AC motor SCR speed controller problem

G

glen

Guest
i built a controller and the SCR never shuts off.
here is the link to the schematic:
http://home.maine.rr.com/randylinscott/aug99.htm

i can get the scr to conduct by changing the pot setting,
but it never turns off. just by looking at the diagram, it seems to
me that i would need a negative voltage on the anode to turn it off,
but with the full wave bridge in there, i never get a negative
voltage. does anyone have a clue how to cycle this scr? killing all
power stops it, as well as a moment. short of the andode and cathode.
gate voltage seems to be zero in the off state and cathode voltage in
the on state. what am i missing here?
thanks
glen
 
glen wrote:
i built a controller and the SCR never shuts off.
here is the link to the schematic:
http://home.maine.rr.com/randylinscott/aug99.htm

i can get the scr to conduct by changing the pot setting,
but it never turns off. just by looking at the diagram, it seems to
me that i would need a negative voltage on the anode to turn it off,
but with the full wave bridge in there, i never get a negative
voltage. does anyone have a clue how to cycle this scr? killing all
power stops it, as well as a moment. short of the andode and cathode.
gate voltage seems to be zero in the off state and cathode voltage in
the on state. what am i missing here?
thanks
glen
Click to expand...
This circuit might work for a purely resistive load (like an
incandescent lamp). Since you are using it for an inductive load, the
current never falls below the SCR holding current before the rectified
AC voltage pops back up and starts increasing the current, again.
Inductance tends to act like a current fly wheel. Besides, the output
of this circuit is DC, not AC, so I don't see any way you are going to
get variable speed AC motor operation out of it, unless the AC motor
is a universal (series) wound (works on AC or DC) type. I guess this
is what you are using, or you would have blown a fuse by now.

You may be able to get the SCR current to go to zero twice a cycle by
adding an RC snubber across the motor. Something like a 100 2 watt
resistor and a 1 uf 400 volt film capacitor in series. The cap will
see a big blast of current each time the SCR fires, but this is
limited ot a safe value (perhaps) by the series resistor. But as the
rectified voltage heads toward zero, the motor draws a bit of current
out of the capacitor, instead of through the SCR, allowing it to reset
to the off state before the voltage comes back up. A better approach
might be to switch to a TRIAC in place of the SCR and put it on the AC
side of the rectifier, Of course if you have a universal motor, you
don't then need the rectifier at all. The RC snubber might still be a
good idea.

--
John Popelish
 
"John Popelish" <jpopelish@rica.net> wrote in message
news:40BC0662.40A2D695@rica.net...
glen wrote:

i built a controller and the SCR never shuts off.
here is the link to the schematic:
http://home.maine.rr.com/randylinscott/aug99.htm

i can get the scr to conduct by changing the pot setting,
but it never turns off. just by looking at the diagram, it seems to
me that i would need a negative voltage on the anode to turn it off,
but with the full wave bridge in there, i never get a negative
voltage. does anyone have a clue how to cycle this scr? killing all
power stops it, as well as a moment. short of the andode and cathode.
gate voltage seems to be zero in the off state and cathode voltage in
the on state. what am i missing here?
thanks
glen

This circuit might work for a purely resistive load (like an
incandescent lamp). Since you are using it for an inductive load, the
current never falls below the SCR holding current before the rectified
AC voltage pops back up and starts increasing the current, again.
Inductance tends to act like a current fly wheel. Besides, the output
of this circuit is DC, not AC, so I don't see any way you are going to
get variable speed AC motor operation out of it, unless the AC motor
is a universal (series) wound (works on AC or DC) type. I guess this
is what you are using, or you would have blown a fuse by now.

You may be able to get the SCR current to go to zero twice a cycle by
adding an RC snubber across the motor. Something like a 100 2 watt
resistor and a 1 uf 400 volt film capacitor in series. The cap will
see a big blast of current each time the SCR fires, but this is
limited ot a safe value (perhaps) by the series resistor. But as the
rectified voltage heads toward zero, the motor draws a bit of current
out of the capacitor, instead of through the SCR, allowing it to reset
to the off state before the voltage comes back up. A better approach
might be to switch to a TRIAC in place of the SCR and put it on the AC
side of the rectifier, Of course if you have a universal motor, you
don't then need the rectifier at all. The RC snubber might still be a
good idea.
Click to expand...
What would happen if you rearranged things such that rather than the
"AC" socket in series with the SCR on the DC side of the bridge, Which
does as John says, to nothing but the SCR right from + to - of the bridge,
and the socket in series with the AC line, preferably the neutral? (I'm
not sure how D1 would fit in in that case)

That way, at least there's be AC at that "AC" socket. :)

And I'd still use a snubber. :)

Cheers!
Rich
 
glen wrote:
i built a controller and the SCR never shuts off.
here is the link to the schematic:
http://home.maine.rr.com/randylinscott/aug99.htm

i can get the scr to conduct by changing the pot setting,
but it never turns off. just by looking at the diagram, it seems to
me that i would need a negative voltage on the anode to turn it off,
but with the full wave bridge in there, i never get a negative
voltage. does anyone have a clue how to cycle this scr? killing all
power stops it, as well as a moment. short of the andode and cathode.
gate voltage seems to be zero in the off state and cathode voltage in
the on state. what am i missing here?
thanks
glen
Click to expand...
The circuit is cluelss dabbling. The only way the author got it to
work was to use a super large SCR with a super large holding current and
a flea power electric drill- most of which use universal motors. Forget
that crap circuit and get a surplus variac.
 
On 31 May 2004 20:36:00 -0700, the renowned grhess@comcast.net (glen)
wrote:

i built a controller and the SCR never shuts off.
here is the link to the schematic:
http://home.maine.rr.com/randylinscott/aug99.htm

i can get the scr to conduct by changing the pot setting,
but it never turns off. just by looking at the diagram, it seems to
me that i would need a negative voltage on the anode to turn it off,
but with the full wave bridge in there, i never get a negative
voltage. does anyone have a clue how to cycle this scr? killing all
power stops it, as well as a moment. short of the andode and cathode.
gate voltage seems to be zero in the off state and cathode voltage in
the on state. what am i missing here?
Click to expand...
A good design to copy.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top