Due to the combined effects of potential energy and gravity, the motor of a downconducting belt conveyor is in a power generation state, and the issue of energy flow must be addressed, which is commonly referred to as the energy feedback problem.
To address the feedback issue, a common practice is to use a four-quadrant frequency converter with an AFE (Active Front-End) rectifier. Besides hardware solutions, the key challenge lies in resolving the control software issues. A crucial principle in software development is to tailor the code to the conveyor's operating process, effectively managing its start-up, shutdown, and overall process control.
Since the end of 2008, our company has provided a total of 31 sets of variable frequency speed control electrical control systems for downlift belt conveyors to mining areas including Guotou Xinji, Wanbei, Shaanxi, Shenyang Shenglong, and Lu'an Mengjia Pharmaceutical. Of these, 19 sets operate under vertical height conditions ranging from 17° to 28°. Based on our experience, reliable operation of downlift belt conveyors requires addressing the following four issues:
1. Effective measures must be taken to prevent the vehicle from slipping during heavy-load starting.
Due to the negative torque, the heavy-load start-up of the downward-facing belt conveyor is not a problem; the key is to effectively prevent runaway. Practice has shown that during startup, in addition to controlling the working sequence of each piece of equipment according to the system startup requirements, it is also necessary to control the acceleration (i.e., the rate of change of speed) during its operation, as shown in Figures 1 and 2. Based on the mathematical model of speed and acceleration, we added an acceleration control envelope for the frequency converter during startup, shutdown, and its operation, ensuring that 0.1 m/s² ≤ a0 ≤ 0.3 m/s², thus ensuring from a software perspective that the frequency converter does not run away.
Figure 1 Control Curve
Figure 2: Schematic diagram of acceleration/deceleration control envelope of a four-quadrant frequency converter
2. Energy can be fed back smoothly and effectively during operation.
During the operation of the downconductor belt conveyor, after adopting the AFE four-quadrant frequency converter, at any frequency, as long as the synchronous speed is exceeded, the excess energy can be fed back to the power grid, which is something that other speed control devices cannot do.
When the system also includes equipment such as coal feeders and coal bunkers, monitoring the motor speed and current of other equipment is crucial during reliable energy feedback; therefore, proper timing coordination is essential. In short, using a frequency converter achieves true redundancy in the control of the downconductor belt conveyor. On one hand, the frequency converter provides online control and protection for the motor; on the other hand, the PLC controller exchanges data with the frequency converter via the PROFIBUS-DP bus, monitoring the operating status of the mine frequency converter while independently collecting motor operating data. When either of these occurs abnormally, the PLC controller will quickly take various actions (such as stopping the coal feeder or the frequency converter).
3. Able to stop smoothly when heavily loaded.
When the mining belt conveyor is under heavy load and requires a shutdown, the frequency converter uses a variable frequency stop method, which involves stopping according to a predetermined stopping curve. During normal operation and shutdown, the PLC controller sends a stop command to the frequency converter (which can be a contact signal or a 4-20mA inverted "S" curve). The frequency converter will then automatically decelerate and stop according to the given stopping curve and time. The total stopping time can be determined according to system requirements (as shown in Figure 3). Throughout the entire stopping process, the current and shaft encoder signals must be continuously monitored to achieve dual closed-loop control.
The problem with heavy-load shutdown of a conveyor belt is the sudden power outage in the coal mine's power supply system. What to do? Due to UPS capacity limitations, the only controllable mode for heavy-load shutdown is currently emergency shutdown. Although we have added a UPS power supply, it only powers the PLC controller. Theoretically, if a three-phase high-capacity UPS power supply is available, it is possible to achieve a soft shutdown under heavy load, provided the capacity is sufficient.
Figure 3 Mechanical characteristics of AC asynchronous machine
4. Power balance during the start-up and operation of multiple motors (see Figure 4).
Figure 4 Power balance during startup
When multiple machines are driven, the frequency converter can achieve full-range torque or speed tracking based on the arrangement of the belt conveyor's mechanical drive unit, thus achieving power balance. In the operating system, one frequency converter needs to be designated as the main unit and set to torque control (e.g., a scraper conveyor with a rigid connection), while the others are set to speed control, in order to achieve high transmission efficiency, good speed regulation performance, and no dead zone across the entire starting range.
Normally, when the master unit is set to torque control, it is independent of speed control. All operating parameters of the master unit are transmitted to the slave unit via PROFIBUS-DP bus communication, serving as the slave unit's input. When the slave unit is set to speed control, it is independent of torque control. The slave unit only needs to adjust and track the master unit's speed according to the speed value given by the master unit to maintain synchronization. The key here is that the master unit's shaft encoder sampling signal must be accurate, and the PLC must monitor the inverter's operating frequency. When the shaft encoder and inverter frequency data are inconsistent and exceed the specified value, the PLC commands a shutdown.
