Who this page is for: plant heads, utility managers and maintenance teams in India who are replacing or adding a screw compressor and need to decide, with their own numbers rather than brochure claims, whether fixed speed or a variable speed drive (VSD) will cost less to run.
Key facts
- An unloaded fixed-speed screw compressor typically draws 20–35 % of full-load power and delivers no air. On a 63 kW package that is 13–22 kW of overhead.
- VSD drive and filter losses are about 2–4 %, so at full flow a VSD package is usually slightly less efficient than a fixed-speed machine running fully loaded.
- Most VSD screw packages have a minimum speed of roughly 20–40 % of rated flow. Below it they load/unload or stop/start like any fixed-speed machine.
- With this page’s default inputs, a 10 m³/min (353.1 cfm) machine working 6,000 h a year saves 14,376 kWh (5.2 %) with VSD. A steady 90 % plant uses 10,314 kWh more with VSD.
- Each extra 1 bar (14.5 psig) of generation pressure costs about 6–7 % more energy.
- With a 0.5 bar (7.3 psig) band and 1 m³ of storage, a 10 m³/min load/unload machine cycles about every 12 seconds at 50 % demand, too fast to reach its true unloaded power.
- Collect at least 14 days of logged power, pressure and load state before you decide.
How each control method actually works
A screw airend delivers air in proportion to rotor speed. A fixed-speed machine has only one speed, so it has to match demand in some other way.
Load/unload. The machine runs loaded up to the upper setting, for example 7.5 bar(g) (108.8 psig). The inlet valve then closes and a blowdown valve vents the separator vessel. The motor keeps running, but the rotors compress almost nothing. At the lower setting, say 7.0 bar(g) (101.5 psig), it reloads. The sump takes tens of seconds to blow down, and during that time power stays well above its settled unloaded value. If the machine reloads before blowdown is complete, the real unloaded power is higher than the datasheet figure. A run-on timer stops the motor after a set unloaded period.
Modulation. The inlet throttles to hold one pressure. Throttling raises the compression ratio, so power falls only slightly with flow. A modulating screw at 50 % flow can still draw 80 % or more of full-load power.
Stop/start. The motor stops when pressure is satisfied. Part-load efficiency is excellent, but motor starts are limited, often to around 6–10 per hour for packages of tens of kW, and every star-delta start stresses contactors. It suits small machines with large receivers.
VSD. An inverter varies motor speed so that output follows demand while pressure stays within about ±0.1–0.2 bar (1.5–2.9 psig) of setpoint. Power falls roughly in proportion to flow, but drive losses apply at every speed, and specific power rises somewhat at low speed. At minimum speed the machine can go no slower. Below that it behaves like a small fixed-speed unit.
| Control method | Pressure stability | Typical power at 50 % demand (share of full load) | Best suited to |
|---|---|---|---|
| Modulation | Very steady | About 80–90 % | Short periods near full load only |
| Load/unload, adequate storage | Band 0.5–1.0 bar (7.3–14.5 psig) | About 60–70 % | Steady high load; base duty in a fleet |
| Load/unload, small receiver | Band plus overshoot | Often above 70 % | Nothing; a fault to correct |
| Stop/start | Band 0.5–1.0 bar | Close to 50 % within start limits | Small machines, large storage |
| VSD within speed range | ±0.1–0.2 bar | About 50–55 % | Variable demand; trim duty |
Unloaded power: the cost nobody invoices
Take a 63 kW package with unloaded power of 25 %, which is 15.75 kW. If it spends 3,000 hours a year unloaded, that is 47,250 kWh. At an illustrative tariff of ₹9 per kWh, the bill comes to ₹4,25,250 a year for rotors that compress nothing. Three common site habits make this worse:
- Run-on timers left at long defaults, so the machine idles through every tea break and shift change.
- Undersized receivers, which cause short cycling. Blowdown never completes, so average unloaded power sits well above the datasheet figure, often by 10 percentage points or more.
- Leakage of 20–30 %, which keeps the machine cycling all night. Fix the leaks first. The profile you design for then becomes smaller and steadier, and that can change the answer.
