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3.12.2: Construction and Construction and Purpose of CoPurpose of Components in DC Genemponents in DC Generatorrator In a practical dc generator we obtain high voltage outputs by:

 Using a large number of coils of many turns instead of single loops.

 Rotating the coils at high speed.

 Using electromagnets to provide a strong magnetic field and mounting the coils in which the voltage is to be induced on a soft iron core: the air gap between this core and the electromagnet pole pieces is very small. The electromagnets used to provide the magnetic field require a dc voltage source to pass current through the winding. In small machines such as those used in aircraft, the design of the machine is simplified by using the output voltage of the generator itself to provide this current.

DC Generator DC Generator

Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3

Construction Construction

A dc generator consists of two main assemblies:

 THE STATOR OR FIXED PORTIONTHE STATOR OR FIXED PORTION. This carries the FIELDFIELD MAGNETMAGNET SYSTEM

SYSTEM, the BRUSHBRUSH GEARGEAR and the BEARINGSBEARINGS. The Brush Gear Assembly and end frame may be considered as a separate major sub

assembly.

 THE ROTOR OR ARMATURE ASSEMBLYTHE ROTOR OR ARMATURE ASSEMBLY. This carries the COILSCOILS, COMMUTATOR

COMMUTATOR and often COOLING FAN BLADESCOOLING FAN BLADES.

Since the generator converts mechanical energy into electrical energy, mechanical energy must be supplied to the generator to turn it. The ‘prime mover’ used to drive aircraft generators is usually the engine.

The frame or yoke is the main chassis of the generator and it also serves to complete the magnetic circuit between the pole pieces. The pole pieces are laminated to reduce eddy current losses, and the field coils or windings are mounted on the pole pieces. The end housings contain the bearings for the armature which rotates at high speed, and one of these housings also holds the brush gear.

The armature (the rotating part of the machine) is made up of shaft, armature core, armature windings or coils, and commutator. The armature core is laminated to reduce eddy current losses, and the armature windings rest in slots cut in the core, but insulated from it.

The commutator is made of copper segments insulated from each other, and from the shaft. The ends of the armature windings are hard soldered to their appropriate commutator segments.

The brushes ride on the commutator and carry the generated voltage to the load. They are usually made of carbon and are held in brush holders in such a way that they can slide up and down against a spring so as to follow the small irregularities in the surface of the commutator.

EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals

The Yoke The Yoke

Field Magnet System Field Magnet System

Typical Generators Typical Generators

Except for very small machines in which permanent magnets are used, the magnetic field is produced by electromagnets in such a way that the armature conductors pass under North and South poles alternately. The poles may be salient, in which case the armature emf wave form has a flat top, or may be flush pole, low reluctance which gives an almost sinusoidal wave form. Salient poles are the most common in aircraft DC generators.

The salient pole piece may be laminated to prevent eddy current heating, or it may be solid, with a laminated pole‘shoe’ fitted to the end.

It will be noted from the diagram that the yoke is an essential part of the magnetic circuit, and must therefore combine permeability with structural strength. It is normally of cast or rolled steel.

Field Assembly Field Assembly

The heavy iron or steel housing that supports the field poles is called the field frame. It not only supports the field poles but also forms part of the magnetic circuit of the field. Small generators usually have two to four poles while larger generators can have as many as eight main poles and eight interpoles. (Interpoles will be dealt with later). The pole pieces are rectangular and in

Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3

The Brush Gear The Brush Gear

Brushes are made of specially treated carbon which is self lubricating; therefore causing little commutator wear. They are carried in small open ended boxes called BRUSHBRUSH HOLDERSHOLDERS. Brush pressure is maintained on the commutator by SPRINGSSPRINGS. Connection to the external circuit is made by copper braid.

Electro-graphitic brushes of normal design, although generally reliable in performance when used in ground equipment and low-altitude aircraft generators, tend to wear very rapidly at high altitudes. This wear can be of the order of 12mm per hour and is because of the following factors:

 At ground level and low altitudes the moisture content of the atmosphere gives a substantial degree of lubrication between the contact surfaces of the brushes and the commutator or slip-rings on which the brushes are bearing.

 At high altitudes the moisture content of atmospheric air is negligible, and with little or no lubrication at the ‘rubbing contacts’ there is considerable friction. Rapid wear of the soft electro-graphitic brushes is, in

consequence, inevitable.

Normally the contact resistance between brush-faces and commutator (or slip-ring) surfaces is fairly high because of the existence of a resistive film formed on the metallic surfaces by the electrolytic decomposition of the moisture content of the atmosphere. At high altitudes this film is removed by frictional wear, and cannot be made good because of the dryness of the atmosphere. Hence the contact-resistance between brush surfaces and metallic surfaces becomes small. This reduction in contact resistance, in the case of a DC generator, gives rise to heavy reactive sparking which, in turn, accelerates brush erosion.

Lack of lubrication of the brush-to-commutator contact surfaces at high altitudes and the reduction of brush-contact resistance experienced at increasing altitudes, are largely eliminated by using brushes which have been especially developed for high-altitude operation.

Two distinct categories of high-altitude brushes are in general use:

 Brushes which form a constant resistance semi-lubricating film on the commutator or slip-rings.

EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals

Film Forming Brushes Film Forming Brushes

The make-up of these brushes includes such chemicals as barium fluoride which builds up, progressively, a constant-resistance semi-lubricating film on the surfaces of the commutator or slip-rings.

