PlantmarginVariable speed drives on fans and pumps: a practical payback guide
Many fans and pumps in tea, dairy, milling and water plants run at full speed. The flow is cut back by a damper or valve. The motor works hard against a restriction.
The physics in one line
For a fan or centrifugal pump, power rises with the cube of speed. At 80% speed the power is about 0.8³ = 51% of full-speed power.
A damper at 80% flow saves far less, because the motor still spins at full speed.
Estimating the saving from measured data
- Measure. Log the motor's kW, the damper or valve position and the running hours over at least two weeks.
- Estimate the flow that the damper position allows. Use the damper curve, or a conservative 1:1 ratio.
- Work out the VSD power at that flow:
P_vsd ≈ P_rated × (0.1 + 0.9 × q^2.6) / η_drive
The 0.1 term covers fixed losses. Using 2.6 instead of 3 keeps the estimate conservative for systems with static head.
- Work out the yearly saving:
(P_now − P_vsd) × running hours × all-in USh per kWh
Example
A 75 kW withering fan runs at 64 kW with the damper 72% open, for 8,760 hours a year. With a VSD at the same airflow it would draw about 42 kW.
22 kW × 8,760 h × USh 21 per kWh ≈ USh 4.0 million a year
That compares with about USh 1.6–3 million installed.
When a VSD is not the answer
- The motor already runs near full load all the time.
- The system needs a constant pressure set by static head (some pumping systems). Model these carefully.
- Very short running hours.
Measure before and after
Plantmargin logs kW before the drive goes in and compares the same production conditions afterwards, so the saving is proven rather than promised.