Re: PWM Using 555's, Comparators, and Hex Inverters

[Dave]

--- On Sun, 8/5/12, Jerome Hollon <jerome...@gmail.com> wrote:

> From: Jerome Hollon <jerome...@gmail.com>
> Subject: PWM Using 555's, Comparators, and Hex Inverters
> To: coll...@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

[Jerome Hollon]

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:

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[Dave]

--- On Mon, 8/6/12, Jerome Hollon <jerome...@gmail.com> wrote:

> From: Jerome Hollon <jerome...@gmail.com>
> Subject: Re: PWM Using 555's, Comparators, and Hex Inverters
> To: coll...@googlegroups.com
> Date: Monday, August 6, 2012, 6:40 AM
>
> 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.

I'm guessing that it's a universal motor, but strange things sometimes happen. :-/

If it is a DC (or universal) motor, then the PWM technique should work. You'll need
a big "free-wheeling" diode across it to handle the inductive spikes during the
switching period. For a high speed PWM, you'll need some fast, and big, diodes,
probably stud mounted.

Oh, you may also need an RC snubber.

Dave

[Jerome Hollon]

It appears to be an AC motor. Specs online say it's a single phase, 1.1kW (1.5 h.p.), 230 Volt motor. It's also a motor made in the UK to run on 50 Hz instead of our 60 Hz.

[Dave]

--- On Mon, 8/6/12, Jerome Hollon <jerome...@gmail.com> wrote:

> From: Jerome Hollon <jerome...@gmail.com>
> Subject: Re: PWM Using 555's, Comparators, and Hex Inverters
> To: coll...@googlegroups.com
> Date: Monday, August 6, 2012, 3:02 PM
>
> It appears to be an AC motor. Specs online say it's a single phase, 1.1kW (1.5 h.p.),
> 230 Volt motor. It's also a motor made in the UK to run on 50 Hz instead of our 60 Hz.

Very likely, it's an induction motor:

http://en.wikipedia.org/wiki/Induction_motor

Those are pretty common on machine tools of that size. Since it's a single phase motor
(The larger ones are usually three phase, but almost no residences have three phase
power supplied. Some businesses do, but it's not universal. Most industrial
buildings do.). it's likely a capacitor start (or capacitor run) type of induction motor.
Some of the smaller induction motors may be shaded pole type, but that usually
implies too much loss for a larger motor.

http://en.wikipedia.org/wiki/Induction_motor

As such, your options for varying the speed are somewhat limited. The two methods
commonly used are either a phase controlled triac, or a variable frequency inverter.
Both have their advantages and disadvantages.

http://en.wikipedia.org/wiki/Induction_motor#Speed_control

As for the 50 Hz versus 60 Hz issue, it should be fine running a motor designed
for 50 Hz on a 60 Hz circuit. The motor may have a slightly less amount of
power, and a slightly higher speed. However, it's not safe to go the other way,
since running a motor designed for 60 Hz on a 50 Hz supply may push the operation into
a region of magnetic saturation, which is where very bad things happen.

Dave