A variable torque load is a mechanical resistance that increases with the square of motor speed, characteristic of centrifugal pumps and fans in HVAC systems. These loads require less torque at lower speeds and exponentially more torque as speed increases, following the affinity law relationship where torque is proportional to speed squared.
Technical Characteristics
Variable torque loads include centrifugal chillers, cooling tower fans, and variable-volume chilled water pumps. Motor current draw remains proportional to load, meaning a 50 percent speed reduction requires only 12.5 percent of full-load current. This efficiency makes variable frequency drives (VFDs) highly effective for these applications.
HVAC System Impact
Most HVAC equipment operates as variable torque loads, including air handlers and secondary chilled water circuits. Energy savings from VFD control can reach 20 to 40 percent at reduced loads, as cubic feet per minute (CFM) or gallons per minute (GPM) reduce proportionally with speed.
Design Considerations
Variable torque equipment typically uses standard AC induction motors without oversizing requirements. System designers select VFDs specifically for variable torque applications, avoiding unnecessary soft starters that provide no energy benefit compared to full-speed operation.