The need for crane modernization poses the following questions to the maintenance personnel:
- should we use frequency converters or thyristor/contactor systems?
- should we implement a new short-circuit motor or short out the currently used rotor circuit?
- how should we handle mechanic and electric mounting/dismantling?
- who should be addressed for development of a Technical Specification for crane reconstruction?
- who can implement all the works from dismantling the old equipment to crane reregistration?
These questions can be answered if you look at the table Comparative analysis and selection of crane systems. However, in case of crane modernization, any change to the control system brings about significant mechanical alterations.
A motor replacement requires a Technical Specification for the reconstruction works coordinated with the producer or an authorized organization, installation of new motor supports or even replacement of the intermediary couplings. The process will incorporate a revision of the current and laying of new cable ways. The lifting motor will need to be equipped with a feedback sensor and an encoder.
Why does the frequency converter require an encoder for the lifting motor?
Based on our experience, we recommend reconstructing older cranes using thyristor control systems. In the absolute majority of cases the capabilities and advantages of frequency converters are not required or used. The training expenses for crane operators and the complexity of frequency converter maintenance often become a headache for the maintenance personnel. Frequency converter installations in place of the obsolete control systems may raise the following demands for the new drive:
- increased velocity controlling capability (over 1:20)
- crane positioning over the workshop objects and load sway prevention
- firmly established electric synchronization of several mechanisms
- round-the-clock, uninterrupted crane operation and heavy-duty operation conditions
- motor capacity over 11-15 кW
If the modernized crane does not have to fulfill the above-mentioned requirements, it will be most rational to use TEDF thyristor control panels.
Modernization using TEDF thyristor control systems allows to:
- renovate the crane within a very short period of time (7-20 days)
- avoid the necessity to introduce mechanic changes and develop a Technical Specification
- retain the currently used motor, motor resistance configuration, brake blocks, reducing units, cable passages, if they are operational
- maintenance costs for thyristor cranes are higher than for the frequency-based ones, but in combination with the modernization costs it will take 8-10 years for them to reach the costs incurred by a similar frequency-based crane.
- менять двигатели на новые короткозамкнутые или закоротить ротор имеющихся?
The motors working with frequency converters are different from the generally used industrial and crane motors. The primary difference is related to the class of winding insulation and bearings. In the frequency-regulated motors the winding insulation is of temperature class F (and higher) with increased insulation durability against “phase-to-phase” and “phase-to-ground” voltage peaks.
Apart for the insulation materials, the motors using frequency converters are equipped with insulated bearing units to break the sneak currents within the “rotor-stator” circuit that produce corrosion on the bearing balls and rollers, on the rolling bearing rings, etc.
The operation period of a simple wound rotor engine is significantly reduced when working with a frequency converter due to the increased wear-out of winding insulation, the passage of sneak currents through the bearing units that lead to destruction and jamming. This bigger the motor capacity, the more obvious is the mentioned effect. The influence is significant for engine capacities of 15 kW and higher.
- how should we handle mechanic and electric mounting/dismantling?
- who should be addressed for development of a Technical Specification for crane reconstruction?
- who can implement all the works from dismantling the old equipment to crane reregistration?
Currently Dvesta performs the entire scope of works related to modernization and reconstruction of cranes. More than 40 cranes have already been redesigned and equipped with TEDF and TEDK thyristor control systems exclusively by our employees.
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