How a variable-speed compressor works
On a variable-speed screw compressor, the drive converts fixed 50 Hz plant supply into variable frequency, so the motor and airend speed follow air demand. The controller compares system pressure with a set point and asks the drive for more or less speed. Within the machine’s turndown range, output tracks demand instead of cycling between load and unload.
The drive itself has three main stages:
- Rectifier – converts incoming AC to DC. Sensitive to supply phase loss, unbalance and surges.
- DC bus – capacitors store energy and smooth the DC. Bus voltage rises when the motor decelerates and falls with supply dips.
- Inverter – power semiconductors switch the DC to create the variable-frequency output. They are sensitive to heat, motor-side faults and overcurrent.
Around these sit cooling fans, heat sinks, filters, reactors or EMC filters, and the control board that talks to the compressor controller. Any of them can be the source of a trip.
Why a drive trip is not always a drive fault
The drive is the most heavily instrumented part of the machine, so it is often the first to notice trouble elsewhere. Common examples:
- A tight airend bearing or high discharge pressure raises torque, and the drive trips on overcurrent or overload.
- A blocked cabinet filter or failed fan in a 45 °C compressor room causes an over-temperature trip.
- A supply dip, often during a DG changeover, triggers DC-bus undervoltage.
- Fast deceleration settings push regenerated energy into the bus and cause overvoltage.
- A noisy or drifting pressure transducer makes the drive hunt in speed, which looks like a control fault.
- A damaged motor cable or failing winding insulation causes earth-fault trips.
Replacing a drive in any of these cases removes the messenger and leaves the problem. The new drive will trip too.
Fault codes and what they usually mean
Fault names differ between drive makers, but most fall into the families below. Always read the maker’s manual for the exact meaning on your model.
| Fault family | What the drive saw | Check first |
|---|---|---|
| Overcurrent | Output current exceeded a fast limit | Motor cable and insulation, mechanical load, acceleration setting, output short |
| Overload (motor or drive) | Current above rating for too long | Working pressure, airend and motor bearings, oil cooler, motor data entered in the drive |
| DC-bus overvoltage | Bus voltage too high | Deceleration time, supply surges, high incoming voltage |
| DC-bus undervoltage | Bus voltage too low | Supply dips, phase loss, DG changeover, loose input terminals, weak capacitors |
| Input phase loss | Ripple from a missing or weak phase | Fuses, MCCB, input terminals, supply |
| Earth fault | Current leaking to earth on the output | Motor insulation resistance, cable damage, moisture in terminal box |
| Heat-sink or drive over-temperature | Power section too hot | Cabinet fans, filters, room temperature, dust on heat sink |
| Motor thermistor | Motor windings too hot | Motor cooling, low-speed duty, ambient, thermistor wiring |
| Communication fault | Lost link to compressor controller | Cable, connector, screens, earth, control supply |
Record whether the fault happens at start, at full speed, at minimum speed, during deceleration or at random. That single fact narrows the list more than any other.
A step-by-step VFD diagnostic sequence
- Isolate safely. Lock off the supply and any remote start. Wait for the DC bus to discharge for the time the drive maker states, then confirm with a rated meter. Vent the compressor to zero pressure.
- Capture the evidence. Photograph the active fault and the drive’s fault history, with timestamps, speed, current, bus voltage and temperature if the drive logs them. Note the compressor controller’s alarm log at the same time.
- Establish the operating state. Was the machine starting, accelerating, at maximum speed, at minimum speed or stopping? What was the system pressure and room temperature?
- Check the supply. Line voltages and balance at the drive input, fuses and terminal tightness. If the plant has DG sets, find out whether faults coincide with changeovers.
- Check cooling. Cabinet fans running, filters clean, heat sink free of dust and oil mist, enough ventilation in the room.
- Test the motor and cable. Insulation resistance with the drive disconnected (never megger through the drive), winding balance, terminal box condition, thermistor circuit.
- Check the mechanical load. Discharge pressure against set point, oil temperature, cooler condition and airend noise. A healthy drive cannot compensate for a failing airend.
- Prove the pressure feedback. Compare the transducer reading with a calibrated gauge and look for noise or drift.
- Review parameters. Compare motor data, acceleration and deceleration times, speed limits and pressure control settings with the last known-good record and the compressor maker’s values.
- Decide on the drive. Only when everything around it is proven healthy should the drive be judged faulty and repaired or replaced.
Speed hunting, poor pressure control and low turndown
Not every VFD problem is a trip. Plants often complain that a variable-speed machine “hunts”, runs at full speed when demand is low, or keeps stopping and starting.
| Behaviour | Possible causes |
|---|---|
| Speed oscillates, pressure swings | Noisy or badly placed transducer, control loop settings, too little storage, another compressor fighting the VSD |
| VSD runs at maximum while plant pressure is low | Demand above machine capacity, leaks, pressure drop in treatment or piping, wrong set point |
| VSD frequently stops at minimum speed | Demand below the machine’s turndown, oversized VSD, base-load machine supplying too much |
| Fixed-speed and VSD machines both modulating | Overlapping pressure bands; the sequencer or settings need review |
A variable-speed machine saves energy only when it is the trim machine, running within its useful speed range. If a fixed-speed compressor’s band overlaps the VSD set point, the two fight and most of the benefit disappears. Our VSD integration and base, trim and VSD optimisation pages cover this in detail. Remember the energy link too: every extra bar of generation pressure costs about 7% more power, so set points matter as much as speed control.
