Electro-mechanical voltage regulators are generally used in

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generators

Explanation: Electro-mechanical voltage regulators, such as the Tirrill regulator or carbon-pile regulators, are traditionally used in generators to maintain a constant output terminal voltage. They function by modulating the excitation field current of the generator based on sensed deviations in the output voltage.

Working Principle: The regulator consists of a sensing element (solenoid) that monitors the generator voltage and a set of contacts connected to the field circuit. If the output voltage deviates, the solenoid moves, opening or closing the contacts to insert or remove external resistance in the field circuit, thereby adjusting the excitation current. This negative feedback loop ensures the terminal voltage remains within a specified narrow band despite variations in load or prime mover speed.

Key Formulas: $E_g = K_a \Phi \omega$ — Induced EMF in a generator where $\Phi$ is field flux and $\omega$ is angular velocity; $V_t = E_g - I_a R_a$ — Terminal voltage relation showing dependence on excitation control

Key Points: Electro-mechanical regulators provide automatic field control for synchronous and DC generators.; They rely on physical contacts and solenoids, making them prone to mechanical wear compared to modern solid-state AVRs.; The primary goal is to ensure stability in the presence of varying load conditions.; Common historical examples include the vibrating contact regulator (Tirrill type).

Advantages: Simple construction and robust operation.; Does not require complex semiconductor cooling or high-frequency switching circuits.

Disadvantages: Contact sparking leads to pitting and regular maintenance requirements.; Slower response time compared to modern electronic Automatic Voltage Regulators (AVR).; High mechanical wear on moving parts.

Applications: Small-scale DC power plants.; Older automotive charging systems.; Remote industrial synchronous generators.

Additional Info: Modern power systems have largely replaced these with Solid-State AVRs using thyristors or MOSFETs.. Option A (Transformers) uses Tap Changers, not electro-mechanical voltage regulators for field excitation.. Option B (Reactors) are passive devices and do not require active excitation control.

Answer: D is correct — Electro-mechanical voltage regulators are used to maintain constant output voltage in generators by controlling their field excitation current.

Exam Tip: Always distinguish between field control in generators (via voltage regulators) and transformer voltage control (which uses On-Load Tap Changers or reactors).

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