Transcript Chapter 16
Chapter 16 DC Generators Objectives After studying this chapter, you will be able to: • Explain the theory of electromagnetic induction • Describe the basic operation of the AC generator • Describe the construction and operation of various types of DC generators Objectives (cont’d.) • Describe the uses and operating characteristics of various types of DC generators • Discuss the methods of connecting DC generators and the basic troubleshooting procedures Electromagnetic Induction • Takes place when a conductor moves across a magnetic field or when a magnetic field moves across a conductor – Electromotive force induced in the conductor – Direction of emf determined by using left hand rule for a generator Generator Construction • Basic generator – Consists of a permanent magnet mounted on a frame (yoke) – Coil of insulated wire mounted on iron core (rotor or armature) is positioned between magnet poles – Armature rotates through magnetic field Generator Construction (cont’d.) • Amount of emf produced depends on – Strength of main field flux – Number of loops of wire on armature – Angle at which armature coils move across lines of force (right angles produce most voltage) – Speed of coil rotation Generator Construction (cont’d.) • All rotating generators produce an alternating emf • Armature construction – Two types of windings: lap and wave winding – Lap winding is used to obtain high current capacity – Wave winding is used to obtain high voltage output Generator Construction (cont’d.) Generator Construction (cont’d.) – Core is made of soft iron or steel disks called laminations – Disks are dipped in insulating varnish and mounted on the rotor shaft – Armature core slots are lined with insulation called fish paper – Commutator made of copper segments Generator Construction (cont’d.) • Brushes – Connect the commutator to load conductors – Made of graphite and carbon • Frame and field poles – Field cores are attached to the frame which provides support and forms part of circuit – Field coils wound with cotton covered wire with a baked enamel insulation Generator Construction (cont’d.) • Field excitation – In all generators field flux is produced by current flowing in coils placed on the field cores • Except magnetos – Separately excited generator • Field is excited from a separate source – Self-excited generator • Current is obtained from machine’s own armature Generator Operation • Effect of armature current – Armature reaction results when main field is distorted as a result of the interaction between the two fields • Neutral plane – When the armature coils are moving parallel with the lines of force • Moving through the neutral plane • No emf is induced Generator Operation (cont’d.) • Armature self-induction – When current increases, magnetic field increases, expands and moves across coil loops, inducing an emf into the coil – Voltage of self-induction opposite to the applied voltage • Interpoles (commutating poles) – Method of adjustment to changing loads Generator Operation (cont’d.) • Compensating for armature reaction – Compensating windings placed in main pole faces to eliminate armature reaction • Other effects of armature current – Magnetomotive force that opposes the main field flux is produced, decreasing the generated emf – Reversed torque developed in the rotor Generator Voltage • Equation used for average emf produced by a generator Generator Voltage (cont’d.) • Saturation curve – Beyond a certain number of ampere-turns, all electromagnets become saturated – Near saturation point • Large increase in current causes only a slight increase in voltage Self-Excited Generator • Three types: shunt, series and compound – Difference is how the armature and field windings are connected • Self-excited generator may fail to build up a voltage, causes include – Loss of residual magnetism – Break or opening in the field – Loose brush connections or contacts Self-Excited Generator (cont’d.) • Shunt generators – Field and armature connected in parallel • Series generators – Field connected in series with the armature • Compound generators – Combines certain features of shunt and series generators into one machine – Better for varying loads Separately Excited Generator • Magnetization current for field coils is supplied externally – From DC generator, batteries or rectifier • Field current is independent of armature emf – Field flux is less affected by load changes than in the self-excited generator • High cost and large physical size Voltage Control Versus Voltage Regulation • Voltage regulation determined by machine design – How well the generator maintains constant output voltage under changes in load • Voltage control takes place outside the generator – Rheostat controls the current through the shunt field Parallel Operation of Generators • Shunt generators in parallel Parallel Operation of Generators (cont’d.) • Compound generators in parallel Generator Efficiency • Three major losses: mechanical, electrical and magnetic • Mechanical losses – Friction at bearings and between brushes and commutator, and winding losses • Electrical losses – Resistance of the field and armature conductors Generator Efficiency (cont’d.) • Magnetic losses – Reluctance in the magnetic circuit: eddy currents and hysteresis – Hysteresis can be reduced by selecting core materials with good permeability Summary • A generator is constructed of a magnet, yoke, coil, and core (armature) • Shunt, series and compound generators are types of self-excited generators • A separately-excited generator is less impacted by load changes than a selfexcited generator • Armature current affects the voltage Summary (cont’d.) • Generator voltage may reach a saturation point in which large increases in current produce only small increases in voltage • Generators may be connected in parallel to increase the generated voltage • Generator efficiency is influenced by mechanical, electrical and magnetic losses