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 :-))