According to international practice and my country's national standards for voltage level classification, voltage levels ≥10kV are called high voltage, and those between 1kV and 10kV are called medium voltage. We also customarily refer to motors with a rated voltage of 6kV or 3kV as "high voltage motors".
(1) Voltage-type frequency converters are classified according to different converter components:
1. An AC-DC-AC frequency converter first converts AC power of a fixed frequency into AC power of a fixed frequency.
The current is rectified into direct current and then inverted into three-phase alternating current with an adjustable frequency.
2. An AC-AC converter directly converts AC power with a fixed frequency into AC power with an arbitrarily adjustable frequency (the number of phases is the same before and after conversion).
(2) Classification based on the energy storage element (filtering method) of DC circuits:
1. Voltage-type frequency converters use capacitors as their energy storage element. Medium and small capacity frequency converters are mainly voltage-type frequency converters.
2. The energy storage element of a current-source inverter is an inductor coil.
(3) Voltage-type frequency converters are classified according to the voltage modulation method:
1. The voltage of a pulse width modulation (SPWM) frequency converter is achieved by adjusting the pulse duty cycle. Almost all medium and small capacity general-purpose frequency converters use this type of converter.
2. The voltage of a pulse amplitude modulation (PAM) frequency converter is controlled by adjusting the amplitude of the DC voltage.
(4) Classification based on the number of phases of the input power supply:
1. Three-phase input and three-phase output frequency converter: Both the input and output sides of the frequency converter are three-phase AC power. The vast majority of frequency converters belong to this category.
2. Single-phase input, three-phase output frequency converter: The input side of this frequency converter is single-phase AC power, and the output side is three-phase AC power. Frequency converters used in household appliances typically fall into this category and are usually small in capacity.
