AUTO and lead / lag: two compressors on the receivers
CHIEF. LEARN · Guide 3 of 5
Two main air compressors on the receivers: pressure switches at 27/30 and 24/30 bar, lead and lag, the STOP button trap in AUTO and the restart after a blackout.
The example throughout is No.1 main air compressor of a ship: a 30 kW motor on a 440 V, 60 Hz supply with a relay starter panel (SP-MAC1) and 220 V control voltage, charging two 3 m³ receivers to 30 bar.
- AMS
- alarm and monitoring system
- HMI
- human–machine interface: the display on the panel door
AUTO and lead / lag
AUTO between 27 and 30 bar
- Mechanics
- Nothing starts: the receivers are above the start setting of the lead switch.
- Electrics
- S1 AUTO puts wire 7 on wire 9. S2 LEAD passes it to wire 10 → X1:6 → PS1. PS1 is open: it last opened at 30 bar and closes again only below 27.
- What you see and hear
- No change.
Pressure falls below 27 bar
- Mechanics
- The compressor starts and loads after 15 s, as in MAN.
- Electrics
- PS1 closes: wire 12 → KA1 coil. KA1 11-14 puts wire 9 on wire 14 → KM1. KA1 21-24 energises K12: AUTO DEMAND to the AMS.
- What you see and hear
- RUNNING lit; the AMS sees DEMAND and RUNNING.
30 bar: stop
- Mechanics
- Stopped by pressure; the drains open and unload the stages.
- Electrics
- PS1 opens at 30 bar: KA1 drops, KM1 drops.
- What you see and hear
- Drains blow briefly, RUNNING off.
The STOP button trap
- Mechanics
- It stopped while STOP was held and started again when it was released.
- Electrics
- STOP S4 sits before S1, so it also cuts KA1. On release PS1 is still closed: KA1 and KM1 pull in again. To stop a compressor in AUTO, turn S1 to OFF.
- What you see and hear
- RUNNING off for a moment, then on again. In this design, a classic watchkeeping surprise.
Lead or lag
- Mechanics
- As lag, No.1 waits for 24 bar: it only helps when the lead compressor cannot keep up.
- Electrics
- S2 LAG moves wire 9 to wire 11 → X1:8 → PS2 (start 24 bar, stop 30 bar) → X1:9 → wire 12. No.2 has the opposite S2 position.
- What you see and hear
- At 26 bar nothing starts.
After a blackout
- Mechanics
- When power comes back, a compressor in AUTO with pressure below the setting starts by itself, after the delay of the ship's sequential restart.
- Electrics
- Nothing latches the stop: when 220 V returns, PS1 is still closed and KA1, KM1 pull in. Limit: no more than 6 starts an hour for the motor; a leak in the air line shows up as more starts per hour on the HMI.
- What you see and hear
- Back to RUNNING after the supply returns.
For training only. Models are simplified; class rules and the maker’s manual are the authority.