Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation 9.0 GENERATOR, EXCITER, AND through a magnetic field. Figure 9-1 shows VOLTAGE REGULATION the principles being discussed in this section. As the conductor passes through This chapter presents the major the magnetic field, in this case downward, it components of the electrical generator, the cuts each of the lines of magnetic force exciter, and the voltage regulator and (flux) which causes a current to be explains how they relate to the "induced" in the conductor. Because the development of power by the diesel engine conductor has a resistance, it is known driven generator unit. from 'ohms law' that the voltage is equal to the current times the resistance. Learning Objectives Therefore, a voltage is also 'induced' between the two ends of the conductor. If As a result of this lesson, you will be able to: the conductor is connected to a closed electrical circuit, this voltage would cause a 1. Describe the functions of the generator, current to flow. The amount of current flow exciter, and voltage regulator. is a function of the voltage induced and the electrical resistance of the load in the 2. Identify the major components of the circuit. generator and how they inter-relate. 9.1.2 Induced Voltage 3. Describe how diesel engine output power relates to the power demands of The actual voltage induced in the conductor the generator. is determined by the number of lines of flux cut per unit of time. Two key factors affect 4. Describe the function of the excitation the magnitude of voltage induced. system and the associated voltage regulator. • The speed at which the conductor moves through the fixed magnetic field 5. Identity the major components of the and the strength of the magnetic field exciter and voltage regulator system. determine the output voltage. This speed is a function of the rotational 9.1 Generator Principles speed (RPM) of the generator/engine. As the speed of the engine the The following is a brief discussion of generator increases, the voltage generator operation and its relationship to produced also increases. the mechanical load placed on the diesel engine. Since the operating speed of the engine and generator is constant in order to 9.1.1 Electromagnetic Induction maintain the desired frequency, another method of voltage control must be Electromagnetic induction, the basic employed. principle of generator operation, involves the movement of an electrical conductor • Generator output voltage is most often Rev 1/11 9-1 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation controlled by regulating the strength This is shown in the part of the diagram (flux intensity) of the magnetic field. labeled 'Start.' As the armature is turned to This is accomplished by the generator a position 90 degrees from the first, the two excitation system. The excitation ends of the loop is acted upon in a manner system monitors the generator output wherein the voltages generated at each and regulates the magnetic field to end of the loop are additive, as shown in maintain the desired voltage. As the the '1/4-cycle' diagram. Peak output load on the generator is increased, an voltage is generated at each cycle point. increase in current flow causes the As the armature continues to rotate, it voltage to drop. The excitation system again gets to a position of no voltage senses this decrease in voltage and generation, shown in the ‘½-cycle diagram’. increases the strength of the magnetic As the rotation continues, a voltage is again field to return the voltage to the desired generated. A close examination of the level. wiring out of the armature reveals that the connections have become inverted. This 9.1.3 How the generator works results in the opposite polarity of voltage. Figure 9-2 shows the principles discussed The diagram at the bottom of the figure above implemented into a machine to shows the resulting build-up and decay and produce a voltage. In this implementation, opposite polarity build up and decay again a ‘U’ shaped form is provided with a ‘gap’ through two cycles (two rotations of the between the open ends of the ‘U’. A coil of armature). The resulting voltage build-up wire is wrapped about the legs of this form and decay forms a sinusoidal wave that is to produce a magnetic field across the gap. defined as 'Alternating Current' or AC. In the gap, an armature is formed by a loop of wire. The loop exits the armature onto This is the basis for a single phase two slip rings. The slip rings are contacted alternator. Two other sets of coils offset by by brushes that connect the generator to 120 degrees and connected to slip rings the outside electric circuit. An engine or would form a machine to generate 3-phase some other prime mover is connected to AC power. This is the basis for all AC 3- the armature causing it to rotate inside the phase generation. gap. When the ‘field’ coil is energized to establish a magnetic field/flux in the gap If instead of using two slip rings a single and the armature is then rotated, a voltage ring that is split into two segments were is generated in the armature. The slip rings used, as the armature rotated, there would and brushes conduct this voltage out to be a buildup and decay of voltage as some load "A." before; but the split slip ring would reverse the connection on each half revolution. Figure 9-3 shows a blowup of the armature The split slip ring configuration is commonly in the gap. As the armature is rotated in its referred to as a 'commutator.' This would initial position, no voltage is created result in a machine that puts out a pulsating because the magnetic flux is equal but DC current. By combining a great many opposite on both branches of the loop. poles and the same number of segments Rev 1/11 9-2 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation on the armature commutator, an almost voltage. This is the case when the power steady DC output would be produced. This factor is 1.0 (unity). The real power (KW) in is the principle of the DC generator or that case is equal to the apparent power motor. (KVA). These terms will be discussed later. When the current is not in phase with the The AC alternator described above has a voltage, there is a lesser power factor, and number of problems. The armature and its the KW is less than the KVA. The KW is slip rings have to handle all the load current still in phase with the voltage, but the KVA that is produced by this generator. The has shifted slightly due to the shift in the brushes and slip rings restrict the amount current. This introduced KVAR, and will be of current that can be handled. To explained later. eliminate this problem, design and construction were changed such that the 9.2 Generator Construction small excitation current now goes through the brushes and slip rings to the rotating Figure 9-5 shows a cutaway of a typical armature field. The large AC current generator. The generator consists of a induced into the stationary stator windings shaft on which is mounted a hub, more is transmitted to the loads by solid often called the spider. The spider may be connections. This same principle also attached to the shaft by a press fit, with or applies to AC synchronous motors as there without keys, or by a flange and bolting. is little difference between an AC generator The spider has slots into which the field and an AC synchronous motor. It is a pole pieces are attached. Together, this matter of what is driving the system – an makes up the ‘rotor’. The rotor assembly electric motor or an engine-driven usually also includes slip rings used to generator. convey the field current into the field windings. These windings are wrapped This also simplifies the construction of the around the pole pieces. A view of a rotor generator. These machines very often also and shaft assembly is shown in Figure 9.7. are called 'alternators' in as much as the voltage and current are alternating. A The stator core consists of a special steel three-phase generator/alternator is simply stampings, called laminations, with slots to three single phase machines interlaced hold the stator windings. The stator core with one another, sharing the same rotor has spaces between some of the assembly, with wiring brought out to laminations through which air is force by connect each phase into the electrical fans on the rotor assembly. This is to system. The result of the interlacing of the provide cooling for the stator core and the alternator windings is shown on the voltage windings. A steel framework supports and trace shown at the bottom of Figure 9-4. aligns the stator core assembly. The steel framework also usually supports the The upper part of Figure 9-4 shows how bearings of the generator. Some the current of each/any phase acts to generators are supplied with two dedicated produce the power. The left-most diagram bearings, one at each end of the generator shows the current in phase with the rotor assembly. Others are supplied with Rev 1/11 9-3 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation an outboard bearing at the far end of the F is frequency in hertz (Hz) generator; the engine end bearing supports P is the number of poles the other end of the generator.