Re: Monitor mains voltage with transformer


"Chris" <cfoley1064@xxxxxxxxx> wrote in message
news:1148272485.207507.222930@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

fssg wrote:
Hi,

I made a microcontroller circuit to control speed of a small fan with
TRIAC
phase control. It works fairly well, but the fan never completely stops,
among other things, so I am wondering if the TRIAC is being triggered the
way I expect.

I don't have a CRO, so I want to sample the output voltage from the TRIAC
using the microcontroller's ADC.

I could do this using a simple resistor divider, but then I have to
connect
circuit ground to Neutral, and circuit ground is my computer ground.
Neutral is supposedly connected to earth, which in theory would make this
safe (I would test first), but it seems like bad practice to do this...

I have a transformer, 240V->24V 20VA 50/60Hz. If I put it's primary on
the
TRIAC output and a resistor divider on the secondary, that would probably
be
safe, but will it work? Will I be able to see the sharp edges where the
TRIAC turns on in a half cycle?


Another question about whether the transformer will pass the signal: To
detect zero crossing, I am using an optocoupler, but other circuits show
using the AC output of the power supply transformer. Because the
transformer is inductive, wouldn't there be a delay between the actual
zero
crossing and when the zero crossing occurs on the secondary?


Thanks,
David

Hi, David. Did you expect a good answer without providing much of any
pertinent information? Maybe you could start by answering a few
questions:

* What type of motor do we have here? Is it an AC induction motor?

I want to make this for a hobby community I am involved in, I don't know
what fan people might use but it will probably be in the 30-200W range.
Capacitor run will probably be the most common, but some people use shaded
pole types.


* What's the current rating (or watts rating) on the nameplate?


I am testing with 3 different fans. One is '240VAC 0.25A 50Hz' shaded pole.
One is '230V 50Hz 173W' capacitor run. One is a self-oscillating room fan,
'240VAC 50Hz ONLY 50W', not sure what motor type but it has 3 speeds and
there are 4 wires going up into the motor.

* Are you using an optocoupler to drive your triac? (Hope so!) If
so, which one? Is it a zero-crossing triac trigger? (If it is, throw
it out and get one that isn't.)


I am using a MOC3021, which is nto a zero-crossing type.

* Are you timing a delay from the zero crossing of the AC line?


I detect zero crossing with a 4N25 and start a timer in my microcontroller,
driving the MOC3021 at a variable time after the zero crossing.

* Have you provided a snubber (R-C or other) across the triac? Could
you sketch out your circuit, using an ASCII Circuit program? (possibly
use Andy's ASCII Circuit v1.28 http://www.tech-chat.de )


This schematic doesn't show it, but I am using a 1/2W 68 ohm and a 100nF X2
across MT1 and MT2.

'------o------o------o--- Active
| | | |
470R.-. | | |
| | | | |
| | | | |
MOC3021 360R '-' | | |
___ | | | |
-----' .----|___|-----o | | |
| _|_ | ' .-.M |
V -> V_A | _|_ | |O ---
- / | | V_A | |V --- 100nF
| '--------------)--- / | '-' |
----' | | | |
| | | |
--- | | |
47nF --- | | |
| | | ___ |
'------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)



To start with, there are a few things here. First, you should know
that phase control of AC induction motors is difficult, and sometimes
it just can't be reliably done. Problems do tend to occur at the low
end, and can actually be more difficult with smaller motors.

However, there are several things you can try here.

* If you haven't already, put an R-C snubber across the triac (start
with 220 ohms 1/2 watt in series with 0.1uF line service self-healing
cap).

I didn't have one when I first started because there appears to be one in
the schematic above, which I copied from a MOC3021 data*** appnote.
Apparently that is something to do with the TRIAC gate so now I have a 68R
and 100nF extra across MT1 and MT2.

