A 75 kW pump motor, currently running throttled at reduced flow — the affinity laws show why VFD speed control saves dramatically more energy than a valve ever could.
| Motor rated power | 75 kW |
| Required flow reduction | 30% (operating at 70% of full flow) |
| Current control method | Throttling valve (baseline power ≈100% of rated, since a valve barely reduces motor power) |
| Operating hours | 6000 hours/year |
| Electricity rate | $0.12/kWh |
For a centrifugal pump/fan against a quadratic system curve, speed reduction tracks flow reduction, and power scales with the cube of speed.
A throttling valve reduces flow by adding restriction, not by slowing the motor — the motor keeps drawing close to its full rated power even though less fluid is actually moving.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| VFD power at 70% flow | n/a (this is the computed result) | 25.73 kW, down from ≈75 kW throttled | ✓ PASS |
| Annual cost savings | n/a (this is the computed result) | $35,478/year | ✓ PASS |
Key insight: The cube law is what makes VFD retrofits so consistently attractive for variable-flow applications — a modest 30% flow reduction, which might seem to warrant only a modest power reduction, actually corresponds to a much larger 65.7% power reduction (0.70³ = 0.343, so power drops to 34.3% of rated) once speed is actually allowed to drop along with flow, rather than being held constant and throttled.
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Open Pump/Fan VFD Energy Savings calculator →A throttling valve reduces flow by adding artificial resistance to the system, forcing the pump to work against a higher effective head at the same speed — the motor still has to do nearly as much work overcoming that added resistance, so throttled power stays close to full-load power even though delivered flow has dropped. This is exactly the wasted energy a VFD retrofit eliminates by letting the pump simply slow down instead.
It applies specifically to centrifugal-type loads against a system curve dominated by friction/velocity losses (a 'quadratic' system curve) — positive-displacement pumps, and systems dominated by static head rather than friction, don't follow the same cubic power-vs-flow relationship, and the baseline throttled-power assumption itself is a conservative simplification; a bankable savings estimate should use actual trended power data or a real pump/fan curve rather than this default assumption.