VFD Overload: Normal Duty vs Heavy Duty Ratings
What is a VFD overload rating? A variable frequency drive's overload rating is the extra output current it can deliver above continuous nameplate current for a fixed time, split into two duty classes — Normal Duty (ND, 110% for 60 s) and Heavy Duty (HD, 150% for 60 s, with some frames adding 200% for 3 s) — per the duty-cycle tables manufacturers publish alongside IEC 61800-2 ratings. Pick the wrong class and the drive trips on overcurrent the first time a strainer clogs, a conveyor jams, or a crane picks up a rated load, even though the same frame runs a lighter duty all day without complaint. This article covers how to read ND/HD current tables on a drive nameplate, why HD continuous current is always lower than ND on the same power module, the overload current formula behind the numbers, what happens electrically once the overload window expires, and how to choose a rating by load type — pumps and fans versus conveyors, cranes, and positive-displacement loads.
Reading the Duty Rating on a VFD Nameplate
Drive catalogs list two current ratings per frame size, not one. Each frame carries a Normal Duty continuous amp rating and a Heavy Duty continuous amp rating, each paired with its own overload percentage and time window. A 45 kW frame might show 92 A continuous in ND with 110% (101 A) available for 60 s, then drop to 88 A continuous in HD with 150% (132 A) for 60 s on the identical power module. That is not a printing error. It is thermal bookkeeping — the HD table reserves more IGBT and heatsink headroom for the larger overload pulse, so the continuous number falls to make room for the spike. Selecting a drive from the variable frequency drives range means checking which of these two tables actually covers the motor's full-load amps, not just the frame's headline kW.
ND vs HD at a Glance
| Criteria | Normal Duty (ND) | Heavy Duty (HD) |
|---|---|---|
| Overload current | 110% of continuous rating | 150% of continuous rating (some frames add 200% for 3 s) |
| Overload duration | 60 s, once per 10 min | 60 s (or 3 s at 200%), once per 10 min |
| Continuous current on same frame | Higher | Lower |
| Torque profile | Variable torque (falls with speed²) | Constant torque, shock-loaded |
| Typical loads | Centrifugal pumps, fans, blowers | Conveyors, cranes, mixers, positive-displacement pumps |
Normal Duty: Variable-Torque Loads
ND covers centrifugal pumps, fans, and blowers — loads where torque demand falls as speed falls, roughly with the square of speed while power falls with the cube. That cube relationship is why running a fan at 80% speed uses about half its full-speed power, the core payback argument for affinity-law energy savings on variable-flow systems. A pump or fan rarely spikes torque suddenly; a partially closed valve or a fouled coil raises load gradually, so 110% for 60 s covers the transient without needing HD headroom. Sizing an ND drive for a VFD for pumps application usually means matching motor FLA to the ND continuous current column and stopping there.
What we see in the field: some panel builders default every job to HD current tables out of habit, even on straightforward pump and fan duty. That is not wrong, but it forces a larger, more expensive frame than the load needs — the ND column on a smaller frame often covers the same motor FLA with room to spare.
Heavy Duty: Constant-Torque and Shock Loads
HD covers conveyors, cranes, hoists, mixers, and positive-displacement pumps — loads that need close-to-full torque across the speed range and can slam the drive with a step load. A conveyor starting under a full hopper, a crane taking up slack on a hook, or a screw pump against a closed valve all draw near-instant torque spikes that 110% cannot absorb without tripping. That is why HD tables allow 150% for 60 s, and why some frames add a 200% for 3 s tier for the sharpest transients — a crane hook snapping tight, for instance. Industrial-class frames built for regenerative or active-front-end options, the kind used on overhauling crane and hoist loads, are typically specified from the HD table first, then checked against ND only if the application later proves lighter than expected.
The Overload Current Formula
Formula: Duty-Rated Overload Current — Source: IEC 61800-2, manufacturer duty-cycle rating tables
Iol = IN × OL%
| Symbol | Description | Unit |
|---|---|---|
| Iol | Permissible overload output current for the rated duration | A |
| IN | Continuous current for the selected duty class (ND or HD column on the frame's table) | A |
| OL% | Overload percentage for the duty class — 110% ND, 150% HD, up to 200% HD short-duration | % |
The formula looks trivial until the cycling limit is added. Manufacturers cap overload events to once per 10-minute rolling window, and the drive's thermal model tracks IGBT junction temperature continuously, not just during the overload pulse itself. Two overload events inside the same window will often derate or trip even though each event individually stayed inside the 60 s limit — the drive is protecting silicon that has not finished cooling from the first event. Comparing this current table against the motor nameplate is exactly the check covered in VFD voltage and current ratings.