A VSD earns its keep by removing unloaded hours. A plant with few unloaded hours gets little back and still pays the drive losses.
Choose the control that matches the hours your plant really spends at each demand level: the cheapest kilowatt is the unloaded one you never draw, and the costliest is the drive loss you pay at full load every day.
Demand profiles that favour each machine
Average demand alone cannot decide this. A plant sitting at 65 % all day and one swinging between 95 % and 30 % have the same average but can reach opposite answers.
| Demand pattern (share of one machine’s flow) | Likely winner | Reason |
|---|---|---|
| Steady 85–100 % most hours | Fixed speed | Few unloaded hours; drive losses every hour |
| Swinging 40–90 % across shifts | VSD | Most hours fall inside the speed range |
| Long periods below 25–30 % | Small night machine, or fix leaks | The VSD also cycles below minimum speed |
| Rapid large swings (presses, blasting) | VSD plus adequate storage | Steady pressure; storage covers fast peaks |
| Demand above one machine | Fleet with base-plus-trim | The question becomes which machine trims |
With the defaults below, the model’s crossover is about 78 % demand. Above it the fixed-speed machine draws less, and below it the VSD draws less. At 50 % demand the figures are 39.38 kW against 34.65 kW. At 25 % they are 27.56 kW against 17.74 kW. Your crossover moves with unloaded power, specific power and minimum speed, so calculate your own.
Base-plus-trim in multi-machine plants
Where demand exceeds one machine, run fixed-speed machines fully loaded as base load and let one VSD trim the swing. The base machines then hardly unload, and the VSD absorbs the variation.
The VSD’s span from minimum to maximum flow must be at least as large as the largest base machine it trims. Suppose a 10 m³/min base machine unloads and the trim can only cover 3–9 m³/min (105.9–317.8 cfm). Some demands then fit neither combination, and the system hunts. This control gap is common where a VSD was added to an existing fleet without a check. One controller should decide the loading order. Hand-staggered pressure bands drift and typically push fleet pressure up by 0.5–1.0 bar (7.3–14.5 psig). See base and trim control and sequencers.
Receiver size and pressure band
For load/unload, the shortest cycle occurs at 50 % demand. A simple isothermal estimate is cycle time (min) = 4 × V × ΔP ÷ (1.013 × Q), where V is storage in m³, ΔP the band in bar and Q the FAD in m³/min. For a 10 m³/min machine with a 0.5 bar (7.3 psig) band:
- 1 m³ of storage gives a 0.197 min cycle, about 12 s
- 2 m³ gives 0.395 min, about 24 s
- 5 m³ gives 0.987 min, about 59 s
At 12 seconds the machine never finishes blowing down. Some VSD “savings” are really the benefit of storage the old machine lacked. See receivers and storage.
The band matters as well. A load/unload machine set at 7.0–7.5 bar(g) (101.5–108.8 psig) averages about 0.25 bar (3.6 psig) above a VSD holding 7.0 bar(g), which costs roughly 1.5–1.75 % more energy. Bands that have drifted to 1 bar (14.5 psig), or setpoints raised after a single complaint, widen the gap. Correct the setpoint before costing either option.
Harmonics, heat and dust on Indian sites
- Harmonics. An unmitigated six-pulse drive typically draws input current with 80 % or more total harmonic distortion. A DC choke or line reactor usually brings it down to about 30–45 %, and active or multi-pulse front ends to below about 10 %. Several drives on one transformer can overheat capacitor banks and trip power-factor panels. Ask for input current distortion at full load, and have your electrical consultant check it against your supply conditions.
- Heat. Many drives are rated for full output up to about 40 °C and derate above that. Compressor rooms in Telangana and Andhra Pradesh often reach 45–50 °C in April and May. A derated drive gives low plant pressure on the hottest afternoons. Duct hot air out, and design for peak summer temperature.
- Dust and monsoon humidity. Cement, foundry and textile dust chokes cabinet filters within weeks. A cold, idle cabinet in humid weather can collect condensation. Clean filters weekly and keep anti-condensation heaters powered.
