BASIC PRINCIPLE OF FAIL SAFE ELECTRIC ACTATORS
An electric actuator in the valve automation field is a device that converts electrically generated high speed low rotary
motor torque into low speed high rotary torque which, when coupled to a part-turn or multi-turn valve, drives the
In basic mechanical electric valve actuator types, the motor is stopped at the desired position when cams mounted to
the actuator’s output drive shaft make mechanical switches. Without electrical power the electric actuator cannot
move and stays put.
A fail safe electric actuator has a way of driving the actuator to the desired fail safe position when external power is
unavailable. There are 2 basic ways of achieving fail safe functionality:
1) Stored electrical energy that can be used in place of the external power to power the motor
2) Stored mechanical energy to physically drive the gear box independently from the motor
ACTUATOR FAIL SAFE FUNCTION – STORED ELECTRICAL ENERGY
Two methods of storing electrical charge that can be used to drive the electric
actuator’s motor when external power is unavailable are in common use, a fully
re-chargeable battery back-up system, or capacitors.
Battery back-up systems use industrial strength rechargeable batteries that are
kept at full charge by a trickle-charger when external power is available. An internal
switch draws charge from the batteries when external power is lost, and
the trickle charger replaces the charge used when external power is restored.
The security of the battery needs to be considered as it ‘s capacity can be affected
by insufficient recharge and battery degradation, and these factors can
limit it’s use in some applications.
A capacitor is an electronic component that fully charges within a few seconds
as soon as external power is applied to it and it typically holds its charge indefinitely.
This makes the capacitor the safer of the two stored electrical energy fail
safe methods, however they can only store relatively low amounts of charge or
they become excessively large and impractical.
ACTUATOR FAIL SAFE FUNCTION – STORED MECHANICAL ENERGY
Electro-mechanical spring actuators store kinetic energy in an internal spring
system and external power holds the springs in their compressed state whilst
available. When external power is lost, the hold is released and the springs
discharge their energy to physically drive the gears in the actuator’s gearbox,
completely independently of the actuator’s electrical system eg: circuitry and
The clear advantage of a mechanical fail safe system in electric actuators is
this elimination of the actuator’s electrical system and the fail safe system is
100% mechanical, and specifiers feel more secure with this than trusting batteries
or electronic components, which can fail.
The disadvantage of a mechanical spring system is it’s physical size and
weight, and cost. Typically the weight is 10-15 time heavier than a failsafe
electric actuator with battery or capacitor, and the cost can be similar.
The final choice will be dictated by the application and if the valve simply
must fail safe, the mechanical spring system will generally be selected.