Abstract: Generally, wound-rotor asynchronous motors are started by using a series liquid resistance starter to achieve reduced voltage starting, which can effectively limit the starting current and reduce the impact of mechanical torque during motor start-up.
This article starts with the starting technology requirements of high-power vertical wound-rotor asynchronous motors. Through the structural design of high-power liquid resistance starter products, it aims to greatly reduce mechanical torque impact and starting current during startup, while also meeting the starting requirements of high-power wound-rotor asynchronous motors and ensuring long-term continuous and reliable operation.
Keywords: wound-rotor motor, ultra-high power, transmission mechanism, continuous start, different speeds
With the development of the social economy and the increasing size of production equipment, the capacity of electric motors in industrial equipment such as ball mills, crushers, fans, and pumps is getting larger and larger. Wound-rotor AC asynchronous motors, with their high starting torque, are widely used in high-power transmission equipment; however, their inherent characteristics have revealed starting problems.
◆The starting current of a motor is 5 to 7 times the rated current, which causes great damage to the power supply equipment, the motor and the mechanical equipment it drives;
◆During startup, the excessive starting current generates an excessive voltage drop, which seriously threatens the safe operation of nearby electrical equipment.
1. History and Development of Liquid Resistance Starters
To address the difficulty of starting large and medium-sized wound-rotor induction motors, in the 1930s, developed countries first developed the use of liquid resistance to reduce mechanical torque impact in medium-voltage wound-rotor motors. Its excellent starting performance completely replaced other starting methods. However, due to limitations in its application environment and the rapid development of electronic technology, the field of motor starting abroad was completely dominated by solid-state soft starters or frequency converters, while the real development of liquid resistance starters occurred in China. As early as the late 1970s, early liquid resistance starters were produced by imitating Japanese technology. Compared with foreign products, our manufacturing costs were lower, the devices were simpler, and maintenance was easier; their performance and price were very suitable for my country's national conditions. In the 1990s, this technology was widely adopted for starting large and medium-sized wound-rotor induction motors. Analysis of motor starting performance shows that a good starting process should have minimal impact on the power grid, short starting time, small inrush current impulse, small mechanical impact force, low noise, and small motor temperature rise.
2. Overview of Technical Requirements for High-Power Liquid Resistance Starters
To improve the starting performance of large and medium-sized wound-rotor asynchronous motors, liquid resistance starters are typically used. Their working principle involves inserting a liquid resistor in series in the motor rotor circuit. This reduces both the starting current and the mechanical torque impact. The motor drives a transmission mechanism to automatically and steplessly adjust the resistance value, gradually decreasing it until it reaches zero, thus achieving a smooth, shock-free start. This product overcomes many shortcomings of traditional starters, such as high starting current, inconvenient operation, and inability to start continuously, completely solving the problem of difficult starting of large and medium-sized motors.
When the starting requirements of high-power vertical wound-rotor induction motors are particularly demanding, we need to design special solutions to meet the diverse needs of users. The following are the specific technical requirements of a particular user.
2.1 Motor parameters: Rated power: Pe=8500KW (wound-rotor vertical structure); Rated voltage: Ue=6000V; Rated current: Ie=985A; Secondary voltage: U2e=2498V; Secondary current: I2e=2062A; Rated speed: ne=490rpm;
2.2 The motor needs to be started continuously, with a starting frequency of ≥40 times per day;
2.3 The motor start-up time is 8 to 20 minutes, and the speed is adjusted in a fixed cycle; the start-up time can be adjusted according to requirements.
Based on the technical requirements, the company's R&D personnel thoroughly studied the advantages and disadvantages of various starting methods, the limitations on motor starting temperature rise and the number of starts, and absorbed the essence of modern liquid resistance technology to develop a new generation of liquid resistance starters. These starters are then used in series in the rotor circuit of high-power motors to achieve smooth, shock-free starting of the motor, reducing the mechanical impact of the starting torque on the unit, greatly improving the mechanical characteristics of the controlled motor, and enabling multiple start-stop operations. The starters have successfully passed user acceptance and resolved other special technical requirements.
3. Structural Design of High-Power Liquid Resistance Starter
3.1 Technical Requirements Analysis
Analysis of the above-mentioned special technical requirements reveals that, firstly, the product structure must have a sufficiently large electrolyte tank to meet the requirement of starting up no less than 40 times per day; secondly, the structure of the electrolyte tank must not only accommodate the installation of electrode (moving and fixed plates) assemblies with series resistors, but also facilitate heat exchange during internal and external circulation processes; thirdly, a large frame is required to accommodate the installation of all parts; and fourthly, one or more transmission mechanisms are needed to meet the above-mentioned time requirements and accommodate different speeds.
3.2 Product Structure Scheme
Based on the aforementioned technical and functional requirements, the structure of a liquid resistance starter for high-power wound-rotor motors needs to consist of five main parts: an electrolyte tank assembly, a support frame assembly, an electrode assembly, a transmission mechanism, and a circulation system. The electrolyte tank assembly is located at the bottom of the product structure, the support frame assembly is above it, the transmission mechanism is at the top, the electrode assembly is located inside the electrolyte tank assembly, and the circulation system is located at the rear right side of the electrolyte tank.
The electrolyte tank assembly of this product includes an external welded steel electrolyte tank and an internally insulated electrolyte container. An electrode assembly is placed inside the insulated electrolyte container, and an electrode guiding mechanism is provided. A lifting frame for raising the moving electrode is installed within a support frame, and the moving electrode is raised and lowered by a transmission mechanism driven by a top-mounted variable frequency motor. A counterweight is located at the front of the support frame to balance the moving electrode assembly and maintain balance during use.
