Variable speed in HVAC equipment: inverter compressors, VFD fans and EC motors

Variable speed saves energy only when the selection, the drive and the control strategy are all stated.

"Inverter", "variable frequency" and "EC" appear on many Chinese HVAC datasheets. Each means the equipment can run below full speed, which matters because most plant spends most of its hours at part load. The words do not tell you how far it can turn down, how efficient the drive is, or whether the published efficiency was measured at the speeds your project will use.

Three ways to vary speed

An inverter compressor is driven through a frequency converter, so capacity follows the load instead of switching on and off. Scroll, rotary and centrifugal compressors are all built this way; our guide to compressor types explains how each unloads.

A fan with a variable frequency drive (VFD) keeps a standard induction motor and changes its supply frequency. Several fan pages quote 25 to 50 Hz as an operating range, which shows the drive range rather than a selected duty.

An EC (electronically commutated) motor combines a permanent-magnet motor with integrated electronics. It is common in small fans, fan coils and data-centre cooling, and makes speed control part of the fan itself.

Efficiency rules for motors and drives

In the EU, Regulation (EU) 2019/1781 has applied since 1 July 2021. It covers induction motors from 0.12 kW to 1,000 kW, requires three-phase motors from 0.75 kW to 1,000 kW to reach IE3, and requires motors from 75 kW to 200 kW to reach IE4 from July 2023. For the first time it also covers variable speed drives, which must reach efficiency class IE2. Fans built into products are also covered by the separate fan rules described in our page on conformity documents.

Variable speed on this site

Compressor-side examples include the Hanyan commercial inverter modular heat pumps, the Envicool CyberMate V inverter CRAC, the Envicool outdoor cabinet air conditioners with a DC inverter compressor and the Envicool EIW micro chillers. In large chillers, the Dunham-Bush DCLC(D)M magnetic-bearing family and the DCLCDZ oil-free centrifugal chiller both describe variable-speed operation, and the DCLCDX family lists variable-frequency forms. Small refrigeration compressors are paired with their controllers in the ZEL variable-speed R600a packages.

Packaged and room units combine both: the AIRSYS UNICOOL wall packages, the AIRSYS OPTIMA2 variable-speed scroll offer and the AIRSYS CHILLROW in-row units with an EC fan and electronic expansion valve.

On the fan side, the Qili cabinet centrifugal fans and the Qinye SWF mixed-flow range both mention variable-frequency operation, while the Bigfans Fengrui PMSM ceiling fans use permanent-magnet motors.

Questions to ask

What is the minimum and maximum speed or frequency, and the corresponding capacity or airflow? Was the published efficiency measured at those speeds, and under which standard? Is the drive supplied, mounted and commissioned by the manufacturer? What efficiency class do the motor and drive meet, and is that stated on the nameplate? What harmonic and EMC measures are included, and what cable lengths do they assume? What does the unit do if the drive fails?

Chiller offers with variable-speed compressors should be compared at matching part-load points, as our chiller quotation guide explains. For fans, a lower speed moves the operating point along the system curve; our duty-point guide and our page on fan types show what to request.

Common questions

Does variable speed always save energy? Usually at part load, but drive losses and minimum-speed limits matter. Ask for figures at the loads your plant actually runs.

Is an EC fan the same as a VFD fan? No. Both vary speed, but an EC fan integrates motor and electronics, while a VFD drives a separate motor.

By HVACDeck Editorial · Updated 2026-11-06