Digital Control of an SCR (Thyrister) DC Motor Control Board?

[[email protected]]

Dear Group,

I currently have a 120/220VAC SCR DC motor controller to power a 90VDC
motor which is connected to a milling machine spindle.
It uses a varible resister / pot to ajust motor RPM.

I wish to control this motor via this board digitaly from the parallel
port of a PC possibly using PWM and Linux EMC2 CNC Software.
How / what could I use to replace the Varible resister / pot?
Also I beleive the pot connection are floating.

Any advice / pointers would be very much appreciated

Regards
SRG

 

[default]

Dear Group,

I currently have a 120/220VAC SCR DC motor controller to power a 90VDC
motor which is connected to a milling machine spindle.
It uses a varible resister / pot to ajust motor RPM.

I wish to control this motor via this board digitaly from the parallel
port of a PC possibly using PWM and Linux EMC2 CNC Software.
How / what could I use to replace the Varible resister / pot?
Also I beleive the pot connection are floating.

Any advice / pointers would be very much appreciated

Regards
SRG

The motor probably already has a PWM controller - more efficient,
lighter, and less costly than other types.

You have three options, ditch the controller altogether and add a
switching transistor to modulate power using your PWM signal from the
computer. (switching transistor plus other parts)

Or you can convert the PWM to a voltage level and use that to set the
speed through the original pot input. AND if it is reliable that's
what I would do - having seen what happened to a coil winding lathe
where the speed control shorted and the out of balance mandrel flew
out of it.

Or find some software and D to A board that will output a DC level
then use that to directly control a resistive output optical coupler.
http://www.fairchildsemi.com/ds/H1/H11F1M.pdf (One such coupler)

This guy shows his circuit for controlling a light using a PWM signal
from a controller:
http://www.epanorama.rackhost.net/schematicsforfree/Lights/An_AC_Dimmer_for_Use_with_the_Stamp.PDF

The pwm is converted to a voltage level which is used to adjust the
output resistance of an optical coupler which sets the speed (in his
case light intensity).

You'd want that sort of isolation to keep from having ground loop
problems between the mill and computer.

 

[Tim Wescott]

The motor probably already has a PWM controller - more efficient,
lighter, and less costly than other types.

Usually when you go buy a "90V DC" motor it's a brushed, DC motor with no
electronics -- the only silicon will be in the lamination iron.

You have three options, ditch the controller altogether and add a
switching transistor to modulate power using your PWM signal from the
computer. (switching transistor plus other parts)

Or you can convert the PWM to a voltage level and use that to set the
speed through the original pot input. AND if it is reliable that's
what I would do - having seen what happened to a coil winding lathe
where the speed control shorted and the out of balance mandrel flew
out of it.

The OP just said that the pot is floating -- moreover, if it's a typical
SCR-controlled circuit there's no isolation from the 120V line, so the
experiment of connecting it to the PC will be a short (albeit exciting)
one.

Or find some software and D to A board that will output a DC level
then use that to directly control a resistive output optical coupler.
http://www.fairchildsemi.com/ds/H1/H11F1M.pdf (One such coupler)

This, assuming voltages and resistance levels match, would work.

This guy shows his circuit for controlling a light using a PWM signal
from a controller:
http://www.epanorama.rackhost.net/schematicsforfree/Lights/An_AC_Dimmer_for_Use_with_the_Stamp.PDF

The pwm is converted to a voltage level which is used to adjust the
output resistance of an optical coupler which sets the speed (in his
case light intensity).

You'd want that sort of isolation to keep from having ground loop
problems between the mill and computer.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[default]

Usually when you go buy a "90V DC" motor it's a brushed, DC motor with no
electronics -- the only silicon will be in the lamination iron.

The OP just said that the pot is floating -- moreover, if it's a typical
SCR-controlled circuit there's no isolation from the 120V line, so the
experiment of connecting it to the PC will be a short (albeit exciting)
one.

Place I worked at had several of the (90VDC) controllers and they did
have isolated LV transformers and were intended to be run with either
a voltage in 0-5 VDC or 10 K potentiometer. These did use PWM and
switching supplies similar to computers.

You are quite right about it if he has something with no isolation,
essentially a lamp dimmer. Things would smoke if he tried to make a
direct connection. SCR controller - should have caught that. SCR
implies phase control.

That limits the good choices to an opto isolated control via PWM to DC
level to the speed control, or a PWM DC supply from scratch which does
sound easy enough.

This is for a mill which may mean it has torque control not just
unregulated speed control.

 

[Ecnerwal]

It uses a varible resister / pot to ajust motor RPM. ....
Any advice / pointers would be very much appreciated

Look up "digital potentiometer" - several places make them, might do
what you want directly, might require a step of opto-isolation somewhere
on the way to do what you want.

 

[[email protected]]

Look up "digital potentiometer" - several places make them, might do
what you want directly, might require a step of opto-isolation somewhere
on the way to do what you want.

Thank you all for your advice....

I have attached a link to information regarding this motor drive,
which may help in explaining what I am trying to achieve. Page five
has the pinout for the pot to the board:

http://www.beel.ca/smcmanual.pdf
http://www.beel.ca/smc-00.html

Thanks again..

SRG

 

[[email protected]]

Dear Group,

I currently have a 120/220VAC SCR DC motor controller to power a 90VDC
motor which is connected to a milling machine spindle.
It uses a varible resister / pot to ajust motor RPM.

I wish to control this motor via this board digitaly from the parallel
port of a PC possibly using PWM and Linux EMC2 CNC Software.
How / what could I use to replace the Varible resister / pot?
Also I beleive the pot connection are floating.

Any advice / pointers would be very much appreciated

Regards
SRG