DC Generator
Introduction
The generators is a device which converts mechanical power to electrical power.
There are two types of generators
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AC Generator
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DC generator
An AC generator produces alternating power. A DC generator produces direct power. However it is important to not that the current generated inside both type of generators is alternating in nature, the output is AC or DC depends on the method of collection of the current.
Both of these generators produce electrical power, based on same fundamental principle of Faraday's law of electromagnetic induction. According to these laws, when an conductor moves in a magnetic field it cuts magnetic lines force, due to which an emf is induced in the conductor. The magnitude of this induced emf depends upon the rate of change of flux (magnetic line force) linkage with the conductor. This emf will cause an current to flow if the conductor circuit is closed.
Hence the most basic two essential parts of a generator are :
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Magnetic Field
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Conductors which move inside that magnetic field
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Note : For the condutor to product electric current there has to be a relative motion between the magnetic field and the conductor i.e. either the conductor should move or the magnetic field should move.
Single Loop DC Generator
A single loop of conductor of rectangular shape is placed between two opposite poles of magnet. The loop is opened and connected it with a split ring as shown in the figure below. Split rings are made out of a conducting cylinder which cuts into two halves or segments insulated from each other. The external load terminals are connected with two carbon brushes which are rest on these split slip ring segments.
BASIC DC GENERATOR
Fleming's Right Hand Rule
This rule says that is you stretch thumb, index finger and middle finger of your right hand perpendicular to each other, then thumbs indicates the direction of motion of the conductor, index finger indicates the direction of magnetic field i.e. N - pole to S - pole, and middle finger indicates the direction of flow of current through the conductor.
FLEMING'S RIGHT HAND RULE
Let's us consider, the rectangular loop of conductor is ABCD which rotates inside the magnetic field about its own axis.
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When the loop rotates from its vertical position to its horizontal position, it cuts the flux lines of the field. As during this movement two sides, i.e. AB and CD of the loop cut the flux lines there will be an emf induced in these both of the sides (AB & CD) of the loop.​ As the loop is closed there will be a current circulating through the loop.
The direction of the current can be determined by Fleming’s right hand Rule.
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Now if we apply this right hand rule, we will see at this horizontal position of the loop, current will flow from point A to B and on the other side of the loop current will flow from point C to D.
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Now if we allow the loop to move further, it will come again to its vertical position, but now upper side of the loop will be AB and lower side will be CD (just opposite of the previous vertical position). At this position the
tangential motion of the sides of the loop is parallel to the flux lines of the field. Hence there will be no question of flux cutting and consequently there will be no current in the loop.
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If the loop rotates further, it comes to again in horizontal position. But now, said AB side of the loop comes in front of N pole and CD comes in front of S pole, i.e. just opposite to the previous horizontal position as shown in the figure beside.
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Here the tangential motion of the side of the loop is perpendicular to the flux lines, hence rate of flux cutting is maximum here and according to Fleming's right hand rule, at this position current flows from B to A and on other side from D to C.
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Now if the loop is continued to rotate about its axis, every time the side AB comes in front of S pole, the current flows from A to B and when it comes in front of N pole, the current flows from B to A. Similarly, every time the side CD comes in front of S pole the current flows from C to D and when it comes in front of N pole the current flows from D to C.
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If we observe this phenomena in different way, it can be concluded, that each side of the loop comes in front of N pole, the current will flow through that side in same direction i.e. out the conductor and similarly each side of the loop comes in front of S pole, current through it flows in same direction i.e. into of the conductor.
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If we see the output the generated wave is Alternating in nature, with the connection of split ring a DC output can be achieved.
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Note : The direction of the current can be determined by the Fleming's right hand rule, the direction of the current is the conventional direction of the current i.e Positive terminal to the Negative terminal. However there are certain book which consider the the flow of electrons as the direction of flow of current i.e. from the negative terminal to the positive terminal in such cases the direction of the current in a generator can be found out by Left Hand Rule (not the Fleming Left Hand Rule). The Left Hand Rule is similar to the Fleming Right Hand Rule except that the direction of the current gets reversed.
