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Hi Ray.
Amazingly (to me) I actually understood everything
you wrote to me below Ray.
I believe I understand now how pwm works. At least
I think I do. From what I
believe you are telling me, it sounds like the pwm
circuit board causes the electricity
to turn on and off very rapidly in milliseconds at
a certain level of draw. The potentiometer
sends a signal to the circuit board which
determines how much time lag to incorporateat a given draw,
which then determines how long in milliseconds the
electricity is on or off to the motor.
The inertia of the moving parts in the motor keeps
the motor going during those milliseconds
that the electricity is off.
From what you Ray, and Ian have told me, it appears
that using a pwm does not eat up
valuable battery electricity by burning off
electricity via a resistor heat coil or heat sink.
So I am assuming if I did not want to waste
electricity that pwm is the way to go.
According to what I just wrote, is my understanding
of the above correct?
I have two questions Ray. You
wrote.....
"One last note.
A motor will not run over the full 0%
to 100% duty cycle range. I think a
motor will still
be stalled up to 80% range. Above that the speed can
be
varied by PWM."
1. Ray, are you
telling me that even with using a pwm to control my motor, that my motor will
not run until the pwm gets a signal that is equal to 80% of the maximum
power that my motor
will run at? If this is true, then how was I able to run my 24 volt motor
at half speed by hooking
it to a 12 volt battery? Did I miss something here?
2. When I am looking for a pwm for my 24 volt
minnkota, do I only need to look for the voltage description
of the pwm? How about watts? I am wondering if the
amp differences in my batteries when I change them out from time to
time would have any effect on what pwm I should
get. You know how some batteries have more cold cranking
amps than others? Basically I am just trying to
figure out what technical description I should look for in shopping
for a pwm.
Thanks very much for your helpful
info.
Bill Akins.
----- Original Message -----
Sent: Wednesday, February 08, 2006 1:50
PM
Subject: Re: [PSUBS-MAILIST] link
Hi Bill,
From what I remember here is a description of
how PWM
works.
PWM turns the voltage fully on and fully off at
a set
rate for a set amount of time. By fully on I mean the
voltage is
at your supply voltage level. If you have a
12VDC supply then the fully on
voltage will be 12VDC.
If a 48VDC the fully on voltage will be 48VDC.
The
fully off voltage will be 0VDC or ground.
The on and off
transistions are square wavish more
then sine wavish, sharp trasistions.
Assume you have a
48VDC supply the wave shape is as follows with
the
voltage either fully on, at 48VDC or fully off,
at
0VDC.
48VDC -----
-----
-----
| | |
|
0VDC
----- -----
The on and off
trasistions occur at a set frequecy,
for purposes of discussion lets use
1000 hertz (1000
clock cycles or one clock cycle per 1
milli-second).
This is the Clock Cycle.
The Duty Cycle is how much
of the time between clock
cycles the circuit on on. A potentiometer in the
PWM
curcuit will set the duty cycle of some where between
0% and 100%.
The percentage is the amount of time the
DC voltage is fully on. The off
duty cycle is the time
the voltage is fully off, or 100% - duty cycle.
Below
is a representation of what you see.
Clock Cycle (assume 1000
hertz clock)
|_________|_________|_________|
10% duty
cycle
-_________-_________-_________-
20% duty
cycle
--________--________--________-
30% duty
cycle
---_______---_______---_______-
40% duty
cycle
----______----______----______-
50% duty
cycle
-----_____-----_____-----_____-
60% duty
cycle
------____------____------____-
70% duty
cycle
-------___-------___-------___-
80% duty
cycle
--------__--------__--------__-
90% duty
cycle
---------_---------_----------_
100% duty
cycle
-------------------------------
For the 80% duty
cycle, for example, the voltage is on
for 0.8 milli-seconds and off for 0.2
milli-seconds.
Now with the voltage being on part of the time and
off
part of the time the effect to electromachanical
devices, a.k.a
electric motors, is that the motors see
the the equivelant voltage of some
portion of the
supply voltage. Say for the 80% duty cycle and a
48VDC
supply the motors see an equivalent of 38.4VDC.
Now you can do
the on and off transistions with a
switch by manually flipping it on and
off. You could
get a 80% duty cycle by turning on a switch for
8
seconds, count 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8, then
turn the switch off
for 2 seconds, count 1 - 2 then
repeat.
But that would get boring
and tiring. Most important
is that the motor would really see on for 8
seconds
and off for 2 seconds. For the PWM to really work
clock cycles
need to happen up in the 1000 hertz (1k
hertz) and faster ranges. That way
the motor is not
fully on or off but some where in between with
the
intertia of the motor parts keeping things moving at a
less then
full speed rotation.
One last note. A motor will not run over the full
0%
to 100% duty cycle range. I think a motor will still
be stalled up to
80% range. Above that the speed can
be varied by
PWM.
Regards,
Ray
--- Akins <lakins1@tampabay.rr.com>
wrote:
> Hi Ray.
>
> Your e mail confirmed a nagging
thought I had about
> the operating differences between DC and AC
voltage.
> I was wondering if the ac and dc voltage would both
>
work in the dimmer switch, but wasn't sure.
> Your e mail confirmed for
me that it doesn't. Rats!
