Protective functions of electric actuator

The main components of an electric drive are a reduction mechanism and an electric motor, but depending on the specific application, electric drives for regulating valves can be equipped with additional devices:

Limit switches are necessary for disconnecting the drive when the valve reaches its extreme positions.
A force limiting device is needed to prevent damage to the equipment or overloading of the electric drive.
A manual override allows the regulating valve to be opened or closed manually in case of power failure or failure of the electric drive.
A valve position indicator allows for visual monitoring of the regulating valve's operation with the electric drive.

The protective function of electric drives determines the behavior of the stem when the power is disconnected and is necessary to protect the equipment and systems, not the electric drive itself.

A vivid example of the need for the protective function of an electric drive is a water heater of an air heating supply unit, the flow of the heat transfer medium through which is determined by a valve with an electric drive. In case of a power failure, the protective function of the electric drive should open the valve and allow the maximum flow of the heat transfer medium through the heater. In the absence of a protective function in the electric drive, a power failure may occur when the flow of the heat transfer medium is completely blocked and the influx of cold air has not stopped, which will cause the heater to thaw.

Variants of implementing the protective function of electric drives:

The stem stops when the power is disconnected.
The stem moves upwards when the power is disconnected. The upward movement is usually done by a return spring.
The stem moves downwards when the power is disconnected. The downward movement is usually done by a return spring.

Note that depending on the valve design, a stem of an electric drive lowered downwards may detach the passage section of the valve or close it.