1 Introduction
For a long time, DC speed control systems held a significant share in mechanical equipment speed control systems due to their superior speed regulation performance: smooth operation, high low-speed torque, and wide speed range. However, the complex structure of DC motors and the commutator (a weak point) resulted in high maintenance requirements and limitations on single-unit capacity, overload capacity, maximum voltage, and maximum speed. With advancements in science and technology, AC variable frequency technology has made substantial progress. The mature application of vector control technology in the 1990s, in particular, led to a qualitative leap in AC speed control technology. Because SVC (sensorless vector control) can achieve near-closed-loop control performance, its system adaptability and robustness have been continuously improved. Therefore, AC speed control systems have reached a level comparable to DC speed control in many applications. Furthermore, ordinary asynchronous motors are simple in structure, robust, durable, and easy to maintain, with many performance indicators superior to DC motors. Consequently, in many new applications, DC speed control systems have been replaced by AC variable frequency speed control, and some outdated DC speed control systems are gradually being converted to variable frequency speed control. This article presents an example of replacing a DC speed control system with a variable frequency speed control system.
2. Status and load characteristics of the original DC system
The original DC speed control system was used on the honeycomb panel production line of a honeycomb products company to drive the winding machine on the production line. This production line can first wind a whole roll of corrugated base paper into a set number of layers and then cut it into honeycomb panels for producing honeycomb paper cores. The winding machine is a large rotating wheel load with a wheel circumference of 12m. This type of load is similar to a paper winding machine and is a constant power load. Its working process is divided into two parts: first, winding the corrugated base paper onto the rotating wheel under a certain tension, with a designed maximum production speed of 200m/min and a normal production speed of 160m/min, the production process takes about 14 minutes; second, after winding, cutting the wound material into blocks, this process requires positioning the rotating wheel, so the minimum speed is about 4m/min, and the cutting process lasts for 6 minutes. The speed during the entire working process is adjusted by a 0-10V analog output from the PLC. The rotary wheel is equipped with a pneumatic-hydraulic brake mechanism to eliminate mechanical inertia and assist in positioning during cutting. The brake mechanism operates similarly to the brake on a motor. When the motor rotates, the brake is released; when the motor stops, the brake mechanism locks the load to prevent the load from rotating freely due to uneven gravity.
This DC speed control system is configured with a German brand Movret 365 DC speed controller and a 2-pole 11kW DC motor. A tachogenerator is used for negative feedback, and a powerful air cooler is also provided to cool the main motor. The motor is connected to a winding machine via a chain and a gear reducer.
2.1 Problems encountered by the original speed control system
DC motors are complex in structure, require significant maintenance, are expensive, and have long repair cycles. The DC motor used in this equipment, after four or five years of use, suffered severe commutator burnout due to poor-quality replacement carbon brushes, resulting in frequent sparking during operation. It underwent one repair, which was not only costly but also required nearly a week of downtime. Later, a brake mechanism malfunction caused the brake pads to fail to fully disengage from the brake disc, leading to excessive load and damage to the motor stator windings, thus presenting another challenge of prolonged downtime for repairs. To address this problem, we proposed replacing the DC speed control system with AC speed control. To change the original system, a thorough understanding of its characteristics and load conditions is essential. The removed DC motor is shown in Figure 1.
2.2 Characteristics of the original system load
2.2.1 Characteristics of DC speed control systems
The original DC speed control system is a type of DC speed control system that changes the armature power supply voltage. Its characteristic is that the stiffness of the motor's mechanical characteristics remains unchanged when the armature power supply voltage is changed. Therefore, even when the motor is running at low speed, the speed changes little with load variations, meaning it has good speed stability and smoothness, and also produces high torque at low speeds. Its DC speed control mechanical characteristics are shown in Figure 2.
