Inst i t ut eofManagement & Techni calSt udi es
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IMTS (ISO 9001-2008 Internationally Certified) ELECTRICAL MACHINES II
ELECTRICAL MACHINES II
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CONTENT Page No.
Sl.No.
8.
Transformer
01-16
9.
DC Generator
17-31
10.
DC Motor
32-40
11.
AC Generator (Alternator)
41-53
12.
AC Motors
54-71
13.
Motor Starters
72-78
14.
Electronics
79-98
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Mica insulation
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In practical generator, the number of coils are large and are accommodated on the surface of the armature. Instead of split rings the commutator (with large number of segments) is provided at the one end of the armature. For example if there are two coils. Then the number of commutator of large number of segments may be less pulsating as shown in figure. 9.9.
E.M.F. Equation ofD.C. Generator:Fig. 9.9. The e.m.f. generated in a direct current generator is proportional to the speed rotation of the armature, total number of armature conductors, total flux available in the field and the type of winding adopted in the armature. Let, P =No. of poles.
o = flux per pole, in webers. Z =total no. of conductors inthe armature (number of slots inthe armature x number of conductors per slot). N = Speed of rotation of armature in r.p.m. A =No.
of parallel paths in armature
Eg = e.m.f induced in any parallel path is armature. The EMF Equation ofD. C. generator (Eg) = 0zn x ÂŁ volts
60 A
Where, A = P in case oflap wound generator, A = 2 in case of wave wound generator. Types ofD.C. Generators: D. C. Generators are classified according to the manner in which their field windings are connected. The process of giving D.C. voltage to the field winding for producing magnetic field is called field excitation. The generators are classified as follows:
D.C. generator
Seperately Excited D.C. generator Self excited D.C. generator
Series Generator Shunt generator Compound generator
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SEPARATELYEXCITED GENERATORS:Long shunt Compound Generator Short Shunt Compound generator In this type of generator, the field winding is excited by a separate D.C. source. The schematic diagram of a separately excited D.C. generator is shown in figure. 9.10.
SELF EXCITED GENERATORS:Inthis type of generator, the field winding is excited by the same machine. When the armature is rotated some e.m.f is generated due to the presence of residual magnetism. Thus some induced currents is produced and this passes through the field coils. This induced current produce more flux. This action is repeated and thereby sufficient current passes through the field coils to generate the rated induced EMF. The self excited generators are further classified in to three types, according to the way of their field winding connections to the armature as follows:
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3. COMPOUND GENERATORS a)
Level Compound: Used where reated terminal voltage is required at full load conditions.
b)
Over Compound: Used where power is to be transmitted to a long distance. In this case, the voltage at load remains constant.
c)
Differential compound: - Used for D.C. welding sets since they have an inherent character to limit the short circuit current.
4. SEPERATELY EXCITED GENERATORS These generators are used for (l) Supplying D.C. motors whose speed in to be varied widely (2) where a wide range of D.C. voltage is required for testing purpose. Questions
Part A choose the correct answer 1.
The e.m.finduced in the d.c generator is alternating one and this is converted in to direct e.m.fby a. slipring
b. corbon brush
c. commulator 2.
d. end rings.
The direction of induced e.m.fis determined by using a. fleming's right hand rule
c. Kirchoff's law b. fleming's left hand rule d. Mutual induction. 3.
4.
The Yoke or frame of the d.c machine is made of a. Copper
b.Aluminium
c. Cast iron
d.Bronze
Inter poles are provided to a. Generate the e.m.f c. economical basis
b. operate in over load. d. improve commutation 5.
Armature lamination is about a. 1 Cm thick
c. 0.5 mm thick b.1 mmthick d. 0.5 inch thick 6.
Compensating windings are provided inslots made on the a.armature
b. mainpole faces
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11. A.C. GENERATOR (ALTERNATOR) 11.0. Introduction In power supply system alternating current is supplied to a much greater extent than direct current supply, because of the following advantages. Advantages ofAC GenerationÂť (1) AC power can be generated in bulk quantitywithoutmuch difficulty. (2) AC requirements are cheaper in cost. (3) AC voltage can be step up or step down to any level of our requirement. (4) We can convert in to AC into DC, in case of atmost need for DC supply. The machine which generates alternating current is called as Alternator (or) Synchronous generator. 11.1. Principle of Alternator
The alternator works on the principle of "Electromagnetic Induction". According to Faraday's Laws of electro magnetic Induction, when there is a cutting of magnetic flux by a conductor or when there is a change in flux linkage by a coil, an emf is induced in the conductor or coiL Fig. 11.1.shows the simple arrangement of an alternator. In the fig. 11.1. shown an open ended loop or coil of wire is rotated between the poles of an electromagnet. An e.m.f. is generated in the loop.
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4. Shaded pole motors.
1.RESISTANCE-START, INDUCTION-RUN MOTORS As the starting torque of this type of motor is relatively small and its starting current is high, these motors are most commonly used for rating upto 0.5 HP where the load could be started easily. The essential parts are shown in Fig: 12.2. • Main winding or running winding. • Auxiliary winding or starting winding •
Squirrel cage type rotor.
• Centrifugal switch. lit
The starting winding is designed to have a higher resistance and lower reactance than the main winding. This is achieved by using small conductors in the auxiliary winding than in the main winding. The main winding will have higher inductance when surrounded by more iron, which could be made possible by placing it deeper into the stator slots, it is obvious that the current would split as shown in Fig: 12.2(b). The starting current "1" start will lag the main supply voltage "V" line by 15 degree and the main winding current, "I" main lags the main voltage by about 80 degree. Therefore, these currents will differ in time phase and their magnetic fields will combine to produce a rotating magnetic field.
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14.2.1. TYPES OF SEMI路CONDUCTORS:
In every elements, the atoms, are tied together by the bending action of valence electrons. Si and Ge atoms contain only 4 valence electrons. These electrons have a tendency to fill the last outermost orbit. Inthis way, the electrons placed in the last orbit of an atom share the electrons with their neighbouring atoms. Similarly, all electrons are tied together with their neighbouring atoms. For this, they form a band called covalent bond. The semi conductors are classified as follows: Semi -conductors
Intrinsic Semi conductors
N. Type Semi -conductors
Extrinsic Semi conductors
P.Type Semi -conductors,
. 14.3. INTRINSIC SEMI CONDUCTORS
A pure semi conductor is called intrinsic semi conductor. The silicon and germani urn atoms contains only four electrons in the outermost orbit. So they are called tetravalent atoms. The co-valent band structure of germanium atom is shown in the Fig: 14.3. At low temperature (0 K), the semiconductor behaves as a perfect consulator. Now no electrons get away from the co-valent band. So the current flow (electron flow) is zero. At room temperature, some ofthe valence electrons may acquire sufficient energy. The bonds may be broken, the electrons become free and are shifted to the conduction band as shown in the Fig: 14.4.
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I ns t i t ut eofManage me nt& Te c hni c alSt udi e s Addr e s s:E4 1 , Se c t o r 3 , No i da( U. P) www. i mt s i ns t i t ut e . c o m| Co nt a c t :9 1 +9 2 1 0 9 8 9 8 9 8