Controlling a motor in presence of phase shift

[fssg]

I am trying to control speed of a single phase fan motor using triac phase
control, across a optoisolated boundary. The motor could be of shaded-pole
or capacitor-run type, between 30 and 200W.

My circuit looks like this:


4K7
1N4001 47K 4N25 .-|___|----- 5V
___ |
Active --|<--|___|------------' '------------ To uC INT
- |/
^ -> -|
Neutral ------------' |>
'---- Gnd




From uC 120R
-----. '------o-----o----o------o--- Active
| | | | | |
| 470R.-. | | | |
| | | | .-. | |
| | | | | | | |
| MOC3021 360R '-' | | | | |
| ___ | | '-' | |
| .----|___|-----o | | | |
| _|_ | ' | .-.M |
V -> V_A | _|_ | | |O ---
- / | | V_A | | |V --- 100nF
| '--------------)--- / | | '-' |
----' | | --- | |
| | --- | |
--- | 100N| | |
47nF --- | | | |
| | | | ___ |
'------o-----o----o--UUU-o------- To load
100uH
T405-600
or
BT137
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


The 4N25 optocoupler detects the zero crossing, a microcontroller turns on
an optotriac LED after the required delay, and holds it on until about 20deg
before the end of the half cycle.

I have noticed that 100% power (where the voltage across triac is almost
always zero) is at about 25deg for a 173W capacitor-run motor, 60deg for a
50W shaded pole motor. This is giving me two problems:
1. If I turn on the triac eariler than this point, I start to get
'half-waving' (?) but I can't see why this happens? Shouldn't it just
continue to run at 100%?
2. Since I don't know what motor my users will plug into it, how do I
prevent this happening? My zero crossing comes straight from the mains -
could I put the zero crossing detector across the triac to detect when the
triac turns off and have my timing referenced to that point instead?

I also notice that on the shaded-pole motor, the circuit doesn't seem to
turn on the triac properly at high firing angles. Starting at 140deg, I can
see where the triac is being turned on, the voltage across the triac dips to
zero briefly but then rises back up to mains voltage. At about 125 deg it
starts to do the negative half cycles properly, then positive half cycles
start working
from about 92deg. Does anyone know why this might happen?

Thanks for any help.

 

[Ban]

I am trying to control speed of a single phase fan motor using triac
phase control, across a optoisolated boundary. The motor could be of
shaded-pole or capacitor-run type, between 30 and 200W.

My circuit looks like this:


4K7
1N4001 47K 4N25 .-|___|----- 5V
___ |
Active --|<--|___|------------' '------------ To uC INT
- |/
^ -> -|
Neutral ------------' |>
'---- Gnd




From uC 120R
-----. '------o-----o----o------o--- Active
| | | | | |
| 470R.-. | | | |
| | | | .-. | |
| | | | | | | |
| MOC3021 360R '-' | | | | |
| ___ | | '-' | |
| .----|___|-----o | | | |
| _|_ | ' | .-.M |
V -> V_A | _|_ | | |O ---
- / | | V_A | | |V --- 100nF
| '--------------)--- / | | '-' |
----' | | --- | |
| | --- | |
--- | 100N| | |
47nF --- | | | |
| | | | ___ |
'------o-----o----o--UUU-o------- To load
100uH
T405-600
or
BT137
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


The 4N25 optocoupler detects the zero crossing, a microcontroller
turns on an optotriac LED after the required delay, and holds it on
until about 20deg before the end of the half cycle.

What you want is IMHO impossible with your circuit. You will need a variable
frequency synthesized from a DC intermediate voltage similar to a switched
mode power supply. SCRs are absolutely unsuitable for this. I do not think
your electronic understanding will be up to this task.
Your zero x-ing detector should not be done with a 1N4001, but with a 1N4004
to -7. You might consider doing an incandescent light control with your
circuit instead.

I have noticed that 100% power (where the voltage across triac is
almost always zero) is at about 25deg for a 173W capacitor-run motor,
60deg for a 50W shaded pole motor. This is giving me two problems:
1. If I turn on the triac eariler than this point, I start to get
'half-waving' (?) but I can't see why this happens? Shouldn't it just
continue to run at 100%?
2. Since I don't know what motor my users will plug into it, how do I
prevent this happening? My zero crossing comes straight from the
mains - could I put the zero crossing detector across the triac to
detect when the triac turns off and have my timing referenced to that
point instead?
I also notice that on the shaded-pole motor, the circuit doesn't seem
to turn on the triac properly at high firing angles. Starting at
140deg, I can see where the triac is being turned on, the voltage
across the triac dips to zero briefly but then rises back up to mains
voltage. At about 125 deg it starts to do the negative half cycles
properly, then positive half cycles start working
from about 92deg. Does anyone know why this might happen?

Thanks for any help.