- Voltage dips. Drives can trip on DC-bus undervoltage during dips that a direct-on-line motor would ride through. Check the auto-restart settings, a point also tested on the standby readiness page. Drive ageing and cooling are covered in VSD reliability.
Running on a diesel generator
A star-delta start puts a heavy transient on a DG set, while a VSD ramps up gently. Against that, drive harmonics heat the alternator and can unsettle voltage regulation, so a generator feeding mainly drives may need more kVA than the kW suggests. Confirm with the generator supplier. A 63 kW fixed-speed machine loading and unloading every 15–60 seconds hits the governor with repeated steps of about 47 kW, the gap between its loaded and unloaded power. Its output also falls with frequency: at 48.5 Hz it delivers about 3 % less air, whereas a VSD holds its setpoint. See electrical supply for rental air.
How to log demand before deciding
- Log three-phase power, not current, at 1–10 second intervals on each machine. Power factor collapses when a machine is unloaded, so current alone misleads. See power and flow logging.
- Log pressure at the discharge and at the critical or furthest user, so that pipe drop is not mistaken for shortage. See pressure profiling.
- Record load state (loaded, unloaded or stopped). Loaded share multiplied by FAD gives delivered air.
- Cover 14 days or more, including all shifts, a Sunday and the month-end peak.
- Measure weekend leakage with production stopped, and decide whether repairs come first. See leak detection.
- Build a duration curve and group it into three bands of hours and average demand. Use the duration curve tool and the demand profiling method.
- Verify FAD on old machines. A worn airend can deliver 10–20 % less than its nameplate, and every kWh figure in the comparison scales with the air actually delivered.
The calculation method
The calculator uses a transparent model that you can check by hand.
Inputs: rated FAD Q (m³/min); fixed-speed full-load package power Pf (kW); unloaded power u (% of Pf); VSD specific power at full flow s (kW per m³/min); VSD minimum speed m (% of Q); tariff (₹/kWh); and three demand bands, each with hours per year and demand as a percentage of Q.
Fixed speed at demand fraction d: the machine is loaded for a share d of the time and unloaded for the rest, so P = Pf × (d + (1 − d) × u).
VSD at or above minimum speed: P = Q × s × d × (1 + 0.10 × (1 − d)). The bracket is a part-load penalty for specific power rising as speed falls. It is zero at full flow and adds 5 % at half flow.
VSD below minimum speed: the machine runs at Pmin = Q × s × m × (1 + 0.10 × (1 − m)) for a share d/m of the time and unloaded otherwise, so P = Pmin × (d/m + (1 − d/m) × u).
Annual figures: kWh is the sum of each band’s power multiplied by its hours. Cost is kWh times tariff. Saving is fixed-speed cost minus VSD cost, and it can be negative. In that case the VSD costs more to run on your profile, and the calculator reports it.
Simplifications: the model ignores blowdown time, receiver size and short cycling, pressure-band overshoot and start limits. Each of these usually makes real fixed-speed part load worse, so on small receivers the true VSD saving is generally larger. Both options are assumed to deliver equal air at equal pressure. Take s and Pf from datasheets at the same pressure and to ISO 1217 conditions.
Compare the two for your plant
Enter your logged figures. The defaults reproduce the worked example.
Worked example (illustrative)
Inputs: Q = 10 m³/min (353.1 cfm) at 7 bar(g) (101.5 psig). Pf = 63 kW, u = 25 %, s = 6.6 kW per m³/min, m = 25 %, tariff ₹9 per kWh (illustrative). Demand bands: 2,000 h at 85 %, 3,000 h at 60 % and 1,000 h at 35 %, a total of 6,000 h.
