Choosing a VSD for Commercial HVAC and Cooling Towers

The best VFD for HVAC is not a brand. It is the drive whose environmental rating, harmonic performance, fan and pump functions and building management system (BMS) protocol match the plant it will serve. For a cooling tower fan, that means a variable-torque drive installed out of the weather or in an IP55 enclosure, with skip frequencies and catch-on-the-fly, a motor and cable checked for inverter duty, and speed controlled from condenser-water temperature. Yes, you can use a VFD on a cooling tower fan motor, provided you check the motor, cabling and tower mechanics first.

This guide covers specifying variable speed drives (VSDs, also called VFDs or inverters) for air handling units (AHUs), cooling towers and chilled- and condenser-water pumps in Singapore, using data for the ranges we supply and our own HVAC projects.

In this article

• What makes a VFD suitable for commercial HVAC?
• IP rating and humidity: designing for Singapore’s climate
• Can I use a VFD on a cooling tower fan motor?
• What type of VFD is best for a cooling tower fan?
• Harmonics and power quality
• BMS protocols: what to specify
• Do variable speed drives reduce electricity bills for HVAC systems?
• Which VFD brand is most reliable for air handling units?
• What we check on site
• Get a drive selection for your HVAC plant
• Frequently asked questions about VFDs for HVAC and cooling towers
• Sources

What makes a VFD suitable for commercial HVAC?

Fans and centrifugal pumps are variable-torque loads whose power falls roughly with the cube of speed. The IEA 4E Electric Motor Systems Platform reports that motor systems used 36% of electricity in buildings worldwide in 2023. NEA’s Motor Driven Systems reference manual shows a typical fan system reaching only about 40% overall efficiency once every component is counted. HVAC also brings humid plant rooms, rooftop towers, BMS integration and fire safety interfaces. The features we look for:

IP rating and humidity: designing for Singapore’s climate

The Meteorological Service Singapore gives a mean annual relative humidity of around 82%, with readings above 90% before sunrise. Drive datasheets assume non-condensing air:

The Hydraulic Institute and Europump guide to variable speed pumping warns that drive electronics cope less well with damp and corrosive locations than conventional starters. In practice:

• Plant or AHU room: an IP20/IP21 drive in a ventilated or air-conditioned panel, away from dripping pipework.
• Beside a cooling tower: better avoided; keep the drive in the switch room (mind the cable-length limits below), or use an IP55 drive such as the ATV212 W version or a rated, shaded enclosure.
• Condensation during shutdown: the HAVSP lists a frost and condensation protection function; the panel design should address it too.

Can I use a VFD on a cooling tower fan motor?

Yes. Our projects at Singapore EXPO, Seletar Mall and Silicon Box all include cooling tower drives. Check these before you specify:

1. Motor insulation and age. Fast-switching PWM drives put voltage spikes on the motor terminals. US DOE Motor Systems Tip Sheet #15 says general-purpose motors may work with PWM drives if peak voltage stays below 1,000 V; above that, turn-to-turn failures can occur. At Singapore EXPO, the motors were over 10 years old.
2. Cable length. Runs of 50 feet (about 15 m) or more almost always need a load reactor or dV/dt filter, which rooftop towers often exceed.
3. Bearings. The tip sheet recommends a 2–5 kHz carrier frequency, a dedicated low-impedance ground and symmetrical cable; the Hydraulic Institute recommends insulated non-drive-end bearings above 100 kW.
4. Minimum speed. Take gearbox lubrication and tower airflow minimums from the manufacturers and set them as the drive’s minimum frequency.
5. Resonance. Sweep the speed range at commissioning and set skip frequencies at vibration peaks.
6. Windmilling. Enable catch-spinning-load so the drive can pick up a fan turned by neighbouring cells.
7. Control. Modulate speed from condenser-water temperature rather than switching fans on and off.

What type of VFD is best for a cooling tower fan?

A normal-duty, variable-torque drive with fan and pump software. A heavy-duty or closed-loop industrial drive adds cost without benefit. This matrix is a guide to fit, not a ranking:

The WEG ADV200 and Himel HAVXS Expert Series suit machinery needing precise speed or torque (see how the HAVXS minimises motor wear); for HVAC fans and pumps, the ranges in the first four rows fit better.

