Mechanical properties and testing for squirrel cage asynchronous magnetic coupler

Abstract: In order to effectively solve the problem of the motor's power margin too adequate to ensure the stable operation of the equipment which widely exists in the current mechanical transmission system, a kind of squirrel cage asynchronous magnetic coupler (SCAMC) is proposed based on the principle of three-phase asynchronous motor. The SCAMC has the advantages of the simplicity of mechanical structure, high reliability in successive operation, simple maintenance, weak skin effect of the squirrel cage rotor and adjustable slip ratio and so on. According to the specific characteristics of the SCAMC structure, the mathematical model of air gap magnetic flux density is established by using the scalar magnetic potential method and two-dimensional field boundary conditions. The air gap magnetic flux density generated by the permanent magnet in SCAMC can be decomposed into radial component and tangential component, and it is only the former one that cuts the cage bar and generates induction current and electromagnetic torque. As a consequence, the effect of tangential air gap flux density can be ignored when studying the operating characteristics of SCAMC; only the radial air gap magnetic flux density needs analysis. The time variable is introduced into the expression of radial air gap flux density, and thus the expression of induced current changing over time is deduced and the transformation rule of induced current with space phase and electrical angle is revealed. Based on the current's superposition characteristics, the current of the cage bar is converted onto the rotor surface and the Lorentz force produced by induced current is integrated along the circumference, and thus the electromagnetic torque model is established. On the base of the above theoretical and technological foundation, a 37 kW SCAMC experimental prototype is designed and produced. After installing the prototype on a 37 kW, 6-pole drive motor, the theoretical calculation, simulation verification and experiment measurement on its mechanical behavior are performed. The results show that the simulation value, experimental value and theoretical value of the electromagnetic torque and the slip ratio of SCAMC are all consistent, and the prototype possesses fine overload characteristics. On condition of identical slip ratio, the error rate between simulation and experimental data and theoretical value is less than 5%. In the process of the load torque increasing from 0 to the rated torque of drive motor (366 N·m), the change of slip ratio of SCAMC is 0-4.8%, which can meet the need of a relative fixed output speed during common machinery producing. However, under the rated load, the maximum slip ratio of the 37 kW, 6-pole drive motor is 2%, while the ratio of SCAMC is 4.8%, about 2.5 times that of the former. That is to say, the mechanical characteristic of SCAMC is softer than the asynchronous motor, which means its impact on the motor can be released when the load is changed. When loaded to the maximum experimental torque, which is corresponding to the drive motor overloading by 10%, the slip ratio of SCAMA is 5.7%, which is increased by 9% compared with that under the rated load, but the operation still falls in the linear area, proving the strong overload capability of SCAMC. The simulation and experimental results verify the validity of the model established in this paper, and provide the theoretical and technological reference for the use of magnetic coupler in machineries with large inertia, difficult starting and frequent overload.