Industrial frequency converter: useful information

Technical guide: What it is, how it works, and how to choose a frequency converter for industrial fans

When anindustrial fanneeds to vary its flow rate based on operating conditions, thefrequency converteris the component that makes this adjustment possible, controlling the speed of theelectric motorwithout wasting energy in artificial pressure drops.

Understanding how it works and which parameters guide selection is the starting point for choosing the right one for your system. This guide answers the technical questions a system manager asks before making a purchase.

What is a frequency converter: correct definition

A frequency converter, also known as a speed inverter, VFD (Variable Frequency Drive), or variable frequency drive, is an electronic device that controls the speed of an AC electric motor byvarying the frequency and voltageof the power supply.

The electrical grid supplies alternating current at a fixed frequency 50 Hz in Europe. A three-phase asynchronous motor connected directly to the grid rotates at a speed determined by the number of poles and the grid frequency and cannot vary. The frequency converter breaks this constraint and converts the grid power, recreates it at a variable frequency, and supplies it to the motor, causing it to rotate at the desired speed from 0 to the nominal speed and beyond, if necessary.

In an industrial ventilation system, this means being able to continuously vary the air flow from 0 to 100% without opening dampers, without bypassing, and without wasting energy.

How it works: the three stages of conversion

Stage 1: Rectifier (AC → DC)
The AC mains supply is converted to DC through a diode bridge or controlled rectifier. The resulting DC voltage forms the converter’s “DC bus.”

Stage 2: DC Bus (Filter and Storage)
The DC bus filters DC voltage fluctuations through smoothing capacitors. This stage ensures a stable DC voltage from which the subsequent inverter can generate the desired output.

Stage 3: Inverter (DC → AC with variable frequency)
The third stage reconverts the DC voltage into three-phase AC current with variable frequency and voltage, using IGBT transistors controlled with PWM (Pulse Width Modulation) technology. Varying the frequency of the signal varies the output frequency and therefore the motor speed.

The result is a synthesized sinusoidal power supply at any frequency between 0 and typically 400 Hz, with voltage proportional to frequency (constant V/f ratio) to maintain the motor’s magnetic flux at nominal values.

Frequency converter vs. inverter: are they the same thing?

In the industrial field, the two terms are used synonymously. More precisely, the inverter is the third stage of the converter, the one that converts direct current into variable alternating current, but in current technical jargon, the term “inverter” refers to the entire device composed of the three stages described above. It should not be confused with:

• The voltage transformer (varies the voltage but not the frequency, does not control the speed)
• The AC/DC static converter (converts alternating current to direct current for other purposes)
• The soft starter (reduces the starting current but does not vary the operating speed)

When we talk about inverters or frequency converters in the context of industrial ventilation, we always mean VFDs, i.e., three-stage devices for controlling motor speed.

Single-phase vs. three-phase: when to use which

Three-phase converters cover motors from 0.37 kW to hundreds of kW and are the standard configuration for all large-scale industrial fans.

Single-phase controllers, on the other hand, are suitable for single-phase motors up to 2.2 kW, typically used on small extractor fans, residential exhaust towers, and air circulators in semi-industrial or residential applications. They are easy to install, compact, and suitable for panel or wall installations.

The choice between single-phase and three-phase is not a matter of preference but it depends on the available power supply and the type of motor installed.

Technical parameters for selection

Motor power (kW)
This is the primary parameter. The converter must be sized at least for the nominal power of the motor. For fans, which have a quadratic load where the power increases with the cube of the speed, it is often possible to use a converter of the same size as the motor without oversizing.

Rated motor current (A)
The nominal nameplate current of the motor must be lower than the nominal current of the converter. If in doubt between two sizes, always choose the higher one.

Supply voltage (V)
Single-phase 230V or three-phase 400V in Europe. Check the mains voltage available in the installation panel before proceeding.

Load type
Fans are quadratic loads: the resistive torque increases with the square of the speed. Some converters have a specific mode for quadratic loads that optimizes efficiency in these applications.

IP protection rating
IP20 is suitable for closed electrical panels in clean environments. IP55 is required in dusty and humid environments. IP66 is required for applications with water washdown, such as in the food and dairy industries.

Communication protocol
If the converter is to be integrated into a supervision system or BMS, it is necessary to check the availability of Modbus RTU, Profibus, CANopen or other protocols required by the system.

Engine protection functions

A well-configured frequency converter protects the motor on several fronts:

• It monitors the temperature via PTC or NTC probe and automatically stops the engine in case of overheating.
• It protects against short circuits and prolonged overloads, against voltages generated by the motor during braking, against the loss of a power phase and against rotor blockage before the heat reaches the windings.
• It also intervenes in the event of prolonged mains voltage drops.

These functions are integrated into modern converters but must be configured correctly during installation. A converter with the protections set to factory defaults is not configured for the specific motor: this is one of the most common mistakes in hasty commissioning.

Invertek Optidrive E3 and CIMI RV Series

The Invertek Optidrive E3
is a three-phase inverter for industrial fans and pumps, available from 0.37 to 160 kW. It includes an EMC filter, Modbus RTU communication, and a specific ECO mode for quadratic loads that automatically optimizes energy efficiency as standard. The standard protection rating is IP55, with an IP66 option for harsher environments. The parametric display with keypad is removable, useful for on-site configuration. It is compatible with WEG W21 and W22 motors (IE3/IE4) without the need for line chokes in standard installations.

The CIMI RV Series
are single-phase speed controllers for motors up to 2.2 kW. Compact and quick to install on a DIN rail or wall, with manual adjustment via potentiometer. The ideal solution for small extractor fans, roof extraction units, and circulators in semi-industrial and residential applications where digital communication or advanced control is not required.

Both products are in stock in Fisciano for immediate shipment.

Compatibility with WEG IE3/IE4 motors

The combination of an inverter and a high-efficiency motor is now the standard for new systems and retrofits that must comply with EU Delegated Regulation 2024/1834 on the minimum efficiency of variable-speed drive systems for fans, effective from 2026.

WEG motorsW21 (aluminum, IE3) and W22 (cast iron, IE3/IE4) motors are designed to operate with inverters: they feature reinforced winding insulation, bearings suitable for eddy currents generated by PWM technology, and compatibility with the frequency range of industrial converters. They do not require additional output filters in installations with motor cables up to 20 meters.ori industriali. Non richiedono filtri di uscita aggiuntivi nelle installazioni con cavi motore fino a 20 metri.