Looking at the schematic for my harbor freight power feed, looks like
universal motor, with SCR control, with no full wave rectifier in
circuit. So it's half wave only control. The SCR is before motor
(which is connected to neutral return), so I believe it is sensing
back EMF from motor which would improve speed control. However it
just does not do a good job of regulating constant feed.

Has anyone seen a simple circuit, or some other off the shelf device
that can replace the internal control board with a much improved
design?

ignator

why don't you look on ebay for a Minarik DC controller - I used to have a
lot of them but they are all gone - I've had good luck with Minarik - you
can look at specs on their web site - should be able to get one for $25 to
$35


"ignator" wrote in message
...
Looking at the schematic for my harbor freight power feed, looks like
universal motor, with SCR control, with no full wave rectifier in
circuit. So it's half wave only control. The SCR is before motor
(which is connected to neutral return), so I believe it is sensing
back EMF from motor which would improve speed control. However it
just does not do a good job of regulating constant feed.

Has anyone seen a simple circuit, or some other off the shelf device
that can replace the internal control board with a much improved
design?

ignator

Feed drive units have been discussed here in RCM before, and IIRC, someone
modified the original circuit for a lower speed setting, but I don't recall
any other changes.

Probably the simplest electronic variable speed control for a universal
motor is the triac and diac ciruit. While this circuit provides varaible
speed, it doesn't regulate the speed. The HF router speed control is a
triac/diac circuit.

Next might be a simple SCR and PWM pulse width modulation IC, but again, no
precise regulation of speed. Similar to a variable speed drill circuit in a
quality drill.

As circuit complexity increases, more parameters come into play (work), and
very good speed regulation becomes realized.

You should try to determine the motor's full voltage, and then the amp
and/or watt load that the motor produces, to decide which circuit would be
best suitable for the feed motor.
Since the original controller is a SCR circuit, the motor brushes are
possibly for DC (no way to assume, since it probably came from China).

Bill's suggestion of a Minarik controller is good advice since they are
reliable and fairly inexpensive.
One thing you can probably be sure of is that a better control circuit is
going to need to be connected externally, so you'd be wise to locate a
sturdy metal enclosure to protect the speed control and yourself.

There are various eBay sellers that sell small, inexpensive regulator
modules, but I haven't used any of them, so I can't make a recommendation.

A couple of safety devices should be considered, in case they're not inluded
with a control that you might find; a fuse to protect the circuit, and a
fuse (at least) or other device to protect the drive motor.
A thermal protector for the motor would provide an extra level of
protection/insurance, that usually isn't provided in many motor driven
devices.

--
WB
..........
metalworking projects
www.kwagmire.com/metal_proj.html


"ignator" wrote in message
...
Looking at the schematic for my harbor freight power feed, looks like
universal motor, with SCR control, with no full wave rectifier in
circuit. So it's half wave only control. The SCR is before motor
(which is connected to neutral return), so I believe it is sensing
back EMF from motor which would improve speed control. However it
just does not do a good job of regulating constant feed.

Has anyone seen a simple circuit, or some other off the shelf device
that can replace the internal control board with a much improved
design?

ignator

On Mar 13, 9:24 am, "Wild_Bill" wrote:
Feed drive units have been discussed here in RCM before, and IIRC, someone
modified the original circuit for a lower speed setting, but I don't recall
any other changes.

snip

I searched and could not find anything on power feeds except
installation questions.
One complaint I do have is the very small usable range of the speed
pot. I do see that the 150 inch pound schematic shows a dual SCR for
full wave operation. I'll try Bill Noble idea of a controller off
ebay. What I really would like would be a controller that has a shaft
encoder input, but gosh that turns a half day task into weeks to get
it right. There's quite a bit a gear reduction from the motor to the
shaft drive, I guess if all else fails I'll design my own, but that
turns into more effort then I want.
ignator

Thanks for the replies.

The limited speed range sounds like what the old post/discussion was about,
now that you mentioned it.

It's been a while since I looked at the diagrams of the internal parts of
the feed drives, so I don't remember where the circuit board is located, but
modifying or upgrading the existing circuit board might be an option if
there is enough space, and if you can identify and locate some suitable
components.

The DC motor drive controllers such as the Minariks and several others, may
have several settings on their circuit boards. This generally allows them to
be used in more types of applications.
Getting the manufacturers' setup instructions and specifications (can often
be found online as a PDF file) should be helpful for setting the adjustments
such as current limit, min/max speed, etc.

BTW, there are two common DC motor drive output voltages.. 0-90VDC and
0-130VDC. Generally, the 90V output is a 120VAC input drive, and the 130V
drive often requires 240VAC input.

--
WB
..........
metalworking projects
www.kwagmire.com/metal_proj.html


"ignator" wrote in message
...

I searched and could not find anything on power feeds except
installation questions.
One complaint I do have is the very small usable range of the speed
pot. I do see that the 150 inch pound schematic shows a dual SCR for
full wave operation. I'll try Bill Noble idea of a controller off
ebay. What I really would like would be a controller that has a shaft
encoder input, but gosh that turns a half day task into weeks to get
it right. There's quite a bit a gear reduction from the motor to the
shaft drive, I guess if all else fails I'll design my own, but that
turns into more effort then I want.
ignator

Thanks for the replies.