Why a compressor needs a soft start
A compressor is one of the hardest loads to start. The motor starts against system pressure, shaft resistance is high from the first revolution. Direct start draws 600-800% inrush current and creates a mechanical shock. A soft starter limits current to 300-400% and ramps up smoothly over 5-15 seconds.
Compressor starting specifics
- Loaded start — residual pressure in the system
- High inertia — flywheel, screw pair
- Restart — after emergency stop, pressure remains
- Starts per hour limit — soft starter allows more frequent starts
Selection: +30-50% current margin
| Power | I_rated | Min. soft starter | Recommended |
|---|---|---|---|
| 7.5 kW | 17 A | 22 A | 25 A |
| 15 kW | 32 A | 40 A | 45 A |
| 22 kW | 45 A | 60 A | 65 A |
| 37 kW | 75 A | 100 A | 110 A |
Settings
- U start: 60-70%
- Ramp: 8-15 sec
- Current limit: 300-400%
Compressor types
- Reciprocating — hardest start, +50% margin
- Screw — easier, +30%
- Scroll — medium start
- Centrifugal — light start, +20%
Soft starter vs VFD
Constant load — soft starter. Variable load — VFD. Full comparison.
FAQ
Can a compressor start without a soft starter?
Up to 4 kW — yes. Larger — not recommended.
Why the bigger margin?
Compressor starts against pressure — higher torque needed.
Needed for screw compressor?
Recommended from 15 kW.
Danfoss or Schneider?
MCD 500 — adaptive start. ATS22 — simplicity.
Cost for 30 kW?
With margin need 45-50 kW rated: 15,000-25,000 UAH.
Summary
30-50% current margin, U start 60-70%, ramp 8-15 sec. Catalogue: Danfoss and Schneider.
