Investigating Triac SSR Commutation With an Inductive Load

An AC solid state relay using a triac can remain conducting or retrigger during an inductive load transition even though its input command has ended. Current and voltage do not necessarily cross zero together. Review commutation behavior separately from ordinary off-state leakage or a persistent input signal.

Identify the output technology

The Omron technical explanation of solid state relays distinguishes output technologies and discusses release problems associated with rapid voltage changes. The acceptable load and waveform depend on the selected relay’s documented characteristics.

Confirm whether the output uses a triac, inverse parallel thyristors, or MOSFETs. A connection suitable for one type may not provide the same switching behavior with another. Check the intended AC supply waveform and load category.

Observe current and voltage together

Capture load current, relay output voltage, and input command using equipment appropriate to the circuit’s voltage and isolation requirements. Identify when load current approaches zero and how quickly voltage then appears across the output.

Do not infer turnoff timing from supply voltage zero alone. An inductive load can keep current flowing after that point, changing the conditions under which the switching element must stop conducting.

Distinguish the failure mechanism

Verify that the input has actually reached its specified inactive condition. Then compare the observed output behavior with the relay’s commutation and voltage slew requirements. Repeated conduction after a current zero is different from a small continuous off leakage current.

Record the load’s actual operating state. A motor, solenoid, or transformer can present different conditions during startup, steady operation, and release. A resistive test load may not reproduce the problem.

Evaluate the complete suppression network

Use the relay and load manufacturers’ guidance when considering snubbers, clamps, or a different switching technology. Check the effect on leakage, component dissipation, and load release, as well as on voltage slew.

A suppression change that helps commutation can alter off-state current and therefore affect a small load. Verify both requirements rather than accepting one improved waveform as a complete correction.

Save output technology, load identity, supply waveform, synchronized captures, and the reviewed correction. Repeat across the relevant load and temperature conditions. A useful conclusion explains why the output fails to release and demonstrates proper commutation under the intended conditions. It does not generalize a relay’s resistive current rating into approval for every inductive load or every non-sinusoidal supply waveform. Record the elapsed time between switching events because the load state may depend on previous operation.