Variable Frequency Drive (VFD) / Inverter

Compact Variable Frequency Inverter (VFD)

The compact frequency inverter is intended for simple applications where V/F or vector control control is sufficient.
The simplicity of this frequency converter translates into ease of adjustment and robustness / operational reliability.

Variable frequency drives / vfd / inverters FLC500

How does a variable frequency drive / VFD / inverter work

Its operation is based on the principle that the motor torque of the electric motor depends on the voltage and frequency offered. When the frequency is increased the motor wants to run at higher rpm. When raising the voltage the more current will run through the windings and the more magnetic field will be created which results in more torque. The windings will have a higher impedance (” special kind of resistance) when the frequency is higher. In order to get the same current at a higher frequency a higher voltage in needed. When the frequency is increased, the motor will tend to run at a higher speed. When increasing the voltage, more current will flow through the windings and more magnetic field will be created, resulting in more torque. The windings will have a higher impedance (“special kind of resistance”) if the frequency is higher. To get the same current at a higher frequency, a higher voltage is required.

By varying these two variables, the desired behavior of the electric motor is achieved.

For simple applications it is sufficient to use a fixed voltage/frequency ratio over the entire speed range, also called V/F control / characteristic. Usually this control method is somewhat optimized with a boost function and some pre-programmed V/F characteristics.

For applications where the frequency converter has to keep more control over the behavior of the electric motor, the “motor model” is used, also known as the field oriented model. Basically, with this method of control, the desired speed of the motor is compared with the actual speed and, based on this, the voltage/frequency ratio is adjusted in such a way that the electric motor delivers the desired torques and runs at the correct speeds.

With an encoder on the motor, the frequency converter can determine the exact speed of the motor even more accurately. As a result, the combination of frequency converter, electric motor and encoder feedback provides the best results in terms of speed accuracy, torque precision and positioning.

Why applying a variable frequency drive / vfd / inverter

In principle, all drives for which one of the following points are desirable:

• The application you want to vary in speed. The frequency converter provides a smooth change in speed by varying the frequency and voltage.
• The application has many starts and stops. The frequency converter provides a “soft start and stop” and also limits the starting currents and thus the thermal load on the motor. a soft start and stop also extends the life of the mechanical components.
• You want to limit the starting currents, but you must start immediately with a lot of torque (star-delta is not possible and a soft starter will still draw large currents). The frequency converter can ensure that your application starts with full torque, at 100%-150% of the rated current.
• You want to save energy. With the frequency converter you can easily reduce the speed of your application and save a lot of energy. The frequency converter can also reduce the field strength in the electric motor when the torque demand of the motor is low.
• Simplifying your control box. By using the options in the frequency converter (including PLC, analog and digital inputs and outputs), you can in many cases greatly simplify your control cabinet.