After a close inspection of the crane working cycle it can be concluded that often half of the working cyclogram of a weight-lifting mechanism is performed without any loads. This constitutes a significant period of time, which can be reduced and also help increase the crane productivity and operating effectiveness.
When the crane hook is lifted, it is not necessary to generate significant dynamic moment for the motor shaft and there is no danger of load fall, this is why the engine can conveniently operate within the weakness field generated by it. The maximum velocity in this case is limited by the mechanic characteristics of the equipment used. When the operator is not required to control the state of the load suspended and no particular movement accuracy is needed, an accelerated (above the nominal value) hook rise would significantly reduce the overall crane operation cycle. An empty hook can be lifted above the nominal speed, thus allowing diminishing the crane cycle time by 20%.
The operating principle of the rapid load hook rise lies in detection of the actual engine load. In case the system detects that the motor operates in the idle mode, there is an opportunity to increase the output frequency of the frequency converter. However, it is important to remember that the operator is used to particular accelerations and slowdowns, here is why it is important to further decrease the acceleration and braking characteristics while working in this mode, or provide an additional pedal for the operator.
When the engine operates within the weakness zone, the shaft dynamic moment falls down, and that is the reason why this principle of operation can be used only when the load hook is empty. Nevertheless, even such a restricted applicability results in significant crane productivity improvements, especially for high-altitude cranes.
