Chapter 4 DC Generators

航空发表于 2011-9-20 08:15:44

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航空发表于 2011-9-20 08:16:24

Chapter 4 DC Generators Common terms Terminal Voltage Terminal voltage, as applied to DC generators, is defined as the voltage that can be measured at the output of the generator. Counter-Electromotive Force (CEMF) In a generator using a rotating armature, the conductors cut the magnetic lines of force in the magnetic field. Voltage is induced in the armature conductors. This induced voltage acts counter to applied voltage; therefore, it is called counter-electromotive force (CEMF). Applied Voltage Applied voltage is defined as the voltage that is delivered across the load. This voltage should be the same as terminal voltage; however, various circuit faults and losses may reduce the terminal voltage. Commutation commutation is the mechanical conversion from AC to DC at the brushes of a DC machine Armature • The purpose of the armature is to provide the energy conversion in a DC machine • the armature converts mechanical energy to electrical energy. Armature • The purpose of the field in a DC machine is to provide a magnetic field for producing either a voltage (generator) or a torque (motor). • The field in a DC machine is produced by either a permanent magnet or an electromagnet.
 Summary • The purpose of the armature is to provide the energy conversion in a DC machine. • The purpose of the rotor is to provide the rotating element in a DC machine. • In DC machines, the purpose of the stator is to provide the field. • The purpose of the field in a DC machine is to provide a magnetic field for producing either a voltage or a torque. A basic DC generator has four basic parts: • A magnetic field; • A single conductor, or loop; • A commutator; and • Brushes The magnitude of the voltage produced i s dependent on a number of factors: • The strength of the magnetic field • The speed at which the conductor cuts the magnetic field • The length of the conductor within the magnetic field • The angle at which the conductor cuts the magnetic field
 
 DC from Four Armature Loops Eg = KN (4-1) Where Eg = generated voltage; K = fixed constant; = magnetic flux strength; N = speed in RPM The magnitude of the EMF induced in a conductor by electro-magnetic induction is dependent upon the following factors: • The rate at which the conductor is cut by the lines of magnetic flux (in this case speed of armature rotation). • The length of the conductor (determined by the number of turns in the armature winding). • The flux density (the strength of the magnetic field). • The aircraft generator, whether AC or DC, is driven by the aircraft engine and therefore its speed of rotation is variable, especially in the case of the piston engine aircraft. • The EMF induced in the armature windings of a generator will vary directly with the speed of rotation of the armature By this means the EMF induced in the generator armature, and therefore the generator output voltage, can be controlled regardless of generator speed or electrical load by varying the current supplied to the core winding of the electro-magnet. When the generator field current is supplied from an external source of direct current, as in this case, this is known as external, or separate excitation. Figure 4-9. Self-excitation Generator Residual magnetism • The soft iron of the electro-magnets retains a small amount of magnetism, known as residual magnetism, even when there is no field current. • This residual magnetism is sufficient to induce an EMF in the armature of the generator when it first starts to rotate, which initiates a current flow from the generator. Residual magnetism • Residual magnetism may be lost, or its polarization reversed, due to excess heat, shock or reversal of field current flow. • The residual magnetism can be restored by briefly passing a current through the field. This is known as field flashing, or flashing the field. Parameter of DC Generator Terminal Voltage DC generator output voltage is dependent on three factors • The number of conductor loops in series in the armature • Armature speed, and • Magnetic field strength. A DC generator contains four ratings. • Voltage • Current • Power • Speed DC Generator Construction • The Yoke is a cylinder of cast iron, which supports the pole pieces of the electromagnetic field. • The Armature is driven by the aircraft engine, and holds the windings (in which the output voltage of the machine is induced) and the commutator. • The Commutator changes the AC voltage induced in the armature into DC voltage. • The Quill Drive is a weak point, which is designed to shear and protect the engine if the generator seizes. • The Suppressor reduces radio interference, which may result from sparking between the brushes and commutator. DC Generator Construction TYPES OF DC GENERATORS • Shunt-Wound DC Generators • Series-Wound DC Generators • Compound Generators Shunt-Wound DC Generators Shunt-Wound DC Generators The shunt-wound generator, running at a constant speed under varying load conditions, has a much more stable voltage output than does a series-wound generator. Series-Wound DC Generators Series Generator A series generator has poor voltage regulation, and, as a result, series generators are not use for fluctuating loads. Compound Generators The change in output voltage from no-load to full-load is less than 5 percent. A generator with this characteristic is said to be flat-compounded . For some applications, the series winding is wound so that it overcompensates for a change in the shunt field. The output gradually rises with increasing load current over the normal operating range of the machine. This type of generator is called an over-compounded generator. The series winding can also be wound so that it undercompensates for the change in shunt field strength. The output voltage decreases gradually with an increase in load current. This type of generator is called an under-compounded generator. ALTERNATORS Alternators • Alternators used in many light single and twin-engined aircraft • Alternators are lighter than DC generators • Alternators do not suffer from the problems of arcing produced by commutation • The armature winding is in the stationary casing of the machine and the generator field windings and their electro-magnets are on the rotor. • Only the relatively small field current need be passed through brushes and slip rings to the rotating field windings. In aircraft alternators, the rotating magnetic field cuts through the stationary conductors of the armature winding, inducing EMF. The armature winding is connected to the output terminals of the alternator, from which the load current is supplied to the distribution bus bars through a rectification system that converts the AC output to DC. Simple Alternator Rectifier A rectifier is a static semiconductor device that permits current flow in one direction only and thereby converts bi-directional AC into unidirectional DC. Alternator Circuit END OF CHAPTER 4

linairm 发表于 2011-10-22 20:28:57

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Chapter 4 DC Generators