What is Frequency Converter

Explained in the simplest way possible, a frequency converter is a device used to change alternating current of one frequency into alternating current of another frequency. The device can be either electronic or electromechanical. Besides its principal purpose, a frequency converter sometimes may change the voltage as well. The reason why you can still find frequency converters in two variants (electronic and electromechanical), is because the first devices used for the purpose, were electromechanical machines known as motor-generator sets. They employed smaller devices with mercury arc rectifiers and vacuum tubes. Today, thanks to solid state electronics, there are completely electronic converters that have a rectifier used to produce direct current which is then inverted to produce the alternating current with the desired frequency.

What do frequency converters serve for?

As their name implies, the principal purpose of frequency converters is to convert bulk amounts of power from one distribution standard into another. Besides this, a frequency converter is used to control the torque and speed in AC motors. For this purpose, the most often used construction for a frequency converter is the three-phase two-level voltage source inverter, which is controlled by a power semiconductor switches and PWD – pulse width modulation. Another usage is in the aerospace and airline industries. In airplanes, frequency converters may be used to provide in-air wall current so passengers could use their laptops.

How do they work?

To explain how a frequency converter works, I'm going to use this picture, which shows the operation principle of a frequency converter fed induction motor. Here you have several motor input voltages. The bold blue curve is the electrical torque- here represented as a function of rotor speed, when the motor is directly connected to a constant supply network. The other curves, which are partly blue, partly black, have other meaning: the blue portion of the curves shows the nominal load region, which results in low slip and power loses, since it's a very steep load region. A behavior like this can also be achieved if you feed the induction motor with a frequency converter and keep the ratio of the magnitude constant. The result of this will be an unchanged torque curve that will remain below the nominal speed, which is constant-flux region -1…+1 [n/nN]. You don't see the bold blue curve in the field-weakening region because here the motor voltage is kept constantly at maximum, which results in a flat torque curve.

The way a frequency converter works depends heavily on the DC circuit structure to voltage-source, direct converters and current source. When you have a voltage source converter, PWM – pulse width modulation is used to produce voltage, and when you have a current-source converter, the voltage is produced by modulating the fixed direct current. When you need low voltage applications, (which are under 1000V) you'll mainly used a frequency converter with a voltage-source topology.