Troubleshooting an Air Compressor That Won’t Start

When an air compressor won’t start, production stops. In most cases, the cause is straightforward to diagnose if you follow a logical process. While a non-starting machine can be alarming, it is often due to a safety control doing its job rather than a catastrophic failure.

A compressor that refuses to start has usually either lost a “start signal” (a permissive), triggered a safety guard, or is mechanically unable to turn. Unplanned downtime can become expensive in minutes, not hours, for most UK SMEs, so rapid diagnosis is key.

This guide details the troubleshooting steps for industrial air compressors, focusing on Atlas Copco GA rotary screw and GX piston models. It will help you isolate whether the fault is in your electrical supply, the control settings, or the machine’s drive train.

Safety Notice: Safe Isolation Procedure

Before removing any panels, you must isolate the machine from the electrical supply. Follow HSE Guidance Note PM73 and Electricity at Work Regulations 1989. Lock off the isolator. Ensure the system is fully depressurised and the air receiver is vented to the atmosphere.

1. The Isolation Test: Check the Supply First

Before blaming the compressor, you should determine whether the fault lies with the site infrastructure or the machine itself. A common issue we see is that the compressor is healthy, but the power supply is unstable.

A competent electrician should disconnect the compressor from the distribution board and test the supply terminals at the isolator under no load.

Voltage Stability: Confirm you have 400V (typically within ± 6 – 10%) across all three phases (L1-L2, L2-L3, L1-L3).
Phase Imbalance: A voltage difference greater than 3% between phases causes motor windings to overheat. If your supply is unbalanced, the compressor’s thermal overload will trip to save the motor.
Voltage Drop: If voltage is present at the isolator but drops significantly when you press the start button, your supply cable may be undersized or a breaker is failing.

If the supply is unstable, this is a facilities issue. If the supply is solid, the fault is internal to the compressor. For urgent assistance with internal electrical faults, contact Control Gear for 24/7 emergency support.

2. Electrical Diagnostics and Protection Relays

Industrial compressors use heavy-duty starters that draw high “inrush” currents, often 5 to 10 times the running current, depending on the starter type, for a fraction of a second when they start. This surge can trip standard electrical protections.

Main Fuse and Breaker Inspection

MCB vs MCCB: Ensure your breaker is motor-rated (Type C or D). A standard Type B breaker is designed for lighting and simple circuits; it will trip instantly when faced with a compressor’s magnetic start surge.
HRC Fuses: Check continuity across all three fuses. A “partial” fuse failure can be tricky, it might allow voltage to show on a multimeter but fail immediately under load. This causes “single-phasing,” where the motor tries to run on two wires and burns out.

Motor Overload vs. Electronic Protection

Thermal Overload Relay: On older electromechanical units, a bi-metallic strip physically bends under heat to break the circuit. You will need to manually reset this once it cools down.
Electronic Protection: Modern Atlas Copco air compressors (like the GA series) use sensors to monitor the current. A trip here usually indicates a locked rotor (the motor is stuck) or a seized airend.
Wye-Delta Timing: If the compressor starts but trips after 5–10 seconds, the Star-Delta timer relay may be failing. In simple terms, the machine is stuck in ‘low gear’ (Star), where motor torque is reduced by roughly one-third, and trips when it tries to shift to ‘high gear’ (Delta).

3. Control Logic and Display Messages

Modern units like the Atlas Copco GA VSD+ are managed by an Elektronikon® controller. This computer performs a safety check, a list of “permissives”, before it allows the motor to turn.

Common Start Inhibits

Phase Sequence Fault: Rotary screw compressors must rotate in a specific direction. If you have recently changed wiring at the board, the phase sequence relay might detect “reverse rotation.” It inhibits the start to prevent the air from running backwards and seizing.
Remote Stop / BMS Signal: If your unit is connected to a Building Management System or SMARTLINK, an open circuit on the remote terminals will hold the machine in “Standby.” The machine is waiting for a signal that never comes.
Fan Motor Fault: On air-cooled units, the cooling fan has its own overload protection. If the fan trips, the controller stops the main motor from starting to prevent it from overheating.

What the Controller Needs to See

Before the start button works, the controller typically verifies:

Emergency Stop is closed (released).
Oil temperature is above 5°C (to ensure flow).
Discharge pressure is below the “Unload” setpoint.
Motor thermal state is “Reset.”

4. Pressure System Mechanics

An electric motor has very low torque when it first starts. It cannot push against high pressure. If your compressor tries to start while the internal chamber is pressurised, it will stall.

Unloader Valve Malfunction

The unloader valve controls the intake and internal pressure relief during start/stop cycles. When the machine stops, this valve should close and vent the internal pressure.

