Keywords : Crane frequency converter speed control circuit design. The crane system in the coal mine workshop mostly adopts double beam AC bridge type 20/5T or 15/3T cranes. The crane is mainly composed of main hook, auxiliary hook, trolley and trolley. The trolley is driven by two motors of the same specifications, while the main hook, auxiliary hook and trolley are each driven by one motor. The main hook is controlled by a master controller and a magnetic control panel, with six gears for lifting and lowering. The auxiliary hook, trolley and trolley are controlled by a cam controller, with five gears for forward and reverse movement. The power supply adopts a three-phase three-wire system with a voltage of 380V. The five motors are wound rotor asynchronous motors. The frequency conversion mechanism is a pole-by-pole short-circuiting rotor resistance method. The control structure is mostly relay type. The braking form of the main hook, auxiliary hook, trolley and trolley is mostly electromagnetic holding brake, and there is also hydraulic holding brake. 1 Relay control structure, series resistor speed regulation method and mine environment affect the performance of the crane (1) Soft mechanical characteristics. Direct start or emergency braking with electromagnetic brake will cause vibration and unstable operation. (2) The equipment is complex, has many control contacts, has a high failure rate, and the operation is unchanging. The speed regulation range is relatively small and the speed regulation performance is poor. (3) Cranes are mostly used for the installation and commissioning of maintenance equipment. The lifting parts move and position frequently, the switching current is large, the series resistor heats up for a long time, the power consumption is large and the efficiency is low. (4) The working environment in the mine is poor. Coal dust and coal ash cause great damage to the slip rings, brushes and contactors of the motor, which can easily cause accidents such as contact burn-out, brush sparking, resistor burn-out and breakage. 2 Modification of cranes using frequency converter speed regulation control system [b][align=center]For details, please click: Analysis of frequency converter speed regulation modification of double beam AC bridge crane[/align][/b]