In industrial production, considering the special nature of the operating conditions, let's assume a system fault causes the potential energy load on the motor to accelerate freely downwards. In this situation, the motor is in a generator-like operating state, and the regenerated energy is fed back to the DC circuit through six freewheeling diodes, causing νd to rise and quickly putting the frequency converter into a charging state. The current at this time will be very large. Therefore, the selected reactor wire diameter must be large enough to handle this current.
Dynamic Braking Units are primarily used in applications where frequency converters require rapid deceleration, positioning, and braking. During braking, the high load inertia converts kinetic energy into electrical energy, causing a rise in the inverter's DC bus voltage. To prevent this from affecting normal operation, a braking unit must be used to dissipate the regenerated electrical energy; otherwise, the inverter will overshoot its overvoltage protection, impacting its normal operation.
The function of an energy feedback device is to effectively and efficiently return the regenerated electrical energy of the motor to the AC power grid for use by other surrounding electrical equipment, resulting in significant energy savings, typically ranging from 20% to 50%. Furthermore, because there are no resistive heating elements, the temperature in the computer room decreases, saving on the power consumption of the computer room air conditioning. In many situations, saving on air conditioning power consumption often leads to even better energy-saving results.
For a variable frequency speed control system to operate in energy feedback mode, the following conditions must be met:
(1) A controllable converter is required on the grid side. When the motor is in energy feedback mode, in order to realize the power feedback to the grid, the grid-side converter must be in inverter mode. Uncontrollable converters cannot achieve inversion.
(2) The DC bus voltage must be higher than the feedback threshold. For the inverter to feed energy back to the grid, the DC bus voltage must be higher than the feedback threshold; only then can it output current to the grid. The threshold value depends on the grid voltage and the inverter's withstand voltage performance.
(3) The feedback voltage frequency must be the same as the grid voltage frequency. During the feedback process, the output voltage frequency must be strictly controlled to be the same as the grid voltage frequency to avoid surge impact.
The mechanical energy (potential energy, kinetic energy) of the load during movement is converted into electrical energy (regenerated electrical energy) through an energy feedback device and fed back to the AC power grid for use by other nearby electrical equipment. This reduces the amount of grid power consumed by the motor drive system per unit time, thereby achieving the goal of saving electricity.
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