Changeover Contact (Changeover / SPDT)
A three-terminal contact structure in which a common terminal shares one normally closed and one normally open terminal. When the relay is not energised the common terminal is connected to the NC side, and when it pulls in to the NO side; this way a single contact can switch between two different circuits.
A changeover contact lets you close one path and open the other without using two separate relays. The common terminal (C) is always connected to only one of the two paths; a short break occurs at the moment of transition, meaning the two paths are never closed at the same time.
Places where this structure is valuable on a crane remote control:
- Direction selection: the common terminal goes to the control voltage, the NO terminal to the forward coil and the NC terminal to the reverse coil. Energising both coils at the same time becomes impossible by the very structure of the contact.
- Source selection: routing the command for the same motion between a wired push button and the radio remote control.
- Status indication: taking an indicator lamp from the NC terminal and a run signal from the NO terminal.
The interlocking logic a changeover contact provides is useful but is not considered sufficient on its own. Against the possibility of contact welding, also place electrical and mechanical interlocking between the contactors in forward-reverse applications. Calculate the contact's current rating according to the load type as well, reducing it for an inductive load.
The context in which this term is used: Wireless Crane Remote Control Selection Guide