One important cause of damage during pneumatic actuator operation is often overlooked: loading. Long-term overload and inadequate protection can shorten actuator life or, in more serious cases, cause sudden equipment shutdown and disrupt production.
When can a pneumatic actuator become overloaded, and what protective measures are effective? This article explains.
1. Common causes of pneumatic actuator overload
Actuator load is not constant. Various factors affect it during operation, and the following five situations are particularly likely to cause overload:
Incorrect torque-limiter settings
Pneumatic actuators usually have an internal torque-limiting mechanism to prevent output torque from exceeding the design range. If the setting is incorrect, for example if the stopping torque is lower than the actual setting, the actuator may continuously deliver excessive torque without reaching the set threshold, causing the motor to stall or even burn out.
This requires particular attention during installation and commissioning. Settings must be precisely matched to the valve's actual conditions, including medium type, pressure rating and operating frequency, rather than simply set to the maximum for every unit.
Torque-limiting circuit failure
The torque-limiting mechanism relies on an electrical circuit to sense torque and provide feedback. Sensor failure, a loose signal connection or a damaged control board can disable torque limiting, leaving the actuator delivering high torque continuously without protection.
Recommendation: inspect the torque-limiting circuit regularly and repair abnormalities promptly. For equipment in long-term service, a functional test every six months is recommended.
High ambient temperature
Excessive ambient temperature directly reduces motor heat dissipation. Pneumatic actuators themselves generate heat during normal operation. If cooling is poor, for example in direct outdoor sunlight, near hot pipes or in poorly ventilated locations, the motor temperature rise reaches its limit more quickly.
An ambient operating temperature no higher than 60℃ is generally recommended for pneumatic actuators. In hot environments, provide shading and ventilation and, if necessary, additional cooling or an actuator rated for high temperatures.
Low supply voltage
This cause is also easily overlooked. If supply voltage is below the actuator's rated operating voltage, the motor cannot produce the required torque but continues running, eventually leading to excessive current and an overheating shutdown.
Common site examples include high voltage drop in long power cables and several devices starting simultaneously on one circuit. Measure actual operating voltage before installation to confirm it is within the rated range. Add voltage stabilization or shorten the supply distance if necessary.
Heat accumulation from continuous use
Pneumatic actuators normally avoid overload when operated within a predetermined duty period. However, continuous high-frequency operation without adequate cooling allows heat to accumulate beyond the motor's permissible temperature rise, triggering thermal protection or damaging the motor.
This is especially relevant to frequent starts and stops over a short period, such as modulating applications. Repeated motor starts produce high starting currents, causing heat to accumulate faster than during continuous operation. Reduce switching frequency where appropriate or select an actuator better suited to frequent starts and stops.
2. Pneumatic actuator load protection
To address these load problems, pneumatic actuators usually have the following protective measures, allowing timely shutdown and limiting damage under abnormal conditions:
- Thermal overload relay: protection against motor stalling
A thermal overload relay is the most common protective component in pneumatic actuators. When the motor stalls or current increases abnormally, heat bends the relay's bimetallic strip, operating its contacts and disconnecting motor power to prevent overheating and burnout.
Relay trip current can be adjusted to the actuator's rated current and is generally set to 1.05-1.2 times the motor's rated current. Too low a setting causes nuisance trips; too high a setting reduces protection, so it must be configured appropriately.
- Thermostat: protection against continuous-duty overload
A thermostat, also called a temperature switch, monitors motor winding temperature. When continuous running or electrical operation raises temperature above the setpoint, the thermostat opens the circuit and reconnects it after the temperature falls.
Unlike a thermal overload relay, a thermostat senses temperature directly rather than current, so it responds more directly and accurately to slowly developing overheating caused by high ambient temperature or inadequate cooling. Using both devices together gives the best protection.
- Fuse or overcurrent relay: prevention of short-circuit accidents
Fuses and overcurrent relays mainly protect circuits from short-circuit currents. A short circuit can instantly raise current to several or even tens of times its normal value. The fuse blows or the overcurrent relay trips rapidly, disconnecting power and protecting the electrical system.
Fuse selection must match operating current and starting characteristics; do not arbitrarily increase its rating. Overcurrent relay settings are generally 130%-150% of the motor's rated current.
3. Recommendations for everyday operation
After understanding overload causes and protection, also consider the following during everyday use:
- Inspect protective devices regularly: thermal overload relays, thermostats, fuses and other protective components may age or fail over time. Check their condition quarterly.
- Maintain cooling conditions: keep the area around the actuator well ventilated. Do not stack objects on it or block cooling passages.
- Keep operating records: record the time and cause of every overload or protective trip to help analyze equipment condition and identify potential problems early.
- Allow a sizing margin: at the selection stage, a 10%-20% safety margin above calculated torque is recommended, so moderate process fluctuations do not readily push the actuator into overload.
Effective protection matters more than the number of protective devices. Set protection parameters appropriately, perform regular maintenance and inspection, and improve operating conditions. These three measures can prevent most load problems in advance.
Zhejiang HES Automatic Instrument & Valve Co., Ltd.
Established in 1989 · Focused on pneumatic actuators for more than three decades
Products include multiple safety measures such as thermal and overload protection