[align=center]Discussion on Certain Questions in the Application of Non-Gear Permanent Magnetism Synchronous Tractor Zhang Jiasheng, Xinjiang Uygur Autonomous Region Special Equipment Inspection and Research Institute[/align] Abstract : This article analyzes and explains key issues of concern in the application of gearless permanent magnet synchronous tractors, such as tractor safety, permanent magnet demagnetization, drive and control devices, and elaborates on the actual application situation. Keywords : gearless, permanent magnet synchronization, tractor, permanent magnet demagnetization, energy-saving braking. 1. Introduction Gearless permanent magnet synchronous traction machines, characterized by low speed and high torque, are increasingly attracting widespread attention in the elevator industry due to their advantages such as energy saving, small size, smooth low-speed operation, low noise, and maintenance-free operation. China possesses abundant rare earth mineral resources and is a major producer of permanent magnet materials globally, giving it a unique advantage in the development and production of permanent magnets. In recent years, many elevator manufacturers have undertaken the development and research of gearless permanent magnet synchronous traction machines. The WYT series gearless permanent magnet synchronous traction machine was the first to pass the inspection of the China Elevator Quality Supervision and Inspection Center in July 2001. It is now in mass production, and the traction machine products have been adopted by more than 50 elevator manufacturers in China, with some products exported to countries such as Japan, South Korea, Malaysia, and Saudi Arabia. The WYT series products are applicable to various elevators with speeds ranging from 0.5 to 40 m/s and load capacities from 450 to 2000 kg, suitable for applications with machine rooms, small machine rooms, and machine room-less elevators. Although elevators using gearless synchronous traction machine technology currently account for only a small portion (approximately 10%) of the Chinese elevator market, its development momentum is extremely rapid. The competition in the elevator market and technological advancements will inevitably further promote the development of gearless permanent magnet synchronous traction machine technology and its applications. This paper analyzes and discusses the structural design features and main performance indicators of the WYT series gearless permanent magnet synchronous traction machine, focusing on key issues of concern in its application, such as traction machine safety, permanent magnet demagnetization, drive and control devices, and summarizes practical application conditions. 2. Structural Design and Performance Characteristics of Gearless Permanent Magnet Synchronous Traction Machine The structural forms of gearless permanent magnet synchronous traction machines can be divided into radial magnetic field structures and axial magnetic field structures. Radial magnetic field structures, depending on the relative positions of the stator and rotor, can be further divided into inner rotor structures and outer rotor structures. Axial magnetic field structures are also known as disc (or dish) structures. Different structural forms of traction machines have different applications and magnetic field distribution patterns. Internal rotor structures have high load-bearing capacity and are suitable for high-load, high-speed elevators, generally used in high-rise residential and office buildings. External rotor structures have relatively smaller axial dimensions and can be used in small machine room or machine room-less elevator applications, but their load-bearing capacity is limited. Disc-type traction machines have even smaller axial dimensions and can be directly installed in the elevator shaft, making them most suitable for machine room-less elevators. Regardless of the structural form of the permanent magnet synchronous traction machine, it must meet the mechanical strength and stiffness requirements corresponding to its load-bearing capacity to ensure safety and reliability in actual use. Specific calculations and analyses are performed on the mechanical stress of the shaft and the overall mechanical strength and stiffness of the gearless permanent magnet synchronous traction machine, thus ensuring sufficient mechanical strength and stiffness in the structural design. 3. Safety and Reliability of Gearless Permanent Magnet Synchronous Traction Machine (1) Problem of Permanent Magnet Demagnetization (Irreversible Demagnetization) The permanent magnet material used in the permanent magnet synchronous traction machine is high-performance neodymium iron boron permanent magnet material. For neodymium iron boron permanent magnet material to undergo irreversible demagnetization, the following two conditions must be met simultaneously: ① The permanent magnet material is at a certain high temperature. At this temperature, the demagnetization curve of the material changes from a straight line at room temperature to a curve, that is, the demagnetization curve bends. The point at which the demagnetization curve begins to bend is called the inflection point, generally represented by bk (see Figure 1). ② The permanent magnet is subjected to a large demagnetizing magnetomotive force at this temperature, causing the permanent magnet operating point to be lower than the inflection point bk. At this time, when the applied magnetomotive force is removed, the permanent magnet operating point will move along the recovery line parallel to the demagnetization curve, and the permanent magnet undergoes irreversible demagnetization. [align=center]Figure 1 Demagnetization Curve and Operating Point of Permanent Magnet[/align] If