question about motor controllers.

[dacflyer]

i have seen in hobby shops, the motor controllers used for R/C cars , the range as high as 100 amps or more. WOW
and they are really small and have barely any heat sink on them, i usually just see the tabs of the FET's , but on a regular motor controller , like one that goes on a kids scooter or wheelchair
, they are big with lots of heat sink,
my question is.. would one of these micro monsters last for a bike or wheelchair? or is there some difference that i do not see??

*some of the R/C cars have voltages up to 24 volts or more. what am i not seeing here?

any control is not a problem as far as servo control is, i just wonder if the controller would last if used on a big motor v/s a micro motor that sucks up to 100amps...i used to think they would melt fast at that many amps draw..

[JPKNHTP]

-JPKNHTP

[dacflyer]

well the big motor controllers like for scooters and wheel chairs use PWM also..

[philba]

I think the assumption is that these devices have a low duty cycle. I'm don't mean PWM duty cycle but rather they get used for short periods of time. The drill example is good - how long do you run the drill for on average? I'd bet it's less than a minute per hole. I have heated up drills when drilling lots of holes through hard material. Also, the batteries probably can't last long enough enough to burn out the motor. I bet in some cases, people have burned it out but that maker probably assumes some rate of return which is cheaper than making it 100% bullet proof.

I'm sure the fact that the vehicle moves is factore into the engineering calcs, as well.

[Chris Smith]

The main factor with FETs is the onboard resistance. [wasted power trying to deliver]

And this is before PWM adds into the benefits of FETs.

My battery cables don’t get hot because they deliver most of the power to the job, not resist it. [for the most]

Low loss cables don’t produce much heat, low loss FETs don’t produce much heat.

FETs with .002 ohms and FETs with 2 ohms are not the same.

2 ohm FETs eat power from the circuit and need heat sinks, where as .002 ohm FETs consume little power for them self in the form of heat doing their job and don’t need as big of a heat sink.

[ohms law]

[dacflyer]

oh, ok,, i see now..that must be why the micro motor controllers are so expencive ...

[Chris Smith]

Low "on resistance" fets do add to the cost.