What Happens When You Exceed the Overload Window
Push past the overload duration and the drive does one of two things, depending on parameter settings: it either derates output current to hold within the thermal model, silently reducing torque and possibly stalling the motor, or it trips on an overcurrent or IGBT-overtemperature fault and shuts down. Neither is a nuisance fault. The trip is the drive refusing to cook its own power semiconductors past their rated junction temperature. Repeated trips on a load that used to run fine usually mean one of three things: the duty class was undersized from commissioning, the mechanical load has increased (a bearing dragging, a conveyor overloaded beyond design), or ambient temperature at the drive has risen enough to reduce the available thermal margin.
This depends on where the drive sits, too — a VFD in a poorly ventilated cabinet during a summer heat spike loses overload headroom well before the motor side changes at all. Checking cabinet temperature is often faster than re-checking motor FLA when overload trips start appearing seasonally.
Selecting the Right Duty Rating
Start from the load, not the catalog page. Variable-torque loads — pumps, fans, blowers — belong on the ND column; constant-torque and shock loads — conveyors, cranes, mixers, positive-displacement pumps — belong on the HD column. Then match motor full-load amps against that column specifically, not against the frame's advertised kW, since the same frame reports a lower kW figure in HD than in ND. The full workflow for matching frame, current column, and motor is covered step by step in how to size a VFD for the motor, and the broader current/voltage rating conventions sit in the VFD engineering guide. General-purpose frames such as ABB ACS580, Schneider Altivar ATV630, and Siemens SINAMICS G120 ship as dual-rated units with both ND and HD current tables selectable by parameter on the same hardware; a side-by-side look at how those three lines differ on overload and other specs is in ABB ACS580 vs Schneider ATV630 vs Siemens G120.
Frequently Asked Questions
What is the difference between VFD Normal Duty and Heavy Duty ratings?
Normal Duty allows 110% overload current for 60 s and targets variable-torque loads like pumps and fans. Heavy Duty allows 150% for 60 s (some frames add 200% for 3 s) and targets constant-torque or shock-loaded equipment like conveyors and cranes. The two ratings live on the same physical frame with different continuous current numbers.
Can a Normal Duty drive run a Heavy Duty load?
Only if its ND current, not its HD current, already covers the motor's full-load amps with margin, and the load never demands a step-torque event beyond 110%. In practice this fails on real conveyors and cranes — the drive will trip on the first jam or hook snag, so the HD table is the correct starting point for those loads.
Why is the HD current rating lower than the ND rating on the same frame?
The HD overload multiplier is larger (150% or 200% versus 110%), so the drive's thermal design reserves more silicon and heatsink headroom for that bigger spike. Reserving headroom for overload means less is available for continuous output, so the HD continuous current column comes out lower than the ND column on identical hardware.
How often can a VFD run at its overload current?
Most manufacturer duty tables allow one overload event per 10-minute rolling window, whether the drive is on ND or HD. Two overload events inside the same window can trigger a trip or derate even if each event alone stayed under 60 s, because the thermal model tracks cumulative junction heating, not just the pulse duration.
Does the overload rating affect voltage as well as current?
No. The ND/HD overload percentage applies to output current on the inverter stage; output voltage tracks the V/f or vector reference and is not raised during an overload event. DC bus voltage can rise separately during regenerative events like fast deceleration, which is a different phenomenon covered under braking.
Conclusion
ND and HD ratings are not marketing tiers — they are two thermally distinct current tables on the same power module, each matched to a different torque profile. Pumps and fans belong on the 110%/60 s ND column; conveyors, cranes, and shock-loaded equipment belong on the 150% (or 200%/3 s) HD column. Size against the correct column's continuous current, respect the once-per-10-minute overload cycling limit, and treat repeat overload trips as a sign to re-check duty class and ambient temperature before swapping hardware. Get the duty class right at commissioning and the drive rarely needs a second look.
