Everything about variable frequency drives (VFDs) – quick answers to your questions

Q: How much energy can a VFD save? (variable frequency drives and electric motors)

The energy savings of variable frequency drives and electric motors (VFDs) depend on the load profile and application. For fans and pumps, power consumption follows the affinity laws: absorbed power scales approximately with the cube of speed (P ~ n³). This means that a relatively small reduction in speed results in a disproportionate reduction in kWh. In HVAC and water applications, 20–50% energy savings are realistic, especially where throttling with valves or dampers was previously used. In applications with nearly constant torque (conveyors, mixers), savings are typically lower, but soft start/stop, more precise control and reduced downtime provide additional benefits (maintenance, quality). In addition to direct kWh reduction, VFDs contribute to lower peak currents, reduced reactive power (especially with AFEs) and suppression of mechanical losses due to oversizing. With internal PID control, the process variable (pressure, flow, temperature) becomes more stable, reducing overshoot and waste. For further integration into energy management, energy registers (kW/kWh) can be logged via Modbus/Profinet in SCADA/EMS systems, allowing continuous optimization of the business case. See the Fluxcon wiki: energy and Wikipedia: affinity laws. Practical measurement and validation: energy registers & data points in the FLC500 manual p.76–80; commissioning (correct parameterization) on p.44–48.

Q: How do the affinity laws work? (variable frequency drives and electric motors)

The affinity laws explain why variable frequency drives and electric motors are such powerful energy savers in turbomachinery (fans, pumps). In simplified terms: flow Q is proportional to speed n (Q ~ n), pressure or head H scales with the square (H ~ n²), and absorbed power P scales with the cube (P ~ n³). Reducing speed to 80% of nominal therefore reduces power roughly to 0.8³ ≈ 0.512, i.e. ~49% less instantaneous power. In practice, system losses, minimum speeds and process constraints apply, but the trend remains: speed control with a VFD is far more efficient than throttling. PID control in the VFD automatically matches supply and demand (pressure/flow control). See Wikipedia: affinity laws and the Fluxcon wiki (energy). Setting PID and ramps (for stable control loops and minimal overshoot): FLC500 p.62–64 and p.47.

Q: How much savings does a 20% speed reduction deliver? (variable frequency drives and electric motors)

For fans and centrifugal pumps, a 20% speed reduction (n = 0.8) results in 0.8³ = 0.512 power according to P ~ n³. This equals approximately 49% less instantaneous power. Over time, depending on operating hours and load profile, this translates into substantial kWh savings. For example: a 15 kW fan operating at 80% speed can save around 7.5 kW during partial load periods. In many buildings and water systems, equipment operates most of the time below peak load, making annual savings significant. Note: real systems include system curves (duct/piping resistance, minimum flow, setpoints). Still, the cubic relation remains a reliable rule of thumb. Combine speed reduction with optimizations such as night setback, sleep/wake, proper PID tuning and maintenance (clean filters/grilles) for maximum benefit. See Wikipedia and the Fluxcon wiki. Monitoring KPIs (kW, kWh, rpm) can be done directly in the VFD: energy registers in the FLC500 p.76–80; commissioning and limits: p.44–48.

Welcome to the Fluxcon FAQ. On this page you will find clear answers about the variable frequency drive — also known as a VFD (Variable Frequency Drive) or frequency converter — and its application with electric motors. Discover how a VFD controls speed and torque, saves energy (e.g. in pumps and fans), reduces wear and stabilizes processes in HVAC, water, compressors and conveyor systems.

Use the FAQ to quickly navigate to basic principles, installation/commissioning, troubleshooting and optimization. Prefer personal advice? Then contact our specialists.

FAQ — Energy & efficiency with variable frequency drives and electric motors

How much energy can a VFD save? (variable frequency drives and electric motors)

The energy savings of variable frequency drives and electric motors (VFDs) depend on the load profile and application. For fans and pumps, power consumption follows the affinity laws: absorbed power scales approximately with the cube of speed (P ~ n³). This means that a relatively small reduction in speed results in a disproportionate reduction in kWh. In HVAC and water applications, 20–50% energy savings are realistic, especially where throttling with valves or dampers was previously used. In applications with nearly constant torque (conveyors, mixers), savings are typically lower, but soft start/stop, more precise control and reduced downtime provide additional benefits (maintenance, quality).

In addition to direct kWh reduction, VFDs contribute to lower peak currents, reduced reactive power (especially with AFEs) and suppression of mechanical losses due to oversizing. With internal PID control, the process variable (pressure, flow, temperature) becomes more stable, reducing overshoot and waste. For further integration into energy management, energy registers (kW/kWh) can be logged via Modbus/Profinet in SCADA/EMS systems, allowing continuous optimization of the business case.
See the Fluxcon wiki: energy and Wikipedia: affinity laws.

Practical measurement and validation: energy registers & data points in the FLC500 manual p.76–80; commissioning (correct parameterization) on p.44–48.

How do the affinity laws work? (variable frequency drives and electric motors)

The affinity laws explain why variable frequency drives and electric motors are such powerful energy savers in turbomachinery (fans, pumps). In simplified terms: flow Q is proportional to speed n (Q ~ n), pressure or head H scales with the square (H ~ n²), and absorbed power P scales with the cube (P ~ n³). Reducing speed to 80% of nominal therefore reduces power roughly to 0.8³ ≈ 0.512, i.e. ~49% less instantaneous power.

In practice, system losses, minimum speeds and process constraints apply, but the trend remains: speed control with a VFD is far more efficient than throttling. PID control in the VFD automatically matches supply and demand (pressure/flow control). See Wikipedia: affinity laws and the Fluxcon wiki (energy).

Setting PID and ramps (for stable control loops and minimal overshoot):
FLC500 p.62–64 and
p.47.

How much savings does a 20% speed reduction deliver? (variable frequency drives and electric motors)

For fans and centrifugal pumps, a 20% speed reduction (n = 0.8) results in 0.8³ = 0.512 power according to P ~ n³. This equals approximately 49% less instantaneous power. Over time, depending on operating hours and load profile, this translates into substantial kWh savings. For example: a 15 kW fan operating at 80% speed can save around 7.5 kW during partial load periods. In many buildings and water systems, equipment operates most of the time below peak load, making annual savings significant.

Note: real systems include system curves (duct/piping resistance, minimum flow, setpoints). Still, the cubic relation remains a reliable rule of thumb. Combine speed reduction with optimizations such as night setback, sleep/wake, proper PID tuning and maintenance (clean filters/grilles) for maximum benefit.
See Wikipedia and the
Fluxcon wiki.

Monitoring KPIs (kW, kWh, rpm) can be done directly in the VFD: energy registers in the FLC500 p.76–80; commissioning and limits: p.44–48.