Variable Torque vs Constant Torque: Picking the Right VFD Duty Class

Selecting a variable frequency drive isn’t simply about horsepower and voltage. An application’s load torque behaviour dictates how hard the drive must work during start-up, steady-state operation, and occasional overloads. That distinction gets captured in the VFD’s duty class — typically labelled variable torque or constant torque. Match the wrong duty to the load and you either pay for unused capacity or compromise reliability. For plant engineers specifying centrifugal pumps, fans, compressors, conveyors, or extruders, the choice between variable torque and constant torque VFDs is one of the earliest and most cost-sensitive decisions in the procurement process. Understanding that choice also shapes how you evaluate solutions built around SLANVERT low-voltage and medium-voltage inverters where duty selection directly influences thermal sizing, harmonics mitigation, and long-term maintenance.

What the torque demand means inside a VFD

A motor delivering mechanical torque draws current proportional to that torque from the drive’s output stage. In a variable torque load — centrifugal pumps, axial fans, blowers — the required torque follows the square of the shaft speed. At 50% speed, the torque demand drops to roughly 25%; at 20% speed, it’s just 4% of the full-load value. This relationship — often called the pump affinity law — means the VFD’s power semiconductors spend most operating hours well below their thermal ceiling. Engineers designing for these loads typically need only a nominal overload margin. NEMA MG1 Part 30.3.4 reflects this by defining a normal-duty (variable torque) rating allowing 110% of rated output current for one minute out of a given cycle — enough to handle a slight process upset or a brief acceleration ramp from standstill.

Constant torque applications present no such relief. Conveyors, positive-displacement pumps, extruders, ball mills, and hoists need essentially the same torque regardless of speed. Start-up can demand 130% to 180% of full-load torque, sustained until the driven machine breaks away from static friction. That current load hits the IGBT modules, the DC bus capacitors, and the heat sink simultaneously. To survive this without derating, the VFD must carry a heavy-duty rating — normally 150% of rated current for one minute, with many high-performance SLANVERT electric drive solutions also handling 180% for two to three seconds to clear a momentary jam. International standard IEC 60034-1 reinforces these overload categories, classifying drives by their duty cycles S1 through S9, where applications with frequent high-torque starts fall into S4 or S5.

Knowing which category your machine fits lets you avoid two common traps: purchasing an oversized constant-torque drive for a variable torque load, which wastes capital and often reduces light-load efficiency, or undersizing a variable-torque unit on a conveyor, leading to overcurrent trips and unscheduled downtime.

Five decision points before you open a datasheet

• Variable torque loads reduce current demand quadratically with speed; constant torque loads maintain demand across the speed range.
• Normal-duty VFDs deliver 110% overload for 1 minute; heavy-duty units provide 150% for 1 minute, typical per NEMA MG1.
• Applications with breakaway torque above 130% and frequent starting cycles always require a constant torque drive.
• Oversizing a VFD for variable torque service does not improve reliability and can lower efficiency at partial load.
• Matching the duty class to the load’s starting profile and overload duration extends bus capacitor and IGBT module life.

How the load family shapes the VFD bus

Centrifugal fans and pumps sit at the gentler end of the duty spectrum. Once they overcome inertia — a fraction of a second for smaller units — the torque required to accelerate fluid follows the square of the speed. A 100 HP pump motor might need only 15 HP of torque at 40% speed, and the drive output stage sees current far below its continuous rating. This is why solar water pump inverter technology built on variable torque platforms can operate reliably with minimal heat-sink mass, reducing enclosure costs for remote installations.

Conveyors and rotary kilns flip that thermal picture. A 75 kW screw conveyor processing wet gypsum not only requires constant torque at every speed but also faces a starting torque that can spike to 160% of full-load until the material begins to slide. That means the drive’s DC link capacitors must supply repetitive high-current pulses, and the IGBT junction temperature swings wider with every start-stop cycle. Duty sizing here moves from “nominal rating” to “thermal stress management.” An undersized drive may not trip immediately — modern VFDs have excellent current-limiting firmware — but the accelerated electrolytic capacitor ageing shows up after two or three years as bus ripple that corrupts the PWM waveform. According to a reliability study published in IEEE Transactions on Industry Applications (Vol. 54, 2018), capacitor temperature rise alone accounts for roughly 40% of field failures in heavy-duty VFD installations that were not sized for the actual overload duty.