WORKING OF A GENERATOR
Working Principle of DC Generator
As we have seen above that the generator produced an alternating current and with the help of split rings A DC output can be achieved.
Let us see how
CASE 1
As we know now thats the first half of the revolution current flows always along ABCD and when connected with the split ring the current flow with will from split ring 1 to ABCD and then to split ring 2. The brush X is connected to split ring 1 and the brush Y is connected to the split ring 2.
So the total current Flow path will be from brush X to split ring 1 and in the path ABCD to the split ring 2 then to the brush Y and through the load back to the brush X. The Direction of current flow is shown in the diagram.
Case 2
In the next half revolution,current flows always along DCBA and when connected with the split ring the current flow with will from split ring 2 to DCBA and then to split ring 1. The brush X is now connected to split ring 2 and the brush Y is now connected to the split ring 1.
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So the total current Flow path will be from brush X to split ring 2 and in the path DCBA to the split ring 1 then to the brush Y and through the load back to the brush X. The Direction of current flow is shown in the diagram.
Hence, the current in the load resistance in both cases flows from brush Y to brush X. Hence current is unidirectional.
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The position of the brushes of DC generator is so arranged that the changeover of the segments 1 and 2 from one brush to other takes place when the plane of rotating coil is at right angle to the plane of the lines of force. It is so become in that position, the induced emf in the coil is zero.
WORKING OF A DC GENERATOR
OUTPUT OF A DC GENERATOR
Parts of DC generator
A DC generator has the following parts
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Yoke or Field frame
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Pole of Generator
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Feld winding
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Armature of DC Generator
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Commutator or Split Rings
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Brushes of Generator
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Bearing
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End frame
PARTS OF A GENERATOR
Field Frame Assembly
Yoke of DC generator serves two purposes,
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It holds the magnetic pole cores of the generator and acts as cover of the generator.
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It carries the magnetic field flux.
In small generator, yoke are made of cast iron. Cast iron is cheaper in cost but heavier than steel. But for large construction of DC generator, where weight of the machine is concerned, lighter cast steel or rolled steel is preferable for constructing yoke of DC generator. Normally larger yokes are formed by rounding a rectangular steel slab and the edges are welded together at the bottom. Then feet, terminal box and hangers are welded to the outer periphery of the yoke frame.
A practical DC generator uses electromagnets instead of permanent magnets. To produce a magnetic field of the necessary strength with permanent magnets would greatly increase the physical size of the generator. The field coils are made up of many turns of insulated wire and are usually wound on a form that fits over the iron core of the pole to which it is securely fastened. The exciting current, which is used to produce the magnetic field and which flows through the field coils, is obtained from an external source or from the generated DC of the machine. No electrical connection exists between the windings of the field coils and the pole pieces. Most field coils are connected so that the poles show alternate polarity.
Since there is always one north pole for each south pole, there must always be an even number of poles in any generator.
Note that the pole pieces project from the frame. Because air offers a great amount of reluctance to the magnetic field, this design reduces the length of the air gap between the poles and the rotating armature and increases the efficiency of the generator.
When the pole pieces are made to project they are called salient poles.
Pole Cores and Pole Shoes of DC Generator
There are mainly two types of construction available:
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Solid pole core, where it made of a solid single piece of cast iron or cast steel.
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Laminated pole core, where it made of numbers of thin, laminations of annealed steel which are riveted together.
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The thickness of the lamination is in the range of 0.04" to 0.01". The pole core is fixed to the inner periphery of the yoke by means of bolts through the yoke and into the pole body.
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The pole shoes are so typically shaped, that, they spread out the magnetic flux in the air gap and reduce the reluctance of the magnetic path.
Due to their larger cross - section they hold the pole coil at its position.
