Motor for a fan — the easiest place to save energy
Fan loads are the most rewarding application for variable speed drives. Power is proportional to the cube of speed: reduce rpm by 20% and consumption drops roughly 50%. That is why fan systems are the first candidates for VFD retrofits. But the motor itself needs to be right too. A wrong choice here is less dramatic than on a compressor, yet the penalty accumulates over years — extra kilowatts every month.
Fan type determines the motor
First question: centrifugal (radial) or axial fan? This dictates both speed and power.
| Fan type | Typical speed | Recommended poles | Power (typical range) |
|---|---|---|---|
| Axial (duct, roof) | 1000–1500 rpm | 4 or 6 pole | 0.25–7.5 kW |
| Centrifugal low pressure | 1000–1500 rpm | 4 or 6 pole | 1.5–30 kW |
| Centrifugal medium pressure | 1500–3000 rpm | 2 or 4 pole | 5.5–75 kW |
| Centrifugal high pressure (ID fan) | 1500–3000 rpm | 2 or 4 pole | 15–200 kW |
Axial fans
For residential and industrial axial fans a 4-pole motor rated 0.37–4 kW is usually sufficient. Mounting is typically flange (B5) or combined (B35), because the fan is often mounted vertically or at an angle.
Centrifugal (radial) fans
Centrifugal fans need more power. For a workshop exhaust at 10,000 m³/h — a 5.5–7.5 kW motor at 1500 rpm. For boiler-room ID fans — 15–55 kW and up. Here WEG W22 or ABB start to make sense — they run around the clock, and the IE1 vs IE3 efficiency gap converts into thousands of hryvnias per year.
Power: formula and common sense
The textbook formula: P = (Q × ΔP) / (3600 × η_fan × η_drive), where Q is flow (m³/h), ΔP is pressure (Pa), η is efficiency. In practice it is easier to check the fan manufacturer catalog — it specifies the required motor power.
If no catalog is available (old fan, nameplate worn off):
- Measure current with a clamp meter on the running motor
- Calculate: P ≈ U × I × √3 × cos φ / 1000 (for three-phase)
- Add 10–15% margin
The margin for a fan is smaller than for a compressor — 10–15% is enough. Fan load is "soft": torque rises gradually with speed, no shock starts.
VFD: fastest payback of any application
A fan follows a quadratic torque curve. Cutting speed by 50% drops power to 12.5% of rated. This means a VFD on a 15 kW fan can save 5,000–8,000 kWh per year — roughly 10,000–16,000 UAH at current tariffs.
Payback period for a VFD on a fan is probably the shortest of any application: 6–18 months depending on power and operating profile. We recommend a VFD for any fan above 4 kW that runs more than 4 hours a day.
What to consider when pairing with a VFD
- Minimum speed: do not go below 20% of rated — motor cooling degrades (self-ventilated motors rely on their own fan)
- Output filter: if the cable between VFD and motor exceeds 50 m — install a dU/dt filter
- Bearing currents: may appear at powers above 30 kW. Solutions include insulated bearings or a shaft grounding ring
Soft starting a fan
Fan load is "light" at start — resistive torque at zero speed is minimal. So a soft starter works well here and costs less than a VFD. But if you need speed control — a soft starter cannot help, only a VFD can.
Direct-on-line start (no soft starter, no VFD) is acceptable for fans up to 5.5 kW with infrequent starts. Above that — soft starting is advisable.
Special cases
Explosion-proof fans
For premises with explosive atmospheres (paint booths, chemical plants) an Ex-rated motor is required. This is a separate category — a standard AIR motor will not do.
High-temperature ID fans
Boiler ID fans handle gases up to 200–400 °C. The motor is mounted via an extended shaft or remote coupling to keep it away from the hot casing. Insulation class — H minimum.
Frequently Asked Questions
Which motor for a 2000 m³/h duct fan?
Usually a 4-pole 0.55–1.1 kW with flange mount B5. Exact power depends on duct system pressure. Check the fan manufacturer catalog for the aerodynamic curve.
Is a VFD mandatory for a fan?
Not mandatory, but strongly recommended for power above 4 kW and operation over 4 hours/day. Payback is 6–18 months. On small fans under 2 kW the VFD may cost more than the annual savings.
Can I use a 2-pole motor instead of a 4-pole?
Only if the fan design allows it. Doubling speed increases power eightfold (cubic law). The impeller may not withstand the centrifugal forces. Verify with the manufacturer.
Why does the fan vibrate after motor replacement?
Most common reason — impeller imbalance or poor shaft alignment. Also check that the speed matched: if you installed a 2-pole motor replacing a 4-pole — speed doubled, and that is critical.
AIR or WEG for an exhaust fan?
For fans running up to 8 hours/day at power up to 7.5 kW — AIR is sufficient. For 24/7 duty above 11 kW — WEG W22 IE3 will save more on electricity than the price difference.
Summary
Fans are the load where variable speed control delivers maximum savings. Choose the motor by fan type, mind the mounting style (B5 for axial, B3 for centrifugal) and seriously consider a VFD for any system above 4 kW. Browse our VFD catalog and motor catalog for selection.
