Direct drive technology is hailed by international industry as an advanced method and technology in modern drive technology, and it is increasingly being applied to various industries. The most important and crucial part of direct drive technology is the direct drive rotary motor ( DDR) and the direct drive linear motor (DDL). It is not simply about moving a rotary motor or a linear motor into a system, but about innovatively designing these two types of motors according to different systems and operating conditions.
1. Overview
Direct drive refers to the direct coupling or connection of a new type of rotary or linear motor to a driven load to achieve drive. By eliminating many intermediate components in traditional systems, such as belts, chains, wire ropes, and gearboxes, the structure is greatly simplified, resulting in the entire system having advantages such as high efficiency and low power consumption, high speed and high precision, high reliability and maintenance-free operation, high rigidity and fast response, no lubrication required, and quiet operation.
Direct drive technology is hailed by the international industrial community as an advanced method and technology in modern drive technology, and it is increasingly being applied to various industries. The most important and crucial part of direct drive technology is the direct drive rotary motor (DDR) and the direct drive linear motor (DDL). It is not simply about moving a rotary motor or a linear motor into a system, but about innovatively designing these two types of motors according to different systems and operating conditions.
This paper presents the innovative design of (DDR) and (DDL) modules, achieving direct drive and significant energy-saving effects. This paper introduces one example of DDR (DDR1) and two examples of DDL (DDL1, DDL2) from a series of achievements accomplished using this technology.
Basic Principles and Structure of Direct Drive Rotary Electric Machine (DDR1)
The basic principle and structure of the direct-drive rotary synchronous motor (DDR1) is based on permanent magnets and a specially designed disc motor. It fully utilizes the space at both ends of the outer rotor structure, fixing the stators of the two disc motors together with the stator of the outer rotor structure. The rotor discs of the two disc motors and the rotor cylinder of the outer rotor structure form a three-dimensional closed outer rotor. Within the same spatial volume, this compound structure can generate greater electromagnetic torque than motors with a single outer rotor structure or a single disc structure. The overall structure of the compound permanent magnet synchronous motor is shown in Figure 1.
As shown in Figure 1, the direct-drive compound three-dimensional permanent magnet motor is composed of three relatively independent stator and rotor groups, which can be regarded as a composite structure of a cylindrical external rotor permanent magnet motor and two disk-shaped axial flux permanent magnet motors. However, the three are not completely independent; they are a unified whole that influences and interacts with each other. For the direct-drive compound three-dimensional permanent magnet motor to rotate and output torque, the three electromagnetic systems formed by the three stator and rotor groups must be coordinated. Based on the structural conditions of the direct-drive pumping unit, the compound three-dimensional permanent magnet motor must meet the drive requirements of low speed and high torque. In the research and development of the compound three-dimensional permanent magnet motor, we solved the following key technical problems:
① Structural design issues of dual-disc motors and external rotor motors
② Stable operation issues during starting, forward and reverse rotation of the composite permanent magnet synchronous motor
③ A novel calculation method and design procedure for a composite three-dimensional permanent magnet motor. The innovation of this motor lies in:
A novel dual-structure motor was developed, maximizing the motor's power density. A new electromagnetic field distribution optimizes the motor's electromechanical energy conversion.
Basic Principles and Structure of Direct-Drive Linear Motors (DDL1, DDL2)
The DDL1 direct-drive linear motor was primarily developed for overhead conveyor systems. This type of motor must meet the following conditions within the system:
1) Flat structure, limiting volume
2) Unidirectional operation, frequent starts, running time in seconds;
3) The starting current should be less than that of a motor of the same capacity, resulting in less impact and faster response;
4) Simple structure, low cost, and light weight;
Single-phase linear induction motors come in various structures suitable for different applications. To meet the above requirements, a simple 2-pole capacitor-driven motor is needed, with only one main and one auxiliary phase coil. Due to the low system speed, the motor pole pitch is small, limiting the slot width. To accommodate the primary iron core of the coil, the slot height needs to be significantly increased, resulting in a slot height/slot width ratio larger than that of ordinary motors, known as a deep slot structure. This extra-deep slot structure also improves starting torque. The motor in this project mainly solves the following problems:
1) Adopting a deep-slot structure and layered windings to increase starting torque
2) The motor design and comprehensive optimization are completed by using the field-circuit combination method to maximize the thrust per unit volume.
3) The motor is integrally encapsulated in plastic, ensuring strong integrity and good insulation. The structure of the direct-drive linear motor (DDL1) is shown in Figure 2.
