Stabilistor

The Westinghouse Stabilistor was an advanced AC voltage stabiliser designed to deliver a stable, undistorted output voltage for single-phase 50 Hz applications, even under wide variations in input voltage (190–260 V) and load (no load to full load). Its design leverages magnetic saturation and a specialised filtering circuit to achieve high performance and reliability.

Key Functional Components

• Double-Wound Transformer:
  The double-wound transformer is central to the Stabilistor’s operation. It is designed to operate near magnetic saturation, allowing it to self-regulate the output voltage in response to input supply or load fluctuations. Working in conjunction with an autotransformer and interconnected windings forms a feedback mechanism that dynamically adjusts the core’s magnetic flux. This enables rapid compensation for voltage or load changes, maintaining output stability and minimising waveform distortion. The double-wound construction also helps suppress third-harmonic currents, especially when paired with the resonant filter circuit, ensuring the output remains sinusoidal and undistorted.
• Autotransformer:
  Interconnected with the double-wound transformer, the autotransformer assists in voltage regulation and further enhances the system’s ability to respond quickly to load and supply variations.
• Filter Circuit (Capacitor and Choke in Series):
  ​This filter is connected across the output and serves two purposes:
  • At 50 Hz, it acts as a capacitive load, drawing a leading current necessary for the stabilising action.
  • At 150 Hz (the third harmonic), it becomes resonant, providing a low-impedance path that effectively short-circuits third-harmonic magnetising currents, thus maintaining a pure sine wave output.

Performance and Operational Characteristics

• Voltage Stabilization:
  Guarantees output within ±2% for all loads and input voltages between 190 V and 260 V. Tighter regulation is possible if either the input or the load is held constant5.
• Waveform Quality: 
  The output waveform remains virtually undistorted across the full load range, thanks to the harmonic suppression provided by the transformer/filter combination.
• Response Time:
  The system responds almost instantaneously to changes in load or input voltage, making it suitable for sensitive equipment.
• Efficiency:
  The Stabilistor achieves an efficiency of approximately 85%.
• Power Factor:
  The input power factor is about 0.7 lagging at full output when supplying a unity power factor load. Loads with inherently low power factor should be corrected to maximise voltage stability and load capacity.
• Frequency Sensitivity:
  Output voltage changes by about ±0.6% for every ±0.2 Hz deviation from the nominal 50 Hz supply frequency.

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