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Pole Coils: The field coils or pole coils are wound around the pole core. These are a simple coil of insulated copper wire or strip, which placed on the pole which placed between yoke and pole shoe
POLE, POLE SHOE AND FIELD WINDINGS
Armature
The armature assembly of a generator consists of many armature coils wound on an iron core, a commutator and associated mechanical parts. These additional loops of wire are actually called windings and are evenly spaced around the armature so that the distance between each winding is the same. Mounted on a shaft, it rotates through the magnetic field produced by the field coils. The core of the armature acts as an iron conductor in the magnetic field and, for this reason, is laminated to prevent the circulation of eddy currents.
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Gramme-Ring Armature
There are two general kinds of armatures: the ring and the drum. a ring-type armature made up of an iron core, an eight-section winding, and an eight-segment commutator. The disadvantage of this arrangement is that the windings, located on the inner side of the iron ring, cut few lines of flux. As a result, they have very little voltage induced in them. For this reason, the Gramme-ring armature is not widely used.
GRAMME-RING TYPE ARMATURE
Drum Type Armature
In Drum-type armature, armature core is in the shape of a drum and has slots cut into it where the armature windings are placed. The advantage is that each winding completely surrounds the core so that the entire length of the conductor cuts through the magnetic flux. The total induced voltage in this arrangement is far greater than that of the Gramme ring.
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Drum-type armatures are usually constructed in one of two methods, each method having its own advantage .The two types of winding methods are the lap winding and the wave winding. Lap windings are in generators that are designed for high current outputs. The windings are connected in parallel paths and for this reason require several brushes. The wave winding is used in generators that are designed for high voltage outputs. The two ends of each coil are connected to commutator segments separated by the distance between poles. This results in a series arrangement of the coils and is additive of all the induced voltages.
DRUM TYPE ARMATURE
Commutator
The commutator is located at the end of an armature and consists of wedge shaped segments of hard drawn copper, insulated from each other by thin sheets of mica. The segments are held in place by steel V-rings or clamping flanges fitted with bolts. Rings of mica insulate the segments from the flanges.
The raised portion of each segment is called a riser, and the leads from the armature coils are soldered to the risers. When the segments have no risers, the leads are soldered to short slits in the ends of the segments
Brush
The brushes ride on the surface of the commutator, forming the electrical contact between the armature coils and the external circuit. A flexible, braided copper conductor, commonly called a pigtail, connects each brush to the external circuit. The brushes, usually made of high-grade carbon and held in place by brush holders insulated from the frame, are free to slide up and down in their holders in order to follow any irregularities in the surface of the commutator. The brushes are usually adjustable so that the pressure of the brushes on the commutator can be varied and the position of the brushes with respect to the segments can be adjusted.
The constant making and breaking of connections to the coils in which a voltage is being induced necessitates the use of material for brushes, which has a definite contact resistance. Also, this material must be such that the friction between the commutator and the brush is low, to prevent excessive wear. For these reasons, the material commonly used for brushes is high-grade carbon.
The carbon must be soft enough to prevent undue wear of the commutator and yet hard enough to provide reasonable brush life. Since the contact resistance of carbon is fairly high, the brush must be quite large to provide a large area of contact. The commutator surface is highly polished to reduce friction as much as possible. Oil or grease must never be used on a commutator, and extreme care must be used when cleaning it to avoid marring or scratching the surface
COMMUTATOR AND BRUSHES
Generator Engine Coupling
Depending on the type, generators can be driven by gearbox or pulley and belt. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both.
A gear coupling is a mechanical device for transmitting torque between two shafts that are not collinear. It consists of a flexible joint fixed to each shaft. The two joints are connected by a third shaft, called the spindle
Bearing of DC Generator
For small machine, ball bearing is used and for heavy duty dc generator, roller bearing is used. The bearing must always be lubricated properly for smooth operation and long life of generator.
Types of DC Generators
Generally DC generators are classified according to the ways of excitation of their fields. There are three methods of excitation.
