Research And Design On Control System Of Special Motor For The Electric Forklift

The requirement of environment protection promotes the development of power technology. Because of high-efficiency energy conversion, no emissions, no noise and other advantages, the electric forklift is the best choice of shifting tool in the indoor environment. The electric forklift motor is special, so general-purpose inverters can't meet the control requirement of electric forklift. The key technology of the electric forklift controller is dominated by a few foreign companies, so the research on high-performance control system for the forklift has realistic economic and social benefits.Firstly, the drive circuit, metal core printed circuit board and the power structure of the inverter are designed based on analyzing the operating characteristics of power MOSFET which is the core device of the inverter in this thesis. Parameter analysis method under the working condition of the power MOSFET and the influence of dv/dt on power MOSFET's operation are also provided. An analysis of the mechanism of mutual interference when the switches of bridge arms are acting is carried out to propose the solution of restrain interference. The solution is verified its correctness by simulation and experiment results.Secondly, a theoretical analysis of voltage vector and the switching state are separated from each other when SVPWM method has dead zone is represented in detail, so a concept of transitional switching state is defined to solve the problems created by dead-zone and combined with traditional SVPWM method to propose a SVPWM based on transitional switching state. This SVPWM method makes the switching state correspond to voltage vector accurate. The simulation results show that current-waveform output of the inverter and the performance of the induction motor operation are significantly improved by the novel SVPWM method based on transitional switching state.Thirdly, the hardware structure and control method of the electric forklift control system are introduced. In the hardware structure, a controller scheme adopted the DSP and CPLD as the core devices is used by the motor control system. The controller ensures the motor can be manipulated by the feedback which is created by detection circuit. The control strategy uses a current closed-loop control scheme for scalar controller based on slip frequency. The current closed loop solves the torque ripple in low frequency under V/F control strategy and speed closed loop improves the unsatisfactory static and dynamic performance under the V/F control strategy.Finally, the control system of electric forklift is tested on experimental platform. The experimental result validates effectiveness and feasibility of the control system, current distortion when low and large current electric forklift motor operates under the low frequency and improves the dynamic and static performance of the motor operation.