Indian site conditions that shorten drive life
- Heat. Drive electronics are rated for a stated maximum ambient. Summer room temperatures in Telangana and Andhra Pradesh often reach or exceed that limit unless the room is well ventilated.
- Dust. Dust on heat sinks and clogged cabinet filters reduce cooling and cause over-temperature trips and early fan failure. Cement, stone, foundry and textile plants are particularly hard on drives.
- Humidity and condensation. Monsoon humidity and machines that sit idle can bring moisture into cabinets and motor terminal boxes, leading to earth faults on restart.
- Power quality. Voltage unbalance, dips and frequent grid-to-DG changeovers stress the rectifier and DC bus. Harmonics from many drives on one transformer can also affect other equipment, which is why input reactors or filters are often specified.
- Ageing components. DC-bus capacitors and cooling fans age with heat and time. Drives that have been in service for many years deserve inspection even if they have not failed.
Replacing or repairing a compressor VFD
A drive replacement is a commissioning job, not a box swap. Before a new or repaired drive goes in:
- Match the drive rating, voltage, control method and interface to the compressor and motor.
- Enter motor nameplate data correctly and run any motor identification routine the maker recommends.
- Set acceleration, deceleration, minimum and maximum speed to the compressor maker’s values, not drive defaults.
- Confirm cooling arrangements, EMC and earthing provisions and cable screening.
- Test the full system: start, load, speed response, pressure control, stop and fault handling.
- Record all parameters so the next engineer has a known-good baseline.
How Reddy Compressors helps
Reddy Compressors, at Tarnaka in Secunderabad, has served industry since 2002 as an independent multi-brand compressor specialist. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. Our engineers diagnose and repair variable-speed compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar, Gardner Denver and many more.
- Site diagnosis of drive trips, speed hunting and pressure-control problems, following the sequence above.
- Cabinet cooling, filter and fan service, connection checks and motor insulation testing.
- Drive and motor parts identified from the nameplate and confirmed for fit before quoting, with commissioning after fitting.
- Settings and sequencing reviews so the VSD works as a true trim machine.
- Rental compressors from our own fleet, 50–2,000 cfm, to keep production running during repairs.
- Service and AMC covering drive cabinets as well as the air side.
Our service teams work across Telangana and Andhra Pradesh, and we supply parts and rental machines across India.
What to send us
- Compressor make, model and serial, and the drive make and model, with nameplate photos.
- The fault code and a photo of the fault history.
- When the fault occurs: starting, full speed, minimum speed, stopping or random.
- Room temperature, and whether the machine runs on grid, DG set or both.
- Any recent change: service, parameter edits, electrical work or a new machine on the network.
- Your location and how critical the compressor is.
Frequently asked questions
Why does my VFD compressor trip on overcurrent?
Overcurrent means output current rose above a fast limit. Common causes are a damaged motor cable or insulation, a short at the motor terminals, an acceleration time set too short, or a mechanical load that suddenly increased, such as a tightening airend. Check the motor and cable with the drive disconnected and review the operating state at the moment of trip before suspecting the drive.
What causes DC bus undervoltage faults on a compressor drive?
Undervoltage usually comes from the supply side: voltage dips, a lost phase, loose input terminals or blown fuses, or a grid-to-generator changeover. On older drives, aged DC-bus capacitors can also contribute. Measuring input voltage at the drive and checking whether trips coincide with power events normally points to the cause.
Why does my variable-speed compressor keep overheating in summer?
The drive’s power section relies on fans, filters and clean heat sinks. In a hot, dusty compressor room, clogged cabinet filters, dust on the heat sink and failing fans cut cooling, and summer ambients near 45 °C leave little margin. Clean filters and heat sinks, check the fans and improve room ventilation before the drive is blamed.
Can I replace a compressor VFD with any drive of the same kW?
Not safely. The replacement must suit the motor, voltage, control method and the compressor controller interface, and it must be set to the compressor maker’s speed, acceleration and protection values. A mismatched or wrongly configured drive can cause poor pressure control, motor stress or trips. Treat replacement as a commissioning job.
Why does my VSD compressor run at full speed all the time?
Either demand exceeds the machine’s capacity, or something stops the plant reaching set point: leaks, pressure drop across dryers and filters, undersized piping, or a set point that is higher than needed. It can also mean the pressure transducer reads low. Measure pressure at the compressor and at the point of use to separate the causes.
Does a VFD compressor always save energy?
Only when demand varies and the VSD machine works as the trim compressor within its useful speed range. At steady full load a VSD has small extra losses compared with a fixed-speed machine. Overlapping pressure bands between machines also erode savings. Demand profiling before purchase shows whether variable speed suits your plant.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.