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Field coils excited by permanent magnets – Permanent magnet DC generators
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Field coils excited by some external source – Separately excited DC generators
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Field coils excited by the generator itself – Self excited DC generators
TYPES OF GENERATORS
Permanent Magnet DC Generator
When the flux in the magnetic circuit is established by the help of permanent magnets then it is known as Permanent magnet dc generator. It consists of an armature and one or several permanent magnets situated around the armature. This type of dc generators generates very low power. So, they are rarely found in industrial applications. They are normally used in small applications like dynamos in motor cycles.
Separately Excited DC Generator
These are the generators whose field magnets are energized by some external dc source such as battery .
A circuit diagram of separately excited DC generator is shown in figure.
SEPARATELY EXCITED GENERATOR
Where,
Eg = Generated EMF
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Ia = Armature Current
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Ra = Armature Resistance
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IL = Load Current
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V = Terminal Voltage
Voltage drop at armature = Ia × Ra
Let, Ia = IL = I (say),
Voltage across the load, V = Eg - (Ia x Ra)
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Power generated, Pg = Eg × I
Power at external load, PL = V × I
Series Wound Generator
In these type of generators, the field windings are connected in series with armature conductors as shown in figure below. So, whole current flows through the field coils as well as the load.
As series field winding carries full load current it is designed with relatively few turns of thick wire. The electrical resistance of series field winding is therefore very low (nearly 0.5Ω ).
SERIES GENERATOR
Where,
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Eg = Generated EMF,
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Ia = Armature Current,
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Ra = Armature Resistance,
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Ise = Series Field Current
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Rse = Series Field Resistance
IL = Load Current,
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V = Terminal Voltage
Voltage drop at Armature = Ia × Ra
Let, Ia = Ise = IL = I (say),
Voltage across the load, V = Eg - (Ia x Ra) - (Ise x Rse) = Eg - I (Ra + Rse)
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Power generated, Pg = Eg × I
Power at external load, PL = V × I
Shunt Wound DC Generators
In these type of DC generators the field windings are connected in parallel with armature conductors as shown in figure below.
In shunt wound generators the voltage in the field winding is same as the voltage across the terminal.
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The effective power across the load will be maximum when IL will be maximum. So, it is required to keep shunt field current as small as possible. For this purpose the resistance of the shunt field winding generally kept high (100 Ω) and large no of turns are used for the desired emf.
SHUNT GENERATOR
Where,
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Eg = Generated EMF
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Ia = Armature Current
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Ra = Armature Resistance
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Ish = Shunt Field Current
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Rsh = Shunt Field Resistance
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IL = Load Current
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V = Terminal Voltage
Here, Armature Current (Ia) is dividing in two parts, one is Shunt Field Current (Ish) and another is Load Current (IL).
So, Ia = Ish + IL.
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Shunt field current, Ish = V/Rsh
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Voltage across the load, V = Eg - (Ia x Ra)
Power generated, Pg = Eg × Ia
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Power delivered to the load, PL = V × IL
Compound Wound DC Generator
In series wound generators, the output voltage is directly proportional with load current. In shunt wound generators, output voltage is inversely proportional with load current. A combination of these two types of generators can overcome the disadvantages of both. This combination of windings is called compound wound DC generator.
Compound wound generators have both series field winding and shunt field winding. One winding is placed in series with the armature and the other is placed in parallel with the armature. This type of DC generators may be of two types- short shunt compound wound generator and long shunt compound wound generator.
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In a compound wound generator, the shunt field is stronger than the series field. When the series field assists the shunt field, generator is said to be cumulative compound wound. On the other hand if series field opposes the shunt field, the generator is said to be differentially compound wound.
LONG SHUNT GENERATOR
Where,
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Eg = Generated EMF
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Ia = Armature Current
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Ra = Armature Resistance
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Ish = Shunt Field Current
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Rsh = Shunt Field Resistance
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​Ise = Series Field Current
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Rse = Series Field Resistance
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IL = Load Current
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V = Terminal Voltage
Here Armature Current (Ia) is equal to Series Winding Current (Ise) and dividing in two parts, one is Shunt Winding Current (Ish) and another is Load Current (IL).