Step 1: Fixed-speed power
- 85 %: 63 × (0.85 + 0.15 × 0.25) = 63 × 0.8875 = 55.9125 kW
- 60 %: 63 × (0.60 + 0.40 × 0.25) = 63 × 0.70 = 44.10 kW
- 35 %: 63 × (0.35 + 0.65 × 0.25) = 63 × 0.5125 = 32.2875 kW
Step 2: VSD power (all bands are at or above the 25 % minimum)
- 85 %: 10 × 6.6 × 0.85 × 1.015 = 56.9415 kW
- 60 %: 10 × 6.6 × 0.60 × 1.04 = 41.184 kW
- 35 %: 10 × 6.6 × 0.35 × 1.065 = 24.6015 kW
Step 3: Annual energy
- Fixed speed: 1,11,825 + 1,32,300 + 32,287.5 = 2,76,412.5 kWh
- VSD: 1,13,883 + 1,23,552 + 24,601.5 = 2,62,036.5 kWh
Step 4: Cost. Fixed speed costs ₹24,87,712.50 and VSD ₹23,58,328.50. The VSD saves 14,376 kWh, or ₹1,29,384 a year, which is 5.2 %.
Step 5: Read the bands. In the 85 % band the VSD draws 1.029 kW more and loses 2,058 kWh over 2,000 h. It gains 8,748 kWh in the 60 % band and 7,686 kWh in the 35 % band. If a leak campaign lifts the low band, re-run the model.
Second case: steady 90 % for 6,000 h. Fixed speed: 63 × (0.90 + 0.10 × 0.25) = 58.275 kW, giving 3,49,650 kWh. VSD: 10 × 6.6 × 0.90 × 1.01 = 59.994 kW, giving 3,59,964 kWh. The VSD uses 10,314 kWh more, an extra ₹92,826 a year, so the saving is negative. Here fixed speed with good storage and a tight band is the right choice. A VSD would need justifying on pressure stability, gentler DG starting or expected future swings, not on energy.
Lifecycle cost
On two-shift duty a screw compressor often consumes its purchase price in electricity within a few years, so energy dominates. A fair ten-year comparison also includes:
- installation, harmonic mitigation and any ventilation needed for drive cooling;
- maintenance: the same airend care for both; drive fans, cabinet filters and eventually DC-bus capacitors for the VSD; inlet-valve and contactor wear from cycling for fixed speed;
- downtime risk from a failed drive board, where a fixed-speed base gives a natural fallback;
- future demand, such as a third shift or new lines;
- a lower setpoint of 0.5 bar (7.3 psig), worth 3–3.5 %, but only if you will actually lower it.
Feed the annual kWh into the lifecycle cost comparison calculator.
Demand profile logging and decision sheet
| Item | Value / observation | Source or method |
|---|---|---|
| Site, compressor house, logging dates | 14 days minimum | |
| Required pressure at critical user, bar g (psig) | Logger at user | |
| Generation setpoint / band, bar g (psig) | Controller | |
| Rated FAD Q, m³/min (cfm); verified? | Datasheet / pump-up test | |
| Full-load power Pf, kW | Power logger | |
| Unloaded power, kW and % of Pf | Logger, after full blowdown | |
| Typical cycle time, s | Power trace | |
| Receiver plus pipe volume, litres | Nameplates, survey | |
| Weekend leakage, % of Q | Production stopped | |
| Band 1 / 2 / 3: hours and demand % | Duration curve | |
| VSD offered: s and minimum speed % | Datasheet, equal pressure | |
| Peak summer room temperature, °C | Logger, April–May | |
| Hours per year on DG | DG log | |
| Annual kWh: fixed / VSD; saving (state if negative) | Calculator | |
| Decision, reason, prepared and approved by, date |
Exceptions and cautions
- Oil-free, two-stage and very large machines have different unloaded and drive-loss behaviour. Enter their actual data.
- If demand exceeds one machine, model the fleet rather than a single machine.
- Specific power figures compare only at equal pressure and equal test conditions.
- A VSD with a control gap, or set higher than the machine it replaced, may save nothing.
- Retrofitting a drive needs an airend, motor insulation, bearings and cooling that suit variable speed. Check the maker’s documents.
- Your electrical consultant should check harmonic limits and power-factor penalties against your supply agreement.
