VOLTAGE REGULATORS

A voltage regulator consists essentially of a voltage-sensitive element and associated mechanical or electrical means to produce the changes necessary to maintain a predetermined, constant generator voltage.

An introduction to or explanation of the many different types and styles of voltage regulators manufactured today surpasses the scope of this chapter. All regulators accomplish the same job, but different solid-state devices and circuits may be used to arrive at the end result-voltage regulation. Two typical voltage regulators used in NCF generating equipment are the SCR voltage regulator and the transistor voltage regulator.

Silicon Controlled Rectifier Regulator

The SCR regulator precisely controls the output voltage of an ac electrical generating system by controlling the amount of current supplied to the exciter (or generator) field. This regulator may be used with brushless rotary exciters, brush type of rotary exciters, or direct excitation machines.

The SCR regulator (fig. 4-9) senses the generator voltage, compares a rectified sample of that voltage with a reference diode (zener) voltage,

Figure 4-9.\SCR voltage regulator block diagram.

and supplies the field current required to maintain the predetermined ratio between the generator voltage and the reference voltage. The SCR regulator consists of five basic circuits. These are a sensing circuit, an error detector, an error amplifier, a power controller, and a stability network. The regulator also has an automatic voltage buildup circuit.

SENSING CIRCUIT.\This circuit consists of sensing transformer(s) T1 (T2), diodes CR1 through CR6, capacitors C1 and C2, and filter choke L1 (fig. 4-10). The sensing circuit will sense the generator voltage, rectify and filter this voltage, and apply the resultant dc signal to the error detector and error amplifier. Transformer T1 (terminals E1 and E2) is used on single-phase application, and transformer T2 (terminals E1, E2, and E3) is added to supply three-phase sensing.

ERROR DETECTOR.\The detector consists of reference (zener) diode VR1 and a voltage

Figure 4-10.\SCR voltage regulator schematic diagram.

divider network, consisting of resistors R1, R2, R3, and R5. This network provides a dc signal that is proportional to the generator voltage. The voltage at the junction of R3 and R5 is compared to the voltage of VR1 to develop the error signal, which is applied to the error amplifier.

ERROR AMPLIFIER.\The amplifier consists of two-stage transistor amplifier Q1 and Q2, unijunction transistor 43, emitter follower 44, and their associated components. The error signal drives Q1, which, in turn, controls 42. Transistor Q2 controls the charging time of capacitor C4 in the emitter circuit of 43, thus providing phase angle control of the firing signal applied to the SCRs in the power, controller. Transistor 44 provides the correct voltage to base 2 of 43 to maintain uniform SCR firing.

POWER CONTROLLER.\The power controller consists of SCRs CR11 and CR12 and diodes CR13 and CR14 in a bridge rectifier circuit. The amount of output current depends upon the conduction time of the SCRs and the exciter field resistance. This circuit can be compared to a variable rectifier placed between the power souce (terminals 3 and 4) and the exciter field (terminals F+ and F-).

INPUT POWER.\The voltage applied to the regulator input power stage (terminals 3 and 4) must be 120 volts for single-phase regulators and either 208 or 240 volts for three-phase sensing regulators. The input power may be taken from any generator phase that provides the correct voltage (line to line or line to neutral). The phase relationship of this input voltage in relation to other circuits is not important. When the generator output voltage is greater than the preceding values, a power transformer must. be used to match the generator voltage to the regulator input.

STABILITY NETWORK.\This circuit provides stable operation under all operating conditions. It consists of capacitors C6 and C7; resistors R27, R28, R29; and variable resistor R4. This resistance-capacitance (RC) network injects a stabilizing signal from the power stage to the error amplifier to prevent oscillations (hunting). R4 determines the amount of stability applied to the error amplifier.

AUTOMATIC VOLTAGE BUILDUP.\ Relay K1 provides automatic voltage buildup from generator residual voltage. Normally closed contacts (relay de-energized) provide a current path around the control rectifiers (CR11 and CR12) to allow the generator residual voltage to be converted to dc by diodes CR13, CR14, CR15, and CR16 and applied to the exciter field. When the generator voltage reaches approximately 75 percent of rated voltage, the relay pulls in, removing the rectifier diodes, thereby allowing the SCRs to take control. A minimum of 3 percent generator residual is required for automatic voltage buildup. If less than 3 percent exists, external field flashing may be required.