Gas turbine and nuclear power

 

The gas turbine engine, essentially a jet engine coupled to a turbine that is geared to a propeller shaft, appeared to have found a niche in commercial ship propulsion about 1970. However, the fuel price increase of the 1970s, which gave diesel its dominance over steam, gave it dominance over gas as well, and the niche for the latter suddenly disappeared. On the other hand, the gas turbine remains the principal propulsion engine among naval combat vessels because of the high power that can be produced from very low weights and volumes of machinery.

Steam propulsion survives in certain naval vessels particularly submarines, where the heat source is a nuclear reactor. Extreme cruising range and independence from an air supply are advantages of using nuclear energy as the heat source in naval propulsion, but these advantages are of little merit in commercial shipping. A few prototype cargo ships with nuclear propulsion were built in the 1960s, but they did not lead to commercial application.

 

Electric drive and integrated machinery plants

 

Power is usually transmitted from propulsion engine to propeller by means of mechanical shafting. If the engine is a steam or gas turbine, or a medium-speed diesel engine, a speed reducer will be essential in order to match the most efficient engine speed to the most efficient propeller speed. The usual means for accomplishing this is mechanical gearing, but electrical transmission, with a propulsion motor running at a fraction of the speed of a propulsion generator, is an alternative.

Direct-current transmission is occasionally used because it allows propeller speed and engine speed to be completely independent. Alternating-current transmission with synchronous propulsion motors also is used, usually in high-powered propulsion plants because it avoids the commutation problems that handicap high-power direct-current machinery. Exact electrical synchronization of motor speed with generator speed is required, but the mechanical speeds need not be the same. The speed ratio between motor and generator is established by the number of poles in each machine, just as the respective number of teeth establishes a ratio between mating gears.

Electrical transmission was rarely applied to ships built between 1935 and 1970, but it enjoyed a revival of popularity after that. The impetus was the development of thyristor-based frequency converters for alternating-current power, along with the continuing recognition that electrical transmission offers a flexibility that is difficult to match with mechanical transmission. As examples of the latter point, power from a propulsion generator can be used for cargo handling, and a single generator can drive motors on several shafts. The frequency converters are a means of varying synchronous motor speed while frequency at the power source remains constant.

The typical electric-drive ship built in the late 20th century is a passenger cruise liner with twin propellers driven by synchronous alternating-current motors and powered by an array of medium-speed diesel engines driving synchronous generators. The engine-generators run at a constant 450 revolutions per minute, feeding 60-hertz current to a single bus. All power needs for the ship come from this bus, giving rise to the term integrated machinery plant. Power for the propulsion motors passes through thyristor-based frequency changers; by changing propulsion frequency, these devices regulate propeller speed while all other power users continue to receive 60 hertz from the main system. (J.B.W.)

 

DC MOTORS AND GENERATORS

 

In these days of rapid electronic advancement, technicians, especially digital electronic technicians, frequently overlook the importance of the electric motor as a key system element. In fact, in some educational systems, digital electronics students are not required to take industrially related courses, such as courses related to motors and generators. In this chapter on DC motors and generators, we have attempted to correct that situation by presenting a condensed version of the topic. While, of course, some topics are omitted here, the important principles are presented in some detail.

Many applications in industry and in process control (see Chapter 8) require electric motors as critical functional elements. Because such motors are electromechanical rather than purely electrical, their response sets the entire system's performance limits. So, technicians who wish to be completely functional in the industrial electronics environment must be familiar with the varieties of motors and generators. In addition, technicians must understand basic operating principles if they are to perform intelligent troubleshooting (and system design).

In discussions involving both DC motors and DC generators, it is convenient to refer to a generalized electric machine, because much of what is said about electric motors is equally applicable to generators. The generalized electric machine is referred to as a dynamo, a machine that converts either mechanical energy to electric energy or electric energy to mechanical energy. When a dynamo is driven mechanically by a power source such as a gas or diesel engine, or a steam or water turbine, and provides electric energy to a load, it is called a generator. If electric energy is supplied to the dynamo, and its output is used to provide mechanical motion or torque (a.force acting through a distance), it is called a motor. Generators are rated at the kilowatts they can deliver without overheating at a rated voltage and speed. Motors are rated at the horsepower they can deliver without overheating at their rated voltage and speed.

Dynamo Construction

 

The dynamo construction consists of two major subdivisions, the rotor (armature), or rotating part, and the stator, or stationary part. We will discuss these two parts in detail in the following sections.

Rotor

 

The rotor consists of armature shaft, armature core, armature winding, and commutator.

The armature shaft is the cylinder on which the rotor components are attached. The armature core, armature winding, and commutator (mechanical switch) are attached to the armature shaft. This entire assembly rotates. Sometimes, fins are attached to the shaft to provide cooling of the dynamo.

The armature core is constructed of laminated (thin-sheet) layers of sheet steel. These layers provide a low-reluctance (magnetic resistance) path between the magnetic field poles. The laminations are insulated from each other and attached together securely. The core is laminated to reduce eddy currents, which are circular-moving currents. The grade of sheet steel used is selected to produce low loss due to hysteresis (lagging magnetization). The outer surface of the core is slotted to provide a means of securing the armature coils.

Two basic armature, or field pole, forms are available in the construction of the dynamo: salient (standing out from the surrounding material) and nonsalient (minimal projection) poles.

The armature winding consists of insulated coils, which are insulated from each other and from the armature core. The winding is embedded in the slots in the armature core face.

The commutator consists of a number of wedge-shaped copper segments that are assembled into a cylinder and secured to the armature shaft. The segments are insulated from each other and from the armature shaft, generally with mica. The commutator segments are soldered to the ends of the armature coils. Because of the armature rotation, the commutator provides the necessary switching of armature current to or from the circuit external to the armature.

The armature of the dynamo performs the following four major functions:

1. It permits rotation for mechanical generator action or motor action.

2. Because of rotation, it provides the switching action necessary for

commutation.

3. It provides housing for the armature conductors, into which a voltage is
induced or which provide a torque.

4. It provides a low-reluctance flux (magneSticlines) path between the field
poles. Recall that flux or flux lines are invisible magnetic lines of force.