Common mistakes
- Deciding on average demand instead of hours per band.
- Comparing a VSD datasheet at 7 bar(g) (101.5 psig) with a fixed-speed one at 8 bar(g) (116.0 psig).
- Using datasheet unloaded power for a machine that short-cycles.
- Crediting the VSD with savings that really came from leak repair and a lower setpoint.
- Choosing a trim VSD with a span smaller than the base machine.
- Putting a drive in an unventilated 48 °C room.
- Ignoring harmonic loading on the DG set.
- Assuming a VSD always saves energy.
How Reddy Compressors helps
Reddy Compressors has been an independent multi-brand compressed-air specialist since 2002. We are suppliers, not dealers, and not an authorised dealer of any make, so we can recommend fixed speed, VSD or base-plus-trim on the evidence of your logs alone. We have tie-ups with all major and reputed brands. We supply new, used and refurbished compressors from Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic and others, along with dryers, filters, receivers and piping. Our service covers AMC, overhaul and spare parts for all major makes. More than 20 service engineers and technicians work across Telangana, Andhra Pradesh and districts within about 300 km of Hyderabad, and we supply across India. Our rental fleet of 50–2,000 cfm (1.4–56.6 m³/min) at 2–30 bar can keep a plant running during a changeover.
What to send us
- Make, model, FAD, pressure and age of each compressor
- Any power, pressure or load-hour logs, or running and loaded hours read a week apart
- Receiver sizes and a sketch of the compressor house
- Shift pattern, Sunday working and planned expansion
- Peak room temperature, hours on DG and your tariff
Use the equipment selection enquiry. Call/WhatsApp 99590 60718 or 92468 88812.
Frequently asked questions
Does a VSD compressor always save energy?
No. It saves energy when demand varies and the machine would otherwise spend many hours unloaded. On steady demand near full load, drive losses of about 2 to 4 percent mean a VSD can use more energy than a well-set fixed-speed machine. In this page’s example, a steady 90 percent plant uses 10,314 kWh a year more with VSD.
Watch
Short explainers on this topic
Slide-by-slide engineering explainers from Reddy Compressors. More in the video library.
How much power does a fixed-speed screw compressor draw when unloaded?
Typically 20 to 35 percent of its full-load power, and more if it short-cycles on a small receiver so that the separator vessel never finishes blowing down. Measure it with a power logger after a full blowdown rather than relying on the datasheet.
What is the minimum speed of a VSD compressor and why does it matter?
Most VSD screw packages cannot run below roughly 20 to 40 percent of rated flow. Below that point they load and unload or stop and start like a fixed-speed machine, so long periods of very low demand erode the VSD advantage.
What is base-plus-trim control?
One or more fixed-speed machines run fully loaded to carry the base demand, and a single VSD machine trims the varying part above them. It keeps fixed-speed machines out of unloaded running and lets the VSD do what it does best. The VSD’s flow range should be at least as large as the largest base machine to avoid a control gap.
Can I run a VSD compressor on a diesel generator?
Usually yes, and its soft ramp-up is gentler on the generator than a star-delta start. However, drive harmonics heat the alternator and can disturb voltage regulation, so the generator may need a larger kVA rating. Confirm the arrangement with your generator supplier before relying on it.
Does Indian summer heat affect VSD compressors?
Yes. Many drives are rated for full output up to about 40 °C and derate above it. Compressor rooms in Telangana and Andhra Pradesh often reach 45 to 50 degrees C in April and May, so ducting hot air out and keeping drive cabinet filters clean are essential.
How long should I log demand before choosing?
At least 14 days, covering every shift, a Sunday or holiday and any month-end peak. Log three-phase power, pressure at the compressor and at the critical user, and load state. Then group the results into hours at high, middle and low demand.
Why might the calculator show a negative saving?
A negative saving means the VSD would use more energy than the fixed-speed machine on the demand profile you entered. This happens when most hours are at high, steady load. The result is reported honestly, and the right choice in that case is usually fixed speed with good storage and a tight pressure band.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.