Some ESC questions for the Nerds!

I'm getting more and more interested of how an ESC works. I'm still sad that I couldn't choose physics at school because my marks weren't good enough, now I have to do 2,5 years economics

Anyway... I've got some questions about ESC's for the nerds around here :)
- What does determines the smoothness (no-cogging) of an ESC? Mainly the software does, right? But very many people also think because of capacitors. Adding capacitors makes an ESC more smooth. I don't understand why capacitors can do this? The amp draw isn't high at low RPM, and a capacitor only sucks up current from the battery and gives it to the ESC if the battery can't offer enough current right? Are there also more things which determine the smoothness of an ESC?
- Capacitors got an "uF" amount. The amount of current they can suck up. My dad told me it's possible to convert uF to mA, but he didn't remember how lol. Can somebody tell me how this is done?
- What do MOSFET's actually do in an ESC?
- Why does an ESC get's warm/hot? Because of the resistance in the MOSFET's? Too low rated electronics?

Just some things that I'm interested in, and just want to know :) Especially the first 2 questions. This questions should be easy for BrianG, ZPB etc... So I hope you can help me :)

[lutach]

Yahoo, Google, Bing (Disease in Chinese) and a huge thread in rcgroups will be your friend.

[What's_nitro?]

Here's some good info to get you started.

Farad

MOSFET

On cogging, it happens whenever the ESC cannot properly sense the position of the rotor. It is far more prevalent on startup because the rotor is not moving quickly enough for the ESC to sense its position and commutate the correct sequence of current pulses for smooth rotation. You are right, at low speed the motor does not draw much power. On startup, however, there is a short high current pulse when starting the motor, as it's basically stalled for that short time. Capacitors help with cogging because they can provide that extra burst of current that the motor needs on startup, and so the ESC better senses the position of the rotor and there is less/no stuttering.

ESC's heat up because, like all other electronic things, they are not perfect conductors. There is a certain amount of voltage lost across the junction of the MOSFETs in the ESC. It isn't much, but losses, in the form of heat, are related to the current passing though the ESC. Say you run a 4s pack (14.8V) and there is a voltage loss of 0.5V after passing through the MOSFETs in the ESC. Ths would mean only 14.3V is getting to the motor. So where does the rest go? Well if your motor at a given time is drawing 30 amps of current from your battery, then by Ohm's law:

0.5V * 30A = 15W

Which means that the ESC is dissipating that 15W as pure heat. So you can see why there is such a fuss over creating superconducting materials. No voltage loss would mean no heat!

[BrianG]

FET heat comes from the rdson component, copper losses, and the fact that the switching action is not perfect square waves.

[What's_nitro?]

[magman]

Originally Posted by What's_nitro?

+1 as well to what Brian said...that's why he's one of the RCM elite!

[BrianG]

Originally Posted by What's_nitro?

Sorry if it seemed like I was correcting you, but rather expanding on what you already said. Hope I didn't come across that way...

[redshift]

No fans OR paint now Brian?

Or no painted fans?

[What's_nitro?]

Nah, it's OK. I used the because there isn't a smiley for "Oh sh*t I forgot!".

[BrianG]

Originally Posted by redshift

No fans OR paint now Brian?

Or no painted fans?

No fans for R/C electronics. No paint for R/C bodies (I wreck them too fast to spend time making it look pretty).

[jsr]

I have a question regarding the input capacitor voltage ratings. Why is the voltage of the input caps typically ~10V above the highest supported input voltage rating?

Ex:
4s 14.8V rated ESC = Caps rated to 25V
6s 22.2V rated ESC = Caps rated to 35V

Typically in other circuits, the input filter cap voltage chosen isn't much higher than the supply voltage as the source shouldn't be able to supply more than the maximum rating. Even at full charge of 4.2V/cell, a 4s pack will only be able to supply 16.8V, no where near the 25V rating of the caps.

Thanks.

[BrianG]

It's always nice to have a little extra headroom for voltage spikes, which can easily exceed the applied battery voltage. Also, caps come in certain standard voltage ratings like 10v, 25v, 35v, 50v, and so on. 25v is just the closest common value.

[_paralyzed_]

A Cool guy here, just checking in to see what the nerds are saying

[simplechamp]

Originally Posted by jsr

I have a question regarding the input capacitor voltage ratings. Why is the voltage of the input caps typically ~10V above the highest supported input voltage rating?

Also, electrolytic capacitors last approx. 8 times longer when operated at half of their rated voltage compared to full rated voltage.