Symptom: The motor hums and trips the overload instantly.
Cause: The unloader piston is stuck open. This means the motor is trying to compress air immediately at full load, which it cannot do from a standstill.

Check Valve Failure

The non-return valve stops air in your receiver tank from flowing back into the compressor.

Rotary Screw: Back pressure forces oil and air backwards through the element. You might see oil leaking from the intake filter.
Piston (Reciprocating): High pressure remains in the cylinder head. The piston physically cannot push up against this pressure to complete its first stroke. See our guide on what is a piston compressor is for more on how these components work.

Transducer vs. Switch Logic

Digital Sensors (GA Series): Sensors can “drift” over time. If your screen reads 7.5 bar even when the tank is empty, the controller thinks the system is full and won’t start.
Mechanical Switches (GX / Piston): Contacts can wear out or weld shut. You should also check the differential setting. If the “cut-in” pressure is set too close to the “cut-out” pressure, the switch might chatter but fail to engage the contactor.

5. Mechanical and Environmental Factors

The environment in your plant room plays a bigger role than many realise, particularly regarding temperature.

Viscosity and Cold Starts

Oil gets thicker as it gets colder.

The 5°C Threshold: In unheated UK plant rooms during winter, standard mineral oil becomes like treacle. The energy required to spin the rotors through this thick oil is higher than the motor can deliver.
Correct Fluid: Switching to a high-viscosity index synthetic oil that remains fluid at low temperatures (e.g., Atlas Copco Roto-Inject or Roto-Xtend) can solve this, as these oils stay fluid at lower temperatures.
Ice Blockage: Condensate in small control lines or solenoid drains can freeze. This physically blocks the valves from moving.

The Free Rotation Test

If you suspect a mechanical seizure, you can perform a “free rotation test.”

Isolate and lock off the electrical supply.
Remove the belt guard only after safe isolation and lock-off, or access the drive coupling where provided.
Attempt to rotate the drive coupling by hand.

It should turn smoothly with consistent resistance. If it is locked solid or feels “graunchy” (grinding metal), the bearings have likely failed. Do not attempt to restart. This is usually the point where a professional engineer should be called in to prevent further damage.

Regular analysis of vibration can predict these seizures before they stop production. Read about the importance of regular air compressor servicing to see how proactive care prevents bearing failure.

6. Model-Specific Failure Modes

Different technologies fail in different ways.

Reciprocating Piston Compressors (GX / LE / LT)

Start Capacitor: Single-phase units rely on a capacitor to get the motor spinning. If this looks swollen or is leaking fluid, it has failed. The motor will hum but won’t turn.
Fan Motor Capacitor: Small units often use a separate fan. If this fails, the thermal overload will trip to protect the machine.

Variable Speed Drive (VSD / VSD+)

DC Bus Overvoltage: You might see this error on the inverter drive. In simple terms, the drive is detecting unstable power from your mains supply (like a spike) and shutting down to protect its internal electronics.
PM Motor or Inverter-Related Faults: Permanent Magnet (iPM) motors on VSD+ units are software-driven. “Inverter error” codes often point to a software or drive logic issue rather than a physical jam.

7. Regulatory Compliance & Duty of Care

Operating a compressor requires adherence to strict UK legislation. It is important to treat these machines with respect.

PSSR 2000 (Pressure Systems Safety Regulations): Any system larger than 250 bar-litres requires a Written Scheme of Examination. Bypassing safety switches to force a start is not only unsafe; it can be a criminal offence. The bar-litre calculation is based on the maximum allowable working pressure of the system, not just the compressor rating.
Electricity at Work Regulations 1989: Only competent persons should perform live diagnostics.
PM73 (HSE Guidance): This defines the safe operation and maintenance of air compressors to prevent explosive failure.

To ensure your system remains compliant and safe, follow the recommended maintenance frequency guide suitable for your specific equipment.

Conclusion and Professional Support

If your isolation test confirms stable power and the free rotation test confirms the machine isn’t seized, the issue likely lies within the control logic or starter components. However, if the breaker trips instantly the moment you try to start, you should assume a winding fault or locked rotor and treat this as a “call-an-engineer” situation, not a quick reset.

For complex faults, Control Gear provides 24/7 emergency call-out support across South Wales and the West, including Cardiff, Bristol, Worcester, and Swindon. Our engineers carry diagnostics for Atlas Copco, HPC, and Ingersoll Rand systems.

Prevent Future Downtime:

Avoid repeat failures by implementing a Guardian Service Plan. This includes SMARTLINK integration for remote monitoring, regular oil analysis, and contactor inspection to ensure PSSR compliance and operational reliability.

Next Step:

Do not risk your production capability. Contact Control Gear immediately to book a diagnostic visit or system audit.