John Stumbles wrote:
> I want to switch an induction motor from a flow switch which is rated too
> low to drive the motor directly, so I'm planning to use a triac and
> would appreciate some advice on how to do it.
>
> The motor is a central heating circulating pump which has a
> single-phase motor with capacitor driving a quadrature-phase winding. The
> unit is rated at up to 82W, 0.36A @ 230V AC
>
> The flow switch is rated at 15VA, max 1A AC, into resistive loads.
>
> I've picked up a couple of BTA16 600B triacs and some 250V AC MOV
> suppressors which seemed like a good starting point (OK a better starting
> point might have been to design the circuit first ... :-).
>
> I'm now wondering about how to connect it all up. From the point of view
> of triggering the triac I'm thinking of something like this:
>
> -------------- LIVE 240V AC
> |
> -------
> | MOTOR |
> -------
> -------------|
> | ------------
> FLOW \ | MT2 |
> SW o | |
> | | TRIAC |
> ------| G |
> | MT1 |
> ------------
> |
> -------------- NEUTRAL
>
> However if the switch closes at peak mains voltage then there's going to
> be a surge of current into the gate before the triac gets fully turned on,
> which could damage the triac and/or reed switch. Therefore I think I
> should put a resistor in line with the switch. The peak gate current of
> the triac is 4A for 20uS. (This is more than the steady-state rating of
> the reed switch but I'm guessing/hoping that a mechanical switch can take
> orders of magnitude bigger transient peaks than a semiconductor can.) I
> don't know how long the triac will take to turn on and therefore the
> voltage across it to drop but 20uS sounds about right, so allowing for 4A
> max gives a resistance value of (340V peak / 4A = 85 ohm) about 100 ohm.
> The triac gate current to turn on is 100mA worst case (quadrant IV) so the
> triac should turn on when the supply voltage has risen to (0.1A * 100 ohm
> =) 10V.
>
> My other concern is switch-off transients. Since the load is inductive
> then, even though the triac should turn off at a mains zero crossing,
> there will still be current flowing in the motor winding which will
> generate a big voltage spike. I'm hoping that the MOV will catch
> this. I'm thinking of connecting it across the triac, since that's what
> I'm trying to protect.
>
> I now have this circuit:
>
> -------------- LIVE 240V AC
> |
> -------
> | MOTOR |
> -------
> 100R |
> --/\/\-------|---------------
> | ------------ |
> FLOW \ | MT2 | |
> SW o | | -----
> | | TRIAC | | MOV |
> ------| G | -----
> | MT1 | |
> ------------ |
> |---------------
> |
> -------------- NEUTRAL
>
>
> OK what do folks reckon? Will it fly, or crash and burn?
> Any better suggestions/improvements etc.
>
> In particular I wonder if I should be using a snubber network as well as
> an MOV? (And if so, why?) If so, what components? I've heard 0.1uF and 100
> ohm mentioned. And what sort of capacitor? 400V polyester OK?
>
>
> A bit of background: I've been out of the electronics game
> for more years than I care to remember and never did much with triacs and
> suchlike even when I was involved. However I do plenty of mains work (so
> please feel free to skip the health warnings :-))
Hi, John. A couple of things:
Apart from the "health warnings", it's considered normal practice these
days to use low voltage for control circuits. In the intervening years
since you've been out of the electronics game, using line voltage for
control circuitry has become very bad form. Not only that, but it
could get you in serious legal trouble if something goes wrong and
someone gets hurt.
Second, your reed switch is rated for 15VA, 1A max. Assuming your
switch makes contact at the peak of the AC line cycle, you're switching
400V, which is a mistake. I'll bet if you look in the fine print,
you'll find the maximum switching voltage of the reed switch is either
100V or 150V. Also, the 100 ohm series resistor means the peak gate
current will be in excess of 1 amp, also a problem. Your circuit will
spot weld closed the contacts of the reed switch in short order.
It might be best to forget the triac. Get yourself a small DC wall
wart, a proper control relay with the same coil voltage, and a
transistor and couple of resistors sized properly to drive the relay.
You can then use the reed switch to apply voltage to the base of the
transistor to turn on the relay. This will give you much better
reliability, and is really what small control reed switches are meant
to do.
This circuit with a 12VDC unregulated wall wart and the components
chosen will drive just about any reasonably sized 12VDC coil control
relay (view in fixed font or MS Notepad):
|
| +12V _
| + / \ L
| | .---------( M )-----o
| .---o--------o---. | \_/
| | | |RY1 |
| o 1N4002| | |
| '\ SW1 - C| |
| \ ^ C| / |
| o \ | C| - - - - --- ---
| | | | -------
| | | | / |CRY1
| | '---o |
| | | | N
| | ___ |/ '---------------------o
| ---o--|___|- -| TIP100
| 10K | |>
| .-. |
| 10K| | |
| | | ===
| '-' GND
| |
| ===
| GND
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
Questions of this type usually get a good reception at
sci.electronics.basics
Good luck
Chris
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