How a VSD Screw Air Compressor Works – Screw Air Compressors Manufacturer

A screw compressor pumps air using two meshing helical rotors. A variable frequency drive adjusts rotor speed. Slower rotors, less air. Faster rotors, more air. The controller watches system pressure and tells the drive what speed to run.

01 The Drive

Six-diode rectifier converts AC to DC. Electrolytic capacitors store it. Six IGBTs chop it back into AC at the frequency the controller requests, using pulse-width modulation. The output looks nothing like a sine wave up close. Averaged over many switching cycles, it approximates one well enough for the motor to run.

DC bus capacitors age by losing electrolyte through the end seal. The rate follows the Arrhenius equation: every 10°C above rated temperature halves remaining life. Danfoss rates the VLT series capacitors at 100,000 hours at 40°C ambient. Compressor enclosures exceed that. They just do. Even in temperate climates, the heat from the compressor itself and from the drive's own switching losses pushes the enclosure air well above room temperature. In warm climates the situation is worse, and in outdoor installations in the Gulf states or equatorial regions the capacitors can be significantly degraded in five to six years.

The degradation is invisible for most of its progression. The compressor runs. Motor current gets noisier. The drive starts faulting intermittently on load transients. These faults look like wiring problems or contactor problems or firmware bugs, and maintenance teams chase those explanations for months.

Separate issue: VFDs stored unpowered for eighteen-plus months develop degraded dielectric on the capacitor foil. EPCOS/TDK documents this. Reforming the capacitors with a controlled voltage ramp before full energization is the correct procedure. It is almost never done on spare compressors.

PWM pulses have roughly 100 ns rise times and reflect at motor terminals, reaching peak voltages up to twice the DC bus voltage on long cable runs. On a 480 V system that means 1,300 V peaks. IEC 60034-18-41 covers insulation requirements for inverter-fed motors. Standard motors are not built for this. Cable runs under 15 meters in packaged compressors make the issue mostly irrelevant for factory-built machines.

02 Motor Control

V/f control maintains a linear voltage-to-frequency ramp. Vector control decomposes stator current into flux and torque components (Id and Iq) and manages them separately through a mathematical motor model. Yaskawa published test data on a 75 kW frame showing about 3% efficiency gain at 30% speed under vector control versus V/f. The gap shrinks at higher speeds and is negligible above 80%.

The motor model depends on an auto-tune during commissioning. If the technician uncouples the motor from the airend to avoid dealing with rotation safety procedures, the identified parameters are wrong because the reflected mechanical load changes the motor's electrical behavior. The compressor still runs. The efficiency penalty is invisible in daily operation and shows up only in the annual energy bill or in a formal ISO 1217 Annex E test if anyone ever runs one.

03 Airend and Leakage

VSD does not fix the factory-cut Vi.

Leakage is what makes or breaks a VSD screw compressor at part load. Every clearance in the airend is a leakage path: rotor tip to bore, interlobe sealing line, end plate gaps. The flow through these paths is pressure-driven. Pressure differential across the clearances does not depend on rotor speed. An airend with 0.3 m³/min total internal leakage bleeds that same 0.3 m³/min whether the rotors are turning at 3,000 RPM or 1,500 RPM. At 3,000 RPM, sweeping 5.0 m³/min, the leakage is 6% of displacement. At 1,500 RPM, sweeping 2.5 m³/min, it is 12%.

The specific energy curve that results from this has a minimum somewhere in the 60% to 80% speed range on most machines. Below that, the leakage fraction climbs fast. Below 30% speed, most airends use more energy per cubic meter of air than they do at full speed.

The interlobe sealing line carries the highest leakage flow of any clearance path. On oil-injected machines, the oil filling this clearance provides a hydraulic seal that supplements the geometric clearance. At reduced speed, the oil has more time to fill the gap and the seal improves. At higher speeds, centrifugal force throws oil out of the clearance faster. This partially offsets the geometric leakage penalty at low speed, and it means the specific energy curve is not as steep in the lower speed range as a purely geometric analysis would predict. Kaeser's technical literature for the SFC series mentions this effect. Published data quantifying it is scarce.

