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 thirty-two 5/8 percent steps so that each change represents a specific increment of voltage.
transformer principle although their construction resembles that of a wound-rotor induction
motor. The shunt winding is connected across the line and supplies excitation for the regulator.
Upon excitation of the shunt winding, a voltage is induced in the series winding. Although it
remains constant in magnitude, the phase relationship of the induced voltage, with respect to the
voltage of the shunt winding, changes as the rotor is turned. The position of the rotor, therefore,
determines how much buck or boost voltage is imparted by the regulator at any given time.
3.3.2.3 Ferroresonant Constant Voltage Transformer. The ferroresonant constant voltage
transformer was patented in 1938. Generally, it consists of a resonant electrical circuit and a high
leakage reactance magnetic circuit. The output voltage remains essentially constant for a wide
range (+/- 15 percent) of input voltage. The principle of operation is a resonant circuit,
consisting of a series coil and a capacitor, tuned to resonate at approximately 60 Hz. A saturable
inductor is the secondary winding of the transformer and operates in saturation in the design
input voltage range of the unit to deliver a constant output voltage. The output voltage is rich in
harmonics (usually 20 percent or more harmonic distortion). For use with loads sensitive to
distortion to less than 3 percent. The FRT's are available in designs up to approximately 15 kVA
and are usually used on low voltage (120 to 240 V) circuits. Higher power and voltage ratings
are limited by the capacitor sizing and other methods of regulation are more economic at the
higher voltage and power levels. These types of units are frequency sensitive. The units
designed to operate at 60 Hz are not suitable for use at 50 Hz, which is the power source
available in many foreign countries.
3.3.3 Ratings. Voltage regulators are rated in terms of the number of phases, capacity in
kVA, primary voltage, percent regulation, and frequency. In addition, information concerning
temperature rise, impedance, insulation level, type of cooling, method of making connections,
and similar data is given on the nameplate.
3.3.3.1 kVA Rating. The kVA rating of a regulator is the product of the load amperes and
the voltage of the series winding in kilovolts. Standard ratings of regulators are available up to
833 kVA single-phase and 2500 kVA three-phase for line voltages of 2500 V to 34,500 V. To
enable regulation of 25,000 kVA circuits, 10 percent regulation units have a kVA regulated
circuit reading ten times higher than that of the regulator kVA.
3.3.3.2 Insulation and Cooling. Standard regulators are oil-immersed and self-cooled.
Units above 500 kVA can usually accommodate fans for forced cooling, adding up to 66 percent
to the self-cooled rating.
3.3.4 Applications. Voltage regulators are installed on distribution systems to keep voltages
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