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Controlling a motor in presence of phase shift

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 --| -| 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

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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.

Reply to
fssg
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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.

You will blow up either the motor or your triac.

--
ciao Ban
Apricale, Italy
Reply to
Ban

"Ban"

** Complete drivel.

** Triacs are widely use to control the speed of AC fans.
** Nor yours.
** Sure - but that detector is dodgy anyhow.

** A cheap pulse fired dimmer does that.

** Very helpful - not.

....... Phil

Reply to
Phil Allison

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.

Reply to
cbarn24050

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

....... Phil

Reply to
Phil Allison

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.

Reply to
fssg

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).

Reply to
fssg

"fssg"

** Then you agree it is dodgy.

** It is a nice CRO - particularly for the money.
** Never use it on mains without the probe switched to 10:1.

Even then, be VERY careful.

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

DSE sell all over the world anyhow.

........ Phil

Reply to
Phil Allison

Here it is in another format

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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.

Reply to
fssg

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 !!! "

Reply to
fssg

"fssg"

** ????

...... Phil

Reply to
Phil Allison

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.

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?

THX, how can you give such advice to this OP

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.

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

**Phil you are an Idiot and Australian FUCKWIT! 
--
ciao Ban
Apricale, Italy
Reply to
Ban

"Ban"

** Fans are dead easy to speed vary.

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

( snip load of porcine wog drivel)

** Yes - d*****ad.

** Fine.

** There is no such cap - you dumb, wog wanker !!!!
** Better go try your room fan with a Variac.

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

** From a slimly DAGO shit like YOU - Ban

That is a compliment indeed.

....... Phil

Reply to
Phil Allison

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!

Reply to
fssg

"fssg"

** 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

Reply to
Phil Allison

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