Capacitor For MM 4S
 

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

http://www.shopatron.com/product/par....56.7400.0.0.0

Any thoughts or alternatives?

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.

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

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...Max=&SUBCATID=

Thanks!

where are you putting these?
phil

Across the power leads close to the MM

Kind of like this...

http://i160.photobucket.com/albums/t...x/CIMG0353.jpg

What does adding a capacitor like that do?

Would this benefit my Hydra 120?

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.

So this would work for any ESC?

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

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

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.

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:

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

Come on USPS!!!

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...

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...

....

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.

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

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:

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

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:

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.

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:

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

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-Capa...QQcmdZViewItem

-Jeff

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.

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-Capa...QQcmdZViewItem

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

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.

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

http://search.digikey.com/scripts/Dk...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... :)

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.

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.

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.

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.