Electromechanical regulators

In older electromechanical regulators, voltage regulation is easily accomplished by coiling the sensing wire to make an electromagnet. The magnetic field produced by the voltage attracts a moving ferrous core held back under spring tension or gravitational pull. As the voltage increases, the magnetic field strength also increases, pulling the core towards the field and opening a mechanical power switch. As the voltage decreases, the spring tension or weight of the core causes the core to retract, closing the switch allowing the power to flow once more. If the mechanical regulator design is sensitive to small voltage fluctuations, the motion of the solenoid core can be used to move a selector switch across a range of resistances or transformer windings to gradually step the output voltage up or down, or to rotate the position of a moving-coil AC regulator. Early automobile generators and alternators had a mechanical voltage regulator using one, two, or three relays and various resistors to stabilize the generator's output at slightly more than 6 or 12 V, independent of the engine's rpm or the varying load on the vehicle's electrical system. Essentially, the relay(s) employed pulse width modulation to regulate the output of the generator, controlling the field current reaching the generator (or alternator) and in this way controlling the output voltage produced. The regulators used for generators (but not alternators) also disconnect the generator when it was not producing electricity, thereby preventing the battery from discharging back through the stopped generator. The rectifier diodes in an alternator automatically perform this function so that a specific relay is not required; this appreciably simplified the regulator design. More modern designs now use solid state technology (transistors) to perform the same function that the relays perform in electromechanical regulators.

Main regulators
Electromechanical regulators have also been used to regulate the voltage on AC power distribution lines. These regulators generally operate by selecting the appropriate tap on a transformer with multiple taps. If the output voltage is too low, the tap changer switches connections to produce a higher voltage. If the output voltage is too high, the tap changer switches connections to produce a lower voltage. The controls provide a deadband wherein the controller will not act, preventing the controller from constantly hunting (constantly adjusting the voltage) to reach the desired target voltage.

Basic design principle and circuit diagram for the rotating-coil AC voltage regulator. This is an older type of regulator used in the 1920's that uses the principle of a fixed-position field coil and a second field coil that can be rotated on an axis in parallel with the fixed coil. When the movable coil is positioned perpendicular to the fixed coil, the magentic forces acting on the movable coil balance each other out and voltage output is unchanged. Rotating the coil in one direction or the other away from the center position will increase or decrease voltage in the secondary movable coil. This type of regulator can be automated via a servo control mechanism to advance the movable coil position in order to provide voltage increase or decrease. A braking mechanism or high ratio gearing is used to hold the rotating coil in place against the powerful electromagnetic forces acting on the moving coil. The overall construction is extremely similar to the design of standard AC dynamo windings, with the primary difference being that the rotor does not spin in this device, and instead is held against spinning so the fields of the rotor and stator can act on each other to increase or decrease the line voltage.