Motor for a compressor: power margin and proper starting come first
A compressor is one of the toughest loads an electric motor can face. Starting against residual pressure, cyclic duty, high inrush currents. Pick the wrong motor and it will overheat and fail within months. We see dozens of such cases every year. Selecting a motor for a compressor follows its own rules — different from a general-purpose drive.
Power rating: calculate with margin
Never size a compressor motor "just right." Add a 15–25% margin. The reason is straightforward: a compressor starts against residual pressure in the receiver, and the load at startup far exceeds rated torque.
| Compressor output | Pressure, bar | Motor power (minimum) | Recommended power |
|---|---|---|---|
| 200 l/min | 8 | 1.5 kW | 2.2 kW |
| 500 l/min | 8 | 3.0 kW | 4.0 kW |
| 1000 l/min | 10 | 7.5 kW | 11 kW |
| 2000 l/min | 10 | 15 kW | 18.5 kW |
| 5000 l/min | 10 | 37 kW | 45 kW |
These figures are for reciprocating compressors — the most common type. Screw compressors load more gently, so 10–15% margin is usually enough.
Why you cannot size "just right"
- Starting against pressure creates resistive torque 1.5–2 times above rated
- At ambient temperatures above +35 °C motor efficiency drops 5–10%
- A dirty air filter increases load by 10–15%
- Worn compressor valves add another 5–10% load
Rotation speed: 1500 or 3000 rpm?
For reciprocating compressors the standard is 1500 rpm (4-pole motor). At 3000 rpm the piston group wears twice as fast, noise and vibration increase. We do not recommend 2-pole motors for reciprocating compressors — unless the compressor manufacturer explicitly requires it.
Screw compressors are a different story. Speed depends on the rotor-pair design, and 3000 rpm is often correct. But in large screw compressors (30 kW and up) a variable frequency drive is increasingly common — it adjusts speed to match actual air demand.
Motor type: AIR, WEG, or something else?
For compressors up to 15 kW, AIR motors work just fine. Affordable, spare parts are everywhere, repair is straightforward. For higher-power compressors (15 kW+) running continuously, we recommend WEG W22 IE3 or ABB M2AA — the higher efficiency pays back in 2–4 years.
One more thing. Compressor motors run in S1 duty (continuous) with frequent starts. A standard general-purpose motor handles this, but if starts exceed 10 per hour consider a motor with class H insulation (180 °C instead of 155 °C for class F).
Starting: do not skip the soft starter
Inrush current of an induction motor is 5–7 times rated. For a 15 kW compressor that means a 150+ amp spike on direct-on-line start. Add pressure in the receiver — and the resistive torque at startup climbs even higher.
What happens without soft starting:
- The circuit breaker or fuses trip
- Voltage sags across the supply — lights flicker, other equipment drops
- Mechanical shock to the coupling and belt shortens their life
- A standby generator (if the supply is autonomous) may stall
The fix: a soft starter or a VFD. For fixed-speed compressors a soft starter is the bare minimum. It limits inrush to 2–3 times rated and costs from about 3000 UAH for 7.5 kW. A VFD costs more but adds capacity control and saves 15–30% on partial loads.
Mounting and enclosure
A compressor means vibration, dust, and heat. Mounting requirements are specific:
- IP55 minimum. IP54 is acceptable in clean rooms, but on a factory floor IP55 is better.
- Foot mount (B3) — standard for most compressors. Flange B5 is rare.
- Cooling. A compressor motor runs hotter than a typical drive. Make sure the cooling fins are clear and the fan is unobstructed.
- Removable end shield — for service convenience if the compressor sits in a tight space.
Frequently Asked Questions
Which motor for a 500 l/min reciprocating compressor?
A 4-pole (1500 rpm) motor rated at 4 kW. The bare minimum is 3 kW, but with margin for pressurised start and wear, 4 kW is more reliable. Mounting type — B3 (foot), protection IP54 or IP55.
Can I start a compressor from a generator?
Yes, but the generator must be at least 3 times the motor rating on direct start. Or use a soft starter — then a generator rated at 1.5–2 times the motor power is enough.
VFD or soft starter for a compressor — which one?
If the compressor runs at one speed — a soft starter is cheaper and simpler. If you need to adjust capacity or the compressor often runs at partial load — a VFD will pay for itself through savings.
Why does a compressor motor overheat?
Most common causes: undersized motor, dirty compressor air filter, too many starts per hour (over 10), blocked cooling fins. Check each item in sequence.
How many starts per hour can a compressor motor handle?
A standard AIR motor is rated for 6–10 starts/hour at powers up to 15 kW. If more starts are needed — use a motor with class H insulation or a VFD, which eliminates the inrush shock entirely.
Summary
Choose a compressor motor with 15–25% power margin, 1500 rpm for reciprocating units, and always with soft starting. For smaller compressors up to 15 kW an AIR + soft starter works well; for larger units go with WEG or ABB plus a VFD. Need help selecting? Browse our motor catalog or message our team with your compressor specs.
