Whenever a motor is started, it draws an initial inrush current. If several motors are to be started simultaneously, these combined inrush currents could cause line disturbances throughout the system and cause nuisance tripping of overcurrent devices.

If two or more motors are controlled from the same pushbutton station, it may be desirable to provide a time interval between their starting. One of the ways that we can accomplish this is to use timing relays.

Timers are similar in construction to control relays in the sense that they have a low-voltage armature and coil that opens or closes a set of contacts when energized. The distinctive quality of timers is the addition of some sort of delay mechanism that prevents the auxiliary contacts from changing their state for some pre-set amount of time.

Timers come in two classes: On-delay timers and off-delay timers.

On-Delay Timers

These are commonly used in circuits where we might want to start two motors from the same pushbutton station but with a slight delay to minimize line disturbances due to inrush. The timing contacts for the on-delay timer can be either normally open, timed to close (NOTC), or normally closed, timed to open (NCTO).

On-delay timers delay their action when the coil is energized and revert to their original state instantly when de-energized. Some timers will have instantaneous contacts in addition to their timed contacts.

When looking at the schematic symbol for timed contacts, the arrowhead indicates the direction of the timing action. Some schematic drawings show timed contacts as instantaneous contacts identified with their timing behavior.

Off-Delay Timers

These are commonly used in circuits where we might want to stop two motors from the same pushbutton station but allow the second motor to run for an additional period of time. This arrangement is commonly used in furnace HVAC systems and conveyor belt circuits. The timing contacts for the off-delay timer can be either normally open, timed to open (NOTO), or normally closed, timed to close (NCTC).

Off-delay timers change their original state instantly when energized. And delay their action when the coil is de-energized. Some timers will have instantaneous contacts in addition to their timed contacts.

When looking at the schematic symbol for timed contacts the arrowhead indicates the direction of the timing action.

Some schematic drawings show timed contacts as instantaneous contacts identified with their timing behavior.