The following criteria must be taken into account when selecting a suitable frequency inverter:

1. the degree of protection suitable for the operating conditions
The standard protection class for frequency inverters is IP20 (also IP00 for higher outputs). These devices can be installed in clean and dry rooms or in switch cabinets. Alternatives for "harsh" ambient conditions are devices with IP54 or IP66. Some of these devices are equipped with extended operating elements (e.g. direction of rotation switch ...) and are considerably more expensive.

2. the available or desired mains voltage
There are frequency inverters for 230V/1~ or 400V/3~ mains voltage. The advantages of 230V devices are the use of three-phase motors on the 230V single-phase mains and the sometimes significantly lower price. The motors used here must have a winding for 230V/3~ (usually in delta connection). Devices with 230V are only available up to max. 4kW.

3. required range of functions
The basic requirements must first be clarified here, e.g. vector control for high starting torques and speed accuracy and/or positioning functionality required? Brake chopper required? You will find a selection aid for initial orientation in the product filter with the "Basic", "Standard" and "High-Performance" function scopes.
For the selection of special and extended functions (STO, fieldbus systems, standards ...), the extended product filter or the comparison matrix "Range of functions" (PDF document) can be used.

The decisive criterion for selecting the power of the frequency inverter is the rated current of the motor. The outputs specified for the frequency inverters are guide values for initial orientation and refer to motor outputs for 2- and 4-pole three-phase asynchronous motors (synchronous rated speeds 3000 and 1500 rpm). For motors with lower rated speeds, the rated currents are sometimes significantly higher for the same rated power. In any case, the rated output current of the FI must always be matched with the rated current of the motor - the following applies:
Rated output current of the FI >= rated current of the motor.

Another criterion for determining the power is the required overload capacity of the application (e.g. high starting currents at high starting torques). In principle, all VFDs are designed for overload in a defined period of time. It should be noted that most modern VFDs have a dual rating function for 2 load characteristics. The power and rated currents for applications with high continuous load or high overload are specified here. Practical example: Setting "high" continuous load - 11kW, rated output current 23.4A, overload capacity 120% for 60s or setting high "overload" - 7.5kW, rated output current 18.0A, overload capacity 150% for 60s. With this function, a "smaller size" can be selected for applications without high overload capacity requirements.
For applications with very high overload requirements, the FI must be oversized accordingly. In this case, devices with one power stage, or in special cases two power stages, are selected.
Note on using the product catalogues:
In the product filters and results lists, the power for high continuous load is always specified; in the product details and data sheets you will find information for both load characteristics.

For frequency inverters with a mains voltage of 230V/1~, a three-phase asynchronous motor with a rated voltage of 230V must be used. Standard motors with lower outputs are usually designed with a nominal voltage of 230/400V/D/Y. In this case, the delta connection is used for operation on the 230V FC. FCs with 230V mains voltage are available for motor outputs up to 4kW. The advantages of the devices are the use of three-phase motors on the 230V single-phase mains and the significantly lower price in some cases.

V/f control is the simplest control concept for a frequency inverter. Motor voltage and frequency are controlled in a constant ratio, frequency and voltage are proportional to each other. This results in a constant torque over a wide speed range. Disadvantages are lower torques at very low speeds and problems with load changes in the lower speed range. V/f control is therefore only sufficient for low speed stability requirements and without heavy starting. Typical applications include drives for fans and pumps.

Vector control or field-orientated control consists of a speed controller based on a subordinate current controller. The instantaneous reactive and active current components are controlled. The motor characteristics are stored in an electronic motor model stored in the FI or, if necessary, even determined and adapted automatically. This has the advantage that there is no need for separate speed measurement and feedback in order to control speed and torque. Instead, the only variable used for control is the current motor current. All required motor states (speed, slip, torque, ...) can be determined based on its magnitude and phase relationship to the voltage. Vector-controlled VFDs are suitable for applications with high demands on speed accuracy and consistency as well as high starting torques.

Most modern frequency inverters have a dual rating function for 2 load characteristics. Power and rated currents are specified here for applications with high continuous load or high overload.
Practical example: "High" continuous load setting - 11kW, rated output current 23.4A, overload capacity 120% for 60s or "High" overload setting - 7.5kW, rated output current 18.0A, overload capacity 150% for 60s. This function means that a "smaller size" can be selected for applications that do not require a high overload capacity.
Note on using the product catalogues:
In the product filters and results lists, the power for high continuous load is always specified; in the product details and data sheets you will find information for both load characteristics.

The following motor types are suitable for operation on the frequency inverter: Three-phase asynchronous motors, permanent magnet motors, synchronous reluctance motors.
In the case of standard three-phase asynchronous motors, all modern makes are suitable for FI operation without any special design. Permanent magnet and reluctance motors are designed exclusively for operation with frequency inverters.

Restrictions due to the insulation system of the existing motor With older motors, it may be necessary to protect the weaker insulation system with intermediate motor chokes or sinusoidal filters. The same may also apply to special motors (e.g. integrated motors of high-performance submersible pumps). The information in the operating instructions for the respective motor must be observed here!

Extended criteria for motor design

• Design of the motor for special torque curve may be required
• Observe the mechanical maximum speed of the motor
• Note reduced cooling capacity at low frequencies (below approx. 20Hz), use external fan if necessary
• For larger motors (> size 250), use insulated bearings to increase the bearing service life

Electromagnetic compatibility (EMC) is defined as the ability of an electrical or electronic device to function satisfactorily or as intended in its intended environment without inadmissibly influencing this environment through self-generated electromagnetic interference.
The operation of frequency converters generates high-frequency harmonics in addition to the fundamental wave. These harmonics can be inadvertently coupled into other devices and systems, e.g. via the mains supply. EMC line filters at the power supply input of the frequency inverter prevent this interference.

In principle, all frequency inverters offered by us can be operated directly on the device. All parameters for function adjustment can be called up and changed via the control panel. Additional components are not absolutely necessary. Free commissioning software for PC is also available for most devices (download only possible with user account). Additional interface cables are required here (see accessories for the respective device). The advantages of the software include clarity, copying and archiving options.
The FI can be controlled via external sources (switches, buttons, potentiometers, analogue signals, etc.) using various digital and analogue control inputs.

A brake chopper or brake transistor is a functional unit that can be used to monitor the DC link voltage of frequency inverters. This may be necessary to prevent overvoltages in the intermediate circuit. This overvoltage can be caused by regenerative operation when braking the drive system. The chopper directs the excess energy to a connected braking resistor. This is where the energy is converted into thermal energy. Dynamic braking processes are possible with brake choppers, even with higher mass moments of inertia. Brake choppers up to a certain power (see technical data) are integrated in the device for drives with an increased range of functions. There are also external brake choppers for connection to the DC link of devices without an integrated chopper.

The use of shielded motor cables may be necessary for reasons of electromagnetic compatibility. Electromagnetic compatibility (EMC) is defined as the ability of an electrical or electronic device to function satisfactorily / as intended in its intended environment without inadmissibly influencing this environment through self-generated electromagnetic interference.
The use of shielded motor power is always advisable and is always recommended for cable lengths of 5 metres or more.