Low-speed high-torque permanent magnet direct-drive motors in ropeway applications

Traditional passenger ropeway drive systems generally adopt the drive mode of motor plus reducer. The reducer, as a power transmission mechanism, can reduce the rotation speed of the output shaft and proportionally amplify the torque of the motor to the output shaft of the reducer. Then, the driving wheel that meshes with the output shaft of the reducer transmits the power to the running rope, thus making the running speed of the ropeway meet the design requirements. However, in the use process, reducers have disadvantages such as oil leakage, vibration, overheating, and large noise, which will reduce the continuous operation ability and reliability of the equipment. Due to the mechanical efficiency loss of the reducer, the system's utilization rate of electric energy is reduced. In the maintenance work of the ropeway, the maintenance of the reducer has always been an important part. The leakage or pollution of the reducer's lubricating oil, the damage of the axle and gear parts may cause the reducer to fail to work normally, causing safety hazards. When working in a high-temperature environment, a circulating cooling system should be set up for the reducer, and when working in low-temperature areas, anti-freezing measures should also be taken for the reducer.

In recent years, direct drive systems have been adopted in international ropeway companies' products. The ropeway using direct drive technology eliminates the bulky reducer and directly connects the low-speed high-torque direct-drive motor to the driving wheel. Compared with the traditional motor plus reducer drive, the direct drive eliminates a series of disadvantages brought by the reducer, thus having many advantages.

1. Direct drive technology

The concept of direct drive was first proposed by H.Asada of the Massachusetts Institute of Technology in 1980 and was initially applied to robots. The purpose of direct drive is to directly couple or connect new rotary motors or linear motors to the driven load to achieve drive, thereby greatly simplifying the structure of the system and making it highly efficient, low-power, high-speed, high-precision, highly reliable, low-maintenance, high-stiffness, fast-response, oil-free, and quiet. Direct drive technology is considered a leading method and technology in modern drive technology by the foreign industry and is increasingly being applied in various industries.

Air conditioners, refrigerators, washing machines, and other household appliances use direct drive motors to achieve direct drive frequency conversion speed regulation. The motor's speed can be automatically adjusted according to the working conditions required by the electrical appliance, thereby improving efficiency, reducing energy consumption and noise. For example, the efficiency of a washing machine using a direct drive permanent magnet brushless motor can be increased by nearly 30%, and the efficiency of a variable frequency air conditioner using a direct drive permanent magnet brushless motor can be increased by nearly 20%.

Direct drive motors are widely used in the field of modern electric vehicles, among which permanent magnet synchronous motors have high efficiency, high control precision, high torque density, good torque stability, and low vibration and noise. Under the same quality and volume conditions, compared with other types of motor drive systems, the permanent magnet synchronous motor direct drive system can provide the maximum power output and acceleration for new energy vehicles.

In the fields of CNC machine tools, textiles, metallurgy, printing, postal machinery, packaging, automated production lines, and special equipment, high-performance servo systems are often required. Using low-speed high-torque direct-drive motors can avoid precision errors brought by intermediate transmission mechanisms, simplify the structure, save space, and meet the requirements of high efficiency, high precision, and high performance.

2. Advantages analysis of ropeway direct drive

1. High efficiency, energy-saving, and environmentally friendly

Direct drive eliminates intermediate transmission mechanisms, simplifies multi-stage conversion systems into a single direct drive system, transforms low-efficiency systems with multiple efficiency multiplications into high-efficiency systems with single efficiency, and reduces energy loss in intermediate processes. Its comprehensive efficiency is 5% higher than that of traditional ordinary motor plus reducer drive. As a kind of transportation tool that needs to run continuously for a long time, the use of direct drive can save electricity and meet the national requirements for energy conservation and emission reduction. Since it does not use lubricating oil, it reduces pollution to the environment.

2. Compact structure, less space occupation

The ropeway adopts direct drive to eliminate bulky reducers and couplings, saving a lot of space in the ropeway station, making daily maintenance more convenient, and the power of the inverter matched with the direct drive motor is reduced, making the electrical control cabinet smaller and the control room more spacious.

3. Improved control accuracy

Direct drive eliminates the transmission gap of traditional gear reducers, reduces the control error of the system, reduces the structural resonance frequency of the system, effectively controls the error of the controlled quantity, and improves the gain of the system.

4. High reliability, reduced maintenance

Since the reducer is eliminated, the troublesome maintenance work of the reducer no longer exists. The emergency driving is usually achieved by the auxiliary motor being directly meshed with the large wheel, ensuring that the ropeway can be started in an emergency. At the same time, the direct-drive motor used as the power source is usually a permanent magnet synchronous motor, which has very low maintenance.

5. Smooth operation, low noise

By adopting direct drive, the vibration of the station during low-speed operation is greatly reduced, making the station quieter. Compared with traditional driving modes, the noise in the station can be reduced by more than 15 dB. Since the power of the inverter is reduced, the control room where the electrical cabinet is located is also quieter.

