Short answer: Size a PM VSD compressor on your duty band, not on peak flow alone: read the average and peak m³/min over a full production week, pick the model whose FAD window contains both, and set the pressure band to the lowest number your tools actually need. AirSpace’s FluxDrive PM VSD line spans 10–180 HP (7.5–132 kW) at 7–13 bar, and under variable-demand profiles it carries a 35% Energy Delta savings benchmark (per AirSpace part-load test data against a fixed-speed baseline of the same rating).
By Johnny Wayne, General Manager, AirSpace Machinery Co., Ltd. — 20 years in industrial compressed air systems. Reviewed by our engineering team.
Most sizing mistakes are not arithmetic errors — they are category errors. A plant buys a fixed-speed machine sized to its peak, then discovers that peak happens for forty minutes a day and the other seven hours are paid for at part-load efficiency the machine never had. In the field, we audited dozens of compressor rooms before we wrote this: the pattern is almost always the same — the machine was sized to a nameplate peak nobody ever re-measured. Sizing a PM VSD unit is a different exercise. You are matching a band to a band, and the pressure setpoint you choose is worth more than the frame you choose.
What does a PM VSD screw compressor do differently?
A fixed-speed screw compressor runs its motor at one speed and answers falling demand by unloading — the airend keeps turning, delivering no air, while typically still drawing 20–30% of full-load power (per manufacturer part-load data for oil-injected packages). A permanent magnet variable speed drive (PM VSD) unit slows the motor instead. Turn-down is real: at 50% flow the motor is doing roughly half the work, not idling at full speed. We measured this on our own test bench before we shipped the first FluxDrive unit, and the gap widens as the duty gets lighter.
Two things follow from that, and both change how you size:
- Peak no longer forces you up a frame for efficiency reasons. You still need the peak to be deliverable, but a VSD unit covering 7.8–9.5 m³/min spends its day at the bottom of that window without the unload penalty a fixed-speed unit pays there.
- The pressure band becomes the dominant variable. Every bar you trim off the setpoint is roughly 6–7% of compression energy on that duty (per compressor power curves across the 7–13 bar band). Choosing the band deliberately usually beats buying a bigger machine.
Which AirSpace PM VSD model covers your duty?
This is the table our engineers size against. All figures are AirSpace published ratings for the FluxDrive PM VSD series at 7–13 bar working pressure, direct-drive, per the current series data sheet:
| Model | Power | Working pressure | Indicative FAD | Drive type |
|---|---|---|---|---|
| FD-PMV-10 | 10 HP / 7.5 kW | 7–13 bar | 1.2–1.5 m³/min | Direct-drive PM VSD |
| FD-PMV-20 | 20 HP / 15 kW | 7–13 bar | 2.2–2.8 m³/min | Direct-drive PM VSD |
| FD-PMV-30 | 30 HP / 22 kW | 7–13 bar | 3.2–4.0 m³/min | Direct-drive PM VSD |
| FD-PMV-50 | 50 HP / 37 kW | 7–13 bar | 5.2–6.5 m³/min | Direct-drive PM VSD |
| FD-PMV-75 | 75 HP / 55 kW | 7–13 bar | 7.8–9.5 m³/min | Direct-drive PM VSD |
| FD-PMV-100 | 100 HP / 75 kW | 7–13 bar | 10.5–13.0 m³/min | Direct-drive PM VSD |
| FD-PMV-150-180 | 150–180 HP / 110–132 kW | 7–13 bar | 16.0–22.0 m³/min | Direct-drive PM VSD |
Two rules we give every RFQ:
- The peak must sit inside the window, not at its top. If your measured peak is 9.4 m³/min, the FD-PMV-75 is technically inside its window and practically out of headroom. Go to the FD-PMV-100.
- Leave the top of the window for growth you have not built yet. Adding a second shift is cheaper than replacing a compressor.
How do you size a PM VSD compressor step by step?
Take five measurements, in this order, and the whole exercise typically takes one production week: log the flow, find the true pressure requirement, subtract what receiver storage absorbs, pick the model from the FAD window, then check ambient against the 50°C rating. Most plants skip the second measurement, and it is the one worth the most money — we have seen 8 bar compressor rooms feeding 6 bar tools for a decade without anyone re-reading the nameplates.
