Using triac to switch 240VAC induction motor from reed switch?



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 :-))


.



Relevant Pages

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