The DDL2 direct-drive linear motor is primarily developed for food slicers and elevator door operators. These systems require direct-drive linear motors with wide speed range, high thrust density, stable thrust, and fast response. However, existing linear motors often suffer from problems such as excessive size, insufficient thrust, large thrust fluctuations, slow response, and excessive temperature rise. These issues place higher demands on the design of direct-drive linear motors. The DDL2 direct-drive linear motor consists of two basic parts: a mover (primary) and a stator (secondary). The stator mainly comprises permanent magnets, magnetic conductors, and stainless steel bushings. The permanent magnets are axially magnetized. The mover consists of a three-phase armature winding, a winding frame, and a housing. The windings use a hollow disc coil form, directly wound within the frame slots. This results in high slot fill factor, no end caps, high copper utilization, low copper loss, and better energy efficiency. The topology of the cylindrical permanent magnet linear synchronous motor is shown in Figure 3.
This motor, through its slotless hollow coil structure, rational arrangement of permanent magnets, and overall optimized design, possesses the following characteristics:
1) Simple structure, easy to manufacture, high reliability, easy to maintain, material-saving, and low cost.
2) The motor speed is adjustable, with a wide speed range and good controllability.
3) The output thrust is stable, the thrust is proportional to the quadrature axis current, the linearity is high, and the controllability is good.
4) The mover has a small moment of inertia and a fast dynamic response.
5) It does not generate radial thrust, has low operating friction, and high system efficiency.
6) The motor housing integrates heat sinks, which can achieve natural cooling under normal operating conditions and operating status.
7) The integrated position detector makes it more convenient for users and greatly reduces costs.
2. Implementation and Application Results
Implementation and application effects of DDR1
Currently, the vast majority of pumping units used in oilfields are traditional beam pumping units (commonly known as "nodding donkeys," Figure 4). These pumping units are driven by ordinary rotary motors (asynchronous or synchronous motors), which, through multi-stage reduction gearboxes and connecting rods, perform reciprocating motions 6-10 times per minute. Due to the complex mechanical structure of traditional beam pumping units, and the need for multi-stage reduction gearboxes and connecting rod mechanisms for motor drive output, the efficiency of the pumping unit is very low, generally only around 30%-40% overall. Traditional beam pumping units consume a lot of energy, are difficult to maintain, and are no longer adequate for the needs of oil wells. Nationwide, the annual electricity cost of traditional pumping units is tens of billions of kWh. Therefore, developing new, highly efficient, and energy-saving oilfield pumping units is of great significance for energy conservation and consumption reduction.
To fundamentally solve the problem of low efficiency in oil pumping unit systems, a new type of high-efficiency and energy-saving motor needs to be developed. This motor should be able to output high torque at low speeds to directly drive the oil pump, thus achieving a direct-drive mode (direct-drive oil pumping unit system). Adopting a direct-drive structure eliminates the need for the multi-stage intermediate transmission mechanism of traditional oil pumping units. This not only eliminates the bulky walking beam, drive shaft, and complex gearbox, but also greatly simplifies the structure, significantly reduces the size, and, most importantly, greatly improves the overall efficiency, achieving effective energy savings.
The core of a direct-drive oil pumping unit system lies in the development of a low-speed, high-torque motor. Currently, both domestic and international low-speed, high-torque motors utilize permanent magnet synchronous motor structures. Compared to traditional electrically excited synchronous motors, permanent magnet motors eliminate the electrical excitation winding, achieving brushless operation and offering advantages such as simple structure, reliable operation, small size, light weight, low losses, and high efficiency. However, some current low-speed, high-torque motors only meet the requirements of certain applications. In most cases, the motor speed still needs to be reduced to the required output speed via a gearbox, resulting in insufficient utilization of the motor's effective area and failing to meet users' requirements for low speed and high torque. The direct-drive composite three-dimensional permanent magnet motor completely changes the structure of existing traditional motors, fully utilizing the motor's three-dimensional space. It possesses high power density output characteristics, better meeting the requirements of electromechanical energy conversion and low-speed, high-torque drive, and can directly drive the load at low speeds. This satisfies users' requirements for low speed and high torque.
The overall efficiency of the directly driven, dual-type three-dimensional permanent magnet motor pumping unit has been significantly improved. It has been applied in dozens of wells in Zhongyuan Oilfield, Dagang Oilfield, Daqing Oilfield, and North China Oilfield, and has passed production trials. Performance testing was conducted by the Zhongyuan Oilfield Branch Technical Testing Center, Dagang Oilfield Group Testing and Evaluation Center, China National Petroleum Corporation Oilfield Energy Conservation Monitoring Center, and the National Oil and Gas Field Wellhead Equipment Quality Supervision and Inspection Center. Comparative testing of the original beam pumping unit and the new dual-type three-dimensional permanent magnet motor pumping unit on oil wells showed that active power savings exceeded 50%, and reactive power savings reached over 90%. In addition to significant power savings, the directly driven dual-type three-dimensional permanent magnet motor pumping unit also reduces the weight and footprint of the pumping unit by over 50%, increases production by over 50%, reduces equipment capacity by over 50%, and reduces noise by 30 decibels. Table 1 compares the power consumption of the dual-type permanent magnet motor pumping unit and the beam pumping unit in well H2-40 of Zhongyuan Oilfield.
