In some project technical specifications, requirements such as motor coding, thermal overload protection, and other electrical protections are specified. According to Section 7.04 "Electrical Protection" of GB/T 3811-2008, this article will explore the 13 electrical protection requirements for cranes. Protection of electric motors 01: The electric motor should have one or more of the following protection functions: instantaneous or inverse time overcurrent protection, with the instantaneous operating current setting value being approximately 1.25 times the maximum starting current of the motor; a thermal sensing element is installed inside the motor; and thermal overload protection.
The specific selection should be determined based on the motor and its control method. In actual projects, the accompanying ordering is also related to the requirements of the technical specifications.
Line protection 02
All external lines should have overcurrent protection functions caused by short circuits or grounding. In the event of a short circuit or grounding in the line, the instantaneous protection device should be able to disconnect the line.
For control or auxiliary circuits with smaller wire cross-sections and longer external lines, when the expected grounding current does not reach the instantaneous tripping current value, a thermal tripping function should be added to ensure that the wire insulation is not burned due to grounding.
Phase misalignment and phase loss protection 03
When phase dislocation and phase loss pose a hazard, protection against these issues should be implemented. Note: Phase loss refers to the loss of one or two phases in a three-phase power supply (due to line disconnection, fuse failure, etc.), leaving the equipment with only two-phase power supply; phase dislocation refers to the reversal of the phase sequence of three-phase electricity (for example, connecting A-B-C instead of A-C-B), which can cause the motor to rotate in the opposite direction.
Zero position protection 04
Each transmission mechanism of the crane should be equipped with zero position protection. If the crane stops running due to a fault or loss of voltage during operation, the mechanism should not automatically operate when the power supply is restored. The controller must be manually reset to the zero position before the mechanism can be restarted. Voltage loss protection 05
When the power supply to the crane is interrupted, all electrical equipment related to safety or unsuitable for automatic startup should be powered off to prevent automatic operation of the equipment after power restoration. Protection against abnormal power loss of the motor stator
06
The motor of the hoisting mechanism should be equipped with a stator abnormal power loss protection function. When the speed regulation device or the forward and reverse contactors fail, causing the motor to lose control, the brake should be immediately applied. Overspeed protection 07
Overspeed switches should be installed for important lifting mechanisms and unbalanced luffing mechanisms, where overspeed can cause danger. The setting value of the overspeed switch depends on the performance of the control system and the rated descent speed, and is usually 1.25 to 1.4 times the rated descent speed.
