An Integrated Approach To Modelling Of Electromechanical System Dynamics And Control Of Overhead Travelling Cranes And Applications To Lifting And Lowering Processes Of Payload

Electric cranes are a kind of vital material-handling equipments used in steel works, manufacturing workshops, yards, power stations, ports and railways during modern manufacturing and transportation. It is very important to quantify correctly the dynamic behaviour of electric cranes during operations, to ensure the safety, reliability and economy of electric cranes in crane design.Crane probabilistic design and fatigue design are approaches which are being actively researched. To implement them, one of the most important problems, which must be solved, is to determine the realistic stress time histories acting on the most vulnerable sections of the components of the cranes during operations. The dynamic modeling is the necessary means to get the time histories. Therefore, it is imperative to build the dynamic models which can reflect correctly the dynamic processes during crane operations.Electric cranes are integrated electro-mechanical systems. A new dynamic modeling philosophy, that deals with the coupling effects among induction motors and their drive systems, the payload, the mechanisms and the steel structrue, is proposed in this paper. In the3D dynamic model, the electromagnetic torque or the rotating speed of the electric motor needn’t be assumed anymore and the realistic dynamic responses of the components of electric cranes are determined naturally in the process of the energy exchange between the mechanical and electric systems.The idea has not been attempted in the published literature on the dynamics and control of electric cranes so far.In this paper, an electro-mechanical system dynamic modeling method of the electric cranes (driven by induction motors controlled by the control system by changing rotor resistance and frequency control system as specific examples) describing the dynamic behaviour during their operations is presented by applying the design theory of cranes, analytical mechanics, structural dynamics, mechanical vibration, motor analysis and drive control theory. In this method, the modified Lagrangians, which include the kinetic energy and potential energy of the crane, the magnetic co-energy of the driving motors and the constraint conditions to describe the operating processes of the overhead travelling cranes, are introduced. The method is not only suitable for dynamic modeling of the electric cranes, but also applicable to all other machineries driven by electric motors. As examples, the numerical simulations of dynamic processes of a32t overhead crane with a mechanical brake in the lifting mechanism are carried out in detail. These include the lifting processes of a payload at full speed and the lowering processes of a heavy payload and half a rated payload at full speed. The exceptional braking processes of the same overhead crane with two mechanical brakes in the lifting mechanism when the two brakes co-work and one of the two brakes malfunctions are also simulated elaborately. The features of these dynamic processes are analysed. The effects of the operation of crane operators, the electric motor control scheme, and the parameters of the mechanical and electric systems during normal lifting and lowering of payload on the dynamic behaviour of the crane are investigated. The effects of the magnitude of the payload, the mechanical braking schemes, the lifting speed and the position of the payload when the exceptional braking of the lifting mechanism occurs on the dynamic behaviour of the crane are discussed as well.It is found that the amplitudes of the system dynamic responses do not decrease monotonously while operators’ operations slow down; the exceptional braking which happens at lower lifting speed might be not always safer than the exceptional braking which happens at higher lifting speed for cranes themselves. It is identified that the exceptional braking of the lifting mechanism when one of two brakes fails should be considered as a significant design conditions to ensure the secure implementation of the redundancy brake function of the lifting mechanism with two-brake.