The short answer. First decide when the motor trips. A trip during starting points to the supply, the starter or a machine that is starting against pressure. A trip after minutes or hours of running points to sustained over-current: low or unbalanced voltage, a lost phase, too high a discharge pressure, a mechanical drag, or an overload relay set wrongly. Measure voltage on all three phases and current on all three lines before changing any setting, and never “cure” a trip by turning the overload relay up.
Who this page is for: maintenance electricians, plant engineers and compressor owners whose electric compressor trips on overload, fails to change over from star to delta, or blows fuses, including sites that run compressors on a DG set.
Key facts
- Induction motors are designed to run within about ±10 % of rated voltage. On a 415 V supply that is roughly 374 V to 457 V, measured at the motor terminals while running, not at the main panel with the machine stopped.
- Voltage unbalance of only 2 % can raise current unbalance to 12 to 20 % and overheat one winding. Above about 1 % the motor should be derated; above 2 % the cause should be found and fixed.
- At full load, running current should be close to, and not above, the nameplate full-load current (FLC). Consistently above about 105 % of FLC means the motor is overloaded or the supply is poor.
- In a star-delta starter with the overload relay in the delta (phase) circuit, the relay is set to FLC ÷ √3, about 0.58 × FLC. In a direct-on-line starter it is set to FLC.
- Star-delta changeover on compressors is typically set somewhere around 5 to 10 seconds, once the motor has reached most of its speed with the machine unloaded. Too short and the delta step draws almost direct-on-line current; too long and the motor labours in star.
- Losing one phase while running raises current in the two healthy lines to roughly 1.7 times normal for the same load. A correctly set relay trips; a wrongly set one lets the winding burn.
Start with timing: when does it trip?
The single most useful fact in motor diagnosis is the moment of the trip. Note it before anyone resets anything, and note what the controller or relay indicated.
| When it trips | What usually trips | Most likely causes |
|---|---|---|
| Immediately on pressing start | Main breaker, fuses, earth-leakage device | Short circuit or earth fault in the motor or cable, a welded or failed contactor, a wrong breaker curve |
| During the first 1 to 10 seconds (in star) | Overload relay, breaker, controller “start failure” | Machine starting against pressure (unloader or blowdown fault), low supply voltage, seized airend or bearing, timer set too long |
| At the star-to-delta changeover | Breaker or fuses, sometimes overload | Changeover too early, failed delta contactor, wrong timer, very weak supply such as a small DG set |
| After several minutes or hours of running | Overload relay, controller motor-overload alarm, thermistor relay | Low or unbalanced voltage, single phasing, discharge pressure too high, high room temperature, mechanical drag, wrong relay setting |
| Randomly, often at the same time of day | Phase monitor, under-voltage relay | Supply dips when other large loads start, grid fluctuation, loose terminations heating up |
Trips at start: the four things to check
1. Is the machine starting against pressure?
A piston compressor must start with its delivery line vented by the unloader or pressure-release valve; a screw compressor must start with its inlet valve closed and its separator vessel blown down. If either fails, the motor tries to accelerate a loaded machine and the starting current lasts far longer than normal. Listen for the vent at the previous stop. On a screw machine, check that sump pressure has fallen before a restart; most controllers will not allow a restart until it has. Our page on pressure switch settings and safety valve lifting covers the unloader checks.
2. What is the voltage doing during the start?
Fit a recording meter, or have one person read voltage at the motor terminals while another starts the machine. A dip below about 85 % of rated voltage during the start, on a 415 V supply below about 353 V, will stretch the start and may drop contactors out. Long, thin cables and weak transformers or DG sets are the usual reasons.
3. Is the star-delta changeover right?
In star, each winding sees 415 ÷ √3 = 240 V, so starting current and torque fall to about one third of direct-on-line values. The changeover to delta should come when the motor is close to full speed. If the timer is too short, the motor reaches delta at low speed and draws a current surge close to a direct-on-line start, enough to trip the breaker. If the delta contactor fails to pull in, the motor stays in star, cannot carry load, and trips on overload a little later. Watch the contactors through the changeover: you should hear star drop out, a brief gap, then delta close.
4. Can the machine turn freely?
With the supply locked off, turn the airend or crankshaft by hand or through the coupling where the design allows. A screw airend with failing bearings, a piston machine with a seized big end, or a motor bearing that has collapsed will show itself here. See motor, drive and coupling reliability.