You will blow up either the motor or your triac.

 

[Phil Allison]

"Ban"

What you want is IMHO impossible with your circuit. You will need a
variable frequency synthesized from a DC intermediate voltage similar to a
switched mode power supply.

** Complete drivel.

SCRs are absolutely unsuitable for this.

** Triacs are widely use to control the speed of AC fans.

I do not think your electronic understanding will be up to this task.

** Nor yours.

Your zero x-ing detector should not be done with a 1N4001, but with a
1N4004 to -7.

** Sure - but that detector is dodgy anyhow.

You might consider doing an incandescent light control with your circuit
instead.

** A cheap pulse fired dimmer does that.

You will blow up either the motor or your triac.

** Very helpful - not.




........ Phil

 

[[email protected]]

I am trying to control speed of a single phase fan motor using triac phase
control, across a optoisolated boundary. The motor could be of shaded-pole
or capacitor-run type, between 30 and 200W.
The 4N25 optocoupler detects the zero crossing, a microcontroller turns on
an optotriac LED after the required delay, and holds it on until about 20deg
before the end of the half cycle.

I have noticed that 100% power (where the voltage across triac is almost
always zero) is at about 25deg for a 173W capacitor-run motor, 60deg for a
50W shaded pole motor. This is giving me two problems:
1. If I turn on the triac eariler than this point, I start to get
'half-waving' (?) but I can't see why this happens? Shouldn't it just
continue to run at 100%?
2. Since I don't know what motor my users will plug into it, how do I
prevent this happening? My zero crossing comes straight from the mains -
could I put the zero crossing detector across the triac to detect when the
triac turns off and have my timing referenced to that point instead?

I also notice that on the shaded-pole motor, the circuit doesn't seem to
turn on the triac properly at high firing angles. Starting at 140deg, I can
see where the triac is being turned on, the voltage across the triac dips to
zero briefly but then rises back up to mains voltage. At about 125 deg it
starts to do the negative half cycles properly, then positive half cycles
start working
from about 92deg. Does anyone know why this might happen?

Thanks for any help.

I cant read the circuit but I can guess what its supposed to look like.
Firstly because the motor is inductive the current crossing point (turn
off) is delayed and not very predictable, it also moves about during
operation. Timing your firing from the point of turn off rather than
mains crossing will improve performance no end. Secondly the triac
turns off at zero current but the voltage is not zero, this causes a
step voltage to be applied to the triac with can cause it to reconduct
again. You need a triac with a very good dv/dt rating and bigger than
normal snubbers. Thirdly at high firing angles the current does not
have time to rise above the triacs latching current so it goes off as
soon as the pulse ends, by using an opto to monitor the triac voltage
you can detect this condition and refire as required, this opto will
also give you your timing signal.

 

[Phil Allison]

Thirdly at high firing angles the current does not
have time to rise above the triacs latching current so it goes off as
soon as the pulse ends,

** The OP is ( allegedly) using continuous firing - not pulse.





........ Phil

 

[fssg]

What you want is IMHO impossible with your circuit. You will need a
variable frequency synthesized from a DC intermediate voltage similar to a
switched mode power supply. SCRs are absolutely unsuitable for this. I do
not think your electronic understanding will be up to this task.
Your zero x-ing detector should not be done with a 1N4001, but with a
1N4004 to -7. You might consider doing an incandescent light control with
your circuit instead.

Sorry I am actually using a suitable diode I just can't remember what it
is exactly so I put 1n4001 in the diagram. That SCR is actually a TRIAC,
maybe the diagram is not clear.

 

[fssg]

"Ban"



** Complete drivel.




** Triacs are widely use to control the speed of AC fans.




** Nor yours.




** Sure - but that detector is dodgy anyhow.

Hi Phil. Why is my detector dodgy? It's cheap, it causes some timing error
but I can remove that in software.

I don't know if you remember my last thread but I bought the CRO you
suggested. It's really bad At least it won't be such a great loss if I
blow it up while working on the mains. BTW how did you know I was in
Australia or was it just coincidence? (I am using a US newserver).

 

[Phil Allison]

"fssg"

Hi Phil. Why is my detector dodgy? It's cheap, it causes some timing
error but I can remove that in software.

** Then you agree it is dodgy.

I don't know if you remember my last thread but I bought the CRO you
suggested. It's really bad

** It is a nice CRO - particularly for the money.

At least it won't be such a great loss if I blow it up while working on
the mains.

** Never use it on mains without the probe switched to 10:1.

Even then, be VERY careful.

BTW how did you know I was in Australia or was it just coincidence? (I am
using a US newserver).

** I used it as an example of what is available.

DSE sell all over the world anyhow.




......... Phil

 

[fssg]

I cant read the circuit but I can guess what its supposed to look like.

