Abstract: Taking the variable frequency speed control of a bridge crane as an example, this paper analyzes the generation mechanism and hazards of harmonics from the frequency converter and proposes common methods for harmonic suppression. Keywords: Bridge crane; Frequency converter; Harmonics Due to its advantages of high efficiency, wide speed range, high speed accuracy, smooth start and stop, stepless speed regulation, and energy saving , variable frequency speed control technology is widely used in bridge cranes in China, with most ports combining it with PLC control for design, installation, commissioning, and research. I. Application of Frequency Converters in Bridge Cranes Variable frequency speed control of bridge cranes is generally used in the hoisting mechanism and the trolley lateral movement mechanism, consisting of a variable frequency motor, frequency converter, control devices, and circuit protection switches. To prevent interference from the frequency converter and improve the accuracy of motor control, AC-CDC reactors, braking units and braking resistors, filters, etc., are installed around the frequency converter. A photoelectric rotary encoder is also installed in the variable frequency motor of the hoisting mechanism to improve speed regulation accuracy and response speed. Simultaneously, the control devices combine PLC and relays to improve the reliability of the control system. II. Structure of Frequency Converters and Harmonic Generation Frequency converters can be divided into two main categories: indirect and direct frequency converters. Indirect frequency converters convert the power frequency current into DC through a rectifier, and then convert the DC into AC with a controllable frequency through an inverter. There are three different forms of indirect frequency converters: (1) using a controllable rectifier for voltage transformation and an inverter for frequency conversion, both of which need to be coordinated in the control circuit; (2) using a chopper for rectification and pulse width modulation for voltage regulation, and an inverter for frequency conversion; (3) using an uncontrolled rectifier for rectification and a PWM inverter for simultaneous frequency conversion. Only when using fully controllable devices with controllable shutdown (such as IGBTs) can the output waveform of this type of frequency converter be sinusoidal. In the inverter circuit, the output voltage of a voltage-type circuit is a rectangular wave; the output current of a current-type circuit is a rectangular wave. Rectangular waves contain more harmonics, which have an adverse effect on the load. [b][align=center]For more details, please click: Application of Frequency Converters in Bridge Cranes and Harmonic Suppression[/align][/b]