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Wednesday, 11 December 2019

DC Generator Parts and their Functions

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The essential parts of a dc generator are :

a. Magnetic frame (yoke)
b. Pole core and field coils
c. Armature
d. End shield and bearings

The constructional details are shown in Figure. The functions of each parts of d.c. generator is dealt in the succeeding paragraphs.

a. Magnetic Frame or Yoke :

For smaller machines the yoke is made of cast iron but for larger capacity machines, it is made of fabricated rolled steel or steel casting. It serves two fold functions; one it acts as a magnetic path and the other to hold the field magnets in position. It is accommodated with two end shields housed with bearings and supports the armature at the centre and also protects the Machine from dust, dirt, moisture, etc. in case of smaller machines the yoke is made in one piece but for larger machines they are split into two halves horizontally. The surface where they meet are carefully machined and the two halves are bolted together by which the difficulty in removing larger armature without damage is eliminated. The frame is with foot rest as far apart as possible is casted to give maximum stability and right support. Tapped holes are provided at the inner side of the frame to accommodate field poles.


b. Pole core and field coils

Pole Core and Pole Shoe:

There exists different forms and shapes for pole core but all serve the same  purpose of concentrating the magnetic field. The most commonly used pole core is shown in Fig. These are made of MS Plates for smaller machine. Present day design use laminated soft sheet steel  punched to suitable shape and size. These laminations are then stacked and riveted to form pole core. Two tapped holes of 1.5 Φ and 30 mm deep are provided on top for fixing the poles inside the frame by means of screws. The field coils are slipped onto the poles before fixing it inside the frame. The pole face serve the purpose of spreading the magnetic flux uniformly in the air gap. The pole face is so curved that it gives a uniform air gap between the armature and the pole face.

Poles are of two types :

a. Main pole
b. Inter pole

Main poles have a wider area of pole face and is intended to spread the main flux uniformly in the air gap. Inter pole provides flux in the air gap between the main pole for

a. neutralising the armature reaction of the field.
b. inducing an e.m.f. in the coil undergoing commutator in opposition to the reaction voltage.


These may be solid or laminated and are made out of wrought iron or mild steel. Figure shows the interpole with its coil. These are fastened to the yoke by means of screws between two main poles and the interpole coils are connected in series to armature. The length of air gap below the inter pole is usually kept about 100% greater than that of the main pole air gap.


Field Coils :

Field coils are used to produce magnetic field. Usually these coils are wound with conductors on a former to suit the pole and the machine capacity. The cross section of the coil is generally rectangular and the number of turns and gauge depend on the type of connection and the capacity of the machine. In small generators where high quality of insulation and reliability is required, the coils are wound on paper stick or tube of paper or core tube either manually or by automatic winding machines.

Each layer is insulated by paper. In the case of cotton inter-woven coils special machine which winds the coils in layers also inter-weaves cotton yearn between the layers separating the layers are used. This type of coil is used where atmospheric conditions are severe. Coils are also wound on bobbins made of insulating material like bakelite or fibre. Where high current capacity coils are required rectangular copper strips are used instead of round wire and are wound in the form of a spiral on the flat side. The layers are insulated with strips of asbestos. The coils after winding is complete are taped with cotton tape and impregnated in varnish before use. In the case of compound machines both the shunt and series windings are done on the same pole. In the case of interpole, the windings are single layered. These windings are insulated and inserted in the interpole before fixing into the yoke.


Interpoles are narrow poles fixed exactly midway between the main poles. It is also known as commutating poles or compoles. The polarity of the interpole is the same as that of the main pole for the generator whereas for motor, the polarity is the same as that of the main pole behind it. Interpoles are fitted to improve commutation. It is of rectangular in section with brass flanges on top and bottom. Coils are well insulated before use.

c. Armature:

Armature forms an important part revolving within the magnetic field. It consists of the following parts built up over the shaft

a. Armature core
b. Armature winding
c. Commutator

d. Shaft and Bearing:

Shaft plays an important role of housing armature core and commutator besides keeping them at the centre of the stator, free to rotate with the support of the bearings. The diameter and length of the shaft vary depending on the machine capacity and design. The shaft is generally made of mild steel with or without case hardening. To keep the armature core in position locking arrangements like lock nut, rivet, cross pin or V washers are used. For small machines the shaft is supported by the end plates through the gun metal bush but for medium and larger machines ball bearings are used. The ball bearings are housed in the end plates with a dust proof covers on the outer side of it. Provision is made for oiling through oil holes for smaller machines and grease cup for larger machines. The size of the bearing varies depending on the shaft diameter and the load.


