Which load type is classified as variable torque loads?

• A. Fans, centrifugal blowers, and centrifugal pumps
• B. Hoists, conveyors, mixers, and reciprocating or rotary compressors
• C. Wheel grinders, large driller machines, lathes, planers, and boring machines
• D. None of the above

Answer: (A)

Loads that are variable torque change the torque the motor must provide as speed changes. Devices that move air or fluids, like centrifugal fans, blowers, and pumps, behave this way because the resistance they overcome depends strongly on speed. As rpm increases, the flow and pressure rise and so the torque required by the motor grows (roughly with the square of speed), while at lower speeds the torque demand falls off significantly. That speed-dependent torque pattern is what makes them classic variable torque loads.

In contrast, hoists, conveyors, mixers, and compressors tend to need about the same torque across a range of speeds, so they’re treated as constant torque loads. Machine tools such as grinders, drills, lathes, planers, and boring machines are typically constant power (constant horsepower) loads, where torque drops as speed increases to keep power roughly constant.