enough to keep speed in the corner and roll the table for the next double. carving such tight lines to the big o too. I can't see spending 4 times the money when $5 worth of caps fixes the heat issue. I am getting about 30 mins runtime too.

I let quite a few guys drive it at the track yesterday. pretty soon all the questions of where, how much started coming up. We just had a new electric only track open up. the property owner has a brushless 8th scale, b44, 10th truck... all brushless. Sat night racing under lights will be awesome. big track

[lutach]

My Mtroniks truck controller has a huge 10,000uF, 85C capacitor and I'm just wondering, what's tha max amount of capacitance do you guys recommend?

[dezfan]

Can someone post up a picture of were to place the Novak Cap, the pic postest earlier in the thread is dead.

Thanks!

[pipeous]

just solder it across the power leads from esc that go to the battery. pos to pos, neg to neg. mount it in the buggy where it won't get hit ... just finished my soldering yesterday to fix mine for the same problem hehe

[Maciolus]

Do I have to change cut-off setting while running with Novak HV cap on A123 4S2P?

[Cain]

I don't know why the cutoff setting would be affected by the cap? I would think it should be the same as what you would normally run.

Anyone got more info no this?

[Maciolus]

To be honest I do know almost nothing about electronics. One of my friends told me that cap can slightly elevate voltage. Is it truth?

[BrianG]

Caps can help maintain a more stable voltage, but they have to be sized correctly. A cap able to help with 100A bursts would have to be VERY VERY large, like in the multi-farad range (as opposed to microfarads). Even Novaks larger 2700uF cap is only a 0.0027 farad capacitor. The ESC input caps help to filter out the AC ripple current, not necessarily to smooth out current peaks.

[GriffinRU]

Capacitors on the pcb (close to FET's) are meant for copper traces overload. (Need to be EE or better guru to get it)

Capacitors on the power leads are for poor connectors.

And none of them are meant to provide currents to power the motor, they are here for ESC.

Capacitor size is also a problem, due to the inductance. The smaller the cap (current loop) the better, but you still need to select it for the job. In that respect shorter wider caps are better, but check datasheet as well, not the price tag.
(Avoid long leads on the caps.)

[lutach]

Capacitors on the pcb (close to FET's) are meant for copper traces overload. (Need to be EE or better guru to get it)

Capacitors on the power leads are for poor connectors.

And none of them are meant to provide currents to power the motor, they are here for ESC.

Capacitor size is also a problem, due to the inductance. The smaller the cap (current loop) the better, but you still need to select it for the job. In that respect shorter wider caps are better, but check datasheet as well, not the price tag.
(Avoid long leads on the caps.)

Artur,

As you might know, my Schulze 18.97 failed on me. From what Mr. Schulze has said, it was because my wires were too long. I never had a problem such as this before. I know they sell capacitors that goes on the power wires, would that make a difference or would the ESC still have failed?

[GriffinRU]

Artur,

As you might know, my Schulze 18.97 failed on me. From what Mr. Schulze has said, it was because my wires were too long. I never had a problem such as this before. I know they sell capacitors that goes on the power wires, would that make a difference or would the ESC still have failed?

Mr. Schulze correct (I can feel that Rene is going to kill me :)), long power leads can cause fatal failure. It is also related to the old FET drivers. And here is the problem if you have built-in protection than you will lose motor sync if you don't you can lose ESC. The best solution is to make that ESC MCU is monitoring such failure, but then you will have cogging as everyone hates. But that is what they trying to say in manuals about proper wire length and quality connections...
At the time when schulze was released such precautions were not a concern and failure to the wire length and poor motor connector were common.
But it is still hard to proof what happen to your particular ESC, because there are so many variables including aging of semiconductives and caps tend to dry over the time.
And, yes capacitors close to ESC can minimize long wires impact. Different gauge, wire length and twisting also helps.

- I can reuse brains from your schulze...

[lutach]

Mr. Schulze correct (I can feel that Rene is going to kill me :)), long power leads can cause fatal failure. It is also related to the old FET drivers. And here is the problem if you have built-in protection than you will lose motor sync if you don't you can lose ESC. The best solution is to make that ESC MCU is monitoring such failure, but then you will have cogging as everyone hates. But that is what they trying to say in manuals about proper wire length and quality connections...
At the time when schulze was released such precautions were not a concern and failure to the wire length and poor motor connector were common.
But it is still hard to proof what happen to your particular ESC, because there are so many variables including aging of semiconductives and caps tend to dry over the time.
And, yes capacitors close to ESC can minimize long wires impact. Different gauge, wire length and twisting also helps.