The benefits of this motor would be even greater if it were further applied to the retrofitting of existing oil pumping units and its use in industries such as chemical, pharmaceutical, food, and machinery.
Implementation and Application Effects of DDL
1) Implementation and Application Effects of DDL1
Postal conveyor systems typically employ rotary motors with intermediate conversion mechanisms such as sprockets and belts. This results in a complex system structure, high noise levels, and low efficiency. The entire conveyor line is driven by a single high-power motor, concentrating power and causing the entire line to move simultaneously regardless of load, leading to very low efficiency under light loads. Furthermore, it lacks flexibility in line configuration, such as adjusting the conveyor line length. Its operating efficiency is far from meeting modern conveyor requirements, and noise levels significantly exceed standards. To solve this problem, intermediate transmission components such as sprockets and belts must be eliminated to improve system efficiency and save energy. The best solution is to adopt linear direct-drive technology.
A large number of low-power linear motors are used to drive the load, employing segmented energization and distributed power distribution to ensure that power is supplied only where the load is located. This achieves high efficiency and energy saving. The Guangzhou Mail Processing Center invested heavily in a direct-drive postal overhead conveyor system (as shown in Figure 5) from the Swiss company GILGEN. The main component of this system is the direct-drive linear motor unit. A basic unit consists of a linear motor and an intelligent control card, resulting in a standardized structure and simple maintenance. 1897 units along the line propel trolleys suspended on rails forward, carrying packages to their destinations. In this technology, the direct-drive linear motor is the key component of the entire overhead conveyor system; their large number and performance and cost are important factors influencing the development of the overhead conveyor system.
Currently, the overhead conveyor systems imported from abroad are expensive, and it is impossible for my country to import such equipment in large quantities. It is essential to independently research and develop such overhead conveyor systems. Therefore, the first step is to overcome the key component of the overhead conveyor system - the direct drive linear motor. This will provide an important foundation for my country's independent research and development of overhead conveyor systems and can also greatly reduce the maintenance costs of existing imported overhead conveyor systems.
The performance of the motor developed in this project has reached, and in some aspects surpasses, that of similar foreign products. (See Table 2.)
With the increasing demands for domestic postal transport, linear motor-driven postal overhead conveyor systems have promising application prospects. Domestically developed direct-drive postal overhead conveyor systems will significantly reduce system costs and enhance product competitiveness. Each conveyor system requires several thousand such motors, and their price and performance are crucial to the system's performance. Therefore, the development of deep-groove single-phase linear induction motors is arguably one of the key technologies in conveyor system research. This motor is primarily designed for postal conveyor systems, but the system can be extended to industries such as machinery, food, pharmaceuticals, and consumer goods, offering broad application prospects. If 20 conveyor systems are produced annually, even at half the price of imported systems, the output value could exceed 200 million yuan. Furthermore, based on an energy saving of 80 kWh per day for 8 hours, the annual energy saving for 20 conveyor systems could reach 500,000 kWh.
2)(DDL2) Implementation and Application Effects
Traditional food slicers and elevator door operators use rotary motors as drive motors, which convert the rotational motion of the motor into the linear motion of the load through a conversion mechanism such as a gearbox, pulley assembly, and connecting rods. Traditional food slicers and elevator door operators suffer from drawbacks such as complex structure, high cost, high failure rate, high maintenance cost, low system efficiency, slow response, high noise, and oil contamination. To overcome these shortcomings, we use a cylindrical permanent magnet linear synchronous motor as the drive motor, eliminating the conversion mechanism. The motor is rigidly connected to the load, greatly simplifying the overall structure of this direct-drive food slicer and elevator door operator and significantly improving performance. Figure 5 shows an elevator door operator with a linear motor as the direct drive.
Currently, there are almost no domestically designed and manufactured direct-drive cylindrical permanent magnet linear motors for food slicers and elevator door operators. Copley is a major foreign manufacturer of similar products. Table 3 compares the performance of domestically produced motors with that of Copley motors.
Through comparison, the thrust constant and back EMF constant of the domestically produced motor are higher than those of Copley's products. Correspondingly, under the same operating conditions, the domestically produced motor draws less current than the Copley motor, while its power and efficiency are also higher. Especially under operating condition 3, the Copley motor's current exceeds its rated current, leading to excessive temperature rise and limiting its continuous operation to only 2 hours, while the Zhejiang University motor, with its lower current, can continue operating without restriction.
Compared to foreign motors, domestically produced motors offer superior performance and are priced at only about one-quarter of their foreign counterparts, undoubtedly giving them a stronger competitive edge in the market. Food slicing machines were already in mass production by the American company ITW in 2008.
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