>
> 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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