Slide
Fail-safe Electric
Actuation
Failsafe devices and actuators operate after line power has been lost using an alternative energy source that allows the actuator to drive to a predetermined position, usually either open or close.
Energy Sources for Failsafe Operation of Electric Actuators
There are typically three types of energy that can be applied to operate an electric actuator without power
  1. Spring Return Mechanism – A preloaded spring forces the actuator to a default position when power is lost.
  2. Capacitor Discharge – Energy stored in capacitors provides enough power to move the actuator when power is lost.
  3. Battery Backup – A built-in or external battery supplies power to drive the actuator to a safe position.
System
Pro
Con
Spring Return actuators are best suited for critical safety applications where immediate and predictable valve or damper closure or opening is essential for safety, such as emergency shutdown systems.
Spring Return
High Reliability
Few suppliers
Simple Design
Torque Output limited by size/weight
Springs Do not need replacement
Heavy/Expensive
Actuators that have capacitor backup are found where reliable fail-safe action is needed, especially in cases where maintenance is not readily available.
Capacitors
Long Life Cycle
Low Specific Energy
Short Charging Times
Linear Discharge Voltage
High Load Currents
High Self-discharge
Excellent Temperature Performance
High Cost per Watt hour
Battery backup is best used where extended operation during power outages and prolonged operation is needed. Routine maintenance is required to change the batteries.
Battery
Power Density
Limited cycle life
Storage Capability
Voltage and Current limitations
Better Leakage Current than Capacitors
Long Charging Times
Constant Voltage
Temperature sensitivity
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