Dc Power Generation 6.3

DC POWER GENERATION 6.3 INTRODUCTION

INTRODUCTION • Depending on the type of aircraft, and the extent to which electrical power is to be utilized for operation of its systems and components, • The primary supply of such power • Direct Current (d,c) • Alternating Current (a.c). •

GENERATOR • A generator is a machine that converts mechanical energy into electrical energy by process of electromagnetic induction. • In both d.c. and a.c. types of generator, the voltage induced is alternating; • The major difference are the method by which the electrical energy is collected and applied to the circuit externally connected to the generator.

• AC generator the output are collected and distributed by Slip Ring . The output are Sinusonal Wave

• To convert the a.c.to dc unidirectional or d.c., replace the slip rings with a commutator.

• To smooth out the pulsations and to produce a more constant output, • Additional wire loops and commutator are provided. The pulsating output is reduced to a ripple.

To smooth DC output many loops are used.

When many loops are used they are called ARMATURE

INTERNAL RESISTOR IN A DC GENERATOR •

D.C machine has resistance due to 1. Armature windings 2. Brushes 3. Brush to commutator surface contact •

This is called internal resistance and can be measured across the terminals of the generator.

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Internal resistance causes the generators terminal voltage changes in the load current. As the load current increases, the voltage dropped across the internal resistance increases and the terminal voltage decreases.

The generated emf E = Ir + V

TYPE OF DC GENERATOR • • • •

There commonly two type of DC Generator i. The Lap Wound Generator Ii. The Wave Wound Generator This generator are constructed for their purpose and function. • The Lap Wound type are large heavy current type used for heavy machine puerpose. • The Wave wound type are smaller in size for low current type. Typically used as aircraft generater APU. Car , etc….

SHELF EXCITED SHUNT WOUND GENERATOR • Shunt-wound generators are used in aircraft d.c. power supply systems. The term “shuntwound” • the high-resistance field winding is connected across or in parallel with the armature as shown • The armature current divides into two branches, one formed by the field winding, the other by the external circuit. • Since the field winding is of high resistance, • Advantage is gained of having maximum current flow through the external circuit

DC SHUNT Field winding connected parallel with armature. Need a residual magnetism on field and increase when rotor rotate until saturation. NOTE. The shunt generators does not maintain a constant voltage with varying loads. Maximum during no loads and decrease as the loads increase.

STARTER GENERATOR • Gas turbine aircraft are equipped with starter-generator systems. Combination starter-generator • Operates as a starter motor to drive the engine during starting, • After the engine has reached speed, operates as a generator to supply the electrical system power.

• The starter-generator unit shown • Basically a shunt generator with an additional heavy series winding. • This series winding is electrically connected to produce a strong field and a resulting high torque for starting. • Starter-generator one unit performs the functions of both starter and generator and the total weight of starting system components is reduced, and fewer spare parts are required.

• Starter Generator

• The starter-generator shown below right has four windings; (1) series field, (2) shunt field, (3) compensating, and (4) interpole. • During starting, the series, compensating, and interpole windings are used. • Direct starter, since all the of the windings used during starting are in series with the source of 24 volts DC and 1,500 amperes is usually required for starting. While acting as a starter, shunt field is not used .

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Starter Generator Schematic

• When operating as a GENERATOR • Shunt Field, compensating Field and Interpole windings are used. • The output voltage is controlled by connecting: a.The shunt field in the voltage regulator circuit. b.The compensating and interpole windings provide almost sparkless commutation from no-load to full-load.

GENERATOR CHARATERISTIC • The characteristics of a generator • relationship between voltage and the load current flowing in the external circuit connected to a generator, • there are two type of charaters The external characteristic or relationship between terminal voltage and load current, The internal characteristic or relationship between the actual electromagnetic force (emf) generated in the armature windings and load current.

TYPICAL GENERATOR CONTRUCTION • The major parts, of a DC generator are : a. Field frame/yoke b. Rotating armature, c. Commutators and d. Brush assembly.

Field Frame • The field frame (or yoke) constitutes the foundation of a generator. • The frame has 2 primary functions: • 1) It completes the magnetic circuit between the poles • 2) It acts as a mechanical support for the other parts.

