The AlternatorThe synchronous machine consists essentially of (a) a field system excited by direct current and (b) an armature. Almost invariably the armature is the stationary member and the field system the rotating member. The induced e.m.f. in the armature winding is a motionally induced e.m.f. and its mode of production identical with that of the D.C. machine. The only difference is that it is the magnetic field which moves whereas the armature conductor is stationary. As with a D.C. machine the e.m.f. induced in an individual armature coil ia an alternating e.m.f. and consequently by bringing the winding out to fixed terminals, the e.m.f. between these will be alternating also. The complete fixed armature, that is magnetic core and windings, is called the stator, and the rotating field system the rotor. The general constructional features of a salient pole alternator are shown below
As the field system rotates and carries its flux with it, each portion of the stator core will experience reversals of magnetisation, and therefore, as in a direct current machine, the core has to be laminated. For ventilation purposes, a series of radial ventilating ducts are provided. Since the field system rotates, its exciting winding has to be fed by means of two slip rings, but as the excitation voltage is low and the power taken by the field winding small, these present no difficulties
A salient pole has one field coil per pole, very like a D.C. machine. For the very high speeds of turbine driven alternators it is necessary to adopt a cylindrical construction for the rotor and in such a case the field winding has to be housed in a number of slots. A simplified form shows the cross section of a four pole turbo alternator , the dispostion of the rotor field of a turbo alternator rotor may be as high as 40,000 ft per min or 200 metres per sec. The stresses due to centrifugal force are exceedingly high. The rotors are thus made from steel forging, or in some cases from thick steel discs bolted together.
high speed rotorThe axial length is normally considerably greater than the diameter. Has the advantage of great strength and stiffness. The exciting current is carried by bar type conductors in the groups of slots shown below. All currents in one group are in the same direction , those on the next group on the opposite direction. Flux produced is distributed over surface approximately according to sine law.
Details of stationary armature alternators.Armature stampings pressed out of sheets of special magnetic iron or steel alloy. In the smaller sizes the stampings are pressed out in compete rings
Section through top stator of salient pole machine. The armature core is built up of laminations which are held tightly together by end clamping rings. Spacing strips inserted at intervals leave ducts for cooling air to pass through. The air is driven through by the fan action of the rotor and escapes via the apertures in the cast iron supporting frame.
Types of armature slot. The filled slot has round wires but it is common to have rectangular conductors to economise slot space.
Sectional simplified diagram of a turbo alternatorThe rotor is turned from a steel forging ans slotted to carry the exciting windings the slots being arranged as shown above. Because of the high running speed, alternators for large outputs have a considerable axial length compared with rotor diameter.
Layouts of A.C. generators
Conventional excitation scheme (Rotary)Separately excited D.C. exciter (Out dated)
Brushless excitation scheme using shaft mounted diodes (Rotary)Indirect self excitation (Error)
Comparison of the value required to control with a fixed value. When the variable differs from a fixed reference value an 'error' exists and the function of the controlling medium is to restore equilibrium e.g. if the voltage output falls on the brushless rotary excited alternator the a.v.r. controls the exciter field to restore equilibrium.
Modern compound scheme (static)Direct self excited (Functional)
Control of the voltage to a set value is achieved by the inherent characteristics of the machine.
A compound wound d.c. generator with a level compound characteristic has additional current in the series field under load conditions. In the self excited compound alternator there is a constant amount of excitation required for no load condition. Additional excitation due to more current form the current transformers is obtained in response to extra external demands
Recovery graphs for 'functional' and 'error' layouts
Shaft driven generating system
Methods of drive
- Belt or chain driven
- Direct coupling engaging the propeller shaft
- Power taken from the main gearbox.
- Power taken from the free end of the engine
For A.C. systems methods used include the use of a D.C. generator with an D.C./A.C. converter, or direct A.C. generation. With the latter either a constant speed drive is required or a frequency converter. With either method the revolutions at which the shaft alternator can be used is limited. In this way direct drive systems will generally be fitted in conjunction with a C.P. system which maintains constant engine speed under full away conditions.
- Saving on fuel costs, allows efficient use of heavy rather than gas oil
- reduced maintenance costs
- Capital saving on reduced number of auxiliary sets
- Reduced space and weight
- Reduction in noise
- Power available for propulsion reduced
- Capitol cost of plant
- Auxiliaries required for manoeuvring, although some medium speed plants are capable of manoeuvring with shaft alternators and C.P. system
- Complicated constant speed or frequency gear required with slow speed engines