2.2.2 Characteristics of the mechanical load of the original system
The original system was designed for a maximum production speed of 200 m/min, with a machine rotation speed set at 4 m/min for slitting and positioning, resulting in a system speed range of 50:1. The winding process is relatively simple, involving only a motor driving a large rotating wheel to rotate, primarily to overcome the tension on the material to meet process requirements. However, stopping the machine after winding presents a more demanding challenge, as the operator must quickly switch the speed setting potentiometer from the current production speed to the zero-speed position. This requires the rotating wheel speed to drop from approximately 160 m/min to zero as quickly as possible. Due to the heavy load and significant inertia, the frequency converter is prone to overvoltage faults during rapid deceleration, posing a challenge to the inverter. Another challenge is the material slitting process. After the material is wound, it needs to be cut into eight pieces from the eight sides of the rotating wheel using an electric saw. The slitting process is as follows: first, the rotating wheel is manually rotated to the starting position at low speed, detected by a stop photoelectric switch. Once in position, the mechanical mechanism positions the rotating wheel, and then the operator switches to automatic operation, at which point the electric saw begins its first cut. After sawing is completed, the positioning mechanism is released, the brake is released, and the wheel begins to rotate slowly (8 m/min). When the deceleration photoelectric switch signal is activated, the wheel speed decreases to 4 m/min and continues to operate. Once the PLC receives the stop photoelectric switch signal, the motor stops rotating, and the braking mechanism immediately brakes. If the stop photoelectric switch signal is still in the "on" state, the positioning mechanism performs the positioning action again and begins the next sawing process. This cycle continues until all the material on the wheel is cut. After each piece is cut, the material falls off the wheel and is transported away by the conveyor belt. When the material wound on the wheel has not been cut, the load is evenly distributed, and the motor rotates relatively easily. However, after several cuts, the material left on the wheel creates a large unbalanced torque. At this time, the resistance torque during rotation is large, while the motor speed is very low. This requires the speed control system to have sufficient low-speed torque. Furthermore, positioning is also required at this time, so the speed control system must operate smoothly at low speeds without shaking or creeping.
3. Selection of motors and frequency converters
3.1 Motor Selection
Before the modification, we consulted some materials and referenced other experiences of converting DC speed control to AC speed control. The conclusion was that the AC motor needed to be one power class larger than the original DC motor to ensure torque at low speeds, or an expensive AC variable frequency motor needed to be selected. The original motor in this system was an 11kW 2-pole DC motor. Based on this experience, the modification required a 15kW 2-pole AC motor. Through calculations of the reduction ratio and production speed of the original drive system, we found that at the maximum design speed of 200m/min, the motor only operated at 2750r/pm, while the actual production speed was only 160m/min, and the motor operating speed was 2200r/pm. It is evident that there is a considerable speed margin with a 2-pole motor. If a 4-pole motor is selected, its torque is effectively doubled. Even though the torque of variable frequency speed control at low speeds is not as good as DC speed control, this choice will definitely be better than the low-speed performance of the original system. Furthermore, when cutting materials at low speeds, the output frequency of the frequency converter can be twice that of a 2-pole motor, making it easier to ensure performance at low speeds. Based on the rated speed of the four-pole motor, a production speed of 160 m/min can be achieved when it operates at 72 Hz. Therefore, it is feasible to use an 11 kW four-pole motor as a replacement.
Based on the installation requirements, we selected the YSJ series low-noise three-phase asynchronous motor for injection molding machines from Dongguan Motor Factory. The YSJ motor is a new series of products. In addition to the advantages of the Y-series motors—high efficiency, energy saving, good performance, low vibration, power rating and installation dimensions conforming to IEC standards, and convenient use and maintenance—it also features strong overload capacity and low noise, especially under rated load and overload conditions. The performance indicators of this series motor, such as efficiency, power factor, locked-rotor torque, locked-rotor current, and maximum torque, are the same as the Y-series motors, while the noise level is significantly lower. More importantly, this motor has two installation methods: horizontal installation and flange installation. Its flange dimensions are exactly the same as the original DC motor; only the output shaft needs to be machined to the original motor's dimensions for assembly with the original gear reducer.
3.2 Selection of Frequency Converter
Based on the characteristics of the load, we compared the parameters of several domestically produced frequency converters. Considering the use of a four-pole motor with a considerable torque margin, there was no need to increase the power rating of the frequency converter. Ultimately, we selected the 11kW V5-H series frequency converter from LanHaiHuaTeng Company. The V5-H series is a high-performance, general-purpose frequency converter, suitable for this application due to the following features:
(1) In vector control mode 1, it has both the excellent performance of vector control and is not sensitive to motor parameters.
(2) High starting and low speed torque: 180% of rated torque can be achieved when starting at 0.5 Hz, and the motor can be controlled to run stably at 150% of rated torque at 0.5 Hz.
(3) The speed adjustment range is large, up to 1:100.
(4) In the vector control mode without speed sensor, the motor can operate in four quadrants, and the torque, current and speed response is fast, and the motor runs smoothly.
(5) Unique fast DC braking capability: within the range of 0 to 60 Hz, the inverter can eliminate the back EMF of the motor within 0.3 s, thus achieving fast braking.