3.3 Key Technology Research
To achieve its functional requirements, the most crucial aspect of the product's structure is ensuring that the transmission mechanism meets the technical requirements; other components are configured around the transmission mechanism and starting requirements to achieve the overall performance requirements. The following mainly focuses on the motion process and design requirements of the transmission mechanism.
Conventional liquid resistance starter products, in order to meet the starting and speed regulation requirements of wound-rotor asynchronous motors, use transmission mechanisms such as belt pulleys driving lead screw rotation and worm gear reducers driving lead screws to achieve the rising and falling movement of the moving plate in the insulating water tank. The transmission is a constant speed transmission form, and the speed cannot be adjusted within a large range.
This high-power wound-rotor asynchronous motor has unique starting requirements, including a long starting time. The moving plate within the series-connected liquid resistance starter moves downwards at speed V1 until the motor reaches its rated speed. After starting, the bypass circuit breaker quickly closes, and the moving plate of the liquid resistance starter rapidly rises at speed V2 to the position corresponding to the motor's speed n, preparing for the next start to the rated speed. When the motor starts again, the moving plate moves at speed V3 until the motor reaches its rated speed. After completing the subsequent starting cycles, the motor rapidly returns to its highest position (corresponding to "0" speed) at speed Vn before each daily shutdown. A diagram illustrating the motor speed and its corresponding movement position is shown below.
First descent motion: When the motor starts from "0" speed to rated speed, the moving plate of the liquid resistance starter connected in series in the motor rotor circuit descends slowly. Through the preset setting of the frequency converter, the variable frequency motor adopts a fixed speed and drives the output shafts at both ends of the commutator to rotate. When the sprocket installed on one end of the output shaft rotates, it drives the chain to rotate, which in turn drives the sprocket installed on the input shaft (worm shaft) of the worm gear reducer to rotate. The rotation of the sprocket drives the worm shaft to rotate and output to the transition sprocket. The rotation of the transition sprocket drives the output pulley shaft of the electromagnetic clutch (which is in the engaged state at this time) to rotate. Through the lifting chain, the lifting frame assembly moves, thereby realizing that the moving plate inside the liquid resistance starter descends together with the lifting frame and the lifting chain at a speed of V1.
The first rapid ascent: When the liquid resistance starter plate needs to rise to the predetermined position, through the preset settings of the frequency converter (in conjunction with electrical control), the frequency converter motor uses a reverse fixed speed to drive the one-way coupling mounted on the other end of the reversing reducer to rotate. The one-way clutch drives the reversing reducer to transmit power, and the transmission output is sent to the sprocket mounted on the reel shaft. This, in turn, drives the lifting frame assembly through the lifting chain, causing the torque limiter with the sprocket to rotate in the opposite direction (at this time, the electromagnetic clutch is disengaged). This allows the plate to rise rapidly at a speed of V2, preparing for restarting. At this time, the electromagnetic clutch mounted on the reel shaft must be de-energized to ensure that the reel shaft rotates in the opposite direction to lift the lifting frame assembly upward.
The second descent motion: When the liquid resistance starter plate descends and moves slowly again, the variable frequency motor, through the preset settings of the frequency converter, adopts a speed of V3 and drives the reversing reducer, chain, worm gear reducer, electromagnetic clutch (at this time in the engaged state), and output reel shaft to rotate, thereby realizing the descent of the moving plate inside the liquid resistance starter at a speed of V3 until the motor starts to the rated speed; then, according to the control requirements, more than 40 rapid ascent and slow descent motions are implemented until the last rapid ascent at a speed of Vn to the position of the first descent motion of the day, waiting for the start-up of the next day.
Based on the above design analysis, the transmission mechanism plays a crucial role in the entire equipment's operation. To prevent transmission failure, protection of the transmission system is necessary. In the event of overload or limit switch failure during the transmission process, the inner and outer friction plates of the torque limiter with sprocket automatically disengage, separating the lifting and rotating parts, thus effectively protecting the transmission mechanism and lifting components. When the overload is released or the power is disconnected, the inner and outer friction plates of the torque limiter automatically engage, achieving rotary transmission and effectively ensuring the reliable operation and protection of the entire transmission mechanism.
Through the structural design of the main transmission mechanism and in conjunction with the design functions of each component, a complete product is assembled. After inspection in the factory according to technical requirements and on-site testing by the user, the product meets all technical requirements.
4. Features and Applications of WYQ10-10000 High-Power Liquid Resistance Starter
4.1 Advantages
1. The equipment can start continuously and is not affected by temperature rise;
2. It has a wide range of applications and can meet the starting time requirements of different high-power motors;
3. The mechanical protection settings of the transmission system are reasonable, safe and reliable;
4. By setting a counterweight beam, the transmission process is smooth;
5. Precise internal and external guidance to meet the area requirements for the alignment of the moving and fixed electrodes.
4.2 Disadvantages
1. The equipment is large in size, requiring a large installation foundation;
2. The electrolyte tank has a large capacity, and electrolyte replacement takes a long time during equipment maintenance;
3. The equipment has many transmission components, multiple speeds, and complex program control;
5. Conclusion
Through the structural design and application of the high-power liquid resistance starter, and after no-load and load tests, trial runs and several months of operation, it has met the user's requirements for various starting parameters and technical requirements for ultra-high power vertical wound-rotor motors. Its excellent starting performance has been unanimously praised by users. At the same time, the use of this high-power liquid resistance starter has also created a record for wound-rotor asynchronous motors in China.
