How fixed-speed control works

A fixed-speed (power-frequency) screw compressor runs the motor at grid frequency. Pressure is managed mainly by load/unload or start-stop cycles. When air demand falls, the machine still tends to operate near full speed until unload logic engages, which can waste energy.

Fixed-speed units remain a strong choice when air demand is high and stable for long periods, and when capital cost is the primary constraint.

How permanent-magnet VFD control works

A PM VFD system converts power and modulates frequency so motor speed tracks real air demand. High demand raises speed; low demand slows the motor and can enter sleep modes. The result is smoother pressure and less unloaded power waste.

Soft starts typically draw about 1.2× rated current versus 6× or more for direct-on-line fixed-speed starts—reducing stress on generators and weak grids.

How to choose the right drive

Choose fixed speed for steady base-load plants. Choose PM VFD when demand fluctuates, energy tariffs are high, voltage is unstable, or quieter average operation is preferred. SPERAN offers both drive types across box, two-in-one, and four-in-one configurations.