Starting a main air compressor: checks, first run, start sequence

CHIEF. LEARN · Guide 2 of 5

Checks before starting a ship’s main air compressor, the first run after overhaul and the start in MAN step by step: contactor, timers, oil pressure and loading.

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.

NC / NO: normally closed / normally open contact
ETO: electro-technical officer
AMS: alarm and monitoring system
HMI: human–machine interface: the display on the panel door
LP / HP: low pressure / high pressure (first / second stage)

Preparing to start

Mechanical checks

Mechanics: Oil level on the dipstick, no water in the oil. Cooling water valves open and flow visible in the sight glass. Discharge valve V3 open: a start against a closed V3 drives HP up to SV2. Manual drains V1, V2 open. Turn it over by hand, nobody near the machine.
Electrics: Nothing switched yet. After work on the motor or its cable the ETO checks insulation with a 500 V megger: more than 1 MΩ is acceptable, a healthy motor shows well over 100 MΩ.
What you see and hear: At the compressor: valve handles, the sight glass, the oil gauge at zero.

Power on

Mechanics: Still standing.
Electrics: Lock removed, Q1 on: T1 gives 220 V on wire 1 (X1:1) and wire 2 (X1:2). POWER ON lights. The motor heater HM1 is on through KM1 21-22.
What you see and hear: POWER ON lit, HMI STOPPED, ammeter at zero.

First run

Mechanics: A short start checks the direction of rotation after work on the motor or cable. Then start in MAN, close V1 and V2 after a few minutes, check stage pressures, oil and water. After 30 minutes stop, feel the bearings, then put it in AUTO.
Electrics: The MAN start is shown step by step below. In MAN the protections still work: the oil and temperature trips and the overload are not bypassed.
What you see and hear: With V1 and V2 open the compressor cannot load: air blows from both drains.

Start in MAN

Standing, ready

Mechanics: Stopped. Both stages at zero, YV1 and YV2 open.
Electrics: 220 V on X1:1. The protection chain is closed: wire 3 (after S0) → F4 95-96 → wire 4 → KA2 11-12 → wire 5 → KA3 11-12 → wire 6. KM1 21-22 feeds the motor heater on wire 32.
What you see and hear: POWER ON lit, HMI STOPPED.

S1 to MAN

Mechanics: Nothing moves yet.
Electrics: Through STOP S4 (NC) wire 7 is live, and S1 MAN puts it on wire 8. Wire 8 waits at START S3 and at KM1 13-14, both open.
What you see and hear: No change on the door.

START pressed

Mechanics: The motor starts direct on line, unloaded.
Electrics: S3 puts wire 8 on wire 14: KM1 coil pulls in. KM1 13-14 closes in parallel with S3 (self-hold), 43-44 starts KT1 on wire 20, 53-54 makes wire 26 live: RUNNING H2, K10 to the AMS, the running-hours meter, KT3, and through F5 the water solenoid YV3. KM1 21-22 opens: motor heater off.
What you see and hear: Contactor click, RUNNING lit, the ammeter kicks to full scale.

START released: self-hold

Mechanics: The motor runs up in about a second. Both drains whistle: the compressor turns but does not pump.
Electrics: KM1 holds itself through 13-14. The starting current, about 310 A on the 8 % voltage dip, falls to about 17 A once the motor is up to speed.
What you see and hear: Whistling drains, the ammeter drops to 17 A.

Oil pressure builds

Mechanics: The gear pump on the crankshaft only delivers once the shaft turns: oil pressure rises to 2.2 bar within a few seconds.
Electrics: PS3 opens above 1.2 bar. It is not in the trip circuit yet: KT1 keeps the oil trip bypassed for 20 s.
What you see and hear: Oil gauge to 2.2 bar.

15 s: loading

Mechanics: YV1 and YV2 close; the compressor starts pumping. LP rises to 4–5 bar, HP to the receiver pressure.
Electrics: KT3 15-18 closes after 15 s: wire 27 → wire 28 → KT2 15-16 (closed) → wire 29 → YV1 (X1:16) and YV2 (X1:18) get 220 V.
What you see and hear: The whistle stops, the sound of the machine changes, the current rises.

20 s: oil trip armed

Mechanics: Pumping.
Electrics: KT1 15-18 closes: from now on PS3 closing (oil below 0.8 bar) energises KA2 through wire 21 → X1:10 → PS3 → X1:11 → wire 22, and the compressor trips.
What you see and hear: Nothing visible: the bypass ends quietly.

Pumping in MAN

Mechanics: It pumps the receivers up about 0.3 bar a minute against normal consumption.
Electrics: Current 46–48 A at 27–30 bar. In MAN it does not stop on pressure: only STOP or S1 to OFF stops it. The protections still act in MAN.
What you see and hear: Ammeter about 47 A, HMI LOADED.

For training only. Models are simplified; class rules and the maker’s manual are the authority.