Light Duty vs. Heavy Duty in Variable Frequency Drives: Understanding Your Load Before You Select

When selecting a variable frequency drive (VFD), the two power ratings printed on the nameplate — "light duty" and "heavy duty" — often cause confusion. Why does the same drive carry two different power figures? This isn't marketing obfuscation; it's a practical rating scheme manufacturers use to accommodate different load characteristics. Correctly understanding the distinction between the two is the first essential skill in VFD selection.

Start With the Mechanical Characteristics of the Load

To understand light duty versus heavy duty, you first need to look at the mechanical characteristics of the load the motor drives.

Light duty, in the context of VFDs, generally refers to variable-torque loads such as fans and pumps. Their torque increases in proportion to the square of the speed. At startup, dampers or valves are closed, so the load is extremely light, and the equipment rarely runs at full speed even during normal operation. As a result, these applications don't demand much overload capacity from the drive — an overload margin of 1.2 times rated current for 60 seconds is generally sufficient.

Heavy duty, on the other hand, corresponds to constant-torque loads such as extruders, belt conveyors, and mixers. Regardless of speed, torque remains essentially constant, the equipment starts under full load, and it must also withstand frequent shock loading during operation. This demands a drive capable of 1.5 times rated current or even higher overload capacity.

Why the Nameplate Shows Two Power Figures

Once you understand load characteristics, the meaning of the two power ratings on a VFD nameplate becomes clear. The drive's hardware is fixed, and its heat dissipation capacity is limited. Manufacturers adapt a single piece of hardware to two different applications by setting different rated currents and overload thresholds.

• In light-duty mode, the drive is allowed to output a higher baseline current, but the overload multiplier is reduced.

• In heavy-duty mode, the baseline current is reduced, which allows the overload multiplier to be increased.

This is exactly why the light-duty power figure is larger, and the heavy-duty power figure is smaller — the drive is essentially trading current margin for overload headroom.

Selection Principles

The selection rule is therefore straightforward:

• For fans and pumps, simply match the drive to its light-duty power rating.

• For constant-torque loads, the drive's heavy-duty power rating must be no less than the motor's power rating.

For example, for a 30 kW extruder, you should choose a drive rated at 37 kW light duty / 30 kW heavy duty — that is, step up one capacity size.

The most common — and most costly — mistake is simply matching drive size to motor size without considering duty type. In heavy-duty applications, this leads to nuisance tripping at startup, frequent protective shutdowns during operation, and in the worst cases, burned-out IGBTs. If an undersized drive has already been installed, temporary remedies such as extending the acceleration time, lowering the maximum frequency, or adding a reactor can help — but all of these come at the cost of performance, and the drive will ultimately still need to be replaced.