On Sunday 13 January 2008, Colin MacKenzie wrote:
>EMC2 has 3 outputs for spindle control (that I see) PWM Output, CW and CCW
> on the Parallel port. I have a simple relay board I made using grayhill
> solid state relays. These relays are opto-isolated. Because my relays are
> 3A 140v rated and my rotozip is 5am motor, I used 3 of them in parallel to
> comfortably run the rotozip with lots of amp headroom.
First, the SS relay uses the powerline zero crossing to do the turnoffs. So
regardless of which output you connect, it will turn on with the first pulse
of that half cycle, and stay on till the zero crossing at the end of that
half cycle. The rotozip will for all practical purposes, run wide open
unless totally stopped.
There are dmos H bridge drivers available, the most efficient method so far,
but not at a high enough voltage to run the rotozip at a usable power level.
Now, here is something I don't know. If it is possible to sync the pwn driver
to twice the powerline, and make the speed control a delay from zero crossing
to the on pulse, that could work, albeit with nothing like the low speed
torque that a true pwm running on a full wave rectified and filtered supply
could do. For exactly that reason, I wouldn't bother to even try it. You
want low speed torque, and that is going to need a pwm driver that can handle
those voltages. That can be constructed, or purchased. If building it from
scratch, the single most important characteristic of the circuit will be that
the free-wheeling diodes be faster than the transistors at turnoff time, and
that the on drive to the transistors can be shaped to allow the diodes time
to turn off before the transistors are fully driven.
The problem is that when turning the transistors back on, the diodes are also
on, and for a microsecond or two, represent a total short on things until the
charge carriers can be swept clean in the diodes active conduction materiel.
That stresses the heck out of the transistors when they are asked to handle
several hundred amps worth of this short circuit till the diodes can recover
and turn off a microsecond or less later. Use the fastest diode you can
afford, lots of heat sinks and pray, a lot.
The available bridge driver IC's from Allegro etc, solve this by using
actively driven transistors as the diodes so they can stage the switching and
eliminate this virtual short circuit. That process is limited to about 35
volts however unless someone has come up with a new technology in the last
year.
>I don't know what the effect of connecting my SS relay board to the PWM
> output would do. Since my relay board is SS its not like I have armatures
> that will arc and sieze or something due to PWM switching. But I don't know
> how the rotozip would take to quickly switching power on and off in a PWM
> situation.
If its internal speed control is bypassed, there shouldn't be any other
reasons it wouldn't work just like any other universal motor.
> Anyone here attempted this sort of thing? Is the PWM output
> always duty cycled? Or is there some setup to make it always on or off. I
> was only intending on manual speed control until I find a good deal on a
> real spindle and controller. Should I just connect it to the CW output and
> use that to switch spindle on/off?
The rotozip might make a good tool for this, dependent on whether the output
shaft is a part of the motor, (and I don't know this, not having had mine
apart yet) which should be plenty rigid enough for engraving, or if, like the
dremel, the output shaft is on a separate bearing and only loosly coupled to
the motor, which allows the dremel shaft to wobble and crawl excessively IMO.
The dremel might actually capable of more precise work if the flexible cable
was used, it seems quite a bit more rigid to me.
>Also: If anyone has information on a decent spindle for a good price please
> let me know. (maybe direct email.) I will cut a lot of soft aluminum,
> plastics and some wood. I have a moving table design with solid parts,
> linear bearings and ball screws.
I'd investigate a 'trim router', driven by one of the higher voltage gecko's.
One with PM field magnetics would be a definite plus here as its low speed
torque would be quite a bit more. No idea whats on the shelves at Lowes
though.
In any event, for alu, hydrocarbon fluid coolant, lots of it, to keep the cut
alu oxygen free as long as possible. This will be messy :) I've also used
air, (also messy) but that obviously doesn't keep the oxygen away, so carbide
bits will go away fairly quick. They did for me at 1500 rpm, using about a
0.010" cut depth and 5"/min in the feed direction, all the horsepower my
little micromill can mustern without straining. New 1/4" 4 flute solid
carbide bits were getting pretty ratty in less than an hour.
>Btw: the relay board was cut and drilled using emc2 and it worked well.
>
>Thanks,
>Colin
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
No character, however upright, is a match for constantly reiterated attacks,
however false.
-- Alexander Hamilton
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