VFD Sizing for Constant vs Variable Torque Loads
What is VFD sizing for constant vs variable torque loads? It means matching a drive's overload class — Normal Duty (ND) at roughly 110% of continuous current for 60 s per IEC 61800-2, or Heavy Duty (HD) at roughly 150% for 60 s — to the torque the driven machine actually demands across its speed range, not to the motor's nameplate kW alone. Get the class wrong and a heavy-duty conveyor on an ND-rated frame nuisance-trips the first time it jams, while a fan or pump oversized onto an HD frame carries cost the application never asked for. This article covers the constant-torque/variable-torque split, ND and HD current ratings, the overload-margin formula, duty-cycle and ambient derating, and the sizing mistakes that show up most often on returned drives.
Constant Torque vs Variable Torque: What the Load Actually Demands
Constant-torque loads hold their torque demand roughly flat from zero speed to full speed. Conveyors, cranes, hoists, positive-displacement pumps, mixers and extruders sit in this group. Torque stays close to constant, so power (torque times speed) scales close to linearly with speed. A conveyor running at half speed still needs nearly full torque to move the same load — it just moves it more slowly.
Variable-torque loads answer to different physics. On centrifugal pumps and fans, torque scales with speed squared and power with speed cubed — the affinity laws covered in our piece on VFD energy savings on pumps and fans. Drop the speed to 80% and torque falls to about 64% of full-speed torque, power to about 51%. That is the reason an ND-rated drive on a fan is not undersized in practice: the load itself asks for less at part speed, so the drive never sees the peak current the nameplate kW might suggest.
Normal Duty and Heavy Duty Ratings on the Same Frame
Most drive families sell one physical frame at two current ratings: a higher continuous amp rating in ND, a lower one in HD, because the HD rating reserves more silicon headroom for the 150% overload event. That is why the same enclosure can be labeled 22 kW in ND and 18.5 kW in HD — same hardware, different overload contract. Our detailed breakdown of the two classes, including how manufacturers publish the numbers, is in Normal Duty vs Heavy Duty overload ratings.
| Criteria | Normal Duty (ND) | Heavy Duty (HD) |
|---|---|---|
| Overload rating | ~110% for 60 s | ~150% for 60 s (some 200% for 3 s) |
| Typical loads | Centrifugal pumps, fans, blowers | Conveyors, cranes, extruders, mixers, positive-displacement pumps |
| kW rating on a given frame | Higher | Lower |
| Starting/breakaway torque margin | Limited | Built in for jam and breakaway events |
| Low-speed continuous torque | Not required by the load | Often required — check motor cooling |
Sizing Steps: From Motor Nameplate to Drive Selection
The sequence is short, but skipping a step is where most warranty claims start.
- Read the motor's full-load current (FLA) off the nameplate at the intended supply voltage, not the kW rating.
- Classify the load as constant or variable torque, and note any breakaway or jam-clearing event above steady-state torque.
- Pick the duty class (ND or HD) that matches the load, then find a frame whose continuous current at that class is equal to or greater than the motor FLA.
- Check the peak-current event — a jam, a lift, a screw-conveyor start — against the drive's overload percentage and time window for that class.
- Apply derating for ambient temperature, altitude, and any carrier-frequency increase above the drive's default setting.
Step three is where most sizing tools stop. Step four is where field failures start, because a 90 s jam-clearing cycle exceeds a 60 s ND window even when the peak current itself sits inside 110%. For a worked example with real nameplate numbers, see how to size a VFD to an AC motor.
Checking the Overload Margin
Formula: Overload Margin Check — Source: IEC 61800-2, motor nameplate FLA
Ipeak ≤ kOL × Irated
| Symbol | Description | Unit |
|---|---|---|
| Ipeak | Highest current the load draws during the overload event (jam, breakaway, hoist lift) | A |
| kOL | Drive overload factor: 1.10 for ND (60 s) or 1.50 for HD (60 s), per selected duty class | — |
| Irated | Drive continuous output current at the selected duty class | A |
If the calculated Ipeak exceeds kOL × Irated, move to the next frame size or the higher duty class. If the event duration exceeds the drive's rated seconds at that overload level, the margin check on paper does not protect the drive in service — the trip happens on time, not on current.
Duty Cycle, Ambient and Carrier Frequency Derating
Three factors stack on top of the base current rating, and none of them are optional once the installation departs from the drive's reference conditions. Ambient temperature above the drive's 40°C (or 50°C on some ranges) reference costs several percent of continuous current for every few degrees above it. Altitude above roughly 1000 m costs a few more percent per 1000 m, since thinner air cools the heatsink less. Carrier frequency above the factory default trades acoustic noise for switching losses — raise it for a quieter motor and the drive derates current to hold junction temperature within limits.