- I can reuse brains from your schulze...

I'll send it over to see if it can still be used. Artur, please check this out and see if you can change brain boards and add another power board that I have: http://www.rc-monster.com/forum/showthread.php?t=9624&page=9 (Post #126,127 and 128). Sorry to hijack the thread btw.

[kvrc]

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=P10286-ND

I believe this is the same item...

griffinru or anyone else who has good knowledge of caps. will the abve cap be a good choice for any rc esc application assuming the battery is under 25V? also do you think using 2 instead of 1 will make any difference in cooling down a novak hv esc. they appear to be the same as the novak hv one except for the wires.

has anyone purchased a bunch of these and wants to get rid of a few?

[Five-oh-joe]

Is it possible for the mamba max to run hotter because of an extra 2700uf 25v rated low ESR cap? I just soldered the Novak cap leads directly to the board (got a hot enough iron), and am still testing it. Just wondering if it may hurt performance on say 2s lipo.

[Pdelcast]

Is it possible for the mamba max to run hotter because of an extra 2700uf 25v rated low ESR cap? I just soldered the Novak cap leads directly to the board (got a hot enough iron), and am still testing it. Just wondering if it may hurt performance on say 2s lipo.

No, more capacitance means cooler running, IF the controller has a voltage ripple issue -- more capacitance = lower voltage ripple at the controller. Adding additional capacitance is almost never a bad thing... it just isn't usually necessary.

I DON'T, however, suggest trying to solder to the Mamba Max board -- it's a six layer, heavy copper (six ounces per layer) board -- and soldering to that board is SO difficult that we had to spend about twenty thousand dollars upgrading our soldering equipment at work to handle the Mamba Max board. It requires a very good quality high power iron to get the board hot enough for soldering without over-temp on the chips...

Patrick

[Five-oh-joe]

Yeah....I figured that out the hard way. I broke out the butane soldering iron, let it sit for about 20 minutes on full gas to get it hot, then it still had a little trouble. I'm amazed at how much the traces soak up the heat!

I did get the cap soldered on there though, and no damage to everything. ESC runs at around 110F with a 5v fan and the cap, and around 125F during the middle of the day with the same setup using a 4 pole motor geared high with Mashers. I'm also running way too low of a timing for the 4 pole motor, so it's not working as efficiently as it should and probably heats everything up a little more than it should.

[GriffinRU]

No, more capacitance means cooler running, IF the controller has a voltage ripple issue -- more capacitance = lower voltage ripple at the controller. Adding additional capacitance is almost never a bad thing... it just isn't usually necessary.

I DON'T, however, suggest trying to solder to the Mamba Max board -- it's a six layer, heavy copper (six ounces per layer) board -- and soldering to that board is SO difficult that we had to spend about twenty thousand dollars upgrading our soldering equipment at work to handle the Mamba Max board. It requires a very good quality high power iron to get the board hot enough for soldering without over-temp on the chips...

Patrick

If it is so hard...can you offer me part's to improve your ESC?

And lets not go to capacitor importance business, I had enough with your other partner, I do have respect for you.


kvrcQuote:
Originally Posted by suicideneil http://www.rc-monster.com/forum/images/aria/buttons/viewpost.gif (http://www.rc-monster.com/forum/showthread.php?p=143113#post143113)
http://search.digikey.com/scripts/Dk...name=P10286-ND (http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=P10286-ND)

I believe this is the same item...

griffinru or anyone else who has good knowledge of caps. will the abve cap be a good choice for any rc esc application assuming the battery is under 25V? also do you think using 2 instead of 1 will make any difference in cooling down a novak hv esc. they appear to be the same as the novak hv one except for the wires.

has anyone purchased a bunch of these and wants to get rid of a few?


It is alright, I would go with FM-type. If you need couple send me PM.


Five-oh-joeIs it possible for the mamba max to run hotter because of an extra 2700uf 25v rated low ESR cap? I just soldered the Novak cap leads directly to the board (got a hot enough iron), and am still testing it. Just wondering if it may hurt performance on say 2s lipo.

Yes, it is possible. External cap (boost cap , as I call it and some other manufactures) designed to compensate for long wires and poor connectors (or wearing out connectors, tear and wear... from continues usage).