I. Introduction toŻuraw mostowyElectrical Control and Maintenance Importance
In the current field of lifting and hoisting, the bridge cranes used mostly adopt contact-controlled AC contactors for electrical control. Although this control method appears relatively old-fashioned in the current industrial electrical automation control field, due to its strong adaptability to harsh environments, the excellent working performance of wound rotor asynchronous motors in the lifting field, and cost considerations, it still has a wide range of application prospects.
The contact-controlled AC contactors, affected by their inherent mechanical life, the erosion of harsh working environments, and high operating frequencies, have a relatively high failure rate. Therefore, routine maintenance and repair work are of great importance. Especially in heavy industrial enterprises, where the operation rate requirements of cranes are high, maintenance personnel must strengthen routine maintenance to improve the equipment operation rate. On the other hand, when the equipment fails online, the maintenance time must be shortened as much as possible to ensure the normal production rhythm. All of these require maintenance personnel to possess strong professional skills.
For the professional skills of electrical maintenance personnel, a solid foundation of electrical basic theoretical knowledge is essential. However, for online emergency repairs, the experience and techniques accumulated by maintenance personnel in long-term maintenance practice work are even more crucial. During emergency repairs, it is required that maintenance personnel can quickly judge the fault within the shortest time. The length of the fault judgment time depends on, on one hand, the familiarity of maintenance personnel with equipment performance, and on the other hand, the adoption of certain methods and procedures by maintenance personnel during maintenance.
Based on long-term practical experience in crane electrical maintenance, several relatively fast and effective maintenance methods and techniques are summarized for reference.
The sensory method involves using human sensory organs such as eyes, ears, nose, and hands to perceive the fault condition, obtain fault information, and make a preliminary judgment of the fault. For example, when a certain fault occurs, after understanding the fault phenomenon from the operator, we can observe with our eyes to check if there is an obvious fault point and take corresponding measures. We can also listen to the fault sound with our ears to determine whether it is a mechanical sound or an electrical fault sound in the motor and then take appropriate actions. We can smell with our nose to judge if there is a component burning. We can also feel the temperature of components such as motors with our hands to help us make a correct judgment of the fault. This method requires maintenance personnel to have a certain amount of experience.
The eyes can detect visual cues such as sparks, smoke, loose wires, or damaged components. If there is a visible burn mark on a contactor or a wire is visibly frayed, it can provide an immediate indication of a potential problem area. The ears can pick up abnormal sounds. A grinding noise might suggest a mechanical issue with a bearing or a gear, while a buzzing or humming sound could indicate an electrical problem like a loose connection or a failing capacitor. The nose can sense the smell of burning insulation or overheated components. This is especially useful in detecting if a motor winding or a control circuit component has overheated and started to burn. The sense of touch can help in determining if a component is abnormally hot. For example, if a motor casing is too hot to touch, it could mean that the motor is overloaded or there is a problem with its cooling system.
The emergency method is adopted when we still cannot quickly eliminate the fault after using the above sensory method, and the production line does not allow the equipment to stop for a long time. In this case, we need to take emergency measures to get the equipment running as soon as possible. When taking this method, maintenance personnel are required to have rich maintenance experience and solid electrical theoretical knowledge. Because the premise of taking emergency measures is to ensure safety. Without solid electrical knowledge and rich experience, it is difficult to ensure safety. Without the guarantee of safety, everything else is out of the question! The emergency method is only a temporary measure taken in an emergency situation and cannot be used for a long time. Therefore, after the emergency situation disappears, we still need to carefully check and eliminate the fault.
For example, if a certain control circuit fails and causes a crane function to be unavailable, but the production line cannot be stopped immediately. Maintenance personnel with sufficient experience may bypass a non-critical safety interlock (while ensuring that the overall safety of the operation is not severely compromised) to allow the crane to continue basic operations temporarily. However, this is a very risky move and must be done with extreme caution and only by those who fully understand the consequences and have a backup plan to address any potential issues.
IV. Other Fault Diagnosis and Repair Methods
Test Run Method The test run method is to conduct a power-on test run under the premise that it will not cause the fault to expand. By repeatedly observing the operation of the action mechanism, we can detect abnormalities, eliminate faults, or help further locate faults. For example, when a crane’s lifting mechanism does not work properly, we can carefully start the test run, observing whether the motor rotates, whether the gears engage smoothly, and whether the lifting speed is normal. Any abnormal behavior during the test run can provide clues for further fault diagnosis.
Voltage Method This is a direct, effective, and quick method. When an action mechanism that should act does not act, we can use a multimeter to measure the voltage of each component in the coil and the coil branch circuit starting from the two ends of the coil, and finally find the fault point and eliminate the fault. For instance, if a contactor coil fails to energize, we can measure the voltage across the coil terminals. If the measured voltage is significantly lower than the rated voltage, it may indicate a problem in the power supply circuit, such as a loose connection or a faulty transformer. If the voltage is normal but the coil still does not activate, then the coil itself may be damaged.
Short-Circuit Method When we find that an action element that should act does not act and this element is in a multi-loop circuit, and we cannot obtain an accurate conclusion using a multimeter, we can short-circuit the interlocking control elements in the coil branch circuit of this element and eliminate the fault points one by one. For example, in a complex control circuit with multiple relays and contactors, if one of the contactors does not work as expected, we can short-circuit the related interlocking contacts (while being extremely careful not to cause a short circuit that could damage other components or create a safety hazard) to determine if the problem lies in the interlocking circuit or the contactor itself.
Grounding Method When suspecting that there is a wire break in a long branch circuit and looking for the break point of the circuit, in the case of power-off, we can ground one end of the circuit and measure whether the other end of the circuit is also grounded level by level, so as to obtain an accurate conclusion. This method is useful when dealing with hidden wiring faults. For example, if a crane’s control wire is buried in a conduit and a break is suspected, grounding one end and testing along the wire path can help identify the location of the break.
In conclusion, the maintenance of bridge crane electrical systems requires a combination of theoretical knowledge and practical experience. The various methods mentioned above, including the sensory method, emergency method, test run method, voltage method, short-circuit method, and grounding method, can be used flexibly according to different fault situations. Maintenance personnel need to continuously improve their skills and judgment ability to ensure the safe and efficient operation of bridge cranes.