- Log flow over one full production week, not a day. You want average, peak and minimum m³/min, plus how long the plant sits at each.
- Find the true pressure requirement. Walk the line and read the nameplates at the point of use — the regulator setting at the furthest, highest tool plus line loss is your band, not the number on the old compressor’s gauge.
- Subtract what storage can absorb. A correctly sized receiver handles short spikes so the compressor does not have to. Size the machine to the sustained peak, not the instantaneous one.
- Pick the model whose FAD window contains the sustained peak with headroom, using the table above.
- Check the ambient. AirSpace packages are rated for full output at 50°C ambient (per AirSpace series rating); above that, or in a sealed roof-level room, derate before you commit.
What does a sizing calculation actually look like?
Take a plant with a measured duty of 4.5 m³/min at night, 12.5 m³/min at peak, averaging 8.0 m³/min across a 6,000 h/yr run at 7.5 bar.
Fixed-speed baseline. Peak 12.5 m³/min forces the 100 HP / 75 kW frame (FD-PMV-100 covers 10.5–13.0 m³/min). At 60% average load a fixed-speed unit typically draws about 58 kW of input power once unload time is counted:
- 58 kW × 6,000 h = 348,000 kWh/yr
PM VSD, same frame, same duty. The drive tracks the load, and the same average duty typically draws about 45 kW:
- 45 kW × 6,000 h = 270,000 kWh/yr
Difference: 78,000 kWh/yr — about 22% of the baseline, or roughly 6,240 USD at 0.08 USD per kWh (per 6,000 h/yr, 75% load factor, 7.5 bar duty).
That 22% sits inside our 35% Energy Delta benchmark, which is measured under more strongly variable profiles than this one (per AirSpace part-load test data). A duty with wider swings — deep nights against sharp peaks — moves further up that range; a duty that is genuinely flat most of the day does not.
How does PM VSD compare with a fixed-speed unit on part load?
| Load | Fixed-speed input (typical) | PM VSD input (typical) | What it means for sizing |
|---|---|---|---|
| 100% | ~100% of rating | ~100% of rating | No difference at full load |
| 70% | ~78–82% | ~70–72% | VSD advantage starts here |
| 50% | ~65–70% | ~52–55% | Where plants actually run most of the day |
| 30% | ~50–55% | ~33–36% | VSD avoids the unload floor entirely |
| Unloaded | ~20–30% | n/a — motor slows instead | The fixed-speed penalty a VSD never pays |
(Per manufacturer part-load data for oil-injected screw packages across the 30–100% band; values are indicative and vary with discharge pressure.)
The practical consequence: if your logged duty spends most of its hours between 40% and 70%, a PM VSD unit is the correct category of machine, not an upgrade.
What pressure band should you size to?
Size to the lowest band that holds your worst-case point of use, and no lower. In practice, the bands we see on site are:
- General plant air, tools, actuators — typically 6–7 bar at the point of use (per ISO 11011 assessment practice, measure at the point of use, not at the package).
- PET blowing and higher-pressure process air — a different machine class entirely; the PMV line stops at 13 bar.
- Low-pressure duties at 3–5 bar (glass forming, certain textile and ceramic applications) — these belong on a dedicated low-pressure series rather than a 7–13 bar machine throttled down.
Trim the band and the sizing math changes underneath you: a duty that needed 12.5 m³/min at 7.5 bar often needs materially less once a leaking, over-pressurised distribution system is fixed. Do the leak survey before you buy the compressor — on site, we routinely find 20–30% of generated air lost to leaks before a single kilowatt-hour is discussed.
What did the numbers look like in a real installation?
A 15,000 m² iron-parts fabrication plant in Ho Chi Minh City running 200+ staff across 16 h/day was carrying a monthly electricity bill above 3,200 USD for compressed air, with pressure swinging ±0.5 bar and maintenance running around 850 USD/month. The plant’s duty was strongly variable — heavy machining load in the day, a skeleton night shift. We audited the room first, then installed a 75 kW PM VSD unit; the full measurement set is in the case study.