Tuesday, 10 December 2019

Brush Holder in DC Machine

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Brush and Brush Holders in DC Machine :

The basic function of a brush is to collect current with the least possible voltage drop from the rotating coil to a stationary position. The type of brush used depends upon the speed of rotation of the. commutator and the voltage involved in it. While doing its function it should

a. not have abrasive action on the commutator segments
b. not offer high resistive path resulting in voltage drops between the segment and the brush
c. not wear easily giving rise to carbon coating on commutator segments.
d. not offer high frictional resistance between the surface.
e. not be brittle resulting in breakage of brushes
f. not be costly, and should be easily available.

There are mainly four types of brushes in DC Machine. They are

a. Carbon Brush : These are made out of pulverised graphite carbon with binder material mould to shape and baked at 1200°C.

b. Graphite Brush : These are made out of natural plumbage and are suitable for operation at higher current density.

c. Metal Carbon Brush : In these some amount of copper is mixed with graphite carbon to increase conductivity. Their contact voltage varies between 0.1 to 2.2 V and is suitable for higher current density operations.

d. Electric Graphite Brush: These are more robust mechanically and are generally used in tractions.


The length, width and thickness of the brush vary from machine to machine. For smaller machines, brushes of 20 mm width and thickness 6 to 9 mm are used but for larger machines brush width vary from 30 mm to 40 mm width and thickness from 6 to 18 mm. A common shape of the brush is shown in Figure. A flexible wire is attached to one end of the brush for external connection and is called pig tail.

Brush holders in DC Machine not only keep the brush in position but also in continuous contact with the commutator segments. The application of brush is either radial or tangential. The brush holder with clamping screws keeping the brush in tension with the help of the spring. All brush holders for carbon brushes are radial type. There are mainly four different types of brush holders generally used. They are

a. Plunger type brush holder
b. Box type brush holder
c. Lever type brush holder
d. Composite brush holder

For smaller dc machines where the current density is less plunger type brush holders are used. A sectional view of the plunger type brush holder is shown in Figure. A tube is moulded in brass box either round or square section containing the brush with compression spring as in the case of Tullu pump or mixer grinder. The compression spring which is made of brass is retained by means of an insulating cap with a collar in the inner side of which serves as an attachment for the wire.


For larger dc machines box or lever type is used. The body of the brush top clamps into a round brush pin, a metal finger with a friction roller at the end presses on the end of the brush. The tension is regulated by worm and worm wheel adjustments, the whole forming a neat fitting. An isometric view of the different brush holders are shown in above figure. Excellent continuity is obtained in the lever type between the terminal and the brush but the inertia and weight of the moving parts is much greater than the brush leading to sparking. It is pivoted on one end and the brush is kept in tension with a lever and spring, and provision is made for minute adjustments of the tensions.

For larger dc machines when the current density is more and the area of surface of contact has to be more, a composite brush holder is used. A simple figure of composite brush holder is shown in Figure where more than one brush in parallel will be in operation. Multiple brush box which is bolted to the bracket holding the brushes has a plunger type arrangements with compression springs to keep the brushes in tension. The brushes are separated from one another by a thin division of partition. Separate pressing device is employed for each brush. Terminal box is used for terminal connection.



Armature Construction

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Armature forms an important part revolving within the magnetic field. It consists of the following parts built up over the shaft

a. Armature core
b. Armature winding
c. Commutator
d. Brushes and brush holder

Armature Core :

The primary function of an armature core is to provide magnetic path of low reluctance for the flux which have the north pole of the main field to enter south pole. Necessarily, therefore, it should be made of magnetic material.

