At a power plant, the fan control damper driven by an AUMA electric actuator once behaved strangely in automatic modulation: the command dropped from 41% to 31%, but the feedback did not move at all and neither did the valve locally; the control loop tripped out of automatic. The operators checked the actuator on site and found nothing abnormal. About an hour later, the feedback suddenly dropped to 30% by itself, and modulation worked normally ever after.
- In automatic modulation, the command changed but the feedback did not follow; the valve did not move locally and the loop dropped out of automatic;
- On-site inspection showed nothing wrong with the actuator or its wiring;
- About an hour later the feedback "recovered" by itself and modulation resumed as if nothing had happened.
Experienced I&C maintenance engineers immediately identified the crux: in continuous automatic modulation the actuator motor reverses back and forth, heat accumulates, and the internal thermal switch (motor over-temperature protection) trips, cutting off the control circuit — so the motor ignores the command and the valve stays put. Once the motor cools down below the threshold, the thermal switch contact closes again, the control circuit is restored, and the actuator carries on as if nothing had happened. That stop-and-recover interval matches the observed hour exactly.
The reporter confirmed this explanation on the spot. Similar cases appeared in the discussion: at another plant the same symptom turned out to be a failed feedback potentiometer, and replacing it cured the problem — meaning that for "command and feedback disagree yet nothing moves," both the thermal protection and the potentiometer deserve investigation.
Since the root cause is motor overheating, the fix revolves around "move less, load lighter, cool better":
1. Reduce cycling: retune the PID parameters in the DCS to lower the actuator's response sensitivity, and widen the actuator dead band appropriately — with too small a dead band the actuator hunts on every tiny deviation, and constant cycling inevitably overheats the motor;
2. Check the mechanical load: if the damper/valve sticks or the actuator torque rating is undersized, the motor strains every stroke and overheats faster — remove the sticking or move up one torque size;
3. Improve cooling: one plant fitted instrument-air cooling to frequently cycling actuators with good results;
4. Mind the re-transfer details: after the loop drops out of automatic, first match the command to the feedback before re-transferring, to avoid a sudden jump on recovery.
When an electric actuator in modulating service "suddenly freezes and recovers by itself a while later," the odds heavily favor motor thermal protection doing its job — it is protecting the motor, not a broken actuator. Before swapping hardware: check whether cycling is excessive (PID and dead band), whether the mechanics stick, and whether the torque rating is sufficient. Remove the cause of the overheating and the fault disappears with it.