FIELD FRAME

ELECTROMAGNETIC POLES

• The magnetizing force inside a generator is produced by an electromagnet consisting of a wire coil called a field coil and a core called a field pole, or shoe. • The pole are laminated to reduce eddy currents losses • concentrate the lines of force • There is always one north pole for each south pole, so there is an even number of poles in a generator. • The main advantages of using electromagnetic poles are • (1) Increased field strength • (2) A means of controlling the field strength.

ELECTROMAGNETIC POLES

FIELD WINDING

FIELD COILS AND POLE SHOE •

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Reducing the space of air gap between the poles and the rotating armature. This increases the efficiency of the armature. When the pole piecesare known salient poles. The field coils are made up of many turns of insulated wire. The coils are wound and fastened on an iron core or pole shoes. The current required to produce the magnetic field is self generated by the unit itself.

ARMATURE • The armature assembly consists • armature coils, • the commutator, • The armature is mounted on a shaft which rotates in bearings located in the generator’s end frames. • the armature rotated in the magnetic field They are laminated to prevent the eddy currents. • Since the armature rotates, it is also called a rotor. •

COMMUTATORS • The commutator converts the Alternating current flowing in the armature (Rotor) into DC • The raised portion of each segment is called the Riser, • the leads of the Armature coils are soldered to each riser.

LAP WINDING CONFIGURATION • One end of a single armature coil attaches to one commutator segment while the other end is soldered to the adjacent segment. This is known as LAP WINDING • When an armature rotates at operational speed, the magnetic fields that it produces lags behind the speed of rotation. • Lap winding is a method for stabilizing these armature magnetic fields.

CARBON BRUSH • Carbon brushes act as the electrical contact between armature coils and an external circuit. • A flexible braided-copper conductor, called a pig-tail, connects each brush to the external circuit/load • The brushes are made of high grade carbon and held in place by spring-loaded brush holders that are insulated from the frame.

TERMINAL BLOCK • The lead from the brush and field windings are connected to the terminal posts Secured to a block mounted on the commutator end frame Sometime on the yoke assembly. The terminals and block are enclosed in a boxlike cover also secured to the end frame.

• SECTIONED VIEW OF A GENERATOR

COMMUTATION • Commutation is a process DC voltage output is taken from an armature which is ac voltage induced in it. A DC voltage is applied to the load because the output connections are reversed as each commutator segment passes under a brush. No sparking can occur between the commutator and the brush. Sparking between brushes and commutator is an indication of improper commutation because improper placement of the brushes.

SPARKING SUPPRESSION • Sparking at the brushes of a generator, will results in the propagation of electromagnetic waves which interfere with the reception of radio signals. • Screening and suppressing will reduce the interference from generators • Screening involves the enclosure of a generator in a continuous metallic casing and the sheathing of output supply cables in a metallic tubing or conduit

To prevent interference being conducted along the distribution cable system, The screened output cables are terminated in a filter or suppressor units. • Low or high by pass filters. • These filters are connected between generator casing (earth) and terminals. • The use of internal suppression system will help to save in overall weight of a generator installation.

Types of filters • • • • •

A.High pass filters B. low pass filters C. Band Pass filters D. Band Stop Filters E. Smoothing and Decoupling Circuit

• L offers low reactance to the low frequencies, it pass easily to output terminals, causes high reactance to higher frequencies. • Capacitor C offers a low reactance to the high frequencies, it filtered off through it, but C offers a high reactance to low frequencies and it does not attenuate . • To improves the attenuation of the higher frequencies, number of these filter circuits are used in succession. cascade .

SMOOTHING AND DECOUPLING CIRCUIT • Smoothing and Decoupling circuits are special applications of filters. • A smoothing circuit changes a pulsating d.c. to a smooth d.c. in power supply circuits. The filter circuit offers a high reactance to a.c. and a low reactance to d.c. • A Decoupling circuit removes any unwanted a.c. from a d.c. voltage. The circuit offers a high reactance to d.c. and a low reactance to a.c. •

TYPICAL SMOOTHING AND DECOUPLING CIRCUIT

• This type of circuit a common in rectifier circuit

DC POWER GENERATOR • The DC Power Generation consist of • Aircraft generator output is 28 V DC. • Aircraft battery output is 24 volts DC. • 4 major components of the shunt wound generator: Yoke Armature. End frames. Brush gear assembly.

• A typical aircraft power and generation system