Armature core is made up of stampings punched from annealed onto the shaft and mounted over the spider, axial slots are formed in which conductors are wound. The thickness of lamination varies from 0.5 mm to 1 mm depending upon the frequency of magnetic reversal. The stampings are insulated on both sides before stacking onto the shaft. For small machines they are locked at both ends with the help of lock washer and nut and for larger machines it is mounted on the spider. The stampings are held in position with the help of the key way. Usually these laminations contain perforated air ducts to permit axial flow of air through the armature for purpose of cooling. When the diameter of the armature increases beyond one metre, it would be difficult to handle the cut-out stampings for assembly as they tend to distort. In such cases, they are cut into a number of segments which form part of the ring. Usually four six or eight segmental laminations are required to form a complete set. Two keyways are notched in each segment and are dovetailed to make the lamination self locking in position. Fig. (a) shows the different types of lamination stampings of the armature core while Fig. (b) shows the assembly of the stampings on the shaft. On assembly the slots formed are indicated.



The shaft is made of mild steel, and its dimensions depends upon the various components to be mounted on it. The length of the shaft depends on the length of the core which has a direct bearing on the effective length of the conductor for generation of emf. A key way is formed on one side for locking the pulley. The shaft is supported by two bearings either bush or ball bearing, one on each side to enable the shaft to rotate freely within the stator.

Armature Winding:

The simplest type of winding consists of a series of coils wound in the slots of an armature and are connected in succession to a commutator. There are mainly two types of winding namely Lap Winding and Wave Winding. Here we will limit our blog post to the procedure of winding only. The exact details of the windings will be dealt at a later stage. The slots are first insulated with proper insulating material to prevent the conductors from touching the iron core causing ground fault. The insulation so inserted into the slot should protrude approximately 3 mm outside the slot ends and 6 mm above the slot. This will prevent the winding touching the core. The windings are inserted and the lead connections are soldered to the commutator bars. Wedges made out of fibre, wood etc., are inserted between the winding and top portions of the core slot so that the wires do not fly out of armature slot while rotating at full speed. After the armature is wound, leads are soldered to commutator. It has to be made moisture proof besides the coils should be vibrations proof. It is therefore varnished with either baking varnish or air drying varnish. The length of the conductor inside the slot is the effective length for calculating the induced emf.

Armature windings are generally of two types ,

a. Lap winding
b. Wave winding

In the lap winding the beginning and end of the coils are connected to adjacent commutator segment bars. A wave winding is one in which the beginning and end leads of a coil are connected to a definite number of poles. In wave winding, the number of parallel paths for current is equal to two irrespective of the number of poles. But in the lap winding, the number of parallel path for current is equal to the number of poles. Therefore, the number of parallel paths (A) for

a. Wave winding is A = 2
b. Lap winding is A = P

Commutator :

Commutator is an extension of armature to which the terminal connections of the armature winding is soldered. It is a device which automatically reverses the direction of the current to achieve a single direction in the external circuit. It is cylindrical in shape and consists of a number of copper segments of high conductivity insulated from each other as well as from the shaft. Each segment is insulated from the other with the help of mica and is connected to armature by means of lugs. The thickness of mica varies from 25 to 40 mils. The segments are tapered from top to bottom with a V-cut at the bottom on both sides. The back end of the segment has a raised platform with slots for soldering leads of armature winding.
This arrangement prevents adjacent copper bars from touching each other. Onto the iron shell with V ring at the bottom and threaded at the top several layers of mica insulation is used, The insulated segments are then assembled on the other mica ring to form a cylinder all round. The front V ring is then slipped into position and tightened by a nut which can be screwed to the inner threaded portion of the iron shell. On assembly the commutator should be tight and all segments aligned. It is then tested for segmental insulation.


The basic function of a brush is to collect current with the least possible voltage drop from the rotating coil to a stationary position. The type of brush used depends upon the speed of rotation of the. commutator and the voltage involved in it. 


