How does a VSD screw compressor work?
In a fixed-speed oil-injected screw compressor, the motor runs at one speed whenever it is running. Output is controlled by closing the inlet valve and blowing down the separator tank (load/unload), or by throttling the inlet (modulation). Both methods leave the motor turning while it delivers little or no air.
Representative images of machine types we sell, service and rent. Models and capacities are confirmed against your duty.
A VSD machine places a frequency inverter between the supply and the motor. The controller watches discharge or header pressure against a set point and raises or lowers motor speed so that free air delivery (FAD) follows the plant’s consumption. Because screw airend displacement is roughly proportional to rotor speed, a slower motor delivers proportionally less air, and input power falls with it.
- Demand falls and header pressure starts to rise above the set point.
- The controller reduces inverter output frequency, so the motor and airend slow down.
- Delivery drops until pressure settles back at the set point.
- When demand rises, pressure dips, the controller increases speed and delivery recovers.
- If demand falls below the machine’s minimum speed, the compressor stops or unloads and waits for pressure to fall again.
Each manufacturer has its own control algorithm, speed range and minimum-speed behaviour. Those details belong to the specific model and must be read from its data sheet, not assumed.
Where VSD makes engineering sense
VSD is worth evaluating when the plant’s demand pattern gives speed control a real job to do. Typical signs are:
- Demand varies widely across the day or between shifts, for example a busy day shift and a light night shift.
- The existing fixed-speed compressor spends long periods unloaded, still drawing a substantial share of its full-load power.
- Pneumatic equipment cycles frequently, so header pressure swings and operators raise the set point to compensate.
- The station needs a trim machine to follow the changing part of demand alongside one or more base-load compressors.
- Frequent motor starts are a problem for the electrical supply; an inverter gives a soft, controlled start.
It is usually a weaker case where a single compressor runs fully loaded for most of its hours, where demand is very low compared with the machine’s minimum speed, or where the real problem is leaks, pressure drop or poor storage rather than control.
VSD versus fixed-speed: how do they compare?
| Factor | Fixed-speed screw | VSD screw |
|---|---|---|
| Output control | Load/unload or modulation at constant motor speed | Motor speed varies within the model’s allowable range |
| Pressure band | Wider band between load and unload points | Narrow band around a single set point |
| Efficiency at full load | Typically very good; no inverter losses | Small inverter and harmonic losses; good machines are close to fixed-speed |
| Efficiency at part load | Falls as unloaded running increases | Stays reasonable across the speed range; worsens near minimum speed |
| Starting | Star-delta or soft starter; high current peaks | Controlled ramp-up from the inverter |
| Best role in a station | Base load at or near full capacity | Trim machine, or single machine on variable demand |
| Electrical considerations | Conventional motor protection | Harmonics, cabinet cooling, earthing and cable practice matter |
| Purchase cost | Lower | Higher; justified only by measured energy savings |
Why the demand curve comes first
No honest supplier can give a standard saving percentage for VSD. The saving depends on how many hours the plant spends at each level of demand and what the existing machine draws at those levels. The right first step is a short logging exercise:
- Log compressor power (kW) and header pressure for at least one typical week, covering all shifts and a weekend or holiday.
- Where possible, log flow at the compressor outlet, or estimate it from load/unload times and rated FAD.
- Note production events, such as which lines run on which shift.
- Build a demand-duration profile: how many hours per year the plant spends at 20%, 40%, 60%, 80% and 100% of capacity.
With that profile you can compare the existing compressor’s power at each demand level against the VSD machine’s published part-load power at the same level, and calculate annual kWh for each option. If the plant sits mostly in the middle band, a VSD usually has a clear case. If it sits at full load, the case is weak.
Compare machines on specific power, kW per m³/min of FAD at the stated pressure, using the manufacturer’s test data to the recognised acceptance standard. Ask for the curve across the speed range, not a single best point.
Pressure set point: a hidden benefit
A fixed-speed compressor running between, say, 7.0 and 7.5 bar (about 100–110 psig) must keep its lower switching point above the minimum the plant needs. A VSD holds pressure within a narrow band, so the average generation pressure can often be lowered. As a typical rule, each extra bar of discharge pressure costs about 7% more compressor power, so trimming average pressure by even a fraction of a bar has real value. Lower header pressure also reduces leak flow and unregulated use.
That benefit only arrives if someone actually lowers the set point after commissioning, and if distribution pressure drop is not forcing a high set point in the first place.
Turndown, minimum speed and the control gap
Every VSD screw has a minimum speed below which it cannot run efficiently or safely, because of oil injection, lubrication and cooling limits. Below that, the machine stops or unloads. Turndown, the ratio between maximum and minimum delivery, therefore matters as much as maximum FAD.
In a multi-compressor station a common mistake is a control gap: the VSD’s speed range is smaller than the step size of the fixed-speed machines. When demand sits in that gap, the VSD hits its limit and a fixed-speed machine starts and stops repeatedly. The fix is to size the VSD’s usable range to cover at least the largest fixed-speed step, and to sequence all machines from one master controller. Our page on base, trim and VSD energy optimisation covers dispatch in detail.
