Screw compressor with fixed or variable speed?
A screw compressor can have either fixed or variable speed. Fixed speed compressors provide a steady supply of air whereas a variable speed compressor changes the motor speed according to the air demand.
By knowing the different between the compressors and their advantages and disadvantages, it will be easier to find the right screw compressor that can meet your needs.
Screw compressor with fixed speed
Screw compressors with fixed speed run continuously in an on/off cycle, where they have the same speed and perform efficiently at 100% of their capacity. This means that when the motor is running at maximum capacity and compressed air is being produced, the compressor is running most efficiently.
In general, this type of compressor is suitable for users with a constant, steady need for compressed air, as a fixed speed screw compressor runs inefficient with too many start/stop cycles.
When the compressor is turned off, the motor will continue to run shortly without producing compressed air to protect the motor.
Acquiring a fixed speed screw compressor is typically a smaller investment than a similar variable speed screw compressor, but it can be more expensive to operate.
Benefits and disadvantages with fixed speed
| Benefits | Disadvantages |
| Low investment costs | High energy consumption |
| Stable compressed air |
Screw compressor with variable speed
Variable speed screw compressors are equipped with a different technology where the motor automatically adjustes the air supply to the consumption at any given time.
The compressor has a pressure sensor which reports the pressure to the control unit, which sends a signal to the inverter. The inverter then adjusts the motor speed according to the pressure settings, i.e. the compressed air requirement.
This makes screw compressors suitable for users with varying demand for compressed air, as it only produces the necessary air.
As the demand for air increases, the motor speed will increase, and more compressed air will be produced. Contrary, declining demand will lead the motor to slow down and use only the required energy to provide the appropriate flow.
A variable speed screw compressor is thus both a benefit for the environment and for your own energy bill. On slower production days, at breaks or other stops, a variable screw compressor is particularly suitable because they reduce electrical waste.
The acquisition of a variable speed compressor is often a larger investment than a similar fixed speed compressor. On the other hand, energy consumption will be reduced on the machine, which will most lead to a lower total cost of ownership.
Benefits and disadvantages with variable speed
| Benefits | Disadvantages |
| Adjustes the air supply to demand | Higher investment costs |
| Energy savings | |
| Lower cost of ownership |
How do you choose the motor type?
The choice between fixed and variable speed must reflect how the compressed air is consumed in relation to your production and processes.
The load rate (0-100%) distributed over the day, tells how much the compressor is used at a given time, and what intervals the air is consumed in.
By having an overview of the load rate, it is possible to assess what type of motor your screw compressor should have.
