When the belt conveyor drives multiple rollers, due to the different diameters of the driving rollers, the angular velocity of the driving motor is also different. The motor that drives the large-diameter drum is in the electric state, and the motor that drives the small-diameter drum may be in the power generation state, and the latter is equivalent to the load of the former. The working state of the latter motor also produces braking under the above two situations. In some special working conditions, the two main motors driven by the upper double rollers run in a quadrant, and the main motor driven by the frequency converter also works in a quadrant. In the quadrant, the motor is in a braking state, and the energy feedback of the frequency converter needs to be solved.
The braking method of general-purpose inverters is AC/DC/AC voltage inverters, and the rectification part is composed of uncontrollable rectifier diodes, and the energy transfer of this part is irreversible. When the deceleration of the conveyor is too large or the diameter difference between the two driving rollers is large, the regenerative energy from the motor side is transferred to the DC side, and the resistance and capacitance of the DC circuit have no time to consume the regenerative energy, and the generated pumping voltage may damage the filter capacitance. To choose a suitable frequency converter, you must first understand the braking method and main circuit structure of the frequency converter.
During DC braking, the three-phase AC power supply of the main motor is disconnected, and any two phases of the stator are connected to the DC power supply to form a fixed magnetic field. The magnitude of the braking torque can be controlled by controlling the duration and magnitude of the DC current. The braking energy is dissipated in the rotor of the main motor in the form of heat. Ordinary inverters generally have a DC braking function, which is mainly used in occasions where the braking is not particularly frequent and the braking force is not particularly large, such as fan and water pump loads. It is generally used in conjunction with frequency reduction and deceleration, and can also be used to eliminate the main motor running previous crawl. Due to the heat limitation of the rotor of the main motor, DC braking is not suitable for the feedback braking universal frequency converter of the long-distance and large-inclination conveyor driven by double rollers, and cannot feed energy back to the grid. In order to realize the bidirectional flow of energy, a group of active inverter circuits must be connected in parallel with the rectifier on the grid side. frequency conversion main circuit. One company made the active inverter part a self-contained unit so that the phases could be directly on the DC bus. However, implementing an active inverter in the above-mentioned main circuit has higher requirements on the quality of the power grid. During inverting, if the power supply voltage is low or the power supply is cut off, the active inverter will turn over and the fuse will be blown. In addition, due to the parallel connection of a set of active inverter devices, the cost of the system increases, the size of the feedback device increases, and the power grid is polluted. Therefore, this form of feedback braking does not conform to China’s national conditions.
The frequency conversion circuit controlled by double pulse width modulation is a new technology to deal with energy feedback in recent years. Both the rectifier circuit and the inverter circuit use self-shutdown devices for PWM control, and the four-quadrant operation of the motor can be easily realized without any additional circuits, so that the power factor of the system is approximately equal to 1. Dual PWM frequency conversion main circuit, this type of frequency converter has superior performance, but the price is relatively expensive. Feedback braking is generally used in occasions that require frequent braking and precise control of the braking speed, especially in occasions with potential energy loads, such as hoists, elevators, and large-inclination down-belt conveyors. It can limit four images of the motor, save energy and reduce consumption, realize precise braking, and improve the dynamic performance of the motor.
Energy consumption braking Energy consumption braking is to consume the feedback energy on the high-power resistance of the DC circuit through the built-in or external braking resistor, so as to realize the four-quadrant operation of the motor. This braking mode can be applied to general type inverters. This method is simple in principle, low in cost and high in reliability, but wastes energy and has low dynamic performance. It is mainly used in occasions where the braking is infrequent and the braking characteristics are not strict. The energy consumption control part detects the DC voltage, and when it exceeds a certain threshold, the braking unit controls the access of the braking resistor through chopping. When the internal resistance power of the braking unit is insufficient, a high-power braking resistor needs to be connected externally.
Among the above three braking methods, dynamic braking and feedback braking are only applicable to frequency converters. In fact, the main motor is in the state of generating brake.
Based on the above analysis, energy consumption braking is a relatively simple method for the upward belt conveyor with a large inclination angle. Since the belt conveyor with a large inclination angle does not brake frequently, it is not a potential load and does not require precise braking. If the price factor is put aside, the frequency converter with dual PWM control is the best choice.