Trips while running: sustained over-current
An overload relay is a slow device. It trips when current stays above its setting long enough to heat the windings. So a running trip means that, over minutes, the motor drew more current than it should. The question is why.
| Measured finding | What it means | Next step |
|---|---|---|
| All three voltages low, e.g. 370 V or less while running | Motor draws more current for the same output | Check the supply at the transformer or DG, cable voltage drop, loose joints |
| Voltages differ by more than about 2 % from their average | Voltage unbalance overheats one winding | Check single-phase loads on the same feeder, loose terminations, a blown capacitor bank fuse |
| One line current near zero, others high | Single phasing | Check fuses, contactor tips, cable joints; do not restart until found |
| Currents balanced but above FLC | Genuine mechanical overload | Check discharge pressure against rating, separator differential pressure, cooler and room temperature, bearing condition |
| Currents within FLC, relay still trips | Relay setting or relay fault, or high panel temperature | Check the setting against the rule for the starter type; test or replace the relay; ventilate the panel |
How to calculate voltage unbalance: take the three line voltages, work out their average, find the largest difference from that average, and divide by the average. For example, 410 V, 402 V and 395 V average 402.3 V. The largest deviation is 410 − 402.3 = 7.7 V, so unbalance = 7.7 ÷ 402.3 = 1.9 %, enough to justify finding the cause.
Mechanical causes of high current are often overlooked by electricians and electrical causes by mechanics. On a screw machine, a blocked separator element raises internal pressure and motor load; a discharge pressure set above the rating does the same; a fouled cooler runs the whole package hot, and a motor in a hot canopy reaches its thermal limit at a lower current. On a piston machine, slipping belts actually lower current, while overtight belts, failing bearings and leaking valves that force longer running raise it. The electrical and control diagnostics guide covers the supply-to-controller chain in more depth.
Setting the overload relay correctly
- Direct-on-line starter: set the relay at the motor nameplate FLC for the supply voltage in use.
- Star-delta starter, relay in the phase (delta) circuit, the usual arrangement: set at FLC × 0.577.
- Star-delta starter, relay in the line (before the split): set at FLC.
- Service factor: where the motor nameplate gives a service factor above 1.0, some makers allow the setting to rise accordingly. Without that marking, use FLC.
- Trip class: compressors with unloaded starts normally suit class 10 relays. A machine that needs class 20 or 30 to survive starting has a starting problem, not a relay problem.
- Thermistor protection, where fitted in the windings, is the better guard against overheating from poor ventilation or high ambient. If the thermistor relay trips, the windings really are hot.
The most damaging habit in Indian plants is turning the relay dial up after repeated trips. The trips stop, the winding temperature does not, and the motor fails weeks later.
Worked example (illustrative): a 37 kW screw compressor on a long cable
A 37 kW (50 hp) screw compressor, nameplate FLC 66 A at 415 V, star-delta started, trips on overload on summer afternoons. Its own starter panel, built into the package, is fed from the main panel 120 m away on a 3.5-core 25 mm² aluminium armoured cable, so the cable carries line current.
- Overload relay setting: the relay is in the delta circuit, so it should be set at 66 × 0.577 = 38.1 A. It was found at 34 A, about 11 % low.
- Running voltage drop: using typical values for 25 mm² aluminium at operating temperature, resistance about 1.44 Ω/km and reactance about 0.075 Ω/km, with power factor 0.88: drop = √3 × 66 A × 0.12 km × (1.44 × 0.88 + 0.075 × 0.475) = 13.72 × 1.303 = 17.9 V, which is 4.3 % of 415 V.
- Afternoon supply: the panel reads 400 V when nearby air-conditioning load peaks. At the motor that leaves about 400 − 17.9 = 382 V, close to the lower tolerance limit, so current rises to compensate.
- Starting: star current about 2.3 × FLC = 152 A at a starting power factor near 0.4 gives a drop of √3 × 152 × 0.12 × (1.44 × 0.4 + 0.075 × 0.917) = 31.6 × 0.645 = 20.4 V, about 4.9 %, on top of whatever the supply is already doing.
- Fix: relay corrected to 38 A; cable replaced with 50 mm² aluminium (about 0.77 Ω/km and 0.072 Ω/km), which gives a running drop of 13.72 × (0.77 × 0.88 + 0.072 × 0.475) = 13.72 × 0.712 = 9.8 V, 2.4 %. Afternoon voltage at the motor rises to about 390 V, and the trips stop.
The lesson is in the order: the relay was set wrong and the cable was marginal. Correcting only the relay would have hidden a motor running hot every afternoon.
Compressors on DG supply
Many sites in Telangana and Andhra Pradesh run compressors on a DG set, all the time or during outages. A generator is a much weaker source than the grid: its voltage and frequency dip when a large motor starts, and its AVR takes time to recover.
- As a planning rule, a star-delta compressor motor needs a generator of roughly 2 to 2.5 times the motor kW in kVA if it is the main load; a direct-on-line motor needs more, and a soft starter or variable speed drive less. The generator supplier should confirm motor-starting capability.
- Watch frequency as well as voltage. A dip below about 47.5 Hz during the start makes the motor slow and draw more current, and may trip under-frequency protection.
- When several compressors share a DG set, stagger their starts. Most controllers allow a restart delay after power returns; set different delays on each machine.
- Variable speed drives are sensitive to generator waveform and frequency. Some need parameter changes for generator operation; check with the drive maker.
Exceptions and cautions
- All measurements on live panels must be done by a competent person with suitable meters and protective equipment. Follow isolation and permit-to-work rules.