So, Ia = Ise = Ish + IL.
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Shunt field current, Ish = V/Rsh
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Voltage across the load, V = Eg - (Ia x Ra) - (Ise x Rse) = Eg - Ia (Ra + Rse)
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Power generated, Pg = Eg × Ia
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Power delivered to the load, PL = V × IL
SHORT SHUNT GENERATOR
Where,
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Eg = Generated EMF
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Ia = Armature Current
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Ra = Armature Resistance
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Ish = Shunt Field Current
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Rsh = Shunt Field Resistance
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​Ise = Series Field Current
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Rse = Series Field Resistance
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IL = Load Current
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V = Terminal Voltage
Here Armature Current (Ia) is dividing in two parts, one is Shunt Field Current (Ish) and another is Load Current (IL) which is also equal to Series Field Current (Ise).
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So, Ia = Ish + IL = Ish + Ise
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Shunt field current, Ish = V/Rsh
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Voltage across the load, V = Eg - (Ia x Ra) - (Ise x Rse)
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Power generated, Pg = Eg × Ia
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Power delivered to the load, PL = V × IL
Losses DC Machine
The losses in a DC Machine are as follows:
Copper losses
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Armature Cu loss
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Field Cu loss
Loss due to brush contact resistance Iron Losses
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Hysteresis loss
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Eddy current loss
Mechanical losses
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Friction loss
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Windage loss
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Copper Losses
These losses occur in armature and field copper windings. Copper losses consist of Armature copper loss, Field copper loss and loss due to brush contact resistance.
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Armature copper loss = Ia2Ra
where, Ia = Armature current and Ra = Armature resistance
This loss contributes about 30 to 40% to full load losses. The armature copper loss is variable and depends upon the amount of loading of the machine.
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Field copper loss = If2Rf
where, If = field current and Rf = field resistance
In the case of a shunt wounded field, field copper loss is practically constant. It contributes about 20 to 30% to full load losses. Brush contact resistance also contributes to the copper losses. Generally, this loss is included into armature copper loss.
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Iron Losses (Core Losses)
As the armature core is made of iron and it rotates in a magnetic field, a small current gets induced in the core itself too. Due to this current, eddy current loss and hysteresis loss occur in the armature iron core. Iron losses are also called as Core losses or magnetic losses.
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Hysteresis loss is due to the reversal of magnetization of the armature core. When the core passes under one pair of poles, it undergoes one complete cycle of magnetic reversal. The frequency of magnetic reversal if given by, f=P.N/120 where, P = no. of poles and N = Speed in rpm
The loss depends upon the volume and grade of the iron, frequency of magnetic reversals and value of flux density. Hysteresis loss is given by, Steinmetz formula:
Wh=ηBmax1.6fV (watts)
where, η = Steinmetz hysteresis constant
V = volume of the core in m3
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Eddy current loss: When the armature core rotates in the magnetic field, an emf is also induced in the core (just like it induces in armature conductors), according to the Faraday's Law of electromagnetic induction. Though this induced emf is small, it causes a large current to flow in the body due to the low resistance of the core. This current is known as eddy current. The power loss due to this current is known as eddy current loss.
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Mechanical Losses
Mechanical losses consist of the losses due to friction in bearings and commutator. Air friction loss of rotating armature also contributes to these.
These losses are about 10 to 20% of full load losses.
Stray Losses- In addition to the losses stated above, there may be small losses present which are called as stray losses or miscellaneous losses. These losses are difficult to account. They are usually due to inaccuracies in the designing and modeling of the machine. Most of the times, stray losses are assumed to be 1% of the full load
LOSSES IN A DC GENERATOR
Generator Ratings
A generator is rated in power output. Since a generator is designed to operate at a specified voltage, the rating usually is given as the number of amperes the generator can safely supply at its rated voltage. Generator rating and performance data are stamped on the nameplate attached to the generator. When replacing a generator, it is important to choose one of the proper rating. The rotation of generators is termed either clockwise or counterclockwise, as viewed from the driven end. Usually, the direction of rotation is stamped on the data plate.
