Siemens 3RP25 Manual page 63

Application example based on standard operation (operation 1): For example, use of
3RP2560 for compressor control
Frequent starting of compressors strains the network and the machine, and increases costs
for operators. The new time relay prevents frequent starting at times when there is high
demand for compressed air. A special control circuit prevents the compressor from being
switched off immediately when the required air pressure in the tank has been reached.
Instead, the valve in the intake tube is closed and the compressor runs in "Idling" mode, that
is, in no-load operation, for a specific time which can be set from 30 ... 600 s.
If the pressure falls within this time, the motor does not have to be restarted again, but can
return to nominal load operation from no-load operation.
If the pressure does not fall within this idling time, the motor is switched off.
The pressure switch controls the timing via B1.
The supply voltage is applied at A1 and start contact B1 is open, i.e. there is no control
signal at B1 when applying the supply voltage. The pressure switch signals "too little
pressure in system" and starts the timing via terminal B1. The compressor is started, enters
star-delta (ΥΔ) operation, and fills the pressure tank.
When the pressure switch signals "sufficient pressure", the control signal B1 is applied, the
idling time (coasting time) is started, and the compressor enters no-load operation for the set
period of time from 30 ... 600 s. The compressor is then switched off. The compressor is only
restarted if the pressure switch responds again (low pressure).
Note
The following applies to all operations: The pressure switch controls the timing via B1.
3RP25 Time Relays
Manual, 09/2019, A5E31947765002A/RS-AC/003
6.7 Star-delta (wye-delta) function with coasting time (idling)
Single function devices
63