Compressor starters and electrical protection explained

Why the starter matters on an air compressor

An electric compressor starts more often than most plant motors. A fixed-speed rotary screw machine may start, load, unload and stop many times a shift as demand changes, and a piston compressor on pressure-switch control can cycle even more frequently. Every start draws a high inrush current, heats the motor windings and stresses the contactors, coupling and airend.

The starter therefore does three jobs at once. It limits the electrical and mechanical shock of starting, it sequences the compressor so that the airend starts unloaded, and it carries the protective devices that disconnect the motor when something goes wrong. A starter that is wrongly chosen or wrongly set rarely fails on day one. It shows up later as nuisance trips in summer, burnt contactor tips, a motor that runs hot, or a winding failure that could have been prevented.

In Telangana and Andhra Pradesh, two local conditions make this more important: high summer ambient temperatures inside panels and compressor rooms, and supply voltage that can sag or become unbalanced on long rural and industrial-estate feeders. Both push protective devices towards their limits.

Starting methods compared

The right starting method depends on motor size, supply capacity, how often the machine starts and whether the compressor is fixed-speed or variable-speed. The figures below are typical industry values for standard induction motors and should always be checked against the motor and starter maker’s data.

Star-delta remains the most common starter on fixed-speed screw compressors in India because it is simple, robust and easily repaired. Its weak point is the changeover: if the star-to-delta timer is too short the motor has not reached speed, and if it is too long the motor labours in star. Either way the delta contactor closes into a large current pulse.

Protective devices and what each one does

A compressor panel typically combines several layers of protection. Each has a specific job, and none of them replaces the others.

• Short-circuit protection – fuses or a circuit breaker sized to clear a fault quickly without tripping on normal starting current.
• Motor overload protection – a thermal or electronic overload relay set to the motor’s full-load current, taking the starting method into account.
• Phase failure and phase reversal – a phase monitoring relay that stops the motor on single-phasing and prevents a screw airend running backwards after supply work.
• Under-voltage and voltage unbalance – protects the motor from overheating when one phase sags.
• Earth-fault protection – an earth-leakage or earth-fault relay, coordinated with the site’s earthing system.
• Motor winding thermistors (PTC) – where fitted, these sense actual winding temperature and are the best defence against repeated starts and poor ventilation.
• Compressor safety inputs – high discharge temperature, high pressure, low oil pressure on piston machines, emergency stop and door interlocks, which act through the controller or a hard-wired safety chain.

The emergency stop and high-temperature shutdowns must stay hard-wired and fail-safe. They should never be bridged to “keep the plant running”, even temporarily.

How to set the overload relay correctly

Most nuisance trips and many motor failures trace back to the overload relay. The setting depends on where the relay sits in the circuit.

1. Read the motor nameplate full-load current for the actual supply voltage, not the catalogue value for a different voltage.
2. Check where the overload relay is fitted. On a star-delta starter it is often in the phase (winding) circuit, where the current is about 58% of line current (line current divided by √3). Setting it to line current leaves the motor unprotected.
3. Confirm the trip class. Class 10 suits most compressor motors; heavier starting may need class 20, but only if the motor maker allows it.
4. Check the service factor and ambient correction given by the relay and motor makers, especially for panels that run hot in summer.
5. Record the setting, the reason and the date on the panel record, so the next technician does not “turn it up” to stop a trip.

Raising an overload setting to cure trips treats the symptom. The real cause is usually high discharge pressure, a blocked cooler, low voltage, a weak phase, a failing bearing or too many starts per hour.

Common electrical faults on compressors and their causes

A useful rule is that voltage unbalance causes a much larger current unbalance and a disproportionate rise in winding temperature. Measuring all three phase voltages and currents under load is one of the first checks on any repeated trip.

Changing a starter, motor or drive: what must stay consistent

Replacing an electrical component on a compressor is not like-for-like simply because the kilowatt rating matches. Before any change, the following should be confirmed:

• The motor’s voltage, connection (star, delta, six-lead), full-load current, frame, speed and duty.
• Whether the new starter type suits the airend’s unloaded starting sequence and the controller’s logic.
• Contactor and overload ratings for the actual utilisation category and starts per hour.
• Short-circuit rating of the panel and coordination with the upstream breaker.
• Rotation direction, which must be checked on every screw compressor after any supply or motor work.
• Enclosure protection and cooling for the panel’s location, including dust ingress and summer heat.
• For a VFD retrofit, the motor’s suitability for inverter duty, bearing protection, cable type and length, filtering and the compressor controller’s ability to command speed.