Here it is in another format
http://users.bigpond.net.au/futile/ZeroCrossingAndTRIAC.gif

Firstly because the motor is inductive the current crossing point (turn
off) is delayed and not very predictable, it also moves about during
operation. Timing your firing from the point of turn off rather than
mains crossing will improve performance no end. Secondly the triac
turns off at zero current but the voltage is not zero, this causes a
step voltage to be applied to the triac with can cause it to reconduct
again. You need a triac with a very good dv/dt rating and bigger than
normal snubbers. Thirdly at high firing angles the current does not
have time to rise above the triacs latching current so it goes off as
soon as the pulse ends, by using an opto to monitor the triac voltage
you can detect this condition and refire as required, this opto will
also give you your timing signal.

Aha! Thankyou! Instead of just turning the optotriac on until the end of
the half cycle, I tried pulsing it at 40Hz. Now I can control both fans
over the full range without half-waving.

 

[fssg]

** The OP is ( allegedly) using continuous firing - not pulse.





....... Phil

Oops I didn't pick up on what you said before

"
** The LED inside the MOC3021 must be driven on and off correctly to avoid
" half waving" (*)

Do not pulse the LED - drive it on throughout the time you want the triac
to be on only turning it off 0.5 mS prior to the next zero crossing so the
triac can turn off at the next current minima.


* Half waving is common problem when PULSE fired triacs are used to phase
control an inductive load. The triac may turn off at an unexpected point in
the AC cycle since, at that point, current flow has gone through zero value.
The result is that the output waveform is unsymmetrical and has a large DC
component. Motors and transformers will make a growling noise and burn out
if this continues.

The solution is to continuously fire the triac for the whole time it should
be conducting.

BTW

Get yourself a scope !!!
"

 

[Phil Allison]

"fssg"

Aha! Thankyou! Instead of just turning the optotriac on until the end of
the half cycle, I tried pulsing it at 40Hz.

** ????




....... Phil

 

[Ban]

"Ban"



** Complete drivel.

Shaded pole motors are difficult to control. The already low starting torque
is reduced a lot when you fire at higher phase angles. then they just sit
and hum and get hot because of high inrush current. They will also get
hotter even when running, because the low efficiency will be reduced even
more.

** Triacs are widely use to control the speed of AC fans.

really? in HVAC units? I wouldn't buy one. Or do you mean AC opposed to DC?
But then the current should be monitored to avoid stalled condition.
What do you think of this sentence?

** Nor yours.

THX, how can you give such advice to this OP

** Sure - but that detector is dodgy anyhow.




** A cheap pulse fired dimmer does that.

This circuit is nothing else. do you see the cap in series with the triac
coupler? It won't allow continuous firing, only once every half cycle.

** Very helpful - not.

What will happen when that 200W induction motor stalls because of
brownout/too low speed??
**Phil you are an Idiot and Australian FUCKWIT!

 

[Phil Allison]

"Ban"

Shaded pole motors are difficult to control.

** Fans are dead easy to speed vary.

A simple Variac adjusts the speed *perfectly* over the full range.


( snip load of porcine wog drivel)

really?

** Yes - dickhead.

What do you think of this sentence?

** Fine.

This circuit is nothing else. do you see the cap in series with the triac
coupler? It won't allow continuous firing, only once every half cycle.

** There is no such cap - you dumb, wog wanker !!!!

What will happen when that 200W induction motor stalls because of
brownout/too low speed??

** Better go try your room fan with a Variac.

Be a MAJOR learning experience for a dumb wog shit like you.

**Phil you are an Idiot and Australian FUCKWIT!

** From a slimly DAGO shit like YOU - Ban


That is a compliment indeed.







........ Phil

 

[fssg]

"fssg"



** ????

I am driving the optotriac with a square wave, actually 2kHz (don't know
where I got 40Hz from?). I don't know why it works, but it works perfectly,
for both fans over the full range, using a sensitive gate triac with only
20dv/dt rating, and no snubber!

 

[Phil Allison]

"fssg"

I am driving the optotriac with a square wave, actually 2kHz (don't know
where I got 40Hz from?). I don't know why it works, but it works
perfectly, for both fans over the full range, using a sensitive gate triac
with only 20dv/dt rating, and no snubber!

** Congratulations - you have independently, if belatedly, discovered the
advantages of "continuos pulse firing".

This was the method traditionally used in professional dimming equipment for
high powered stage lighting, before opto- coupler triac drivers came into
vogue.

Typically, a stream of short ( 2 uS) bipolar pulses at about 20 kHz rate was
fed via a tiny pulse transformer into the gate of the triac. The pulse
stream was gated on and off ( relative to zero crossings) to follow the
desired conduction angle. This guaranteed proper operation even with large
and difficult transformer coupled lamp loads.

Your 2kHz pulse train should work out OK with low powered fans.




........ Phil