Re: [oopic] PWM freq choice (was: H-Bridge issue...maybe.)

There are so many posts, replies, and whatnot to this thread at this point
(spread over several months, too), that I am getting lost in them. Frankly,
I'm wondering whether I even quoted the correct piece in my original post,
earlier today. The H-Bridge is working as expected, and I'm mostly wondering
whether I should stick with this one, which is controlled the way I prefer,
but may have issues with overheating, or should I try to come up with a
means to manage the control of the much higher power Sabertooth.

If the H-Bridge I am using is rated to handle 1.1 amps, with a peak (or
surge, or something like that) power of 2 amps, and I am using a motor that
can draw 1 amp with a stall power consumption of 2amps, am I going to melt
something? The robot will have the advantage that it will have plenty of
thinking to do. It is a self-guided, indoor, exploration bot, so I could
alter the behavior to sit and process information such that it is running
for relatively brief periods (such as on for 10s, then sitting still for 10s
recording sensor information).

rtstofer wrote:
>
> --- In oopic@yahoogroups.com, Brian Lloyd <brian-wb6rqn@...> wrote:
>>
>> On Mar 2, 2008, at 12:58 PM, Shaggy wrote:
>>
>> >> What is the frequency of your PWM? The 754410 is happiest at 2KHz or
>> >> lower. At 20KHz I've detonated these chips!
>>
>> When figuring pulse frequency and pulse width, you need to consider
>> the inductance of the motor. As you apply voltage to a winding, the
>> current ramps up until the core saturates and then the current is
>> limited only by the DC resistance of the windings. (BTW, this is why
>> "AC" motors are more efficient then "DC" motors.) By varying pulse
>> width, pulse frequency, and applied voltage you can find the "sweet
>> spot" for any motor.
>
> I guess I don't follow this because, it seems to me, the commutator
> and brushes apply the voltage to the windings. In fact, there is no
> reason I couldn't hang a large capacitor after the MOSFET to integrate
> (filter) the DC voltage.
>
> Of course, if I did put the capacitor in the circuit, I would have to
> consider the peak current through the MOSFET during a smaller
> conduction period.
>
> The problem I have with MOSFETs is not driving the gate hard enough
> and allowing the rise and fall time to be somewhat long. As a result,
> at higher PWM frequencies, the MOSFET spends a larger percentage of
> the time in the transition region. At a high enough frequency the
> device may never get into saturation and the heating is excessive.
>
> Eventually, the magic smoke leaks out.
>
> Richard
>
>
>
>
>
>

--
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