Electrical and site points for Indian plants
- Harmonics: inverters draw non-linear current. Check total harmonic distortion at the point of common coupling, the transformer rating, and whether a DC choke, line reactor or filter is fitted.
- Cabinet cooling: inverter electronics are sensitive to heat and dust. In Telangana and Andhra Pradesh summers, compressor rooms can exceed 45 °C, so ventilation and cabinet filter cleaning are part of maintenance.
- Power quality: voltage dips and unbalance can trip drives. Record the supply before installation and check the drive’s tolerance.
- DG back-up: if the compressor runs on a diesel generator during outages, the generator must be sized and specified for an inverter load.
- Cabling and earthing: follow the manufacturer’s recommendations for motor cable type, length and earthing to limit bearing currents and interference.
Our page on VSD reliability and drive systems covers inverter faults and service practice.
Common VSD compressor problems and what we check
| Symptom | Likely causes | What we check |
|---|---|---|
| Drive trips on overtemperature | Blocked cabinet filters, high room temperature, failed cabinet fan | Room ventilation, filter condition, fan operation, heat-sink cleanliness |
| Compressor runs at maximum speed constantly | Undersized machine, leaks, set point too high | Demand profile, leak level, pressure set point |
| Frequent stops at low demand | Demand below minimum speed, small receiver | Usable storage, minimum-speed delivery, sequencing |
| Pressure hunting | Control tuning, transducer fault, interaction with other machines | Sensor calibration, controller settings, master-control logic |
| Higher energy use than expected | Running near minimum speed for long periods, poor dispatch | Power logging against speed and pressure |
How Reddy Compressors helps
We are an independent multi-brand compressor specialist based at Tarnaka, Secunderabad, serving industry since 2002. We sell new and used/refurbished rotary screw compressors, including VSD and fixed-speed machines, from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makers, without being an authorised dealer for any of them. That lets us compare options on the plant’s demand rather than on one catalogue.
- Demand and power logging to show whether VSD, fixed-speed or a combination suits your plant.
- Neutral comparison of offers on specific power, turndown and part-load data.
- Installation advice covering ventilation, electrical supply and receiver sizing.
- Service, AMC and spares for VSD screw packages, with more than 20 service engineers and technicians covering all of Telangana and Andhra Pradesh.
- Rental compressors from our own fleet while a decision is made or a machine is repaired, with supply across India.
What to send us
- Make, model, rated power and pressure of your existing compressors (nameplate photos help).
- Required minimum pressure at the point of use.
- Shift pattern and running hours per year.
- Any power, pressure or load/unload logs you already have.
- Receiver sizes, dryer type and a rough layout of the compressor room.
Frequently asked questions
How much energy does a VSD compressor save?
There is no fixed figure. The saving depends on how many hours your plant spends at part load and how much the existing machine draws while unloaded. A plant with strongly varying demand can save a useful share of compressor energy, while a plant running fully loaded may save almost nothing. Logging power and pressure for a typical week lets you calculate annual kWh for each option before you buy.
Is a VSD compressor worth it for a single-shift plant?
It can be, if demand within that shift varies a lot or the existing compressor unloads for long periods. If the machine runs near full load whenever the plant works, a good fixed-speed compressor is usually the better buy. The number of running hours also matters, because fewer hours mean a longer payback on the higher purchase cost of a VSD package.
Can a VSD compressor run on a diesel generator?
Yes, provided the generator is suitably sized and specified for a non-linear inverter load. Harmonic current from the drive can affect generator voltage regulation and heating, so the generator rating is normally increased compared with a direct-on-line motor. Discuss the combination with both the compressor and generator suppliers before relying on it during power cuts.
Should the VSD compressor be the base or the trim machine?
In most multi-compressor stations, fixed-speed machines carry the steady base load at full capacity and the VSD acts as the trim machine following the variable part of demand. The VSD’s usable speed range should cover the largest fixed-speed step to avoid a control gap. In single-machine plants with variable demand, the VSD naturally does both jobs.
Why does my VSD compressor trip on high temperature in summer?
The commonest causes are hot compressor rooms, blocked cabinet or cooler filters and failing cabinet fans. Inverter electronics need cool, clean air. Check room ventilation and the exhaust path for hot air, clean filters and heat sinks regularly in dusty conditions, and confirm the oil cooler is not fouled. If trips persist, a service engineer should review drive parameters and fault history.
Does a VSD compressor need different maintenance?
The airend, oil, filters and separator are serviced much as on a fixed-speed screw. In addition, the drive needs clean cabinet filters, working cooling fans, tight power connections and periodic checks of fault logs. Motor bearings may need attention if cabling and earthing are poor. An AMC that includes the electrical side keeps the drive reliable.