Clearances are not constant across the speed range. Centrifugal growth and thermal expansion change rotor dimensions. Airend manufacturers who build VSD-specific products sometimes set tighter cold clearances, optimized for the 50% to 70% speed range where the machine accumulates most of its operating hours. The trade-off: sustained full-speed running in a hot environment closes those clearances further. Rotor tip scoring and bore contact marks are the evidence. Service engineers who work on machines in foundries, glass plants, equatorial climates see this damage pattern. The same airend in a climate-controlled room in Scandinavia or northern Japan runs for years without issues.

Whether a particular compressor uses a VSD-optimized airend or a fixed-speed airend with a drive bolted on is not disclosed on the CAGI datasheet. ISO 1217 Annex C tests specific energy at a single full-speed full-load point. Annex E defines part-load test profiles that expose the difference. Annex E data is not always published. When it is, the gap between a purpose-designed VSD airend and a converted fixed-speed airend shows up clearly in the 40% to 60% speed range, where the leakage fraction penalty hits hardest.

04 Oil and gas residence time

At reduced speed, air spends more time in the compression chamber. Oil absorbs more heat per unit of air. The polytropic exponent drops from the 1.25 to 1.30 range at full speed to roughly 1.10 to 1.15 at half speed. The compression process approaches isothermal. Less work per unit of air delivered. Fixed-speed compressors cannot access this because rotor speed, and therefore gas residence time, is locked.

Between 50% and 70% speed, the heat transfer improvement and the leakage penalty roughly cancel each other, producing a flat specific energy curve across that range. This is where a VSD compressor is at its best, operating in a broad zone where efficiency is stable and insensitive to small speed changes.

05 Oil System

Oil injection rate depends on the pressure difference between the sump and the injection point in the compression chamber. That difference stays roughly constant regardless of speed. Airflow drops with speed. The oil-to-air ratio increases.

The oil temperature problem needs to be discussed at length because it is the failure mode that does the most cumulative damage in VSD compressors and gets the least attention during system design and procurement.

At sustained low speed, heat generation drops. Oil temperature follows. If oil temperature falls below the pressure dew point of the moisture in the air, water condenses into the oil. Emulsified oil has reduced film strength. It attacks the borosilicate glass fiber media in the separator element, which swells and loses coalescing efficiency. It promotes corrosion on cooler tubes, receiver internals, and discharge piping. The process is slow. Days to weeks. The compressor does not alarm on water-in-oil. The first symptom is usually elevated separator differential pressure, or high oil carryover downstream, or bearing vibration during a routine check. By that point, the separator element has absorbed enough water to need replacement, the oil has elevated acid number and water content on analysis, and the bearings may already show early pitting.

Controllers that enforce minimum oil temperature, either by maintaining a speed floor or by stopping the compressor for thermal recovery before restarting, handle this. Some compressors have thermostatically controlled oil sump heaters for exactly this scenario. Machines without these features rely on the demand profile cycling the compressor hard often enough to keep oil temperature up. In facilities with steady low demand during night shifts or weekends, this assumption fails.

Foam. Low air velocity through the receiver tank at reduced speed changes surface foam dynamics. Some oil formulations, particularly mineral oils and certain polyglycol synthetics, foam aggressively at low face velocity. The foam rises into the separator element, clogs the drainage layer, increases pressure drop. The compressor draws more power to push air through the restriction. PAO-based synthetics are generally better. Shell Corena S4 R and Kaeser Sigma Fluid S460, both PAO formulations, exhibit lower foaming tendency in low-velocity conditions. Specifying the right oil for a VSD compressor matters more than for a fixed-speed machine. The default factory fill may not be the best choice for a machine that will spend most of its life at 40% to 60% speed.