- Motors with variable speed drives do not use a separate overload relay in the same way; motor protection is a drive parameter. Do not apply star-delta setting rules to them.
- Repeated resets after an earth-fault or short-circuit trip are dangerous. Insulation-test the motor and cable first.
- Cable figures in the example are typical values for illustration. Use the cable maker’s data for real calculations.
- Motors that have been wet, for example after a flooded compressor room, must be insulation-tested and dried before they are restarted.
Motor trip investigation sheet
| Item | Reading or finding | Limit or target | OK / action |
|---|---|---|---|
| When it tripped: at start / changeover / running (after how long) | — | ||
| Device that tripped and any controller message | — | ||
| Motor nameplate kW, voltage, FLC (A), service factor | — | ||
| Starter type and overload relay location | — | ||
| Overload relay setting (A) | FLC (DOL) or 0.577 × FLC (delta circuit) | ||
| Running voltage at motor terminals R-Y / Y-B / B-R (V) | Within ±10 % of rated | ||
| Voltage unbalance (%) | Below about 1 %, investigate above 2 % | ||
| Line currents R / Y / B at full load (A) | Not above FLC | ||
| Lowest voltage during start (V) | Above about 85 % of rated | ||
| Star-delta timer setting (s) and changeover observed | Typically 5 to 10 s | ||
| Discharge pressure at trip (bar g) | Not above rated | ||
| Separator differential pressure (screw, bar) | Per maker | ||
| Room and panel temperature (°C) | — | ||
| Machine turns freely by hand (isolated): yes / no | — | ||
| Insulation resistance, motor to earth (MΩ) | Per test standard | ||
| Supply source: grid / DG (kVA) | — |
Common mistakes
- Turning up the overload relay to stop trips.
- Setting a delta-circuit relay at full FLC, which leaves the motor effectively unprotected.
- Measuring voltage at the main panel with the machine stopped, which hides cable drop and dips under load.
- Replacing a contactor or a relay three times while the real cause, an unloader or inlet valve that fails to unload, keeps destroying them.
- Shortening the star-delta timer to “start faster”, which turns a soft start into a hard one.
- Restarting after a lost phase without finding the blown fuse or burnt contactor tip.
How Reddy Compressors helps
A motor trip is where electrical and mechanical faults meet, so we look at both. Since 2002 Reddy Compressors has sold, rented, serviced and supplied spare parts for compressors of all makes as an independent multi-brand business. Our service engineers investigate motor trips on site, with measurement at the motor terminals, starter and relay checks, and mechanical checks of the airend, bearings and valves. We supply motors, contactors, relays and controller parts, and our own rental fleet, from 50 to 2,000 cfm (about 1.4 to 56.6 m³/min) and 2 to 30 bar, can keep air on while a motor is rewound or replaced. Site visits cover Telangana, Andhra Pradesh and the districts within about 300 km of Hyderabad, and we supply machines and parts across India.
What to send us
- Photographs of the motor nameplate, the compressor nameplate and the starter panel with the relay visible.
- When the machine trips (at start, at changeover, or after how long running) and what the controller or relay shows.
- Voltage and current readings if you have them, and whether the site runs on grid, DG or both.
Use our compressor repair page or call/WhatsApp 99590 60718 or 92468 88812.
Frequently asked questions
Why does my compressor motor trip only on summer afternoons?
Afternoon voltage is often at its lowest because of air-conditioning load, room and panel temperatures are at their highest, and the compressor works hardest. Together they push current and winding temperature up. Measure voltage at the motor terminals at that time of day, and check cooling and relay settings.
What should the overload relay be set to in a star-delta starter?
If the relay sits in the delta (phase) circuit, as it usually does, set it to the motor full-load current multiplied by 0.577. If it sits in the main line before the star-delta split, set it to the full-load current.
How long should the star-delta timer be on a compressor?
Typically about 5 to 10 seconds, long enough for the unloaded motor to reach most of its speed in star. Too short gives a heavy current surge at changeover; too long overheats the motor in star.
How much voltage unbalance is acceptable?
Aim for less than about 1 %. Above 1 % the motor should be derated, and above 2 % the cause should be found, because current unbalance can be six to ten times the voltage unbalance.
Can a compressor run on a DG set?
Yes, if the generator can carry the motor’s starting current without a deep voltage and frequency dip. As a rough guide, allow about 2 to 2.5 kVA of generator per kW of a star-delta motor when it is the main load, and stagger starts when several machines share a set.
Is it safe to keep resetting the overload relay?
No. Each trip means the windings ran hot. Repeated resets without finding the cause shorten motor life and can end in a burnt winding. Find the cause first.
Related pages
- Electrical and control diagnostics
- Motor, drive and coupling reliability
- Variable speed drive reliability
- Motor input and electrical measurement worksheet
- Compressor troubleshooting and diagnostics
- Motors and drives
- Pressure switch settings and safety valve lifting
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 5 November 2026.