TRIAC control of inductive load. Current ringing at higher power causes TRIAC to remain on

Question

Here is the circuit I am using to control the motor. L2 is meant to represent the motor and C3 is the capacitor that comes with the motor. It is a single phase AC motor found here.

This circuit is taken from LTSpice. This particular simulation is designed to simulate firing the TRIAC at 50% power, at each peak of the sine wave. In the simulation I am seeing motor current ringing which causes the TRIAC to remain on for several AC half cycles.

In the real circuit, I only start to see this behaviour at power level of 65% or more. At 65% the TRIAC fires in the right place for the first half cycle, but for the next half cycle the TRIAC doesn't turn off due to non-zero current, and so 100% of the power is delivered to the motor for that half cycle. After that I guess current does go to zero because the TRIAC turns off. Then it fires again in the next half cycle and the process repeats. So the total average power delivered to the load is not 65%, it is (65 + 100) / 2 = 82.5%.

This is not stalling the project. We can manage it. I am just wondering if any of you have any suggestions for additions to be made to the circuit to help reduce this ringing for more reliable TRIAC control of the inductive load.

Answer 1

Does R1C1 not act as a snubber across the triac already?

Yes, but R1 is too high for triac snubber, actually R1C1 is the snubber for opto triac, also. You should use something like 47 ohm and 22nF, or even your old snubber 39 ohm 10nF.

R1 is the key. Using high R value is good for optotriac (high immunity to dV/dt) but not for triac. Decreasing R1 is good for the triac (high immunity to dV/dt).

Fitting a Rgk also helps.

By the way, your LTSpice circuit is wrong: the motor is not composed by inductor and capacitor in series, this is just an auxiliary winding, there shall be another winding in parallel of them.