My Dad seems to think it's a one horsepower DC motor, but is going to verify that today. I understand that switching AC is a different challenge.
--
Sent from my Android phone with K-9 Mail. Please excuse my brevity.
Dave <wa4...@yahoo.com> escribió:
--- On Sun, 8/5/12, Jerome Hollon <jeromehol...@gmail.com> wrote:
> From: Jerome Hollon <jeromehol
...@gmail.com>
> Subject: PWM Using 555's, Comparators, and Hex Inverters
> To: collexion@googlegroups.com
> Date: Sunday, August 5, 2012, 9:37 PM
> My Dad recently complained to me that the new, fancy models of his
> wood lathe has a knob to control speed, and since his is really old,
> the only way he can change speed is to shut it off, change the
> gearing, and turn it back on. So I thought this would be a perfect
> chance to get to play with a high current MOSFET and a PWM circuit
> that wouldn't require a microcontroller (of course, it'd be easier
> that way).
> However, I see a lot of ways to create a PWM circuit, and I'm a
> little confused as to which one I should go with. I've seen were
> people use a 555 and a potentiometer hooked to a cap to
> slow/accelerate the charging/discharging of the cap, I've also seen
> a very similar (but much less complicated) circuit doing the same
> thing, only with a 74AC14 (Hex Inverter with a Schmitt trigger).
> I've also seen people using a comparator to do PWM but haven't looked
> at it enough to know what they've actually built.
> Which one of these are the best do you think? The goal is to drive
> a MOSFET to switch a very high frequency (>=1kHz).
Umm, none of the above?
The first question to ask would be what kind of a motor is on the
wood lathe? Quite a few machine tools use induction motors. Induction
motors will ONLY work on AC. And, you're not going to be switching
AC with a MOSFET (well, at least, not very well).
Some of the older models may have used a "universal" motor, which will
work off of either AC or DC, and which can vary the speed by controlling
the current to the field.
The approach usually used for controlling the speed of an induction
motor is to use a phase controlled triac. By controlling the delay
of the 60 Hz Ac signal to the gate, you can determine at what point in
the phase of the sine wave that the triac will fire.
The PWM approach will work for a DC motor (e.g., series/parallel
/compound), or a universal motor, but not an induction motor. But,
I rather doubt that many machine tools use DC motors, since it would
be required to convert the AC line voltage to DC, which is not
necessarily an easy task.
Dave
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