The design of the starting circuit in a doubly-fed speed control system focuses on the rotor-side AC-AC converter, which typically operates at a limited output frequency range (usually up to 1/3 to 1/2 of the grid frequency). This limitation makes it impossible to directly start the motor at zero speed. In practical applications, there are two primary methods for starting such systems. One involves adding series resistance to the rotor windings, while the other uses an off-the-shelf AC-AC inverter combined with a modified motor wiring configuration to initiate the startup process.
In this starting method, when the asynchronous motor begins operation, the stator winding is initially closed, while the rotor winding connects to the inverter. As the inverter’s output frequency increases from 0 to half the synchronous speed, the motor gradually accelerates toward half the synchronous speed. At this point, the stator winding is connected to the grid, maintaining the motor's speed at half the synchronous level. Continuously adjusting the inverter’s output frequency from this midpoint to zero allows the motor to smoothly accelerate to full synchronous speed. Changing the phase sequence then pushes the motor beyond its synchronous speed. This approach offers excellent starting performance with minimal current surges. Compared to the traditional series resistance method, it significantly reduces the required equipment size and costs.
For the experiments conducted, the stator was connected to a power frequency supply, while the rotor side utilized an advanced AC-AC converter developed by our research institute, capable of handling up to 200 kW. The test subject was a JR51-4 three-phase wound asynchronous motor with a power rating of 2.8 kW. Its rated voltage and current specifications were: stator—380V, 6.3A. Repeated testing confirmed that when the stator frequency remains constant, the motor successfully starts when the rotor frequency reaches 25 Hz or higher. Below this threshold, continuous adjustment of the rotor frequency results in operation along the synchronous speed characteristic curve, producing a rotor winding voltage waveform that reflects both synchronous and super-synchronous double-fed operation. However, when the rotor frequency falls between 0 and 25 Hz, the motor fails to start.
This experiment highlights the importance of balancing rotor and stator frequencies to ensure smooth motor operation. It also demonstrates how advanced AC-AC converters can optimize starting conditions for doubly-fed induction motors, reducing reliance on traditional resistance-based methods and improving overall system efficiency. Future research could explore further refinements in converter technology to enhance performance across a broader operational spectrum, particularly in scenarios requiring dynamic adjustments under varying load conditions.
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