When the belt conveyor drives multiple rollers, due to the different diameters of the driving rollers, the angular velocity of the driving motor is also different. The motor that drives the large-diameter drum is in the electric state, and the motor that drives the small-diameter drum may be in the power generation state, and the latter is equivalent to the load of the former. The working state of the latter motor also produces braking under the above two situations. In some special working conditions, the two main motors driven by the upper double rollers run in a quadrant, and the main motor driven by the frequency converter also works in a quadrant. In the quadrant, the motor is in a braking state, and the energy feedback of the frequency converter needs to be solved.
The braking method of general-purpose inverters is AC/DC/AC voltage inverters, and the rectification part is composed of uncontrollable rectifier diodes, and the energy transfer of this part is irreversible. When the deceleration of the conveyor is too large or the diameter difference between the two driving rollers is large, the regenerative energy from the motor side is transferred to the DC side, and the resistance and capacitance of the DC circuit have no time to consume the regenerative energy, and the generated pumping voltage may damage the filter capacitance. To choose a suitable frequency converter, you must first understand the braking method and main circuit structure of the frequency converter.
During DC braking, the three-phase AC power supply of the main motor is disconnected, and any two phases of the stator are connected to the DC power supply to form a fixed magnetic field. The magnitude of the braking torque can be controlled by controlling the duration and magnitude of the DC current. The braking energy is dissipated in the rotor of the main motor in the form of heat. Ordinary inverters generally have a DC braking function, which is mainly used in occasions where the braking is not particularly frequent and the braking force is not particularly large, such as fan and water pump loads. It is generally used in conjunction with frequency reduction and deceleration, and can also be used to eliminate the main motor running previous crawl. Due to the heat limitation of the rotor of the main motor, DC braking is not suitable for the feedback braking universal frequency converter of the long-distance and large-inclination conveyor driven by double rollers, and cannot feed energy back to the grid. In order to realize the bidirectional flow of energy, a group of active inverter circuits must be connected in parallel with the rectifier on the grid side. frequency conversion main circuit. One company made the active inverter part a self-contained unit so that the phases could be directly on the DC bus. However, implementing an active inverter in the above-mentioned main circuit has higher requirements on the quality of the power grid. During inverting, if the power supply voltage is low or the power supply is cut off, the active inverter will turn over and the fuse will be blown. In addition, due to the parallel connection of a set of active inverter devices, the cost of the system increases, the size of the feedback device increases, and the power grid is polluted. Therefore, this form of feedback braking does not conform to China’s national conditions.
The frequency conversion circuit controlled by double pulse width modulation is a new technology to deal with energy feedback in recent years. Both the rectifier circuit and the inverter circuit use self-shutdown devices for PWM control, and the four-quadrant operation of the motor can be easily realized without any additional circuits, so that the power factor of the system is approximately equal to 1. Dual PWM frequency conversion main circuit, this type of frequency converter has superior performance, but the price is relatively expensive. Feedback braking is generally used in occasions that require frequent braking and precise control of the braking speed, especially in occasions with potential energy loads, such as hoists, elevators, and large-inclination down-belt conveyors. It can limit four images of the motor, save energy and reduce consumption, realize precise braking, and improve the dynamic performance of the motor.
Energy consumption braking Energy consumption braking is to consume the feedback energy on the high-power resistance of the DC circuit through the built-in or external braking resistor, so as to realize the four-quadrant operation of the motor. This braking mode can be applied to general type inverters. This method is simple in principle, low in cost and high in reliability, but wastes energy and has low dynamic performance. It is mainly used in occasions where the braking is infrequent and the braking characteristics are not strict. The energy consumption control part detects the DC voltage, and when it exceeds a certain threshold, the braking unit controls the access of the braking resistor through chopping. When the internal resistance power of the braking unit is insufficient, a high-power braking resistor needs to be connected externally.
Among the above three braking methods, dynamic braking and feedback braking are only applicable to frequency converters. In fact, the main motor is in the state of generating brake.
Based on the above analysis, energy consumption braking is a relatively simple method for the upward belt conveyor with a large inclination angle. Since the belt conveyor with a large inclination angle does not brake frequently, it is not a potential load and does not require precise braking. If the price factor is put aside, the frequency converter with dual PWM control is the best choice.
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