Normal Duty (ND) vs Heavy Duty (HD) in frequency drives

⚙️ Normal Duty (ND) vs Heavy Duty (HD) in Variable Frequency Drives

With the increasing adoption of variable frequency drives in commercial and industrial applications, technicians and engineers have learned to integrate them seamlessly into their projects. However, a significant number of users still fail to correctly select the drive's operating mode or are unaware of the critical settings required for proper configuration.

In many cases, the drive is chosen correctly in terms of power, but fundamental parameters related to the application and control method are overlooked, which can directly impact the equipment's performance, efficiency, and lifespan.

The difference lies not in the "quality" of the drive, but in two key factors:
👉 overload capacity and
👉 the type of torque required by the load.

Understanding this relationship is crucial for correctly selecting between operating modes such as Normal Duty and Heavy Duty, as well as for safely and efficiently adjusting operating parameters.

🟦 Normal Duty (ND) – Variable Torque

🔹 Technical Characteristics

Typical Overload: 110–120% for 60 seconds

Designed for:
✔ Variable torque loads
✔ Soft starts
✔ Low mechanical stress
✔ Lower average current
✔ Reduced thermal stress on the drive

🔹 Typical Applications (ND)

✔ Industrial fans
✔ Exhaust fans
✔ Centrifugal pumps
✔ HVAC
✔ Cooling towers
✔ Light blowers
✔ Air conditioning systems

🧠 Rule:

If the torque increases with speed → it is usually Normal Duty.

🟥 Heavy Duty (HD) – Constant Torque / High Force

🔹 Technical Characteristics

Typical Overload: 150–180% for 60 seconds

Designed for:
✔ High starting peaks
✔ Heavy loads
✔ Constant torque or impact
✔ Higher thermal capacity
✔ Greater protection margin

🔹 Typical Applications (HD)

✔ Loaded conveyor belts
✔ Mixers and agitators
✔ Extruders
✔ Mills
✔ Elevators
✔ Presses
✔ Screw compressors
✔ Positive displacement pumps
✔ Crushers

🧠 Rule:

If the load requires high torque even at low speed → it's Heavy Duty.

⚠️ Common Mistakes

❌ Using Normal Duty on loaded conveyors.
❌ Undersizing drives for heavy start-ups.
❌ Ignoring duty cycles and starts per hour.
❌ Thinking that “more HP always solves the problem.”
❌ Not changing the Normal Duty and Heavy Duty selection parameters on the frequency converter.

This causes:

• Overheating.
• Overcurrent trips.
• Short lifespan.

✅ Quick Selection Rule

You can use this quick guide:

Remember that at ELECTRO.CR we can help you select the ideal frequency inverter for your application.