Plot of mechanical characteristics
The rigidity is positive if the speed increases with an increase in the load. An example is an unstable part of the mechanical characteristics of the asynchronous engine. In order for the electric drive at normal static loads to develop the required operating speed, and with mechanical overloads, it ensures the limitation of the moment, a special form of mechanical characteristics is necessary.
Within the limits of the load from zero to normal workload (for example, when lifting a loaded bucket), it is advisable to have a stable operating speed of the mechanism, t. e. the corresponding section of the mechanical characteristics should have high rigidity. In the overload zone, it is desirable to decrease in speed up to zero at a load equal to a locking value.
The mechanical characteristic corresponding to the specified requirements is called excavator. In static mode, this characteristic ensures the limitation of the moment during overload. For the drives of all digging mechanisms (lifting and pressure of shovels, dragging traction), when designing and commissioning, relatively low cut -off coefficients are selected.
The more difficult the conditions for limiting the moment during stopping, the less the required filling of the excavator characteristics. So, for the mechanisms of lifting the shovels, cats = 0.7-0.8 are taken, and for the pressure mechanism of cats = 0.6-0.7, and smaller values correspond to the career excavators.
These requirements arise from the features of the excavator during digging: the driver should feel a noticeable decrease in speed an increase in the loading of the mechanism and prevent his possible stopping in time. With a high filling of the characteristics, the transition to a rapid decrease in speed even during overloads in normal digging mode is unexpected for the driver, and the probability of stopping increases. Too frequent stops increase the duration of digging, reduce the performance of the excavator and increase the heating of electric machines.
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