# 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 :-))

.

• Follow-Ups:

## Relevant Pages

• Re: Simple 555 PWM - disappointing performance
... The diode is placed to ensure the motor ... >>>current has somewhere to commutate to when the switch turns off. ... stages then you must reconnect the timer to a lower voltage and omit ...
(sci.electronics.design)
• Re: Simple 555 PWM - disappointing performance
... Further since you are now switching voltage across the motor, ... The diode is placed to ensure the motor current has somewhere to commutate to when the switch turns off. ... for a pnp switch, the pnp collector goes to the motor and the cathode of the ...
(sci.electronics.design)
• Re: Two-speed motors with inverters (again)
... winding is switched damaging whatever is used in the output stage. ... of the switch? ... change in impedance between the supply cables and the motor windings. ... They will limit the output voltage to no more or less than the DC ...
(uk.rec.models.engineering)
• Re: Two-speed motors with inverters (again)
... winding is switched damaging whatever is used in the output stage. ... of the switch? ... change in impedance between the supply cables and the motor windings. ... They will limit the output voltage to no more or less than the DC ...
(uk.rec.models.engineering)
• Re: PWM for dc Motor
... if the motor isn't stalled, wheeling diodes installed or low side ... Suppose you drive it with a square wave, PWM to whatever average voltage output you want. ... It doesn't have "diodes" -- well, the little triangles in the MOSFET symbols are the body diodes doing that, but up to 20A or so, they won't draw appreciable current. ... It likewise follows that current cannot change suddenly (as could happen if you drove it with a single on/off switch), assuring you a roughly triangular, smoothly varying current waveform. ...
(sci.electronics.design)