Monday, 9 December 2019

Flemings Right Hand Rule for Generators

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Flemings Right Hand Rule can be used to find out the direction of induced e.m.f. in a conductor undergoing generation of emf. Fig shows the method of positioning right hand with fingers at right angles to each other. Flemings Right hand rule says that keeping the first finger, second finger and the thumb of right hand at right angles to each other similar to be three axis, if the first finger points out in the direction of magnetic field, the thumb in the direction of motion of conductor, then the second finger points out the direction of induced e.m.f.
(a) Method of positing Right Hand
(b) Direction of Induced EMF
As seen from above, for an e.m.f. to be produced in a conductor there should be a flux and motion between the conductor and the flux. Here the conductor of length 'l' meter is placed in a magnetic field of Φ Wb in A square metres so that the flux density

B = Φ/A Wb/m2 or testla.

The conductor is rotated about the axis with a velocity of 'v' m/sec. According to Faradays Laws of Electromagnetic induction, the e.m.f. induced in the conductor is given by

e = Rate of change of flux linkages
= B.l.v Volts

This value is maximum since the direction of motion is at right angle to the direction of flux. If the conductor were to move parallel to the direction of flux, then the conductor does not cut the lines of force i.e.. no flux linkage. Therefore, no e.m.f. will be induced in the conductor. Hence when the conductor is made to rotate about an axis it moves from parallel to the magnetic flux to perpendicular position. Therefore, the induced e.m.f. would also vary from zero to maximum.

Consider a position between zero angle to the direction of flux to perpendicular position wherein the direction of motion at any instant makes an angle of θo with respect to the direction of flux.
The component of voltage induced perpendicular to the direction of flux at any instant is equal to v sinθ. This will be true since when the conductor moves parallel to the direction of flux θ = 0 and sin 0o = 0. When the conductor moves perpendicular to the direction of flux θ = 90°. Hence re-writing the equation of e.m.f. as

e = B.l.v sin θ volts

Where e = e.m.f. induced in the conductor (Volts)
B = Flux density (Wb/m2)
l = length of the conductor (m)
θ = Angle of incidence between the direction of flux and the direction of motion about an axis

If the coil has N number of turns, then the e.m.f. induced at any instant is given by

e = 2N.B.l.v.sin θ, Since a coil contains two sides.


RS232 Parameters, Start and Stop Bits

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RS232

RS232 (Recommended Standard 232) is the standard developed by EIA (Electronic Industry Association) in the year 1960 for the serial communications of data. This standard is a widely used standard, the PC consist of two RS232 ports Com1 and Com2.  Rs232 is used to connect a DTE (Data Terminal Equipment) to a DCE (Data Communications Equipment). For E.g. ., DTE is pc, printer and DCE is Physical interface. In RS232, data are transmitted serially and Provides full duplex Communication.

RS232 Parameters

When two devices want to communicate, the sender sends data as character by Character. The character is corresponds to a bit. The number of characters is called data bits. The data is appended with one start bit at prefix and stop bit in the suffix. The receiver decodes the data using start and stops bits and receives it. This type of communication is called an asynchoronous communication, because no clock is used. Parity bit is also appended to the data to be sent in order to achieve error correction. For two devices to communicate through RS232 the communication parameters should set in both systems. They are Data bits, data rate, start bit, stop bit, parity bit, flow control.

Data rate

• Data rate is the rate at which the data communication takes place.

• The PC supports various data rates such as, 50,150,300,600,1200,2400,4800,19200,38400,57600 and 115200 bps.

• The oscillator in the RS232 circuitry operates at 1.845 MHZ.

• It is divided by 1600 to obtain 115200 data.

Data bits

• Data bits is the Number of of bits transmitted for each character.
• The character can have 5 or 6 or 7 or 8 bits.
• If ASCII character is send, the number of bits is 7.

Start bits

• Start bit is the bit that is prefixed to the data in order to identify the beginning of the character.

Stop bits

• These bits are appended to the data bits to identify the end of character.
• If the data bits are 7 or 8, one stop bit is appended.
• If the data bits are 5 or 6, two stop bits are appended.

Parity

• Parity bits are appended to the character for error checking.
• The parity can be even or odd.
• For even parity, the parity will make the total no. of bits even.
• For odd parity, the parity will make the total no. of bits odd.

Flow control

• Flow control can be defined as a protocol to stop/resume data transmission. It is also known as hand shaking protocol.

