VFD Investment Return Analysis
When you switch from mechanical throttling (damper/valve) to an AC Drive in your fan and centrifugal pump applications; instantly see the drop in your electricity bill, the reduction in your carbon footprint, and how many months the investment will pay for itself.
Laws of Physics
Changes directly proportional to speed.
Changes proportional to the square of speed.
Reducing speed by 20% reduces energy consumption by approximately 50%!
1 System and Application Data
Average mechanical efficiency of Motor + Drive.
2 Operational and Financial Profile
Throttling (Damper) vs. VFD Power Curve
Financial Analysis Result
Engineering Expert Window
System Dynamics and Retrofit Risks
Static Head (Dead-Head)
Drive speed should not be reduced randomly without analyzing the System Curve. Especially in pumps, a minimum operating frequency (e.g., 30 Hz) is required to overcome gravity or constant system pressure (Static Head). Falling below this speed causes the pump to spin without pumping fluid (Dead-Head); this leads to mechanical seal burns and cavitation.
Bearing Currents and Insulation Class
When modernizing (Retrofitting) older generation (grid-fed) asynchronous motors with a VFD, the high dV/dt (rate of voltage rise) caused by the drive's PWM waves can puncture winding insulation or cause Bearing Currents leading to mechanical failure. If necessary, a Motor Choke should be used.
Frequently Asked Questions
Why does energy consumption drop by the "cube" of speed?
Due to the physics of centrifugal loads, flow rate changes linearly with speed, and pressure is proportional to the square of the speed. Since work done is (Power = Flow × Pressure), Power changes with the cube (3rd power) of speed. Therefore, a fan running at 80% speed draws only about (~51%) of the power it would at full speed.
Is this calculation valid for constant torque loads (Conveyor, Crane, Extruder)?
Is there an efficiency difference between "Bypass" and "Valve/Damper Throttling"?
Yes. In Bypass (Recirculation) systems, fluid is returned from the line, and the motor continues to run continuously at 100% full power; it is the most inefficient method. With Valve/Damper throttling, a physical resistance (blockage) is created in the line, the power drawn by the motor drops slightly, but the fluid hits the valve, wasting energy as heat and noise. A VFD (AC Drive) provides the highest efficiency by reducing energy directly at the source.
How does using a drive affect the compensation system or harmonics?
Modern VFDs meet the motor's reactive power demand via their own DC bus, which raises the Power Factor (Cosφ) drawn from the grid to around 0.98, incredibly lightening the load on the compensation panel. However, drives emit THDi (Current Harmonics) into the grid. Therefore, for high power ratings, ONX drive solutions with Chokes (DC/AC) or Low Harmonic / AFE structures should be preferred over standard drives.