VFD Screw Air Compressors: Sizing and Running Cost
A VFD (variable frequency drive) screw air compressor changes its motor speed to match the air your plant is using at that moment, instead of running flat out and unloading. On a plant whose demand rises and falls through the day, that cuts the energy bill by up to 35%, and energy is about 80% of what a compressor costs over five years. Colt builds VFD screw compressors from 10 HP upwards at its ISO 9001:2015 plant in Sonipat, with an IE4 permanent magnet motor, a compressor-rated drive and soft start. This page gives the model table, how to size one and a running-cost example.
VFD screw air compressor models
Free air delivery is a range because the machine slows down with demand: the lower figure is the minimum output, the upper figure the full output. Measured to ISO 1217 Annex C. 3 phase, 400 V, 50 Hz.
| Model | Motor (HP / kW) | FAD @ 8 bar (CFM) | FAD @ 10 bar (CFM) | FAD @ 13 bar (CFM) | Noise dB(A) | Size L×W×H (mm) | Air outlet (inch) |
|---|---|---|---|---|---|---|---|
| CS 10 VFD | 10 / 7.5 | 19.6 – 42.9 | 19.6 – 35.7 | 19.2 – 29.6 | 64 | 920×650×1050 | ¾ |
| CS 15 VFD | 15 / 11 | 27.3 – 70.2 | 27.3 – 58.8 | 27.3 – 53.2 | 66 | 1320×800×1320 | 1 |
| CS 20 VFD | 20 / 15 | 26.3 – 92.2 | 26.3 – 79.6 | 25.8 – 70.1 | 66 | 1320×800×1320 | 1 |
| CS 25 VFD | 25 / 18.5 | 47.2 – 117.1 | 47.2 – 98.6 | 47.2 – 87.2 | 67 | 1320×900×1500 | 1 |
| CS 30 VFD | 30 / 22 | 52.8 – 134.0 | 52.8 – 115.9 | 52.0 – 102.5 | 67 | 1320×900×1500 | 1 |
| CS 40 VFD | 40 / 30 | 79.6 – 199.2 | 79.6 – 177.8 | 79.6 – 162.0 | 70 | 1500×1000×1450 | 1½ |
| CS 50 VFD | 50 / 37 | 97.6 – 244.0 | 97.6 – 222.6 | 97.6 – 204.2 | 70 | 1400×1000×1580 | 1½ |
| CS 60 VFD | 60 / 45 | 112.8 – 282.0 | 112.8 – 252.4 | 110.8 – 223.2 | 71 | 1500×1000×1450 | 1½ |
| CS 75 VFD | 75 / 55 | 143.2 – 358.0 | 143.2 – 324.6 | 141.6 – 280.4 | 73 | 1800×1210×1830 | 2 |
| CS 100 VFD | 100 / 75 | 191.5 – 478.8 | 191.5 – 414.2 | 190.0 – 362.4 | 73 | 1800×1210×1830 | 2 |
| CS 120 VFD | 120 / 90 | 208.0 – 594.0 | 208.0 – 516.0 | 202.0 – 472.0 | 74 | 2420×1540×2040 | 3 |
| CS 150 VFD | 150 / 110 | 244.8 – 698.2 | 244.8 – 610.0 | 242.0 – 538.6 | 74 | 2770×1890×1940 | 3 |
| CS 180 VFD | 180 / 132 | 292.0 – 832.4 | 292.0 – 682.8 | 290.2 – 592.6 | 75 | 2650×1800×1850 | 3 |
Models above 180 HP, up to 350 HP, are also available; ask us for their data sheets.
Why the VFD saves energy
A fixed-speed screw compressor has two states: loaded, making air at full power, and unloaded, making no air but still turning and drawing 20 to 30% of full power. The more your demand swings, the more hours it spends unloaded. A VFD machine has no unloaded state in normal running; it simply turns slower, and power drops with speed. It also starts softly, so there is no current surge on the supply.
5-year cost comparison, fixed speed vs VFD, from the Colt screw compressor brochure.
Running-cost example
Put your own numbers into the same three lines. The example uses a 75 HP (55 kW) compressor on a two-shift plant.
| Line | Fixed speed | VFD |
|---|---|---|
| Running hours a year | 6,000 | 6,000 |
| Average power drawn (kW) | 55 × 0.85 = 47 | about 30% less = 33 |
| Energy a year (kWh) | 2,82,000 | 1,98,000 |
| Energy cost at ₹8 per kWh | ₹22.6 lakh | ₹15.8 lakh |
| Saving a year | — | about ₹6.8 lakh |
The 0.85 loading and the 30% saving are typical of plants whose demand varies through the day; a plant with steady full-load demand saves less, and one with big swings saves more. Send us your running hours and tariff and we will work it for your machine.
How to size a VFD screw compressor
- Add up the air you use in CFM, for everything that runs at the same time, and add about 20%.
- Pick the model whose full output at your pressure (8, 10 or 13 bar) is at or just above that figure.
- Check your usual demand sits inside the range, not at the very bottom: a VFD machine is at its best between about 40% and 100% of speed.
- Keep the receiver and air treatment: 500 L up to 30 HP, 1,000 L from 40 to 100 HP, 2,000 L from 120 HP, plus a refrigerated dryer and line filters.
Related: all screw air compressors, with the fixed-speed table · fixed speed vs VFD guide · two-stage VFD screw compressors · 16 bar VFD compressors for laser cutting
VFD screw air compressor FAQs
How much energy does a VFD screw air compressor save?
It depends on how much your air demand swings. A fixed-speed machine runs at full speed and unloads when demand drops, and an unloaded screw compressor still draws 20 to 30% of its full power while making no air. A VFD slows the motor instead. On a plant that averages 50 to 70% of the compressor's capacity, the saving is typically 25 to 35% of the energy bill; our brochure figure is up to 35%.
When is a fixed-speed compressor the better buy?
When your demand is steady and close to the machine's full output through the shift, for example a single process that runs continuously. The VFD then has nothing to follow, and a fixed-speed machine costs less to buy. Many plants do best with one fixed-speed base-load machine and one VFD machine to take the swings.
How do I size a VFD screw compressor?
Add up the air used by everything that runs at the same time, in CFM, and add about 20%. Pick a VFD model whose top FAD at your pressure is at or just above that figure, and check that your usual demand sits inside the model's range, not at the very bottom of it. A VFD compressor is at its most efficient between about 40% and 100% of its speed.
What is the pay-back on the extra cost of a VFD?
Work it from your own numbers: motor kW × running hours × tariff gives the yearly energy cost, and 25 to 35% of that is the saving. For a 75 kW machine running 6,000 hours a year at ₹8 per kWh, the energy cost is about ₹36 lakh and a 30% saving is about ₹10.8 lakh a year, so the VFD premium usually pays back within the first year on two- or three-shift plants.
Does a VFD compressor need a smaller receiver?
No. Keep the same receiver as a fixed-speed machine: 500 L up to 30 HP, 1,000 L from 40 to 100 HP, 2,000 L from 120 HP. The receiver still smooths short peaks and gives the dryer steady flow.
Can an existing fixed-speed Colt compressor be converted to VFD?
Often yes, with a compressor-rated drive, a motor check and a controller change. Send us the model and serial number and we will tell you whether it is worth doing or whether a new VFD machine makes more sense.
SEND YOUR AIR DEMAND AND RUNNING HOURS
Model data from the Colt screw air compressor brochure. Last updated: .