> --- 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.
Yes, they do. The trick with an electric motor is to reduce the IR
losses in the windings. If you can you want to control duty-cycle to
keep the magnetics from saturating (if possible). You give up some of
your ultimate torque but you reduce power requirement considerably and
efficiency is much higher.
Better still you can use brushless AC motors that allow you to change
duty-cycle AND timing (as opposed to being tied to the fixed timing of
a commutator). That will let you get more torque at higher RPM by
advancing the timing to allow time for the B-field in the armature to
build up.
> 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.
Precisely. That is hard on your MOSFETs. Nothing likes to drive a
capacitive reactive load.
> The problem I have with MOSFETs is not driving the gate hard enough
Ah, yes. I was forgetting that. It was my problem with the early power
MOSFETs way back when.
> 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.
Yes, you got that right. I used to do analog amp design back when
enhancement-mode power MOSFETs first came out. (Yes, I am that old,
thank you.)
>
--
Brian Lloyd Granite Bay Montessori
brian AT gbmontessori DOT com 9330 Sierra College Blvd.
+1.916.367.2131 (voice) Roseville, CA 95661, USA
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— Antoine de Saint-Exupéry
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