When reading the TECO VFD selection manual, many people get stuck on the two letters ND / HD—same power, yet the ND-rated and HD-rated models differ in both price and load capacity. The TECO VFD ND and HD difference is essentially the "overload rating." Choose wrong and you either get frequent OL trips or overspend. This article explains it in one read.
ND = Normal Duty (variable torque); HD = Heavy Duty (constant torque). They correspond to the VFD's allowed overload capability and continuous current:
| Rating | Overload | Typical Load | Equipment |
|---|---|---|---|
| ND (variable torque) | 120% for 60s | Square-law, current drops as speed drops | Fans, pumps, oil pumps, cooling towers |
| HD (constant torque) | 150% for 60s | Current barely changes with speed | Compressors, conveyors, mixers, extruders, hoists |
Simple rule: fans/pumps → ND; constant-torque heavy load → HD.
Constant-torque equipment (e.g. compressors) has start/load shocks and needs the VFD to survive 1.5× current for 60s without tripping. So HD models use bigger power modules and stronger cooling—hence the higher cost. Fan/pump loads fall with the square of speed, so ND's 120% margin is enough; buying HD there is wasted money.
ND scenario: VFD rated output current ≥ max motor load current, roughly 1:1.
HD scenario: upsize one step—a 7.5kW motor pairs with an 11kW VFD to absorb start shocks and overload.
Fast accel / high inertia: beyond HD 150%, go even bigger.
High temp / high carrier: if ambient is hot or carrier >7.5kHz, the VFD derates—leave more margin.
Rule of thumb: keep at least 15% current margin in any case.
In the TECO VFD model list, a series usually shows both ND and HD ratings (or one frame rated for two powers). Always match "load type," not just the kW number—the line most often missed in the TECO VFD selection manual.
Mistake 1: "ND is cheaper, I'll use it for heavy load too" → frequent overload trips, higher module-failure risk.
Mistake 2: "HD does everything, buy all HD" → 20%–40% extra budget wasted on fans/pumps.
Mistake 3: "Size by motor kW, not current" → same 7.5kW motor can have different full-load current by brand/poles; go by rated current.
Q: One device drives both a fan and a conveyor—ND or HD?
A: Choose HD by the heaviest load and upsize one step, so the weak link doesn't trip.
Q: Does TECO servo also use ND/HD?
A: Servo is sized by inertia/torque—different logic—but the "leave margin for heavy load" principle is the same. Ask us for details.