UNIT 10AC GENERATOR VOLTAGE REGULATION

AIMS OF THE UNIT:- to describe ac generator voltage regulation

- to understand the grammar point ‘

TASKS

1 Do your best to answer the brainstorming questions. 2 Read the text for general understanding.

3 Make up questions to the text. 4 Find the sentences with the new words in the text. Give the Kazakh or Russian

equivalents of the words.

5 Write sentences with the new vocabulary. 6 Do the given exercisesfor better remembering the topic.7 Speak on the topic. Given schemes and questions will help you to remember

and understand the topic.

8Find more information about the topic. Do some research, create slideshow or a

Brainstorming questions

project work and present them.

 

1 What happens if the output of the control amplifier section of a voltage regulator is toolow?

2 In a solid-state voltage regulator, what component supplies the power to the bridgecircuit?

A voltage regulator controls the magnetic field strength. Current generating the magnetic field is known as excitation current. The auxiliary dc generator (called the exciter) or a rotating three-phase rectified ac exciter generator supplies this current. The exciter is on the same as the ac generator to make it an integral part of the generator. The chief advantage of exciter units is that each generator has its own independent source of excitation.

One type of voltage regulator that has no mechanical moving parts (except the exciter control relay) is the solid-state regulator. The ac generator output flows to the voltage regulator, which compares it to a reference voltage. The difference supplies the control amplifier section of the regulator. If the output is too low, regulator circuitry increases the field strength of the ac exciter. If the output is too high, it reduces the field strength. The power supply for the bridge is CR1. CR1 provides full-wave rectification of the three-phase output from transformer T1. The dc output voltages of CR1 are proportional to the average.The negative anode of CR1 supplies power through point B, R2, point C, Zener CR1, point D, and to parallel-connected V1 and R1. Takeoff point C of the bridge is located betweenresistor R2 and the Zener diode. The other leg of the reference bridge (resistors R9, R7, and temperature compensating resistor RT1) connects in series with V1 and R1 through points B, A, and D. The output of this leg of the bridge is at point E. As voltage changes occur, voltage across R1 and V1 (once V1 starts conducting) remains constant, leaving the total voltage change occurring across the bridge. Since voltage across the Zener diode remains constant (once it starts conducting), the total voltage change occurring in that leg of the bridge is across resistor R2. In the other leg of the bridge, the voltage change across the resistors is proportional to their resistance values. For this reason, the voltage change across R2 is greater than the voltage change at point E. If the generator output voltage drops, point C is negative with respect to point E. Conversely, if the generator voltage output increases, the voltage between the two points reverses polarity. The bridge output taken between points C and E connects between the emitter and the base of transistor Q1. With the generator output voltage low, the voltage from the bridge is negative to the emitter and positive to the base. This is a forward bias signal to the transistor, and the emitter to collector current increases. With the increase of current, the voltage across emitter resistor R11 increases. This increase, in turn, applies a positive signal to the base of transistor Q4, which increases emitter to collector current and increases the voltage drop across emitter resistor R10. This gives a positive bias on the base of Q2, which increases its emitter to collector current and increases the voltage drop across its emitter resistor, R4. This positive signal controls output transistor Q3. The positive signal on the base of Q3 increases the emitter to collector current. The control field of the exciter generator is in the collector circuit. Increasing the output of the exciter generator increases the field strength of the ac generator, which increases the generator output. An underspeed switch located near the F+ terminal prevents generator excitation when the frequency is at a low value. When the generator reaches a suitable operating frequency, the switch closes and allows the generator excitation. Resistors R27, R28, and R29 connect in series with the normally closed contacts of relay K1. The coil of relay K1 connects across the power supply (CR4) for the transistor amplifier. When the generator starts turning, electricity from the 28-volt dc bus goes to the exciter generator field to flash the field for initial excitation. When the field of the exciter generator energizes and the ac generator output voltage increases, relay K1 energizes, opening the field flash circuit.

Another type of solid-state voltage regulatoroperates by sensing the voltage existing on thelines. It amplifies the changes in this signal, and varies the average current supplied to the field winding of the integral exciter. The voltage regulator consists of a sensing circuit with input rectifiers, a temperature compensated Zener diode, reference and error-detecting bridge, and a three-stage transistor amplifier. The output of the bridge circuit is a voltage inversely proportional to the difference between generator voltage and regulator set voltage. This output is referred to as the error signal. Transformer T1 in the regulator supplies three-phase, ac generator output. It provides isolation from the generator and delivers correct utilization voltages. The transformer output passes through the full-wave bridge rectifier (CR1) to obtain a dc voltage to supply the comparison circuit. The rectifier output is proportional to the average of the three line voltages. The voltage reference and error-detecting bridge uses this voltage for comparison with the constant voltage across the Zener diode (CR5), which tells whether the generator output is too high or too low. Potentiometer R7 permits adjustment to the desired voltage. The glow tube (V1) serves to increase the sensitivity of the voltage reference and error-detecting bridge. Thermistor RT1provides temperature compensation in the comparison circuit. It offsets the effects of changes in other elements of the circuit that result from temperature variations to maintain a nearly constant voltage. The error-detecting bridge output voltage sawtooth wave shape is due to the ripple resulting from the semi-filtered, three-phase rectifier supply. This sawtooth voltage goes to the input of the first stage of the three-stage transistor amplifier. Overdriving the second and third stages obtains an essentially square wave output. The effect of the error detecting bridge output is to modulate the width of the pulses passing through the amplifier. The power for operating the three-stage transistor amplifier comes through the full-wave bridge rectifier (CR4) from transformer T1. Obtaining amplifier power this way requires special consideration. There are conditions that require excitation when no voltage is available to supply the amplifier. Such conditions exist during initial buildup of system voltage and during three-phase short circuit on the generator. Control relay (K1) connects across the full-wave bridge rectifier (CR4), overcoming these obstacles. When the relay is de-energized,its contacts provide permanent magnet generator (PMG) voltage to the exciter field. When generator voltage is 90 volts line to line, voltage across CR4 energizes control relay (K1), removing the self-excited field circuit. The voltage regulator then supplies the exciter field. The absence of phase shift and fast response characteristics of transistor-type amplifiers eliminates feedback networks and stabilizing transformers in this voltage regulator. Varying the output current to the exciter field varies the width of the square wave impulses. As the voltage rises (shown by the dotted back-to-back sawtooth), the square wave pulse to the exciter field is off longer than it is on. This causes the output of the ac generator to decrease. The decrease in voltage causes the back-to-back sawtooth to drop to its normal value (shown by the solid waveform). This causes the on and off times of the square wave pulse to the exciter field to be about equal. Varying the on and off excitation to the exciter field controls the ac generator output.

EXERCISES FOR BETTER REMEMBERING THE TOPIC