How much each factor costs depends on the enclosure and cooling arrangement — a wall-mounted IP20 unit in a ventilated cabinet loses less headroom for the same ambient rise than the same frame sealed into an IP55 enclosure. Some integrators default to the highest carrier frequency available for a quieter installation, but that trades away exactly the current headroom an HD load needs at its overload point.
Field Examples Across Applications
A belt conveyor feeding a crusher is constant torque with a jam event as its worst case: an HD-rated drive, sized on breakaway current and jam duration rather than steady running current, is the usual choice — Schneider Altivar Machine ATV340 and similar variable frequency drives in this class are built around exactly this duty profile. A crane hoist adds overhauling torque on the way down, so the sizing question extends past the ND/HD table into braking — Siemens SINAMICS variable frequency drives in the G120 family pair with brake choppers for this reason.
A centrifugal water pump is the clean variable-torque case: torque and current fall with speed, so an ND-rated frame at the motor's FLA covers it without the HD margin. ABB's ACS580, sized this way, is a common choice for pump stations — see VFD sizing and setup for pumps for the pump-specific version of this walkthrough. Mixed-duty applications, such as a positive-displacement pump on a batching line, sit closer to constant torque than the pump name suggests, and sizing them as if they were centrifugal is a common mistake.
Common Sizing Mistakes
Five patterns account for most of the sizing complaints that reach a distributor's technical desk.
Oversizing "to be safe." Jumping a frame size without a torque-based reason does not add protection — it adds cost and can push the drive's minimum output frequency and current resolution outside the motor's comfortable operating band at light load. What we see in the field: drives coming back on warranty claims are oversized about as often as undersized, because someone defaulted to the next frame up instead of checking the actual overload event.
Ignoring low-speed cooling on constant-torque loads. A shaft-mounted cooling fan loses airflow as motor speed drops. A constant-torque load running continuously at low speed needs forced external ventilation or a motor rated for it — the drive's overload class does not fix a motor cooling problem.
Missing the breakaway spike. Screw conveyors, positive-displacement pumps and mixers with settled material need more starting torque than their running torque. Size against the breakaway current, not the steady-state FLA.
Comparing overload percentage without checking duration. 150% for 60 s is not the same margin as 150% for an 80 s jam-clearing sequence. Time the actual event before assuming the rating covers it.
Skipping carrier-frequency derating after a site request for quieter operation. Raising carrier frequency post-commissioning without re-checking the current derating table can leave a previously adequate margin too thin for the next overload event.
Frequently Asked Questions
Can I run a conveyor on a Normal Duty rated drive?
Only if the conveyor's peak current, including jam-clearing events, stays within the ND drive's 110%/60 s window and the continuous current still covers the motor FLA. For most conveyors with real jam risk, an HD-rated frame or an oversized ND frame is the safer specification.
What's the practical difference between the overload percentage and the continuous current rating?
The continuous rating is what the drive can output all day without derating. The overload percentage is a short-duration allowance on top of that, capped at a fixed number of seconds — 60 s is typical for both ND and HD, with some HD frames adding a 200%/3 s tier for very short spikes.
Does oversizing a drive protect the motor?
Not automatically. An oversized drive at light load can run with reduced current resolution and, on some designs, a higher minimum switching frequency requirement that the motor never needed. Size to the torque profile first; treat extra margin as a deliberate choice, not a default.
How do I know if my load is constant torque or variable torque?
Ask whether torque changes with speed. Centrifugal pumps, fans and blowers need less torque as speed drops (variable torque). Conveyors, cranes, mixers, extruders and positive-displacement pumps need close to full torque across the whole speed range (constant torque).
Why does carrier frequency change the drive's current rating?
Higher carrier frequency means more IGBT switching events per second, which raises switching losses and heatsink temperature. To hold junction temperature within limits, the drive derates its continuous current output as carrier frequency rises above the factory default.
What derating applies at altitude above 1000 m?
Thinner air removes less heat from the drive's heatsink, so most manufacturers apply a percentage derating for every 1000 m above roughly 1000 m elevation. The exact percentage varies by frame and cooling method, so treat it as a range to check against the specific model, not a fixed universal number.
Conclusion
Sizing a VFD correctly starts with the load's torque profile, not its kW label. Classify the load as constant or variable torque, pick ND or HD accordingly, verify the overload-current and overload-duration margin against the worst real event, then stack ambient, altitude and carrier-frequency derating before locking the frame size. Our VFD engineering guide ties this sizing logic to the rest of the selection process, and the VFD selection checklist turns these steps into a one-page reference for the next specification.