> Back to square one. So if I don't find a
ready
> made, available for sale variable speed waterproof
> and
pressurproof switch, it now appears my only
> choice is to make one
using those circuit boards in
> conjunction with a magnetic proximity
switch
> and waterproof and pressureproof it by encasing it
> in
pvc. Ray, how does the pulse wave modulation
> broken into duty cycles
for a percentage of time
> work? Does that mean I could only run it at
one
> particular speed for just a certain amount of time?
> Or
does that mean it would work as a variable
> speed controller like you
know I want across the
> entire speed range? I've still got to check on
the
> manufacturer NP sent me wherein he said they have
> ready
made exactly what I need. Hope so.
> Hate to have to make this switch.
> I had some doubts about the ac and dc working in the
> dimmer
switch but was going to try and
> experiment with it anyway, but now
you have saved me
> time and effort by confirming those doubts.
>
Thanks for your help and info and saving me from
> wasting my time and
effort Ray. I'm great with
> ballast,
> hands on forming and a lot
of other things, but the
> electrics are my weak point.
>
>
Bill Akins.
>
>
>
> ----- Original
Message -----
> From: Ray Keefer
> To:
personal_submersibles@psubs.org
> Sent: Tuesday, February 07, 2006 2:35
PM
> Subject: Re: [PSUBS-MAILIST] link
>
>
> Hi Bill,
>
> I believe that
household rotary light dimmer
> switches
> rely on the
fact that the voltage is AC.
> Alternating
> between
0VAC and 120VAC as a sine wave. The SCR in
> the
>
switch turns on when the voltage gets to a level
>
set
> by the rotary switch then turns off when the
sine
> wave
> peaks and drops below that point.
>
> With a DC circuit you do not get that
automatic
> voltage change that the the SCR can trigger on.
>
> The way to control DC is either on/off, or
using
> PWM
> where the DC is electronically broken up
into duty
> cycles of being on for a percentage of time
and
> off
> the other percentage of time.
>
> Regards,
> Ray
>
> --- Akins <lakins1@tampabay.rr.com>
wrote:
>
> > Thanks again for thinking of me
Steve. But it
> > wouldn't help it to stay dry even if it
was
> inside
> > my sub. My sub is a completely
wetsub that I use
> > scuba gear
> >
to operate. No dry cockpit in my boat. It
> wouldn't
>
> be practical for me to install it in a pod
>
outside
> > the sub when I need it in the cockpit. It's
not
> > locating the electronics
> >
that is the problem. I can and have found them
> at
>
> many sites. All kinds of circuit boards to
> operate
a
> > DC variable speed motor. Just none
of
> > them I have found thus far are already
>
waterproof
> > and pressureproof with an exposed to the
water
> > switch knob, handle or twistgrip. For
my
> > application I don't need a fancy circuit
board.
> I am
> > using between the voltage
spectrum of 12 volts
> for
> > half speed and 24
volts for full speed.
> > Since a household rotary light
dimmer switch is
> made
> > to handle 115 volts, it
will more than handle my
> max
> > of 24 volts. So
you see I don't
> > need a circuit board. If I wanted to
make the
> switch
> > myself, it would be simplest
and cheapest to
> just
> > use the rotary dimmer
switch and
> > encase it in pvc and use a magnet to
actuate it
> > without using a circuit board for the
switch at
> all.
> > The problem is then installing
the dimmer
> > switch in pvc to water and pressure proof
it and
> > then to make a magnet actuate the shaft to
the
> > switch. I could probably do it by taking the
> > time and trouble to experiment but it would
just
> be
> > so much easier to be able to buy one
like I
> need. If
> > I can find
it.
> > Bill Akins.
> >
> >
> > ----- Original
Message -----
> > From: steve
> > To: personal_submersibles@psubs.org
> > Sent: Monday, February 06, 2006 6:22
PM
> > Subject: RE: [PSUBS-MAILIST]
link
> >
> >
>
> considering they make them for land vehicles,
>
i'm
> > not suprised!
>
> surely it would be contained within the sub or
>
a
> > custom pod outside?
> >
> > steve
>
> -----Original Message-----
>
> From: owner-personal_submersibles@psubs.org
>
> [mailto:owner-personal_submersibles@psubs.org]On
> >
Behalf Of Akins
> > Sent: 06
February 2006 23:33
> > To: personal_submersibles@psubs.org
>
> Subject: Re: [PSUBS-MAILIST]
link
> >
> >
>
> Thanks for the links Steve.
>
>
> > However I didn't see
any waterproof and
> > pressureproof variable speed motor
controllers
> ready
> > made at the
sites.
> > Bill
Akins.
> >
> >
>
> ----- Original Message -----
> > From: steve
> > To: personal_submersibles@psubs.org
> > Sent: Saturday,
February 04, 2006 6:37 PM
>
> Subject: [PSUBS-MAILIST]
link
> >
> >
>
>
> >
just in case it's useful to anyone...
>
>
> > www.4qd.co.uk
>
>
> > make
electric vehicle speed controllers
> and
> > have a
sister site www.4qdtec.com with lots
of
> good
> > info.
>
> pay the small
subscription fee its worth
> it!
>
>
> >
steve
>
>
>
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Yahoo!?
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>
>
>
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