BECs, UBECs etc. capacity/power

[AltechMYOB]

Can't find a better category for this question, so here it is in this one.

I'm putting together a list of stuff for my first build. My first Q of the day...

When it comes to selecting these devices for a specific role, is there any problem with selecting an item that has spare capacity eg. if the application calls for a 40 amp BEC, and presuming there is no space problem, I'd rather buy a bigger one, say 60 amp. This does 2 things for me - 1. Versatility for reuse in another role in the future & 2. I like my electronics (semiconductors) to run cool. We're putting more and more stuff in our models, leaving cramped quarters inside and little convection 'space' to dissipate heat.

So the Q is, will the 60 amp BEC do the job for a 40 amp application... or is there a downside, (other than space) that needs to be kept in mind?

tnx.

[pldb64]

My own rule of thumb (borrowed for those far more knowledgable than myself) is don't exceed 80% of the rated capacity of the device in question.

ie if you're pulling 40A, i'd be putting in 50A capable ESC minimum.

My rationale is simple. You may initially install the ESC and test it with a battery that is not brand new. If you install a 40A ESC and it tests to run at 38A (for example), you may be fooled into thinking you're ok. However what happens to the current draw if you happen to buy a new battery (even of the same C rating as the original)? It's chemistry is that much newer and it may deliver a greater amount of current and now you may be beyond the spec of the ESC. Equally, you may buy a new type of battery with a higher C rating. That may lead to the battery delivery more current into the ESC (because your system is asking for it, but you're previous battery couldn't deliver). Also, what happens if you're out at the field and you break a prop and someone lends you a slightly different prop size so you can keep flying - all of a sudden you've changed the configuration of you're power system.

Don't assume you're power system (and current draw in particular) will stay static and always stay within 80% margins. Leads to returning from the flying field a happy little bunny.

[AltechMYOB]

hmmm...

Sounds logical, thanks

[RCModelReviews]

There are several factors but don't assume that a 30A ESC will work successfully with a motor that draws 30A -- chances are it will only do so for a little while.

Reasons:

1. The Chinese are notorious for over-stating the current capabilities of their ESCs
2. A motor may draw 30A continuous but it might also draw much more for short periods when you ram the throttle open.
3. Heat is a killer. If you ESC isn't well ventilated it will eventually fail.

Higher amp-rated ESCs will run cooler than smaller ESCs -- simply because they use more/better FETs to switch the current and those more/better FETs produce a lower level of voltage drop which, in turn, means that less heat is generated in the first place.

I always try to over-ESC a system by at least 25-35%.

For example -- the motor in my ESC draws 20A at full power so I use a 25/30A ESC. I've only blown one -- and that was because a motor magnet came loose and jammed -- causing the motor to stall and draw far more than 30A for must a moment. Result: instant boom!

[AltechMYOB]

Excellent! Thanks B