Variable Torque VFD AHU Applications

In the 2026 Malaysian engineering landscape, Variable Torque (VT) is the standard VFD configuration for Air Handling Units (AHUs). This specific application is governed by the Fan Affinity Laws, which dictate that for centrifugal loads (like the fans in an AHU), the torque required by the motor varies as the square of the speed.

Understanding this load type is essential for meeting the mandatory Building Energy Intensity (BEI) targets set by the Energy Efficiency and Conservation Act (EECA) 2024, which requires facility managers to optimize motor-driven systems for maximum energy reduction.

1. The Physics of Variable Torque: Centrifugal Fans

The fans in an AHU are "Variable Torque" loads because the resistance they encounter from the air changes significantly as their speed changes.

• Torque Relationship: Torque is proportional to the Square of the Speed ($T \propto n^2$). To double the fan speed, you need four times the torque.

• Power Relationship (The "Cube Law"): Power consumption is proportional to the Cube of the Speed ($P \propto n^3$). This is why VT applications are the most lucrative for energy savings.

• VFD Sizing: Because the torque demand is very low at low speeds, VFDs designed specifically for VT applications (often labeled "HVAC" or "Pump/Fan" drives) have lower overload ratings (typically 110% for 60 seconds) compared to "Constant Torque" drives (150% overload).

2. Statutory AHU Applications (EECA 2024)

Under the EECA 2024 and the new ErP 2026 Fan Regulations, variable torque VFDs are used in AHUs to achieve specific statutory energy outcomes:

3. VFD Tuning for Variable Torque

To maximize the life of the IE5 EC Fan Wall or motor and ensure 2026 statutory compliance, specific VT tuning parameters must be applied:

• V/f (Volts per Hertz) Ratio: Unlike constant torque loads (which use a linear ratio), AHUs should use a Quadratic (Square) V/f curve. This reduces the voltage at lower speeds more aggressively, saving additional electricity where high torque is not needed. 

• Skip Frequencies: Variable torque fans can hit mechanical resonances at certain RPMs. Technicians must "map out" these frequencies in the VFD to prevent mechanical vibration and housing fatigue.

• Flying Start: Essential for AHUs in KL high-rises where duct "windmilling" (air flowing through the fan while off) is common. This allows the VT drive to catch and control a spinning fan without tripping on overcurrent.

4. Why VT Matters for EKG (Malaysia) SDN BHD

At EKG (Malaysia) SDN BHD, we prioritize Variable Torque VFD integration because it offers the fastest ROI for our clients. Under the 100% Green Investment Tax Allowance (GITA), the capital expenditure for VT drives is fully claimable, provided they are part of a Professional Engineer (PE) endorsed energy conservation project.