Capacitor For MM 4S

[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-Capa...QQcmdZViewItem

[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]

Originally Posted by 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-Capa...QQcmdZViewItem

-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/Dk...name=P10286-ND

I believe this is the same item...

[GriffinRU]

Originally Posted by suicideneil

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... :)

[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.

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]

Originally Posted by BrianG

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.