If no direction is stamped on the plate, the rotation may be marked by an arrow on the cover plate on the brush housing. It is important that a generator with the correct direction of rotation be used; otherwise, the voltage will be reversed. The speed of an aircraft engine varies from idle rpm to takeoff rpm; however, during the major portion of a flight, it is at a constant cruising speed. The generator drive is usually geared to revolve the generator between 1-1/8 and 1-1/2 times the engine crankshaft speed. Most aircraft generators have a speed at which they begin to produce their normal voltage. Termed the “coming in” speed, it is usually about 1,500 rpm.
Armature Reaction
Current flowing through the armature sets up electromagnetic fields in the windings. These new fields tend to distort or bend the magnetic flux between the poles of the generator from a straight-line path. Since armature current increases with load, the distortion becomes greater with an increase in load. This distortion of the magnetic field is called armature reaction. Armature windings of a generator are spaced so that, during rotation of the armature, there are certain positions when the brushes contact two adjacent segments, thereby shorting the armature windings to these segments.
When the magnetic field is not distorted, there is usually no voltage being induced in the shorted windings, and therefore no harmful results occur from the shorting of the windings. However, when the field is distorted, a voltage is induced in these shorted windings, and sparking takes place between the brushes and the commutator segments.
Consequently, the commutator becomes pitted, the wear on the brushes becomes excessive, and the output of the generator is reduced. To correct this condition, the brushes are set so that the plane of the coils, which are shorted by the brushes, is perpendicular to the distorted magnetic field, which is accomplished by moving the brushes forward in the direction of rotation. This operation is called shifting the brushes to the neutral plane, or plane of commutation. The neutral plane is the position where the plane the two opposite coils is perpendicular to the magnetic field in the generator.
On a few generators, the brushes can be shifted manually ahead of the normal neutral plane to the neutral plane caused by field distortion. On non adjustable brush generators, the manufacturer sets the brushes for minimum sparking. Compensating windings or interpoles may be used to counteract some of the effects of field distortion, since shifting the brushes is inconvenient and unsatisfactory, especially when the speed and load of the generator are changing constantly.
EFFECT OF ARMATURE REACTION
Compensating Windings
The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are also connected in series with the armature. Consequently, this series connection with the armature produces a magnetic field in the compensating windings that varies directly with the armature current. The compensating windings are wound in such a manner that the magnetic field produced by them will counteract the magnetic field produced by the armature. As a result, the neutral plane will remain stationary any magnitude of armature current. With this design, once the brushes are set correctly, they do not need to be moved again
COMPENSATING WINDINGS
Interpoles
An interpole is a pole placed between the main poles of a generator. An example of interpole placement is shown in Figure. This is a simple two-pole generator with two interpoles. An interpole has the same polarity as the next main pole in the direction of rotation. The magnetic flux produced by an interpole causes the current in the armature to change direction as an armature winding passes under it.
This cancels the electromagnetic fields about the armature windings. The magnetic strength of the interpoles varies with the load on the generator; and since field distortion varies with the load, the magnetic field of the interpoles counteracts the effects of the field set up around the armature windings and minimizes field distortion. Thus, the interpole tends to keep the neutral plane in the same position for all loads on the generator; therefore, field distortion is reduced by the interpoles, and the efficiency, output, and service life of the brushes are improved
INTERPOLES
DC Generator Maintenance
Inspection
In general, the inspection of the generator installed in the aircraft should include the following items:
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Security of generator mounting.
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Condition of electrical connections.
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Dirt and oil in the generator. If oil is present, check engine oil seal. Blow out dirt with compressed air.
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Condition of generator brushes.
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Generator operation.
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Voltage regulator operation.