3. Analysis of the feasibility of ropeway direct drive

In the field of industrial automation, low-speed high-torque motors are often required as power sources, such as high-performance CNC machine tools and injection molding machines. The ropeway, as a special device, has strict requirements for the running speed according to the "Safety Standards for Passenger Ropeways". The driving and turning stations of the ropeway have a certain height difference, so there are four working conditions: empty, heavy up empty down, heavy down empty up, and heavy up heavy down. The drive system of the ropeway must meet the speed and torque requirements under different working conditions. This requires the direct-drive motor and its drive used in the ropeway to not only provide sufficient output torque but also safely return the negative power generated during the heavy-down working condition to the grid. The permanent magnet synchronous motor can realize low-speed and large torque power output when designed with a multi-level structure. This type of motor has good torque output characteristics at low speeds, so from a theoretical point of view, low-speed high-torque permanent magnet synchronous motors are the core components of the first choice for realizing direct drive in ropeways. This article analyzes the feasibility, safety, and practicality of using permanent magnet synchronous motors as the power source for ropeway direct drive from the structure of the permanent magnet synchronous motor.

The structure of the permanent magnet synchronous motor used in the ropeway mainly includes the stator, rotor, detection components, and cooling system.

3.1 Stator

The stator of a permanent magnet synchronous motor mainly includes the stator winding and stator iron core. The stator winding is fixed in the stator slot according to a certain arrangement, and is internally connected to the corresponding terminals in the motor connection box. Apart from the conductive material, various insulating materials are used to insulate the coils from each other and from the iron core, while also providing initial fixing for the coils. The stator has a temperature sensor to prevent the motor from overheating. This type of motor stator block is easy to replace, greatly reducing the maintenance of the motor. When one or several stator blocks are damaged, the motor can still run, but the running speed does not reach the design requirements. However, for passenger ropeways, it can still safely transport passengers back to the station at low speed, providing another safety barrier except for the emergency drive motor. Currently, there is also a series-excited dual-stator permanent magnet synchronous motor that has been applied in ropeways. This type of motor can be understood as two motors connected in series. Each motor is connected to its own drive. When one motor is damaged, the other motor can still drive the ropeway to run.

3.2 Rotor

The rotor of a permanent magnet synchronous motor consists of a permanent magnet, rotor iron core, shaft, and bearing. According to the position of the permanent magnet in the rotor iron core, the rotor can be divided into surface

type and embedded type. According to the magnetic circuit structure, the surface type rotor can be further divided into protruding type and embedded type. The embedded type rotor can be divided into radial type, axial type, and hybrid type according to the relationship between the magnetization direction of the permanent magnet and the rotating direction. The rotor is supported by bearings, and the temperature of the bearings is monitored by temperature sensors. The maintenance workload of the bearings is relatively low.

3.3 Detection device

In order to improve the stability of the permanent magnet synchronous motor, a position sensor is usually used to detect the position of the motor rotor for high-performance control of the motor. The position sensor used here is usually a rotary encoder, which can be divided into magnetic encoding and optical encoding according to its working principle. According to the different output signals of the rotary encoder, they can be divided into absolute value encoder and incremental encoder.

At present, the rotary encoder that is widely used in permanent magnet synchronous motors is the rotary transformer, which is a type of encoder based on magnetic principles and is essentially a micro-motor. The rotary transformer can convert the mechanical angle into an electrical variable with a specific function. The output coil of the rotary transformer provides two-phase high-frequency alternating current voltage signals that are modulated by the position of the rotor, and the absolute position information of the rotor can be obtained through the decoding circuit. The direct drive motors used in ropeways usually use two sets of independent encoders to calculate the speed and position, and the two sets of encoders directly transmit the signals to the control system of the ropeway, reflecting the design idea of redundancy in the safety field.

3.4 Cooling system

The ropeway is a type of transportation that needs to run continuously for a long time. It is unavoidable to produce various losses during the conversion of mechanical and electrical energy in the motor, such as iron loss and mechanical loss, which eventually emit heat to the motor, causing the temperature of the motor to rise. Therefore, it is necessary to cool the motor through the cooling system to ensure that the motor can reliably operate within the normal temperature range. The cooling of permanent magnet motors can be divided into water cooling and air cooling. Water cooling can be further divided into shell cooling, end cover cooling, and axial cooling according to the structure. The direct-drive motors used in ropeways usually use air cooling, using a forced cooling system with multiple fans.

4. Control of ropeway direct drive motors

4.1 Control strategy

At present, there are two high-performance control methods for permanent magnet synchronous motors: vector control technology (also known as magnetic field orientation control technology) and direct torque control technology. The basic principle of vector control is to realize the decoupling of torque current and excitation current through coordinate transformation, so that the torque current and excitation current can be controlled separately like a DC motor, thereby achieving good static stiffness and dynamic response performance. The direct torque control technology directly controls the magnetic field and torque of the motor by controlling the voltage space vector output by the voltage type inverter. At present, most of the drives of permanent magnet synchronous motors on the market are based on vector control technology, which has become mature and can meet the control requirements of direct drive motors used in ropeways.

4.2 Introduction to the control system based on PLC and inverter

The permanent magnet synchronous motor direct drive ropeway can accurately control the motor torque and speed through PLC and inverter. The PLC reads in the set instructions and related parameters through the control panel and sends speed and torque control instructions to the inverter. When the ropeway fails, the inverter and other peripheral detection components send the fault signals back to the PLC, and the PLC processes the fault information and issues corresponding instructions.

5. Conclusion

This article summarizes the advantages of direct drive compared to traditional driving modes from multiple perspectives and introduces the structure and control of the core component of direct drive - low-speed high-torque permanent magnet synchronous motors in detail. Currently, four direct-drive ropeways have been built and put into use in China, and with the development of the ropeway industry, the application of direct drive technology in ropeways will become increasingly widespread.