• For Hardware handshaking, two signals are used:

• RTS-REQUEST TO SEND

• CTS-Clear To Send

• The software handshaking is known as XON/XOFF.

RS232 Connector Configurations

• RS232 specifies two types of connectors:

25-pin connector
9-pin connector

• Only few pins are used in 25-pin connector.

• Important pins are 2 (Transmit), 3 (Receive) 7 (Signal Ground)

• RS232 uses unbalanced transmission and is susceptible to noise.


For the 25-pin Connector

Pin Number
Function (Abbreviation)
1
Chassis Ground
2
Transmit Data (TXD)
3
Receive Data (RXD)
4
Request To Send (RTS)
5
Clear To Send (CTS)
6
Data Set Ready (DSR)
7
Signal Ground (GND)
8
Carrier Detect (CD)
20
Data Terminal Ready (DTR)
22
Ring Indicator (RI)

For the 9-pin Connector

Pin Number
Function (Abbreviation)
1
Carrier Detect (CD)
2
Receive Data (RXD)
3
Transmit Data (TXD)
4
Data Terminal Ready (DTR)
5
Signal Ground (GND)
6
Data Set Ready (DSR)
7
Request To Send (RTS)
8
Clear To Send (CTS)
9
Ring Indicator (RI)




Saturday, 7 December 2019

Working of Simple Loop DC Generator

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Working Principle of Simple Loop DC Generator

For the purpose of study, a single loop generator is taken. In figure, the single loop generator having a coil with two sides connected to a commutator to which two brushes slide.


The coil is placed in between two permanent magnet in such a way that it can rotate about its axis cutting the flux produced by the permanent magnet.

Five distinct positions of the coil are chosen for the purpose of study. in Fig, the coil side ‘ab' and 'cd' rotate between the north pole and the south pole of a permanent magnet. Assuming the initial position of the coil sides moving parallel to the direction of flux as shown in Fig, no linking of the flux takes place as it does not cut the flux. Hence, no e.m.f. is induced in the conductor.
Conductor moving Parallel to the Direction of Flux
Rotating the conductor by 90o clockwise, the conductor travels along the path of a circle and takes a position as shown in Fig. The conductor coil sides 'ab' moves down perpendicular to the flux while the coil side 'cd' moves upwards again perpendicular to the flux. Maximum e.m.f. is induced in the coil and since the sides of the coil arc connected in series, the total e.m.f. is the sum of the e.m.f. of each side of the coil.
Conductor moving at Right Angle to the Direction of Flux
If external circuit is complete, the current, in the coil will be in the direction 'b' to 'a' in coil side 'ab’ and 'd' to 'c' in coil side 'cd'. Since the coil sides 'ab' and 'cd' are connected in series, the total e.m.f. is the sum of the e.m.f. induced in each coil side and the direction will be from 'dcba'. Again rotating the coil by another 900 (i.e., 180° from initial position), the coil occupies a position as shown in Fig. The conductor coil sides 'ab' and 'cd' moves parallel to the magnetic flux. Since it does not cut the magnetic flux, no e.m.f. is induced in it. Hence, the e.m.f. which was maximum in the previous position drops down to zero.
Conductor moving Parallel to the Direction of Flux
If the position of the coil sides 'ab’ and 'cd' are moved by rotating it further by 90o (i.e., 270° from initial position), it occupies a place as shown in Fig. The coil sides having rotated by 90° and the motion of the coil side 'all' is upwards perpendicular to the magnetic flux while the coil side 'cd' is downwards perpendicular to the magnetic flux. The e.m.f. induced in the coil sides are maximum but since the direction of motion of the conductors are opposite to that shown in Fig, the sign of the maximum e.m.f. induced and the resultant current is also opposite. The direction of current is from 'a' to 'b' in conductor 'ab' and similar from 'c' to 'd' in conductor 'cd'.
Figure
Rotating the coil sides by another 90° (i.e., 360° from the initial position) bringing the conductors to the position shown in Fig, the coil travels parallel to the flux and as such no e.m.f. is induced in it. This position also refers to the initial position. The e.m.f. which was negative maximum would drop down to zero. It should be noted that since the conductor moves in the path of a circle and the change in the magnitude of e.m.f. also is smooth following sine law. Since the e.m.f. has a positive quantity and a negative quantity, the e.m.f. is said to alternate from a positive to negative quantity. Therefore, it is called alternating e.m.f. or alternating current. If a graphical representation of the e.m.f. produced in the coil is shown, it would resemble as shown in Fig.

