Static Voltage Regulators

Automatic regulators of various kinds have been devised for keeping comparatively small amounts of power at a constant voltage.

Fig. 197. Resonant-circuit voltage regulator.

Figure 197 shows one circuit for a resonant-reactor voltage regulator. Inductance L₁ is linear. Inductance L₂ and capacitor C₂ are parallel-resonant at the supply line frequency and rated voltage. The pair draws very little current, so that the reactive voltage in L₁ is low. Output current flows through its secondary winding which is of such polarity as to maintain rated voltage. Inductance L₂ is partially saturated at this voltage. If line voltage falls below rated value, less current is drawn by L₂, and the L₂C₂ combination becomes untuned. Total current to the parallel circuit is then capacitive, and this capacitive current, drawn through L₁ raises the output voltage. Conversely, if line voltage rises above rated value, the L₂C₂ combination becomes untuned on the inductive side, and the output voltage falls below the line value. Output voltage variations of ±1 per cent are obtained with ±10 per cent line voltage variations in this manner, and with load changes from zero to full load.

Constant supply frequency is a condition for resonance at rated voltage; with the good frequency control of modern power systems this condition is generally fulfilled. Load power factor variations cause output voltage to change. Some regulators are provided with taps to minimize this effect. Output wave form contains a noticeable third harmonic, because the large magnetizing current of L₂ must flow through appreciable impedance in L₁. Owing to the partial saturation of reactor L₂, it tends to operate at a high temperature and requires good ventilation. Practical regulators are in use with ratings from 25 v-a to several kva.

Electronic voltage regulators make use of a gas-filled regulator tube, which has a v-a characteristic such as that shown in Fig. 198.

Fig. 198. Voltage regulator characteristic.

Current drawn by this tube changes between wide limits with virtually no change in voltage. A series resistor is ordinarily used to limit the current. When output current in excess of the "V-R" tube rating is desired, it may be used as a voltage reference for a current amplifier.

Some voltage regulators amplify the difference between a voltage reference and the output voltage of a rectifier or generator. This difference is called the error voltage. The amplifier output reduces the generator field if generator output voltage is high and increases the field if the output voltage is low. Likewise, motor speed may be regulated by the difference between tachometer output and a voltage reference. Or the angular position of a motor may be controlled electrically as desired by remote means. These means are discussed in books on servomechanisms.⁽¹⁾ If a thyratron amplifier is used as part of a servo system, one thyratron may produce an effect opposite to that of the other, such as reversing current in the load. This amplifies the power controlled by error voltage.

In many modern regulator and servo systems, magnetic amplifiers are used. These devices are described in Magnetic Amplifiers.

(1)	See Principles of Servomechanisms, by G. S. Brown and D. P. Campbell, John Wiley & Sons, New York, 1948.