 Brushes of this category do not wear abnormally at altitudes of up to some 35,000 ft providing that generators to which such brushes are fitted are previously run at low altitude for some hours to allow the formation of the protective film.

 This film, once it has been formed, is very dark in colour and to the inexperienced eye it may well give the impression of a dirty commutator or slip-rings.

Non Film Forming

Non Film Forming BrushesBrushes

Brushes in this category contain a lubricating ingredient such as molybdenum disulphide: this lubricant is often packed in cores running longitudinally through the brush.

 Since the brush is itself self-lubricating there is no question of preliminary formation of film, hence there is no necessity for running generators fitted with these brushes at low altitude before entering into high-altitude operation.

 Against this advantage of immediate availability for high-altitude operation must be set the disadvantage of appreciably shorter life, due to somewhat more rapid wear when compared with film-forming brushes.

The following precautions MUST be observed when using high-altitude The following precautions MUST be observed when using high-altitude brushes:

brushes:

 Film-forming brushes must not be used at high altitudes until the generator has been in operation for a specified period after fitting the brushes to a machine with a ‘Clean’ commutator or slip-rings – this period is essential to allow the film produced by brush action on the commutator or slip-ring surface to attain a serviceable thickness.

 Under no circumstances should non-film forming brushes be run on films created by film-forming brushes, nor should film forming and non-film- forming brushes be used simultaneously in the same machine. When changing from film-forming to non-film forming brushes the existing film must be completely removed by cleaning the commutator or slip-ring with a rag moistened in lead free gasoline, or other approved cleaning agent.

Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3

The Armature The Armature

The rotor or Armature Assembly consists of the shaft, the iron core, the output windings and the commutator, as shown.

 The Iron Core provides a low reluctance path between the field pole pieces giving increased flux density, ensuring that the largest emf possible is induced into the output windings.

 The core is constructed as a laminated soft iron drum with longitudinal slots into which the output windings are fitted.

 The core is laminated to reduce eddy currents and thus heat.

 The Output Windings are placed in longitudinal slots in the iron core to reduce the magnetic circuit air gap. The armature and coil windings are vacuum impregnated with silicone varnish to maintain insulation resistance under all conditions with the coils also insulated with p.t.f.e. [Poly-tetra- fluoro-ethane].

The windings are wedged into the slots with insulating material to prevent them from being thrown out by centrifugal force.

All coil connections are silver soldered to withstand local hot spot temperatures.

A Typical Armature Assembly A Typical Armature Assembly

EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals

Wave Winding Wave Winding

Another feature of multi-pole machines is the manner in which the Coils of the armature winding are connected together to provide the required output conditions. One method, called wave winding, provides increased output voltage by arranging for the voltages induced by each pair of poles to be added in series.

 Therefore, the output voltage is twice (four pole) and three times (six pole) that of the equivalent two pole machine. With wave winding the output voltage may be obtained across one pair of brushes.

Lap Winding Lap Winding

The other armature winding method is called lap winding and this method is most useful when high output current is required.

 In lap winding, groups of series connected coils are connected in parallel by the provision of additional brushes at points around the commutator which are equal in potential.

 In a four pole machine this results in the provision of four parallel current paths from the two positive brushes to the two negative brushes.

 In a six pole machine there are six parallel current paths from the three_{positive brushes to the three negative brushes.} The provision of additional parallel paths makes the lap wound generator suitable for high output current.

Wave winding is u

Wave winding is used for DC sed for DC generators of generators of high output vohigh output voltage. ltage. LapLap winding is used for DC generators of

winding is used for DC generators of high output current.high output current. The Commutator

The Commutator

This is a cylinder mounted at one end of the armature and consists of a large number of copper segments. The segments are wedge-shaped and a large number are assembled side by side to form a ring, each being insulated from the other by a mica insulating strip.

Each segment forms the junction between two armature coils, the wires being soldered into risers at the ends of the segments.

Generator Cooling Generator Cooling

The maximum output of any generator, assuming no limit to input mechanical power, is largely determined by the facility with which heat (arising from

Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3

natural processes of heat radiation from the extensive surfaces of the machine case may well provide sufficient cooling effect, but such ‘natural’ cooling is hopelessly inadequate for the lightweight high output generators used for aircraft electrical supply, and must, therefore, be supplemented by forced cooling.

The majority of aircraft generators in current use are blast-cooled by slipstream air.

Generators fitted to the modern aircraft are oil cooled.

Adequate cooling may, therefore, be self induced, separately induced, a ram air function, or oil heat exchanger system.

EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals

Generator Drives Generator Drives

The fundamental requirements of the element through which torque is transmitted to the rotor shaft of the generator may be summarized as follows:

 Effective transmission of torque up to a specified maximum.

 Effective interruption of torque transmission if the torque-demand of the generator exceeds the permitted maximum, this condition can arise as a result of seizures of the generator rotor, etc.

 Quick and simple removal and replacement of the torque-transmission element.

The requirements quoted above are satisfied almost entirely by ‘weak -link’ devices known as quill drives. The device is basically a ‘necked’ metal shaft with serrations or splines (These may be either male or female) at one or both ends. The serrations or splines mate with corresponding formations on the driven rotor shaft to transmit the torque delivered by the drive unit, and the ‘necked’ portion is designed to shear in the event of rotor seizure, etc, thus interrupting the drive and protecting the components against further possible danger.

Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3

Notes: Notes:

EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals

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