4. Control wiring of frequency converter
Because AC speed control systems are simpler, wiring is greatly simplified compared to the original DC system. Figures 3 and 4 show a comparison of the wiring between the two speed control methods. The frequency converter only needs one operating signal and the speed signal from the original speed control system to operate normally. Simultaneously, the frequency converter's fault output is connected to the PLC for effective system protection. In Figure 4, 85K4 is the operating signal of the original DC speed controller. Ra, RB, and RC are the relay output terminals of the frequency converter, defined as the frequency converter's fault output. Ra and RC are connected to the PLC's COM and X5 input points respectively as fault feedback for the frequency converter.
5. Parameter settings of frequency converter
Based on the application characteristics here, in addition to inputting the parameters in group P9 according to the motor's chrome plate data, the parameters listed in Table 1 must also be set and adjusted according to the actual situation. Other parameters not mentioned can be set to factory defaults. The parameters to be input in group P9 are shown in Table 2. After the parameters are input, if possible, allow the inverter to perform rotational self-tuning (or dynamic motor parameter self-learning) on the motor. This allows the inverter to identify as many key motor parameters as possible, achieving better control performance. In this example, because the load is difficult to disconnect, only static self-tuning of the motor parameters was performed.
6. Problems encountered and solutions during debugging
6.1 Determination of deceleration time
Because positioning is required during the slitting process, the deceleration time should be as short as possible for accurate positioning. Initially, it was set to 2 seconds, but when the inverter switched from high-speed operation to shutdown after winding production, it reported an EOV2 (overvoltage during deceleration) fault. After repeated testing, the deceleration time was set to 6 seconds, and everything worked normally. This variable frequency drive system was not equipped with a braking resistor. The initial plan for the modification was to add a braking resistor to improve its deceleration performance if the stopping speed was too slow and did not meet the requirements.
6.2 Application of Automatic Torque Boost Function
Considering the need for low speed and high torque during material cutting, the torque boost function of P0.16 was initially set to 10. However, during manual testing, the inverter reported an EOC1 (overcurrent during acceleration) fault as soon as it started. Adjusting the value to a lower setting did not resolve the issue. Finally, the value was reset to the factory default of 0, and the automatic torque boost function resolved the EOC1 fault.
6.3 Setting the startup frequency
During the cutting process, once two pieces of material are cut off from the octahedron, the reverse potential energy of the octahedron is significant. Without this setting, when the brake is released and the motor just begins to rotate, it will first be dragged slightly in the opposite direction by the load before being driven by the motor. Due to the chain drive, there is a large backlash during forward and reverse rotation, which will cause noticeable vibration of the large roller, a problem that must be overcome. Our analysis shows that this occurs because the motor output torque is too small during the low-speed start-up of the frequency converter, insufficient to overcome the potential energy of the load, thus causing it to be dragged in reverse by the load. The original DC speed control system did not have this problem, which proves that the dynamic response of the negative feedback DC system is very fast, and the low-speed torque is very large. To solve this problem, we first increased the starting frequency of the frequency converter to increase its output torque during startup, and secondly, we modified the PLC program to delay the release of the brake. That is, we first give the frequency converter a start signal, and only release the external brake after the system generates a certain torque, thus avoiding this problem.
6.4 Application of DC braking function during shutdown
During the slitting process, the rotating wheel needs to be positioned. The original system's positioning process was as follows: when the PLC detected a stop signal, it output a signal to stop the motor, and the braking mechanism applied the brakes simultaneously. Therefore, the original system had a persistent problem: after the motor stopped, before the brakes fully engaged, the rotating wheel would be pulled forward or backward by the load, causing the positioning position to deviate after the brakes engaged. This required manual repositioning, resulting in low production efficiency and high operator workload. To solve this problem, we fully utilized the inverter's DC braking function, setting the stop mode to deceleration stop plus DC braking mode. During the stop process, when the inverter's output frequency drops to 1Hz, the inverter initiates DC braking, clamping the load with the motor and preventing the rotating wheel from rotating freely due to the unbalanced torque of the material. This ensures accurate positioning after the braking action. With this setup, this AC speed control system has a significant advantage in positioning function compared to the original DC speed control system.
7. Conclusion
After the system upgrade was completed and put into production, we observed and tracked the inverter's output during operation. During the winding process, the inverter operated at 72Hz, with an output current of only around 10A, indicating a very light load. When driving an unbalanced load at low speed, the inverter's output frequency was 5.2Hz, and the maximum current reached 23A, but these were instantaneous and generally within 15A. Furthermore, the entire operation was very smooth, demonstrating that the AC system's load capacity far exceeded the mechanical load's power requirements. Therefore, we removed the forced-air cooling fan from the original DC speed control system, further reducing system complexity, making maintenance easier, and thus more efficient and energy-saving. Subsequent observations showed that the motor's operating temperature remained below 50℃. This demonstrates that under certain conditions, the AC speed control system has significant advantages over DC speed control.