A motor rewound or replaced with a different design can invalidate the old starter schedule even when air capacity is unchanged. Treat the electrical drawing, the panel labels and the actual wiring as three records that must agree before settings are changed.

Safety boundaries

Compressor panels carry lethal voltages and stored energy. Isolation, lock-out and tag-out, proving dead, and any live testing must be done by competent, authorised electrical personnel under the site’s own procedures. A compressor also holds stored air pressure, and some controllers restart automatically when power returns, so electrical isolation and pressure release should be part of the same permit. Where a change affects the site’s installation, earthing or protection coordination, the site’s electrical engineer or licensed contractor remains responsible for it.

How Reddy Compressors helps

Reddy Compressors sells, rents and services compressors of all major makes, including ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan and Kirloskar, as an independent multi-brand specialist rather than an authorised dealer. Our service team of more than 20 service engineers and technicians works across Telangana and Andhra Pradesh from our office at Tarnaka, Secunderabad.

On electrical and starter issues we diagnose repeated trips by measuring voltages, currents and temperatures under load, check overload settings against the motor nameplate, inspect contactors, timers, phase relays and terminations, and trace whether the true cause is electrical, mechanical or thermal. We supply matching starter components, contactors, overload relays, phase monitors, controllers and motors after identifying the machine from its nameplate, and we can provide a rental compressor while a panel or motor is being repaired. Where the work touches the site’s own installation, we work alongside the site’s electrical team.

What to send us

• Compressor make, model and serial number, with a nameplate photo.
• Motor nameplate photo (kW, voltage, full-load current, connection).
• Panel photos showing the starter, overload relay and its setting.
• The fault or alarm message and when it happens (start, changeover, afternoon, full load).
• Measured supply voltage, if available, and any recent supply or motor work.

Frequently asked questions

Why does my air compressor keep tripping on overload?

Repeated overload trips usually mean the motor is genuinely drawing more current than it should, or the relay is set or placed wrongly. Common causes are a hot compressor room, blocked oil cooler, pressure set higher than needed, low or unbalanced supply voltage, worn bearings or too many starts per hour. Check the setting against the motor nameplate first, then measure phase currents under load before changing anything.

Is star-delta or soft starter better for a screw compressor?

Both work well when chosen and set correctly. Star-delta is simple, robust and easy to repair, and suits most fixed-speed screw compressors that start unloaded. A soft starter gives smoother, adjustable acceleration and a smaller current step, which helps on weaker supplies, but its thyristors generate heat and it has a starts-per-hour limit. For fluctuating demand, a variable-speed drive is often the better choice.

What should the overload be set to on a star-delta starter?

It depends on where the relay sits. If the overload relay is in the motor winding circuit, as is common on star-delta starters, it should be set to about 58% of the motor’s full-load line current, because winding current is line current divided by the square root of three. If it is in the line circuit, set it to full-load current. Always confirm against the starter maker’s diagram.

Why is phase reversal protection important on a screw compressor?

A rotary screw airend must turn in one direction only. If two supply phases are swapped after cable, transformer or panel work, the motor will run backwards, which can damage the airend, empty the oil system or blow oil from the inlet. A phase sequence relay stops the machine starting in the wrong direction. Rotation should still be checked by a brief bump test after any electrical work.

Can I fit a VFD to my existing fixed-speed compressor?

Sometimes, but it needs proper engineering. The motor must be suitable for inverter duty, the airend must run safely and efficiently across the intended speed range, cooling must work at low speed and the controller must be able to command the drive. Cable length, filtering, earthing and bearing currents also need attention. In many cases a purpose-built variable-speed compressor or better sequencing gives a cleaner result.

How many times an hour can a compressor motor start?

There is no single number; the motor and starter makers set the limit for each design, and it falls as motor size increases. Exceeding it overheats the windings and wears contactors quickly. On screw compressors the controller usually enforces a minimum run time or stop delay for this reason. Frequent starting is often a sign of an undersized receiver, a narrow pressure band or a machine larger than the demand.

Related pages

• Compressor electrical and control diagnostics
• Compressor VFD diagnostics and drive faults
• Motors and VFD spare parts
• Compressor controllers and HMI parts
• Compressor breakdown diagnostics
• Motor input and electrical measurement worksheet