Harmonics and power quality

A drive’s diode rectifier draws current in pulses, adding harmonic distortion. IEEE 519 is commonly used to set limits, and specifications may add a per-drive THDi (total harmonic current distortion) limit. Two published approaches:

• Schneider says its ATV212 achieves a THDi below 35% without a line or DC choke, compared with the 48% THDi level imposed by IEC/EN 61000-3-12.
• Himel’s X Series catalogue describes a built-in DC reactor and says THDi can reach under 35%. Check which ratings include the reactor.

On our ST Aerospace Aviation Centre project at Seletar Aerospace Park, every chilled- and condenser-water drive had to stay below 35% THDi, and we measured each one during testing and commissioning. With a similar clause, ask for THDi at the actual rating and budget for measurement.

BMS protocols: what to specify

Settle network or hardwired control early, because options differ:

• Schneider ATV212: Modbus, Metasys N2, Apogee FLN P1 and BACnet are built in as standard, with LonWorks on an option card, according to the Altivar 212 catalogue.
• Himel X Series: option cards for protocols including Profinet, Profibus, Modbus TCP and BACnet MS/TP, according to the X Series catalogue.
• Himel HAVSP: Modbus communication, plus two analogue inputs (0–10 V or 0–20 mA) and programmable relay outputs for hardwired control, according to the HAVSP4T0370P datasheet.

Retrofits need care. At Singapore Polytechnic (about 30 drives) and Republic Polytechnic (about 40 drives), the replacement HAVSP drives were configured to work with each campus’s existing controls and verified at commissioning. Get the existing points list, signal types and protocol before ordering.

Do variable speed drives reduce electricity bills for HVAC systems?

Yes, where fans and pumps can run below full speed. US DOE Motor Systems Tip Sheet #11 notes that with no static lift, power varies with the cube of speed, so a 20% speed reduction can cut input power by about 50%.

Worked example: a 7.5 kW cooling tower fan

A 7.5 kW tower fan motor draws 6.5 kW at full speed for 4,000 hours a year. With a drive, it runs at 70% speed for 3,000 hours and full speed for 1,000. Using DOE’s typical 10 hp drive efficiencies (about 92% at this part load, 96% at full load):

• Fixed speed: 6.5 kW × 4,000 h = 26,000 kWh a year.
• With a drive: (6.5 × 0.343 ÷ 0.92) × 3,000 h + (6.5 ÷ 0.96) × 1,000 h ≈ 7,270 + 6,770 = 14,040 kWh a year.
• Saving: about 12,000 kWh a year, roughly S$3,400 at the SP Group regulated tariff of 28.59 cents/kWh before GST for October to December 2026.

If the fan currently cycles on and off, compare against its actual hours, and measure before and after. Our ATV212 HVAC energy article and VSD efficiency guide go further into the energy side.

Which VFD brand is most reliable for air handling units?

We don’t rank brands: reliability depends more on application fit, installation and commissioning than on the logo. The Carbon Trust VSD technology guide lists the usual causes of drive problems: high ambient temperature, insufficient cooling, water or dust ingress, incorrect parameters and loose terminals.

The ranges we supply for AHUs:

• Himel HAVSP Smart Pump Series: 2.2–315 kW, PID with sleep, Modbus, fire override and multi-pump functions. It is the range used on most of our published HVAC projects.
• Himel X Series: 0.4–185 kW, coated PCBs, built-in DC reactor and wide protocol support including BACnet MS/TP. See our X Series guide.
• Schneider Altivar 212: 0.75–75 kW, dedicated to HVAC, with IP55 versions and BMS protocols built in. See our ATV212 article.
• Schneider Altivar Process ATV630: for larger pumps and fans that need process-grade monitoring.

At GIIS, we supplied HAVSP drives for the Phase 2 AHUs, configured to the operating accuracy needed to maintain cooling capacity for classrooms and laboratories, then tested and commissioned.