The representation at each stage of rotation of conductor is shown in Fig below.

From the above, it is seen that the e.m.f. produced within the coil is alternating in nature and to convert to direct current a commutator is used. A commutator is a device by which current flows in single direction in the external circuit. The- simplest commutator for the above two pole generator is split ring and is represented in Fig. With the use of commutator the negative portion of the induced e.m.f. is converted into positive such that the current in the external circuit is in single direction. In Fig, two distinct positions of the conductor representing the positions shown in Fig and are shown with an external circuit. Note carefully the flow of current from the coil to the external circuit even though the current in coil side 'ab' and 'cd' change for every 180° rotation of the conductor. The graphical representation of e.m.f. in the external circuit is shown in Fig.

Such unidirectional current is called direct current in short d.c. A generator which gives d.c. in the external circuit is called d.c. generator. Necessarily it uses a commutator.

FUNCTIONS OF SPLIT RING AND COMMUTATOR:

When the coil rotates, the e.m.f. induced in the conductor is of alternating in nature. The current in the conductor coil is the result of the e.m.f. produced in the conductor. Since the coil rotates, there should be a method of drawing the current for the external circuit from a rotating coil to, a stationary terminal. There are three methods of achieving this object. They are :

a. By using a split ring when the generator is of two pole
b. By using a commutator when the generator is of multipole
c. By using a slip ring

The split ring and commutator are used in case the current in the external circuit should, be uni-directional, and slip ring is used when the current in the external circuit has to be alternating. Fig. (a) shows a split ring while Fig (b) shows the commutator. In both the cases, two or four carbon brushes slide over it collecting the current from the rotating conductor. Commutators are used for d.c. machine as well as repulsion motors.
(a) Commutator
(b) Split Ring
A commutator consists of number of bars, an equal number of mica segments, and an iron core consisting of two end rims and a connecting shell on which the bars and mica segments are placed. Towards the bottom, the bars are partly cut out on both sides in the shape of a V for holding the commutator together by rings. Mica segments are used between bars to prevent adjacent bars from touching. The armature windings are connected to the commutator segments to enable the e.m.f. generated to be collected to a stationary terminal. Fig. shows the commutator assembly.
Commutator Assembly


Friday, 6 December 2019

Classification of DC Generators

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CLASSIFICATION OF DC GENERATORS

D.C Generators are classified depending on the type of connections made. If the field coils are connected in parallel to armature, it is called shunt generator. If the field coils are excited separately, it is said to be separately excited generator, and if the field coils are connected in series to armature it is known as series generator. There are two combinations possible viz shunt and series and if both are used, then it is said to be compound generator. In compound generator two combinations are possible

(a) Short shunt compound generator. and
(b) Long shunt compound generator.

The generator classification tree is given below.
Separately Excited DC Generator :

When the field winding is excited by a separated d.c. current, it is called separately excited DC generator. In this case the field current can be varied through a variable resistance connected in series to the battery circuit.  The Load is connected directly across the armature. The polarity of the generated e.m.f. depends on the direction of rotation of the shaft and the polarity of battery source. The connection diagram is shown in Figure.

D.C. Shunt Generator :

On the shunt generator the current for the magnetization of the field is tapped from the armature itse1f. The shunt field is connected in parallel to the armature. As such the shunt winding has to take full voltage of the armature and so it consists of thin wire of many turns. The sum of the load current and shunt field current is the armature current. The connection diagram is shown in Figure.



D.C. Series Generator :

In the case of a series generator, the field winding is connected in series to armature and the current in the armature, series field and the line are same. Since the voltage drop iii the series field should be as least as possible and also be capable of taking full load current, the field winding consists of less number of turns with thick wire. The total current is the same as that of series field or the armature. The connection diagram is shown in Figure.


