AC motor SCR speed controller problem

[glen]

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

 

[John Popelish]

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.

 

[Rich Grise]

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.

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

 

[Fred Bloggs]

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

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.

 

[Spehro Pefhany]

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?

A good design to copy.

Best regards,
Spehro Pefhany

 

[glen]

thanks for all the input.
i ran across this schematic a number of different places online while
doing research on building a low cost speed controller. it seemed
like the simplest way to get the job done. i already bought enough
parts to build 3 of these darn things. here are some additional links
thay may help someone figure out how to get this circuit to work.
both links are from the SCR manuf. site.
one shows the device specs of the scr itself, and the other is an
application note from the design department of the manuf.
the app note shows a dc motor control circuit which is remarkable
similar to the ac one i built. but, alas, no info on the magic box
they call the 'firing circuit'.
http://www.irf.com/technical-info/appnotes/an-312.pdf
http://ec.irf.com/v6/en/US/adirect/ir?cmd=catProductDetailFrame&productID=10RIA120

 

[CBarn24050]

i ran across this schematic a number of different places online while

doing research on building a low cost speed controller. it seemed
like the simplest way to get the job done. i already bought enough
parts to build 3 of these darn things. here are some additional links
thay may help someone figure out how to get this circuit to work.

Hi, this is a real crappy circuit, very unlikely to work reliably. Small
controllers can be got for a small price although there are not so many made
these days since the advent of the cheap ac inverter. The firing circuit is
done with a small micro, it's a quite complex circuit if you do it the old way
with opamps and comparators. There used to be several chips made to do this but
they are all obsolete now.

 

[glen]

ok, fellas, heres an update on my so-called project to build an ac
motor speed controller.
first, i tried the snubber circuit recommendation and found no change
in my drill motor speed. next, tried a normal household fan, and
fried the pot. next, tried a shaded pole ac motor, and fried the
motor windings. finally, tried a capacitor start, 1\4 hp motor (it
has brushes, so i'm thinking its a universal type ac mtr), and this
motor just hummed for 10 seconds, and then bits of the SCR flew across
the work bench. Sooooo....i'm definatly running thru my project parts
at a fairly good clip at this point. maybe start all over with dc
motors???? (and this was supposed to help me grasp the whole
scr trigger, ac motor fundamentals deal....) i'm a little
disapointed, to say the least....because i already knew how to melt
components, that was somthing i picked up years ago...

 

[CBarn24050]

ok, fellas, heres an update on my so-called project to build an ac

motor speed controller.
first, i tried the snubber circuit recommendation and found no change
in my drill motor speed. next, tried a normal household fan, and
fried the pot. next, tried a shaded pole ac motor, and fried the
motor windings. finally, tried a capacitor start, 1\4 hp motor (it
has brushes, so i'm thinking its a universal type ac mtr), and this
motor just hummed for 10 seconds, and then bits of the SCR flew across
the work bench. Sooooo....i'm definatly running thru my project parts
at a fairly good clip at this point. maybe start all over with dc
motors???? (and this was supposed to help me grasp the whole
scr trigger, ac motor fundamentals deal....) i'm a little
disapointed, to say the least....because i already knew how to melt
components, that was somthing i picked up years ago...

This is a DC motor controller, for DC motors only. With such a little grasp on
electronic circuitry you shouls sell your solering iron and do something else.

 

[Activ8]

ok, fellas, heres an update on my so-called project to build an ac
motor speed controller.
first, i tried the snubber circuit recommendation and found no change
in my drill motor speed. next, tried a normal household fan, and
fried the pot. next, tried a shaded pole ac motor, and fried the
motor windings. finally, tried a capacitor start, 1\4 hp motor (it
has brushes, so i'm thinking its a universal type ac mtr), and this
motor just hummed for 10 seconds, and then bits of the SCR flew across
the work bench.

So you got that firing circuit working after all.

Sooooo....i'm definatly running thru my project parts
at a fairly good clip at this point. maybe start all over with dc
motors???? (and this was supposed to help me grasp the whole
scr trigger, ac motor fundamentals deal....) i'm a little
disapointed, to say the least....because i already knew how to melt
components, that was somthing i picked up years ago...

Remember, even if it doesn't say it on a document, always say to
yourself,

"no one makes any warranty, either express or implied of
merchantability or fitness for a particular purpose, nor will they
be liable for any damages."

Which means that a man has got to know his limitations.