Three things changed, in the order the plant noticed them:
- Pressure stability. The ±0.5 bar swing closed to a tight band, which is what actually changed the reject rate on the finishing line.
- Part-load power. The night shift stopped paying unload power for air nobody was using.
- Maintenance interval. Fewer load/unload cycles on the airend and drive.
One customer who had previously run a European premium-brand package on a comparable duty documented 34% savings after we replaced it — that figure is one installation measured on site, not a benchmark, and the case study states the measurement conditions.
When is a PM VSD compressor the wrong choice?
We say so before the RFQ, not after:
- A genuinely flat duty. If your logged flow sits above 85% of rating all shift, a fixed-speed or two-stage unit at its efficient point costs less and puts fewer electronics in a hot room.
- Air demand that is already oversized. If the plant is 30% oversized on flow, fixing the sizing buys more than changing the drive type.
- No one to read the duty. A VSD unit rewards a plant that knows its own load curve. If nobody can produce a week of flow data, spend the money on a data logger first.
Key takeaways
- Size a PM VSD compressor on the duty band — average, peak and minimum over a full production week — not on peak flow alone.
- The AirSpace FluxDrive PM VSD range spans 10–180 HP (7.5–132 kW) at 7–13 bar, with indicative FAD from 1.2 to 22.0 m³/min (per AirSpace published series data).
- Trimming the pressure band by one bar is worth roughly 6–7% of compression energy on that duty — usually more than moving up a frame size.
- The 35% Energy Delta benchmark applies to variable-demand profiles; a flatter duty will land below it, and that is the honest answer to give a buyer.
- AirSpace packages hold full output at 50°C ambient, which matters for roof-level and tropical compressor rooms.
Frequently asked questions
How much can a PM VSD compressor actually save?
Under variable-demand operating profiles, AirSpace’s PM VSD packages carry a 35% Energy Delta savings benchmark against a fixed-speed baseline of the same rating (per AirSpace part-load test data). A duty that is flat for most of the day will land well below that figure — the worked example above comes out at 22% for a moderately variable profile.
What size PM VSD compressor do I need for 10 m³/min?
A sustained 10 m³/min peak with headroom points at the FD-PMV-100 (100 HP / 75 kW, 10.5–13.0 m³/min). If your average is 8 m³/min and the peak is short and absorbable by receiver storage, the FD-PMV-75 (7.8–9.5 m³/min) becomes a question for our engineers rather than an answer from a table.
Is a PM VSD compressor worth it if my plant runs at steady load?
Usually not on energy grounds. If your logged duty stays above 85% of rating, a fixed-speed or two-stage unit at its efficient point is the better buy, and the saving is in capital cost rather than kWh. The VSD case is built on hours spent at part load.
Can a PM VSD compressor run at 50°C ambient?
Yes. AirSpace packages are rated for full output at 50°C ambient, and the rating holds across the 7–13 bar band rather than only at the bottom of it. Above 50°C, or in a sealed room with poor air change, derate before committing to a frame size.
How is PM VSD different from a standard inverter compressor?
Both vary motor speed. PM VSD pairs the drive with a permanent magnet motor, which holds high efficiency at partial load instead of falling away as the motor slows — the part-load numbers in the table above are where that shows up.
Sources and standards
- ISO 11011 — compressed air energy assessments; the measurement basis for the duty logging described above.
- ISO 8573-1 — compressed air purity classes, referenced where air quality constrains the specification.
- ISO 9001 / CE marking — AirSpace quality system and conformity route for the packages covered here.
- EU Regulation 2024/2847 (Cyber Resilience Act) — where a unit is supplied with a network-capable controller; see our product security and vulnerability disclosure policy.
- FluxDrive PM VSD ratings: AirSpace published series data, also in the downloads library.
About the author
Johnny Wayne is General Manager of AirSpace Machinery Co., Ltd., and has spent 20 years in industrial compressed air systems. He has specified and commissioned screw compressor packages from 10 HP to 540 HP across food and beverage, textiles, glass, ceramics and general manufacturing, and writes the sizing guidance AirSpace engineers work from. Company qualifications: ISO 9001 quality system, CE conformity — both verifiable on request.
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