How control responds to demand
The control system monitors pressure and adjusts speed to the required delivery. Every machine has lower and upper control limits. Below its minimum delivery, it must enter another mode, such as unloaded operation or shutdown. The exact range depends on the model, pressure and design; it is not the same for all machines.
When a variable-speed drive helps
VSD can reduce long periods of unloaded operation and stabilise pressure if demand remains within its usable range. In a larger station, it can complement fixed-speed base-load machines. The outcome depends on central control and the load profile. A detailed guide to interpreting the data is provided in the article on the efficiency curve.
What to check in practice
- Measure flow and pressure during representative shifts.
- Compare input power at several delivery points, not just at maximum output.
- Check behaviour at minimum demand, during breaks and when another machine fails.
Example scenario
Demand fluctuates between 2 and 5 m³/min. A candidate with a control range of 3 to 6 m³/min cannot cover part of the operation by continuously adjusting speed. This range is illustrative only; actual selection requires data for the specific machine at the specified pressure.
What to watch out for
- Assigning the same savings to all VSD machines.
- Choosing an oversized machine that will constantly operate below its minimum control range.
- Assuming two variable-speed drives will always complement each other without coordination.
Is variable-speed control better at continuous full demand?
Not automatically. At a stable operating point, compare the input power of the complete packages and the reserve required. The benefit of a variable-speed drive must be assessed for the specific profile, not by the name of the control method.