VFD Basics
Variable Frequency Drives (VFDs) save energy by matching motor speed to demand. Learn how VFDs work, where to apply them, and key installation considerations.
What is a VFD?
A Variable Frequency Drive (VFD) is an electronic device that controls the speed of an AC motor by varying the frequency and voltage of the power supplied to it. Instead of running the motor at full speed and throttling output (wasting energy), the VFD reduces motor speed to match the actual demand.
VFDs are the single most cost-effective energy saving device for pumps, fans, and compressors that operate at variable loads. Savings of 20 to 50% are common.
How VFDs Save Energy
For centrifugal loads (pumps and fans), the affinity laws apply: power is proportional to the cube of speed. Reducing speed by 20% reduces power consumption by nearly 50%. This is why VFDs save so much energy on variable-load applications.
Without a VFD, a pump running at full speed with a partially closed discharge valve wastes energy. The pump generates full pressure, and the valve throttles it down. The energy difference between what the pump generates and what the system needs is wasted as heat and turbulence.
| Speed Reduction | Flow | Power Required | Energy Saved |
|---|---|---|---|
| 0% | 100% | 100% | 0% |
| 10% | 90% | 73% | 27% |
| 20% | 80% | 51% | 49% |
| 30% | 70% | 34% | 66% |
| 50% | 50% | 12.5% | 87.5% |
Where to Use VFDs
- Pumps with variable demand: Water supply, circulating pumps, boiler feed pumps with variable steam demand.
- Fans with variable demand: HVAC supply and exhaust fans, cooling tower fans, boiler forced draft fans.
- Compressors with variable demand: Screw air compressors, refrigeration compressors. VSD compressors are increasingly standard.
- Conveyors with variable speed: Material handling, packaging lines.
Where NOT to Use VFDs
- Constant full-speed loads: If the motor always runs at 100% speed, a VFD adds cost and complexity without saving energy. A direct-on-line starter is simpler and more efficient.
- Very low-speed operation: Most motors are not designed for continuous operation below 30% speed. Cooling fans on the motor shaft don't move enough air at low speed, causing overheating.
- Positive displacement pumps: PD pumps don't follow the affinity laws. Reducing speed saves much less energy than with centrifugal pumps.
VFD considerations
VFDs require inverter-duty motors (reinforced insulation). They generate harmonics that can affect other equipment. Long cable runs between VFD and motor can cause voltage reflections that damage motor insulation. Use VFD-rated cable and follow the manufacturer's cable length limits. Install proper shielding and grounding.
Tip: Before installing a VFD, verify that the motor is inverter-rated. If it is an older motor (IE1 or IE2), it may not have adequate insulation for VFD operation. A motor rewind with inverter-grade insulation or a new IE3/IE4 motor may be needed.