Retrofitting variable-speed drives
A variable-speed drive (VSD) retrofit replaces throttling or on/off control of a motor-driven pump or fan with electronic speed control, so the machine delivers exactly the flow needed. Because the power a pump or fan draws falls steeply with speed, matching speed to demand is one of the largest electrical savings available in industry.
What it is
Many pumps and fans run at full speed continuously while a valve or damper throttles the excess — wasting energy as friction. A VSD retrofit fits a drive that varies motor speed to the real duty, eliminating that artificial restriction and the energy lost across it.
Why it is done
For centrifugal pumps and fans, power scales roughly with the cube of speed, so a modest speed reduction yields a large power saving. Where flow demand varies and is currently met by throttling, a VSD typically pays back quickly — but only on the right duty; constant-load machines see little benefit.
How it is done
Candidate machines are screened for variable flow and current throttling losses. A drive is sized to the motor and duty, harmonic and cabling requirements are checked, and the control signal is linked to the real process variable — pressure, flow or level. The throttling valve or damper is opened up, the control loop tuned, and savings verified against the throttled baseline.
- Screen candidates
- Confirm variable flow
- Size the drive
- Wire to process signal
- Open throttle, tune loop
- Verify savings
What to watch for
Fitting a VSD to a constant-load machine saves little while adding losses and complexity. Leaving the old throttling valve partly closed, or ignoring harmonics and motor compatibility, undermines the result.
Related practices
Running a compressed-air leak survey programme
Retrofitting waste-heat recovery
Sequencing and controlling multiple compressors
Related topics
Pump Efficiency: Where the Energy Goes and How to Cut It · Specific Energy Consumption (SEC)
Common in: Power Generation · Chemicals · Steel & Metals · Cement · Paper & Packaging · Food Processing