What we check on site

Checks drawn from our HVAC projects:

• Existing motor condition: at Singapore EXPO the motors were over 10 years old; check insulation first.
• Existing controls: match the BMS signals and protocol, as at both polytechnics.
• Harmonic acceptance: know the THDi limit and test method, as at ST Aerospace.
• Cabling: at Singapore EXPO the arrangement helped optimise cabling and installation cost.
• One platform: at Seletar Mall, 33 HAVSP drives (0.75–15 kW) covered towers, pumps and carpark fans.
• Energy specification: at Silicon Box, the drives met the energy consultants’ requirements across four systems.
• Efficiency class: IEC 61800-9-2 defines classes for drives where specified.

If a pump only ever runs at full speed, a soft starter may be the better tool. See our guide to soft starter vs VFD for pumps.

Get a drive selection for your HVAC plant

Send the motor nameplates, application, drive location and BMS protocol through our contact page and we will propose drives from our variable speed drive ranges, with datasheets. See also our HVAC and chilled-water solutions.

Frequently asked questions about VFDs for HVAC and cooling towers

Can I use a variable frequency drive on a cooling tower fan motor?

Yes. Check the motor’s insulation and age, keep the motor cable short or add a filter, set the minimum speed from the tower and gearbox makers’ data, programme skip frequencies around any resonance, and enable catch-spinning-load so a windmilling fan can restart.

What type of VFD is best for a cooling tower fan?

A normal-duty, variable-torque drive with fan and pump functions: PID control from condenser-water temperature, skip frequencies, catch spinning load and a suitable IP rating for where it is installed. A heavy-duty industrial drive is not needed.

What is the best VFD for HVAC?

The best VFD for HVAC is the one that matches the plant: a normal-duty rating, HVAC functions such as PID, sleep and fire mode, the BMS protocol the building uses, acceptable harmonics and an enclosure suited to the location. We supply Himel and Schneider Electric drives designed for these duties.

Which VFD brand is most reliable for air handling units?

Reliability depends mainly on correct rating, a dry and ventilated installation, correct parameters and good commissioning. We supply the Himel HAVSP and X Series and the Schneider ATV212 for air handling units, and we test and commission them on site.

What IP rating does a VFD need in Singapore?

Inside a ventilated or air-conditioned panel, an IP20 or IP21 drive is normal. Near a cooling tower or in a damp area, use an IP55 drive or a suitable enclosure, because morning humidity in Singapore regularly exceeds 90%.

Do variable speed drives reduce electricity bills for HVAC systems?

Yes, when fans and pumps can run below full speed. Fan and pump power falls roughly with the cube of speed, so a 20% speed reduction can cut input power by about half. In our worked example, a 7.5 kW cooling tower fan saves about 12,000 kWh a year.

Sources

• Meteorological Service Singapore, Climate of Singapore.
• Schneider Electric, Altivar 212 variable speed drives catalogue (DIA2ED2101102EN).
• Himel, X Series VSD catalogue.
• Himel, HAVSP4T0370P product data sheet (SMART Pump VSD).
• US Department of Energy, Motor Systems Tip Sheet #11: Adjustable Speed Drive Part-Load Efficiency.
• US Department of Energy, Motor Systems Tip Sheet #15: Minimize Adverse Motor and Adjustable Speed Drive Interactions.
• Hydraulic Institute, Europump and US DOE, Variable Speed Pumping: A Guide to Successful Applications (Executive Summary).
• Carbon Trust, Variable speed drives technology guide (CTG006).
• IEEE, IEEE 519-2022 Standard for Harmonic Control in Electric Power Systems.
• IEC, IEC 61800-9-2:2023 Energy efficiency indicators for power drive systems and motor starters.
• IEA 4E Electric Motor Systems Platform, Policy brief: electric motor systems, why are they important?.
• National Environment Agency, Motor Driven Systems reference manual (SCEM).
• SP Group, Tariff information.
• Trans-Digi Global case studies: GIIS, Singapore EXPO, Singapore Polytechnic, Republic Polytechnic, ST Aerospace Aviation Centre, Silicon Box, Seletar Mall and Gateway East.