Consideration must be taken when designing a control system as to what happens when controllers fail or if there is a loss of power. This is referred to as fail-safe or spring return. Devices, like valves and dampers, can be made to fail in a position that provides a minimum amount of comfort control or, more importantly, protects expensive mechanical equipment and building integrity.
One example of this is an outside air damper on an air handler. Most outside air dampers are configured to close when there is a power failure. This protects water coils from cold air. Cold air can freeze coils and cause them to burst, leading to expensive repairs and further system down time.
Another example is on a hot water valve. Hot water valves can be set up to fail open to help protect coils from freezing and provides some degree of comfort control. Conversely steam valves are often set up to be fail closed. This is to protect equipment and, more importantly, people.
Achieving a fail position, being open or closed, is usually achieved by one of two methods: mechanical return or electric return.
Mechanical return usually involves a spring. Basically the electric motor works against a spring. When power is removed from the motor the spring contracts and moves the actuator to the fail position. Fail open or fail closed is usually dictated by the mounting orientation of the actuator.
Electric return, or capacitive discharge return, involves an electric storage device that is built into the actuator. The storage device, most likely a capacitor, discharges when power is lost and drives the actuator to the fail position. Usually the direction that electric return actuators fail in is determined by a selector switch.
Battery backup return is very similar to capacitive discharge return. The main difference is that a battery is used to store the electricity needed to power the actuator to the fail position. Due to their size, battery backup units are usually only found on very large valves.
That is pretty much the basics of spring return, fail-safe electric motors. There is a great amount of debate as to which is best. Spring return is proven technology that many still swear by. Electric return lost a good bit of credibility early on because the early units were unreliable but that has changed. Electric spring return units now benefit from improved design and are available in higher torque ratings than their spring return counterparts.