Side-by-side duty comparison

Reading the rating plate correctly

A 45 kW VFD sold as “normal duty” can handle a 45 kW variable torque motor without any derating. To drive a 45 kW conveyor motor with that same hardware, the manufacturer will often label the drive “heavy duty 37 kW” — essentially applying a 0.8–0.85 derating factor to the power stage. This isn’t a software trick; it reflects the fact that at 150% overload, the continuous output current must stay below the internal bus thermal limit. The IEC 61800-2 standard formalises this with the terms “low overload” (LO) and “high overload” (HO). When you specify an HO drive, you’re telling the supplier that the semiconductor junction temperature calculation must consider a larger I²t integral, which in turn affects the heat-sink design, the fan tray selection, and the capacitor bank arrangement.

Site-level verification is straightforward: run the motor at full load and check the drive’s output current display. If the current runs above 90% of the VFD’s nameplate continuous rating for more than a few minutes in a variable torque application, the duty sizing is marginal. For a constant torque machine, that same 90% might be normal — because the heavy-duty rating already accounted for the overload envelope.

What matters when the application doesn’t fit neatly

Mixers, extruders, and positive-displacement blowers sometimes live in a grey zone. Their torque rises linearly with speed (not squared) but they don’t demand severe breakaway overloads every cycle. Many engineers will still select a constant torque duty VFD to be safe, especially when the driven machine’s viscosity or density changes with temperature. The penalty is a slightly higher upfront cost — maybe 10% to 15% — and marginally lower light-load efficiency. For a machine that operates 6,000 hours a year, however, that efficiency delta matters. In those borderline cases, consulting the motor’s torque-speed curve and running a 24-hour data log of actual load current can reveal whether a variable torque drive with a small oversize factor (e.g., using a 55 kW normal-duty VFD for a 45 kW constant torque motor) is more economical over the life of the machine. A technical note from the U.S. Department of Energy’s Motor Systems Efficiency program suggests that a conservative duty selection, combined with a properly sized line reactor, can extend mean time between drive failures by approximately 20% when the load profile includes frequent acceleration cycles.

Frequently Asked Questions

Can I use a constant torque VFD for a pump or fan?

Yes, it will work. The drive simply has more overload headroom than the load needs. The trade-off is cost — the VFD will be physically larger and typically 10%–20% more expensive — and its light-load efficiency may be slightly lower because the output stage is optimised for higher current delivery.

What happens if I put a variable torque drive on a conveyor?

An overcurrent trip will probably occur during the first heavy start or jam. The VFD’s software may allow limited overload, but sustained operation beyond the 110% rating will raise IGBT junction temperatures until a protective fault stops the drive. Over time, thermal cycling damages capacitors.

Do I need to specify duty class for low-voltage and medium-voltage drives the same way?

The overload concept is identical, but medium-voltage VFDs often use different topologies (multilevel voltage-source inverters or current-source inverters) where the thermal inertia of the power stack influences the overload duration. Always check the manufacturer’s specific heavy-duty ratings, which may allow longer overloads at lower frequency. For example, SLANVERT medium-voltage drive solutions can be configured with a user-adjustable overload profile for conveyor systems requiring 150% for up to 90 seconds in specific SLANVERT electric drive product lines.

How do I know whether the breakaway torque truly exceeds the normal-duty limit?

Measure the motor’s deceleration time with the VFD disabled, then calculate the required breakaway torque from the load’s inertia and the desired acceleration time. If that value stays above 130% of full-load torque, a constant torque VFD is the safe choice. A simple clamp meter test during direct-on-line start also gives a practical baseline reading.