A centrifugal pump is the classic example of a variable torque load. Fans, blowers, and centrifugal compressors also fall into this category. What makes these loads “variable torque” is a specific physical relationship: the torque they demand increases with the square of the shaft speed. Run a centrifugal pump at half speed, and it only needs about 25% of its full-load torque. Double the speed, and the torque requirement quadruples.
How Variable Torque Works
In any rotating machine, the load it drives has a characteristic torque-speed curve. For variable torque loads, that curve is a parabola starting from the origin. The math is straightforward: torque equals a constant multiplied by speed squared (T = k × n²). This means the torque needed at low speeds is very small, and it climbs steeply as speed increases.
The power demand is even more dramatic. Because power equals torque multiplied by speed, and torque itself scales with speed squared, power ends up scaling with the cube of speed. Cut the speed of a fan by 20%, and the input power drops by roughly 50%. This cubic relationship is governed by what engineers call the affinity laws, which describe how flow, pressure, and power relate to speed in pumps and fans. Flow changes proportionally with speed. Pressure changes with the square of speed. And power changes with the cube.
Common Variable Torque Equipment
The following equipment typically behaves as a variable torque load:
- Centrifugal pumps are the most common example. Even with the discharge valve closed, the torque sits at 30 to 50% of full-speed torque, and starting (breakaway) torque requirements are low.
- Centrifugal fans follow the same physics. As the fan spins faster, it builds more pressure. That pressure pushes back against the blades, increasing the torque needed. At higher speeds, both pressure and flow increase, so horsepower rises rapidly.
- Centrifugal blowers behave like fans and require low breakaway torque to start.
- Centrifugal compressors (including axial-centrifugal types) follow the same quadratic torque curve, also with low starting torque.
- Liquid agitators qualify as variable torque loads, though they need moderate breakaway torque compared to pumps and fans. Slurry agitators also fall in this category but demand somewhat more starting torque due to the heavier medium.
The common thread is centrifugal action. Any machine that accelerates a fluid (air, water, or other liquid) outward through spinning impellers or blades will generally follow the variable torque pattern. Positive displacement pumps, which trap and push fixed volumes of fluid, do not. They behave as constant torque loads instead.
How This Differs From Constant Torque
A constant torque load demands roughly the same torque regardless of speed. Conveyors are a good example: a belt carrying boxes needs the same force to move them whether the belt runs slowly or quickly. Mixers processing thick material, presses, and extruders also fall into this category. These applications often require at least 150% overload capacity for short bursts, whereas variable torque loads need far less torque at low speeds and have low breakaway requirements.
The practical difference comes down to the torque-speed profile. At 50% of base speed, a variable torque load demands only 25% of its rated torque. A constant torque load at 50% speed still demands 100% of its rated torque. This distinction matters when sizing motors, selecting drives, and estimating energy costs.
Why Variable Torque Loads Save Energy
The cubic power relationship makes variable torque loads ideal candidates for variable frequency drives (VFDs). Instead of running a pump or fan at full speed and throttling the output with a valve or damper, a VFD slows the motor down to match the actual demand. The energy savings can be substantial.
A U.S. Department of Energy analysis illustrates this well. In one example, a fan operating with a VFD instead of damper control reduced input power by 82.9%, even after accounting for reduced motor efficiency (77.8%) and drive efficiency (86%) at partial load. The fan and VFD together consumed only 2.8 kW compared to the original throttled setup. These savings exist because of that cubic law: small speed reductions translate to large power reductions.
This is why HVAC systems, water treatment plants, and industrial cooling systems are among the most popular applications for VFDs. The building’s heating and cooling demand fluctuates constantly, so pumps and fans rarely need to run at full speed. Letting them slow down, rather than fighting against throttled output, captures savings that compound over thousands of operating hours per year.
Starting Behavior of Variable Torque Loads
One practical advantage of variable torque loads is their gentle starting characteristic. Because torque demand is so low at low speeds, the motor doesn’t have to fight a heavy load to get moving. Breakaway torque for centrifugal pumps, fans, blowers, and compressors is generally low. Compare this to breakaway-type loads (like loaded conveyors or crushers), where starting torque can reach 200% of full-load torque.
This low starting demand means motors driving variable torque loads experience less mechanical stress during startup. It also means the VFDs used on these applications can often be smaller and less expensive than those required for constant torque loads, which need higher overload ratings to handle the surge at startup.