Acquire maintenance skills √ : Maintenance of Tower Crane Electrical Systems and Troubleshooting methods for Common Faults

The electrical system of a tower crane (hereinafter referred to as a tower crane) is divided into five parts: power supply, traveling, slewing, hoisting and luffing. These five parts are connected by the main power supply cable, the operation linkage console and the control system, all of which can operate independently without interfering with each other. Each system is composed of an electric motor, a unit controller (including control components such as frequency converters and DC speed regulators), a series of circuit breakers and protection devices, and is controlled by a linkage console and a PLC (Programmable Logic Controller).

Key points for maintenance and upkeep

During the daily operation of tower cranes, the sudden faults that occur are mostly in the electrical system. This requires us to do a good job in the maintenance and care of the electrical system in advance to prevent electrical faults from happening.

Lightning protection

Lightning strikes can cause severe damage to the electrical system of tower cranes. The highest point of the A-type frame of the tower crane must be equipped with A lightning arrester (lightning rod), down conductor and grounding body. The bottom of the tower crane (the track of the traveling tower crane) must be grounded strictly in accordance with the regulations. When lightning occurs, the entire machine must be powered off and cannot continue to operate.

A K80 tower crane at a certain construction site was not completely powered off due to thunderstorms, which led to the burning out of the traveling frequency converter and the main unit of the moment limiter.

Rainproof

Based on experience, the period after rain is a time when electrical faults of tower cranes occur frequently. Generally, when the rain is not particularly heavy, the tower crane should be operated with power on. The motors of tower cranes are generally installed outdoors. If rainwater enters the motors, it may cause grounding trips at best and short circuits at worst, burning out the motors. Therefore, under the condition of ensuring smooth ventilation, the electrical cabinets and motors of tower cranes should take rain-proof measures, such as adding rain covers, etc.

Prevent pollution

Dust accumulation on the electrical components of tower cranes can lead to poor contact. Therefore, according to the specific conditions on site, regular dust removal should be carried out on the main electrical components such as cable joints, contactors and circuit breakers after power failure. The control cables at the lubrication points of tower cranes are prone to getting contaminated with oil. If not cleaned for a long time, the rubber on the outer surface of the cables will be corroded, exposing bare wires and causing short circuits and grounding. Therefore, after the lubrication and maintenance are completed, it is necessary to check whether there is any oil stain on the cable. If so, it should be wiped clean with cotton yarn in time.

Regular tightening

Tower cranes vibrate significantly during operation, which can easily cause bolts to loosen and wire ends to fall off. Therefore, it is necessary to regularly tighten the cable joints and connecting bolts at all parts.

A certain tower crane experienced a phenomenon where the hook slipped when lifting heavy objects, but the frequency converter did not alarm and all electrical components were working normally. After inspection, it was found that the cause was that the connecting bolts of the rotary encoder on the hoisting motor were loose, resulting in a deviation in the speed feedback.

Common Faults and Troubleshooting methods

After an electrical fault occurs, the first step is to determine which part has the problem. If several mechanisms cannot operate simultaneously, it can be judged that there is a problem with the main power supply, PLC or certain components on the interlocking console. The fault of the rotating electrical system of the large tower crane is manifested on the display panel of the frequency converter as a fault code. The most common faults of frequency converters are external faults. One end of the frequency converter is connected to the motor and resistance box, and the other end is connected to the control system PLC.

The sequence for troubleshooting should progress from simple to complex. First, check the motor and resistance box, and then inspect the PLC and related contactors and circuit breakers. Electrical faults are concealed, and the judgment and troubleshooting of faults can be carried out by working backward. The common faults and troubleshooting methods are as follows.

There is a power supply trip or sparks appear at the main power terminal of the frequency converter

Disconnect the power cord and check whether the input terminals of the frequency converter are short-circuited, as well as whether the DC side terminals P and N of the frequency converter's intermediate circuit are short-circuited. If there is a short circuit, the cause might be a damaged rectifier or a short circuit in the intermediate circuit.

There is no display when power is supplied

Disconnect the power cord and check if there is any phase loss or open circuit in the power supply. If the power supply is normal, power it on again to check whether there is voltage at the DC side terminals P and N of the inverter's intermediate circuit. If the above checks are normal, it can be determined that the internal power switch of the frequency converter is damaged.

The motor does not start if there is no output when the machine is turned on

Disconnect the output motor wire, restart the machine and observe the input frequency displayed on the inverter panel. At the same time, measure the AC output terminals. If everything is normal, the cause might be incorrect Settings of the inverter's start-up parameters or incorrect wiring of the operation terminals, or it could be that the inverter part is damaged or the motor is not correctly connected to the inverter.

The inverter stops outputting.

Excessively high voltage, excessive current, overheating and poor grounding of the motor can all cause the inverter to stop outputting.

If the voltage is too high, it is necessary to check whether the grid voltage is too high or whether the motor load is too large. If the motor load is too large and can only be shut down for a short period of time, a frequency converter with braking function should be purchased and the corresponding braking resistor should be configured.

If the current is too large, first check whether the motor load is too heavy and adjust the parameters of the frequency converter appropriately. If the adjustment doesn't work, it might be that the inverter has aged or been damaged.

The motor overheating might be due to the ambient temperature being too high, exceeding the allowable limit of the frequency converter. At this point, it is necessary to check whether the cooling fan is operating or whether the motor overheat protection device is functioning properly.

Rotation without movement

If the rotation does not move, first observe whether there is any problem with the external wiring. If the wiring is normal, then use a multimeter to measure the resistance values of the resistance box and the motor terminals to determine which part is short-circuited or grounded.