Why Your Air Compressor Keeps Shutting Down

A compressor that keeps shutting down is one of the most frustrating problems in a plant. Every machine on the floor needs air, and when the compressor trips, everything stops. The good news: rotary screw compressors trip for a small number of well-understood reasons, and most of them are fixable without a service call.

Reason 1: High Discharge Temperature

This is the most common compressor trip. The discharge air/oil temperature exceeded the safety limit (typically 100–110°C). The compressor's thermal switch or controller shuts it down to protect the element and oil from degradation.

• Dirty oil cooler — The #1 cause. Blow the cooler fins with compressed air from the clean side. If it's heavily fouled, use a coil cleaner spray.
• Low oil — Check the sight glass. If the oil is below the minimum mark, top off with the correct compressor oil. Never mix oil brands.
• Failed thermal bypass valve — This valve routes oil around the cooler when it's cold and through the cooler when it's hot. If it's stuck in the bypass position, oil never gets cooled.
• Hot room — The compressor room needs adequate ventilation. If the room heats up, the compressor can't reject enough heat. Add ducting to bring in outside air.

Reason 2: High Motor Current / Overload

The motor overload relay or VFD trips because the compressor motor is drawing too much current. This usually means the compressor is working harder than it should.

• Clogged oil separator — High differential pressure across the separator element forces the motor to work harder. Replace the separator when the ΔP exceeds 1 bar (15 PSI).
• Low voltage — Measure voltage at the motor terminals under load. Low voltage = higher current for the same power output. Common with long cable runs or undersized transformers.
• Minimum pressure valve stuck closed — This valve maintains 4.5 bar (65 PSI) in the oil circuit for lubrication. If stuck closed, the motor fights against full backpressure.

Reason 3: Low Oil Pressure / Level

If the compressor trips on low oil pressure, stop immediately. Running a rotary screw element without adequate oil even briefly can cause catastrophic damage to the rotors — a multi-thousand-dollar repair.

Check the oil level first. Then look for external leaks at fittings, hoses, and the separator tank. If the oil is foaming (visible through the sight glass), the wrong oil type is the likely cause.

Reason 4: Phase Loss or Phase Reversal

If the compressor won't start at all (trips immediately on attempt), check for a phase loss or phase reversal fault. A blown fuse, tripped breaker, or loose connection on one phase will cause this. Phase reversal (running backward) happens after electrical maintenance when leads get swapped — the compressor controller detects this and refuses to start to protect the element.