the demagnetization curve of a permanent magnet is a straight line at the operating temperature, then no matter how large the demagnetizing magnetomotive force is applied, the permanent magnet will not undergo irreversible demagnetization. Similarly, if the demagnetization curve of the permanent magnet is curved, and the applied demagnetizing potential is small, the operating point of the permanent magnet will be above the inflection point of the demagnetization curve, and irreversible demagnetization will not occur. To ensure that the permanent magnet synchronous traction machine does not lose magnetism (irreversible demagnetization) during actual operation, on the one hand, when considering the selection of permanent magnet size and winding turns in the design, it is necessary to ensure that the permanent magnet operating point has a high value (much greater than the inflection point) even under the worst operating conditions. The selected permanent magnet material has good high-temperature resistance characteristics, and its demagnetization curve is always a straight line under normal operating conditions; on the other hand, a thermistor is embedded in the winding of the permanent magnet synchronous traction machine. When the temperature reaches 100°C, the traction machine can be stopped by monitoring the thermistor. The above measures ensure that the permanent magnets of the permanent magnet synchronous traction machine will not lose magnetism during elevator operation (including the worst case). (2) Self-generating energy consumption braking When the traction machine is de-energized, the control system can short-circuit the stator winding of the traction machine. If the brake of the traction machine fails at this time (failed to deactivate after power failure), the elevator will experience free "running". Due to the presence of the permanent magnets, a generating braking current will be induced in the stator winding of the traction machine. The braking torque generated by this braking current will prevent the "running" movement of the traction machine. The faster the "running" speed, the greater the braking current and the greater the braking torque generated. Under the action of the generating braking torque, the "running" speed of the traction machine will be limited to a small speed to maintain uniform operation and will not cause danger. This is safer than the traditional geared traction machine. 4. The WYT series gearless permanent magnet synchronous traction machine drive unit possesses excellent low-speed, high-torque characteristics, effectively meeting elevator traction requirements. This performance relies on the good performance and compatibility of the matching drive unit. In practical applications, the following five drive unit models have been used, all achieving good operating results: CT Unidrive-LFT inverter (UK); KEB F4 inverter (Germany); Fuji VG7-S inverter (Japan); Yaskawa 676GL5-IP inverter (Japan); and CEVI inverter (Italy). Due to the low speed of the gearless traction machine (rated speed of 96 r/min for a 1 m/s traction machine), to ensure the accuracy of speed feedback, the encoder used for speed detection must have high precision, generally at least 4096 P/N, and preferably an 8K-10K P/N encoder or a sine or cosine encoder with equivalent resolution. This is crucial for achieving good speed regulation performance of the gearless traction machine. Because gearless traction machines lack reverse self-locking force, they are prone to slippage during startup, negatively impacting elevator start-up comfort. A load detection device is needed for compensation control, ensuring the traction machine outputs torque corresponding to the current load before the brake opens. This prevents slippage after brake release, guaranteeing a smooth start-up. 5. Field Application of Gearless Permanent Magnet Synchronous Traction Machines Currently, nearly a thousand elevators using the WYT series gearless permanent magnet synchronous traction machines are in operation, with speeds ranging from 0.5-4.0 m/s and load capacities from 450-2000 kg. Applications include residential buildings, star-rated hotels, train stations, and office buildings, encompassing both machine-room and machine-room-less elevators. Common features include: Reduced noise. Due to the very low motor speed and the absence of gear transmission noise, coupled with the use of high-precision bearings, the noise in the elevator machine room is significantly lower than that of traditional traction machine elevators. Less elevator vibration. Reduced motor torque pulsation results in very smooth traction machine operation. Furthermore, without the torsional vibration caused by gear rotation, the elevator runs much more smoothly than before. The energy-saving effect is significant. Without the losses from gear rotation, coupled with the high efficiency of the permanent magnet motor, energy savings are approximately 40% compared to traditional geared elevators. 6. Conclusion Although the application of gearless permanent magnet synchronous traction machines is still in its early stages, we have every reason to believe that in the next 3-5 years, with increased production, decreased costs, and improved technology, gearless permanent magnet synchronous traction machines will become the mainstream product for elevator traction devices, eventually replacing most traditional geared traction machines. Its application prospects are very broad. About the Author: Zhang Jiasheng works at the Xinjiang Uygur Autonomous Region Special Equipment Inspection and Research Institute, Director of the Elevator Inspection Department, Senior Engineer. Contact Address: No. 9 Changjiang Road, Urumqi, Xinjiang, Postcode: 830000 Contact Number: 0991-5826632 13579231258