Air Compressor Not Starting? Here’s Why

An air compressor that fails to start indicates a defined fault condition within its electrical supply, mechanical drive, pneumatic system, or control logic. Effective diagnosis depends on first establishing whether the motor is physically unable to rotate or whether a safety or control system is intentionally inhibiting startup. This distinction determines the correct investigative pathway and prevents unnecessary component replacement.

This diagnostic framework applies across industrial compressor platforms, including rotary screw, oil-free, and piston machines manufactured by Atlas Copco. The following guide outlines the principal causes of startup failure within UK industrial operating conditions and regulatory constraints.

Diagnosing the Failure Mode: “Can’t Start” vs “Won’t Start”

Categorising the failure directs the troubleshooting process. See our guide on common air compressor failures for additional diagnostic steps.

• Can’t Start: The motor attempts to turn but is mechanically obstructed. Symptoms include a loud hum, a stalled motor, or an immediate circuit breaker trip. This suggests a mechanical seizure or major electrical failure.
• Won’t Start: The machine remains silent. The controller prevents the start sequence based on sensor inputs or mode selection. This is a logic inhibitor protecting the machine.

Safety and Regulatory Context

Diagnosing industrial equipment requires strict adherence to UK safety regulations.

• PSSR 2000: The Pressure Systems Safety Regulations 2000 (PSSR) prohibit operating a system if safety devices are compromised. A startup inhibit often indicates a safety lockout.
• PUWER: The Provision and Use of Work Equipment Regulations 1998 (PUWER) require equipment to be maintained in a safe state. Only competent persons should perform electrical diagnostics.
• LOTO: Lockout/Tagout (LOTO) procedures are mandatory before opening electrical cabinets or inspecting drive trains.
• Interlocks: Never bypass thermal relays, pressure switches, or electrical interlocks to force a start.

Electrical Supply and Distribution Faults

For 415V three-phase systems, supply integrity is the primary variable.

• Phase Imbalance: If the voltage imbalance exceeds 2–3% between phases, the controller inhibits the start to prevent motor winding damage.
• Blown HRC Fuses: If High Rupturing Capacity (HRC) fuses degrade from thermal fatigue caused by repeated inrush currents, they may fail continuity tests despite appearing intact.
• Circuit Breaker Trips:
  • Instant Trip: If the breaker trips in milliseconds, a direct short circuit or earth fault is present.
  • Delayed Trip: If the breaker holds for a few seconds before tripping, the motor is facing an overload caused by mechanical drag.
• Phase Sequence (K25 Relay): If maintenance work reverses two phases, the K25 relay prevents the motor from starting to avoid reverse rotation damage.

Controller-Monitored Inhibits

Modern controllers, such as the Atlas Copco Elektronikon, manage safety logic and sensor inputs to generate specific startup inhibits.

• Anti-Recycle Timers: If the mandatory blowdown period (typically 20–60 seconds) has not elapsed, or if the starts-per-hour limit is reached, the controller prevents restart.
• Remote Mode: If “LAN” or “Remote” mode is selected, local start buttons are disabled while the machine awaits a master signal.
• Sensor Failures: If a PT1000 temperature sensor or 4-20mA pressure transducer reads as “Open Circuit” (infinite resistance), the logic interprets the value as unsafe and inhibits startup.
• Latched Alarms: If a shutdown alarm occurs (e.g., Motor Overload F21), the fault remains latched and requires a manual reset at the controller even if the condition clears. Anglian Compressors engineers can assist with interpreting obscure fault codes during emergency breakdown support.

Variable Speed Drive (VSD) Faults

VSD compressors depend on sensitive power electronics and dedicated thermal management systems, which introduce specific failure modes.

• DC Bus Voltage: If the DC Bus voltage is too high due to supply surges, or too low due to weak supply, the drive inhibits the start sequence.
• Cabinet Cooling: If cabinet filters are blocked, internal airflow is restricted. This causes IGBT temperatures to rise; if limits are exceeded, the drive locks out.
• Communication Loss: If data cables between the controller and drive are loose, a “Communication Error” shutdown occurs.

Mechanical and Pneumatic Barriers

A rotary screw compressor must start unloaded to minimise torque requirements.

• Stuck Unloader Valve: If the inlet valve sticks open, the compressor attempts to compress air immediately, causing the motor to overload and trip.
• Minimum Pressure Valve (MPV): If the MPV sticks closed, pressure remains trapped in the separator vessel, creating back-pressure that prevents the motor from reaching running speed.
• Drive Train Failure: If a belt snaps or a coupling shears, the motor rotates without load or pressure rise, triggering a controller stop.
• Seized Air End: If oil starvation or bearing collapse occurs, rotors may weld together. The motor hums and trips the breaker instantly. Severe seizures may require you to repair or replace your compressor.

Piston Compressor Specifics (LE/LT Series)

Reciprocating compressors feature distinct electrical and mechanical failure points.

• Start Capacitor: Single-phase motors use capacitors for starting torque. Failure results in a humming motor that does not rotate.
• Start Winding: A burnt-out start winding prevents the motor from initiating rotation.
• Pressure Switch: Burnt contacts or a ruptured diaphragm prevent the circuit from closing.
• Tank Check Valve: A leaking check valve allows tank pressure to push back against the piston, preventing the motor from starting against head pressure.

Environmental Factors

Ambient temperature and humidity influence start reliability in the UK.

• Cold Oil Viscosity: Mineral oils (ISO 46) thicken significantly below 5°C. This drag increases starting torque, causing overload trips.
• Frozen Condensate: Condensate freezes in control lines or drain traps in unheated plant rooms. Ice blocks pneumatic signals required for valve operation.
• Condensation on Electronics: High humidity causes dew point formation on PCBs. Cabinet heaters prevent condensation and electrical faults.

Maintenance-Related Causes

Neglected maintenance creates conditions that trigger safety inhibits. See our air compressor maintenance guide for detailed schedules.

• Blocked Coolers: Dust accumulation on cooler fins results in reduced thermal transfer, leading to high-temperature trips shortly after startup.
• Differential Pressure: Clogged intake filters result in a high pressure drop (dP), increasing the vacuum load on the motor during startup.
• System Leaks: Excessive downstream leaks result in rapid cycling, which triggers anti-recycle timers. Leak detection surveys help identify these inefficiencies.
• Loose Terminals: Vibration results in loosened electrical connections, increasing resistance and causing voltage drops. Professional compressor servicing includes torque checks on all electrical terminations.

Conclusion

A compressor failure to start is a specific engineering state defined by electrical, mechanical, or logic conditions. Accurate diagnosis distinguishes between safety lockouts and component failures. Complex faults involving VSD electronics, safety interlocks, or internal mechanical seizures require investigation by a competent person to ensure PSSR compliance and operational safety.

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