The separator element itself is sized for full-speed airflow, which means it is oversized at part load. Generally this is fine and separation efficiency improves at lower face velocities. The exception is when foam or the elevated oil-to-air ratio at low speed overwhelms the drainage capacity, causing oil to pool on the downstream side of the element and get carried into the air system.

06 Pressure Control

PID loop modulates motor speed to hold system pressure within ±0.15 bar. Load/unload control on fixed-speed machines swings a full bar or more between load and unload setpoints.

Compression energy scales at about 7% per bar. A half-bar reduction in average operating pressure saves 3.5% of compression energy continuously. Over a year of continuous operation on a 75 kW machine, the pressure band contribution alone accounts for roughly 20,000 kWh.

Factory PID tuning is conservative because the factory cannot predict the downstream system configuration. On-site adjustment of PID gains while watching the pressure trace under varying demand conditions tightens the pressure band and reduces speed hunting.

07 PM Motors and Their Consequences

Atlas Copco GA VSD+, Kaeser SFC, CompAir L-Series VSD use interior permanent magnet motors. IE5-class efficiency per IEC 60034-30-2 across the operating range. Higher torque per unit volume than induction motors, which allowed OEMs to reduce package footprint.

Total VFD dependency. If the drive fails, the compressor is offline. PM motors cannot start across-the-line. Replacement drive lead time varies.

Back-EMF during shutdown. The magnets produce voltage whenever the rotor turns. The airend can coast on residual pressure after inlet valve closure. The VFD must manage the regenerated energy. DC bus overvoltage faults during shutdown are common on VSD compressors.

NdFeB magnets demagnetize permanently above the grade's knee-point temperature, around 150°C for N35SH. Overcurrent events or sustained high temperature cause cumulative damage. The motor draws progressively more current for the same torque output. Diagnosis requires comparing measured back-EMF at a known speed against original factory data.

08 Bearing Currents

PWM common-mode voltage induces shaft voltage through stator-to-rotor capacitive coupling. When shaft voltage exceeds the lubricant film breakdown threshold (about 0.5 V), current discharges through the bearing and machines micro-pits in the race. Accumulated damage produces circumferential grooves called fluting. SKF classifies this as electrical erosion. Their countermeasures include hybrid ceramic bearings (Si3N4 elements) and shaft grounding rings (Aegis SGR series). Without mitigation, bearing life under VFD operation is typically two to four years. With ceramic bearings or grounding rings, the electrical erosion mechanism is eliminated.

Whether the OEM includes bearing current mitigation as standard varies. It is one of the most consequential differences between competing VSD compressor packages.

09 System Integration

Receiver sizing: 20 liters per m³/min of compressor capacity is a common starting point. Smaller receivers force rapid speed changes and degrade PID stability.

Cycling refrigerated dryers paired with VSD compressors can short-cycle at low airflow because reduced thermal mass destabilizes evaporator temperature control. Non-cycling or VSD refrigerated dryers match better.

Harmonics from the six-pulse rectifier run about 40% THDi (5th and 7th dominant). IEEE 519-2022 limits injection at the point of common coupling. A 3% input line reactor reduces THDi to around 25%. In facilities with power factor correction capacitor banks, series resonance near the 5th harmonic can amplify distortion and destroy capacitors.

10 Energy Savings Breakdown

CAGI load/unload power curves for fixed-speed compressors show about 18% of annual energy wasted in unloaded running at 60% average demand. The VSD eliminates this. Lower pressure band saves additional percent. Thermodynamic shift toward isothermal compression at reduced speed adds more. VFD parasitic losses subtract about 3%.

Why screw compressors went mainstream traces the 1970s shift away from piston units.

The productive range for a VSD compressor is roughly 40% to 75% average demand. Above 85%, a fixed-speed machine is more efficient. Below 35%, leakage fraction, oil temperature, and foam behavior degrade performance enough that a smaller fixed-speed machine or a different system configuration would serve better.

End of Reference