D.C. Compound Generator :

D.C Compound generators are of two types

(a) Long shunt compound generator and
(b) Short shunt compound generator.

The difference between the two lies in the connection of the shunt winding.

Long Shunt Compound Generator :

Compound generator utilise both the shunt field and the series field. In the long shunt compound generator, the shunt field is connected parallel to the line as shown in Figure. The line current distribution is again a combination of shunt generator and series generator. In this case the armature current is equal to series field current but the line current is the sum of armature current and shunt field current.

Short Shunt Compound Generator :

When the shunt field is connected across the armature of a series generator, it is termed as short shunt compound generator. As in the shunt generator, the shunt field is impressed with armature voltage and series field takes the line current. The line current is the sum of armature current and the shunt field current. The connection diagram is shown in Figure.



Monday, 2 December 2019

Micromachined Antennas

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Micromachine is a technique used in an antenna or a system for improving the performance of the system. The performance improvement includes increasing the gain, bandwidth, efficiency etc.  In other words, we can say that by using a micromachine technique, we can improve the radiation intensity of the antenna. We know that the surface waves will affect the radiation (fringing waves) of an antenna (since it will affect the major and minor lobes). By using micromachine technique, we can reduce the surface waves. Thus the major lobe and minor lobe levels can be increased.   

Micromachine

First we are considering a substrate over which we are creating a cavity. i.e., the removal of some portion of substrate for  providing the patch over the substrate. Micromachine are of two types.

(i) Surface micromachining

Surface micromachining means removal of portion (small) of substrate from the surface of the substrate for producing the pitch.

(ii) Bulk micromachining

Bulk micromachining means bulk amount of substrate can be removed from the substrate. In order to remove the substrate portion, we use the etching process. etching process is of two types.


Dry etching:

In dry etching, an evacuated chamber is used. In the evacuated chamber, the plasma is generated and this plasma is used to remove the silicon substrate.

Wet etching:

In wet etching, a durable mask is used. The region of the substrate which need to be etched is unmasked and all other portions are masked, where we have to undergo chemical reactions.


GaAs ( Gallium Arsenic) Substrate:
ɛr = 12.9

Thickness = 350 μm
Patch length = 1.21 mm
Patch width = 1.91 mm

The permittivity of the substrate determine the performance. The synthesised permitivity can be obtained as


Where,  ɛr_ syntheff is the effective synthesized permittivity and can be obtained as:
Where, K0 is the free space wave number and β is the propagation constant.


The effective synthesized permittivity can be obtained as:



From the graph, it is clear that as the microstrip width increases, the permittivity decreases. As permittivity increases, the performance also increases. Hence, in order to improve the permittivity, we must select the width of the microstrip as very small.

Micromachined slotline Antenna:

The micromachined slotline antenna can be easily integrated with 2 or 3 terminal active devices.
There are mainly 2 types of slot ring antenna.

(i) With trenches
(ii) Without trenches

GaAs FET activated by DC bias.

For fixed resonant frequency, a trenched one is having larger size than the untrenched. So there will be a higher radiation efficiency for the trenched one. E - plane and H - plane radiation power is about 1 to 2 dB higher for trenched.

Gate is used for control of flow of charge carriers from source to drain (S to D). The amount of fringing can be calculated by the applied voltage. Depending upon our requirement, we can increase the cavity by changing the DC applied voltage.




By applying the voltage, the capacitor can be trenched. For backside trenching, anisotropic etchant (KOH) is used. By doing like this, 550 tapered edges are produced. For slot line, isotropic etchant CP4 is used (mixture of nitric acid, hydrofluoric acid and water).

Microelectro mechanical system antenna:

The micro electro mechanical system antenna is used when a single antenna is used for a range of frequencies. This can be applied in telecommunication systems, radars etc. Since a single antenna is used for the entire system, we can reduce the size and cost of the system. This can be achieved by using micro electro mechanical switches or capacitors in antenna. The capacitors used here are cantilever type capacitors and fixed beam type capacitors.



The substrate used is glass type (ɛr = 400). The sputtering is done at 100/3000 A0. The seed layer is Ti/Au.