Related pages
Field guide
Fixed speed or variable speed: Let the load profile decide
A variable-speed drive (VSD) compressor changes motor speed to follow demand. Where demand swings a lot it can cut unloaded running and steady the pressure. It is not automatically the most efficient choice everywhere.
| Fixed speed | Variable speed | |
|---|---|---|
| Best where | Demand is steady and the machine runs near full load | Demand varies widely through the day |
| Advantages | Simple control, robust, predictable maintenance, strong at steady load | Less unloaded running, stable pressure, soft starts, quick response to changing production |
What to check before choosing a VSD
- A measured load profile: minimum, average and maximum demand.
- Annual operating hours and the pressure band you can accept.
- The number of compressors on the system.
- Ambient conditions and maintenance capability.
- Electrical and harmonic requirements.
- The machine’s minimum speed. Below it, a VSD still unloads or stops.
Base and trim
The best arrangement is often a fixed-speed compressor carrying the base load with a VSD machine trimming the variable part of demand. With two or more machines, a master controller sequences them to cut unloaded running and start-stop cycles. See controls and sequencing. Compare running cost with the specific power calculator, entering your own load profile and tariff.
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Specific power comparison
Compare two compressors on what matters over life: the electrical power needed per unit of air delivered, and what that costs a year.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Machine A: input power (kW): 75
- Machine A: delivered flow (m³/min): 11.5
- Machine B: input power (kW): 80
- Machine B: delivered flow (m³/min): 12.6
- Running hours per year: 6000
- Air actually needed (m³/min): 11
Result
- Machine A specific power: 6.52 kW per m³/min
- Machine B specific power: 6.35 kW per m³/min
- Energy to deliver the air needed: A: 4,30,435 kWh per year
- Energy to deliver the air needed: B: 4,19,048 kWh per year
- Energy difference: 11,387 kWh per year in favour of B
- Cost difference: Enter your electricity tariff (cost per kWh) above
Method. Specific power = input kW ÷ delivered flow (m³/min) at the same pressure and reference conditions. Annual cost = kW × hours × tariff. Use the full-load input power on the same test basis for both machines.
What to take from it. A small gap in specific power is multiplied by thousands of running hours. Compare on the same pressure and reference conditions.
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.
Load/unload cycle energy
Turn measured load and unload times (or the logged percentage of time in each state) and measured package kW into average power, annual energy, the energy spent running unloaded, and specific power based on the air actually delivered. An optional second set of times compares a change.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Times entered as: Measured cycle times (seconds)
- Loaded package power (kW): 82
- Unloaded package power (kW): 28
- Loaded time (s per cycle, or percent): 40
- Unloaded time (s per cycle, or percent): 60
- Stopped time, motor off, 0 if none (s per cycle, or percent): 0
- Power while stopped, controls only (kW): 0
- Running hours per year with this cycle pattern: 6000
- Free air delivered when loaded, FAD (m³/min): 12.5
- After change, optional: loaded time, all three 0 = not used (s or percent): 0
- After change, optional: unloaded time (s or percent): 0
- After change, optional: stopped time (s or percent): 0
Result
- Cycle time: 100 s (36 cycles per hour)
- Loaded fraction: 40 %
- Unloaded fraction: 60 %
- Unloaded power as a share of loaded power: 34.1 %
- Average package power: 49.6 kW
- Annual energy: 2,97,600 kWh per year
- Unload energy (buys no air): 1,00,800 kWh per year = 33.9% of the annual energy
- Loaded specific power: 6.56 kW per m³/min
- Average air delivered: 5 m³/min
- Effective specific power over the cycle: 9.92 kW per m³/min
- Energy cost per year: Enter your electricity tariff (cost per kWh) above
Method. Two input modes. Cycle timing: enter measured seconds per cycle. Fraction: enter the share of time loaded, unloaded and stopped (for example 40, 60, 0, from controller loaded/running hours); no cycle time is reported in this mode. Average kW = (P_load × t_load + P_unload × t_unload + P_stopped × t_stopped) ÷ (t_load + t_unload + t_stopped); the states must not overlap. Annual kWh = average kW × the hours this pattern runs. Unload energy = P_unload × unloaded fraction × hours. Average air delivered = loaded fraction × loaded FAD; effective specific power = average kW ÷ average air delivered. Unloaded power is never assumed to be zero: enter measured package kW (unloaded read after blowdown has settled). Scenario comparison: enter the predicted times after a change (bigger receiver, wider band, auto-stop) in the “after change” fields; the same powers, FAD and hours are used and the kWh difference is shown. An auto-stop timer is modelled by moving unload time into the stopped state (check the motor’s permitted starts per hour). For different day and night patterns, run the tool once per pattern and add the results. The rapid-cycling warning below 30 s per cycle is a screening value used by this tool. Money is shown only when you enter your own tariff.
What to take from it. A large unload-energy share, or an effective specific power well above the loaded figure, means a fixed-speed machine is running lightly loaded: look at storage, pressure band, sequencing, auto-stop or a smaller/VSD trim machine.