The output of the frequency converter is a PWM wave, which contains a large amount of higher-order harmonics. The variable frequency power sensor conducts AC sampling of the input voltage and current signals, and then inputs the sampling values through transmission systems such as cables and optical fibers with a digital input variable frequency power analyzer. The digital input variable frequency power analyzer calculates the sampling values of voltage and current, and can obtain voltage effective value, current effective value, fundamental voltage, fundamental current, harmonic voltage, harmonic current, active power, fundamental power Harmonic power and other parameters.

Frequency conversion refers to changing the frequency. The equipment used for frequency conversion is called a frequency converter, while Siemens calls it a vector controller. The initial frequency converter only changed the frequency conversion, but now the frequency converter can modulate voltage, current, and frequency again, which is vector control. A power control device that utilizes the on-off effect of power semiconductor devices to convert a power frequency power supply to another frequency.

The frequency converters currently used mainly adopt the AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). Firstly, the power frequency AC power is converted into DC power through a rectifier, and then the DC power is converted into AC power that can be controlled by frequency and voltage to supply the motor. The circuit of a frequency converter generally consists of four parts: rectification, intermediate DC link, inverter, and control. The rectifier part is a three-phase bridge uncontrollable rectifier, while the inverter part is an IGBT three-phase bridge inverter with a PWM waveform output. The intermediate DC link is filtering, DC energy storage, and buffering of reactive power. To add, before variable frequency technology, motors used direct current speed regulation, which means changing the DC voltage. DC motors have brushes, complex maintenance, and electric sparks (which cannot be used in hazardous situations), which are basically eliminated. Relatively speaking, DC speed regulation is more stable, so it is still used. Figure 1 shows the voltage and current waveforms of the frequency converter input and output captured by a variable frequency power analyzer.