* Obviously the biggest problem here is the inductance. You're going
to have difficulty turning the triac off if the curent is going great
guns while the voltage crosses zero. One good solution for this is
using back-to-back SCRs, with separate optocouplers for each SCR.
Obviously the reverse-biased one won't trigger, and the other one will
stop when the voltage reverses. There are many solid-state relays
which are made like this, and all you have to do is provide the 3mA or
so to drive the opto input (make sure you don't get an SSR with
zero-crossing triggering). One cheaper solution, which *might* help is
to get a Teccor Alternistor, which has many of the characteristics of
back-to-back SCRs. It provides better turn-off behavior than a
standard triac. I've found I had better luck driving inductive loads
with these:

http://www.littelfuse.com/data/en/Data_Sheets/E4AlternistorTriac.pdf


Is an alternistor what ST would call a 'snubberless' triac? If so, don't I
need the ability to trigger in the 4th quadrant to do power control?

I have a very old (Nov 1980 date code) fan speed controller here, which is
almost identical to the circuit in figure AN1003.15 in
http://www.littelfuse.com/data/en/Application_Notes/AN1003.pdf. This
controls all 3 fans perfectly, but not to a complete stop. It is triac
based so I should be able to get away without SCRs. In fact it doesn't even
have a snubber...

* If you're desperate, and the motor's small enough, you can start
trying to change the power factor of the load by adding a power
resistor in parallel with the motor. Adding a capacitor across the
motor is a spectacularly bad idea, because you'll cause di/dt failure
in the thyristor (too fast a change in current causes the triac to
smoke). Of course, you're just burning power this way, but it can
help.

* If worse comes to worst, you can always do it like the dimmer
switches used for ceiling fan controls -- they reduce the speed as you
dial down, and then use a switch to turn off. You can always just use
a relay in addition to your phase control.

Having said all of this, I'd guess that, especially without a scope,
you're going to have an easier time just working on solving your
problem than trying to diagnose it by reading the voltage across the
thyristor or the load with your uC. You're going down a rabbit hole
here -- you just want to control the motor load. Take the steps you
need to do that, and worry about the other stuff later.

Actually, there are many applications where metering the voltage (or
more frequently, the current) on an AC load *is* a good idea. It can
be a good part of the control circuit, and there are many ways to do
this -- but that's actually another problem. In fact, as long as you
don't load the transformer secondary, you can get a pretty good idea
where the zero crossing is, and what's going on with the load voltage.
And there's no lag to speak of between transformer primary and
secondary. You are also adding inductance, and changing the circuit
you're measuring, too. But let's work on the fan first here.

Feel free to take the time to express your problem a little better --
the few minutes it might take to actually describe your problem well is
a lot easier to spend than hours thrashing around in the lab.

Good luck
Chris


OK, well since I first posted I have found a few bugs, mostly in my zero
crossing detection circuit. That part is working properly now and matching
my calculations.

I tested my triac firing output from the microcontroller by putting an RC
circuit on it so that I can get a duty cycle reading. It also matches
expectations, giving 0-100% over 0-90deg firing angles.

Unfortunately fan speed control is not so predictable.

The 173W capacitor run fan:

Starting with 90deg firing angle (zero power), works as expected, fan
stopped, no buzzing and doesn't get hot. Speed increases as expected up
until 45deg when it doesn't get any faster. However, at 16deg, the speed
starts jumping around, 1s at full, 1s at slightly slower, and so on. At 14
deg it slows right down, starts buzzing and generally just gets very hot.
This continues until 2deg, when the fan stops and I cannot hear any buzzing
at all.

Starting at 0deg and increasing power, starts silent, then starts buzzing at
2 deg, buzzes until 19deg, then at 21deg it takes off full speed. Stays
full speed until about 45deg, when I can detect a bit of a slow down (by
ear). Can control speed right down to zero.

The shaded pole 0.25A fan:
Barely worth writing about. At 90deg, no buzzing as expected, but as power
is increased, starts to spin but vibrates soflty at what sounds like about
10Hz. On the way from 90 to 0, fan starts to spin then stops at 3 or 4
places in this range. Eventually burns out snubber resistor.

This ^^^ is with 68R and 100nF snubber.

I have tried 3 different triacs (in brackets, critical rate of rise of off
state voltage):
T405-600 (20V/us)
BT137X-600 (100V/us)
BT139-600 (250V/us)

There must be something wrong with my circuit?

Thanks Chris I really appreciate your help.


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