Triac motor controller

Question

I have build a circuit with the purpose of driving a ~1000VA AC motor. The motor controller fires a triac only at zero crossings. The circuit is as follows:

^{simulate this circuit – Schematic created using CircuitLab}

Both C2 and C1 are ceramic capacitors. R4 is a 5W resistor, the rest is regular 1/8W. The BTA208S-800E has a proper heatsink mounted directly to it.

The problem is this: after a few seconds of operation the circuit gets really hot and a little later starts to malfunction (random firing, loss of speed control).

The motor is from a old vacuum cleaner and therefore I have no datasheet for it (Google didn't yield any results either).

The circuit is one found on the internet, since I do not have the required knowlegde to calculate RC snubbers.

Answer 1

The BT208S is a smallish SMT triac- you will be dissipating 5-6W (assuming European mains) with a 1000VA motor. That's a lot of power.. Probably your heat sink is woefully inadequate for the power dissipation and the triac is overheating.

If just mounted on FR4 board the Rth-ja is 75K/W so 5W would heat it to about 400C -- so maybe your heatsink is better than that, but nowhere good enough.

Edit: For that amount of power, to keep the junction temperature sensible, you either need a BIG heatsink coupled thermally to the MT2 of the triac (might require an aluminum board) or a fairly large heatsink with a lot of air flowing over it.

If you just want to turn the motor on and off, forget about the triac and use a power relay.

Answer 2

R3 = 2.7k, R2 = 220R, C1 = 100n and connects to NEUTRAL, NOT gate. Works fine for me.