The design of the starting circuit in a doubly-fed speed control system is crucial for achieving efficient motor operation. The rotor-side AC-AC converter typically operates within a limited frequency range, usually up to 1/3 to 1/2 of the grid frequency. This limitation means that directly applying an inverter to start the motor at zero speed isn't feasible. In practical applications, there are generally two approaches to starting such systems. One involves adding series resistance to the rotor windings, while the other uses standard AC-AC inverters along with altered motor wiring configurations. These methods are implemented to ensure smooth acceleration.
When starting an asynchronous motor, the stator windings are initially closed, and the rotor windings are linked to the inverter. As the inverter's output frequency increases from 0 to half the synchronous speed, the motor gradually accelerates. Once the motor reaches half of its synchronous speed, the stator windings are connected to the grid, maintaining the motor’s speed at half the synchronous rate. By further adjusting the inverter's output frequency from the initial value down to zero, the motor continues to accelerate until it reaches synchronous speed. Changing the phase sequence then allows the motor to exceed synchronous speed. This starting method ensures that the inverter's maximum steady-state power remains around half of the motor's total power. It offers excellent starting performance with minimal current surges. Compared to traditional series resistance starting, this approach significantly reduces the need for bulky starting equipment, thereby lowering costs.
In our experiments, we connected the stator side to a power frequency supply and used a bi-directional AC-AC converter developed in-house on the rotor side. This converter has a rated capacity of up to 200kW. Our test subject was a JR51-4 three-phase wound asynchronous motor with a power rating of 2.8kW. Its rated voltage and current specifications are as follows: stator—380V, 6.3A. Repeated trials confirmed that when the stator frequency remains constant, the motor can successfully start when the rotor frequency reaches or exceeds 25Hz. Below this threshold, specifically between 0 and 25Hz, the motor fails to initiate. Additionally, during testing, the rotor winding voltage waveform exhibited characteristics typical of synchronous and super-synchronous double-fed operations.
These findings underscore the importance of precise frequency control in optimizing motor performance. While the rotor-side AC-AC converter provides a compact and cost-effective solution, ensuring consistent frequency adjustments is vital for reliable operation. Future research could explore advanced algorithms to enhance the efficiency of these systems, potentially integrating real-time monitoring capabilities to further refine performance metrics.
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