ALL IS VEL: 2013

         Today concept
FUNCTION OF CHOKE IN TUBELIGHT

   The choke has two main functions. It (in conjunction with the starter if it has one) causes the tube to ignite by using the back emf to create a plasma in the tube and it controls the current through the tube when it is ignited.

In a gas discharge, such as a fluorescent lamp, current causes resistance to decrease. This is because as more electrons and ions flow through a particular area, they bump into more atoms, which frees up electrons, creating more charged particles. In this way, current will climb on its own in a gas discharge, as long as there is adequate voltage (and household AC current has a lot of voltage). If the current in a fluorescent light isn't controlled, it can blow out the various electrical components.

A fluorescent lamp's ballast works to control this. The simplest sort of ballast, generally referred to as a magnetic ballast, works something like an inductor. A basic inductor consists of a coil of wire in a circuit, which may be wound around a piece of metal. If you've read How Electromagnets Work, you know that when you send electrical current through a wire, it generates a magnetic field. Positioning the wire in concentric loops amplifies this field.

This sort of field affects not only objects around the loop, but also the loop itself. Increasing the current in the loop increases the magnetic field, which applies a voltage opposite the flow of current in the wire. In short, a coiled length of wire in a circuit (an inductor) opposes change in the current flowing through it. The transformer elements in a magnetic ballast use this principle to regulate the current in a fluorescent lamp.

A ballast can only slow down changes in current -- it can't stop them. But the alternating current powering a fluorescent light is constantly reversing itself, so the ballast only has to inhibit increasing current in a particular direction for a short amount of time.

Magnetic ballasts modulate electrical current at a relatively low cycle rate, which can cause a noticeable flicker. Magnetic ballasts may also vibrate at a low frequency. This is the source of the audible humming sound people associate with fluorescent lamps.

Modern ballast designs use advanced electronics to more precisely regulate the current flowing through the electrical circuit. Since they use a higher cycle rate, you don't generally notice a flicker or humming noise coming from an electronic ballast. Different lamps require specialized ballasts designed to maintain the specific voltage and current levels needed for varying tube designs.

Fluorescent lamps come in all shapes and sizes, but they all work on the same basic principle: An electric current stimulates mercury atoms, which causes them to release ultraviolet photons. These photons in turn stimulate a phosphor, which emits visible light photons. At the most basic level, that's all there is to it!

   ALL IS VEL

EE2355 DESIGN OF ELECTRICAL MACHINES

UNIT – I

1. A laminated steel tooth of armature of a d.c. machine is 30mm long' and has a.taper such the maximum width is 1.4. times the minimum. Estimate the mmf required for a mean flux density of 1.9 wb/m2 in the tooth. B-H characteristics of steel is given below:

B wb/m2 1.6 1.8 1.9 2.0 2.1 2.2 2.3

H A/m 8700 10000 17000 27000 41000 70000 10900

2. Determine the apparent flux density in the teeth of a d.c. machine when the real flux density is 2.15wb/m2. Slot pitch is 28 mm, slot width is 10 mm and the gross core length 0.35 metre. The number of ventilating ducts is 4. Each duct is 10 mm wide. The magnetizing force for a flux density of 2.15 wb/m2,is 55000 H/m. The iron stacking factor is 0.9.

3. Compute the main dimensions of a 2500 KVA, 187.5 rpm, 50 Hz. Three phase, 3 KV salient pole synchronous generator. The specific magnetic loading is 0.6 wb/m2 and the specific electric loading is 3400 ac/m. The ratio of core length to pole pitch= 0.65.

4. State and explain the main factors which influence the choice of specific magnetic loading and specific electric loading in a synchronous machine. Explain the role of digital computes in the design of electrical machines.

5. A 15 kW, 230 V, 4 pole d.c machine has the following data:

Armature diameter = 0.25 m ; armature core length = 0.125 m ; length of air gap at pole centre =

2.5 mm ; flux per pole = 11.7 x 10-3 Wb, ratio Polearc/ pole pitch=0.66. Calculate the mmf

required for air gap.

6. Derive the expression for the specific permeance of slots with double layer windings and for

special purpose induction motors.

7. Compute apparent magnetic flux density in the teeth of a dc machine when the real flux density is

2.15 Wb/m2. Slot pitch is 28 mm. Slot width is 10mm and the gross core length is 0.35 metre. The

number of ventilating ducts is 4, each 10 mm wide. The magnetizing force for a flux density of

2.15 Wb/m2 is 55000 A/m. The iron stacking factor is 0.9.

UNIT –II

1. Derive output equation of a d.c. machine and point out its salient features.

2. State and explain the factors which governs the choice of spe ific magnetic loading in a

d.c. machine.

3. A 5 KW, 250 volts and 4 pole, 1500 rpm d.c. shunt generator is designed to have a square pole

face. The average magnetic flux density in the air gap is 0.42 wb/m2 and ampere conductors per

metre = 15000. Compute the main dimensions of the machine. Assyme full load efficiency = 87%.

The ratio of pole arc to pole pitch = 0.06.

4. Find the main dimensions of .a 200 kW, 250 volts, 6 pole, 1000, rpm DC generator. The maximum

value of flux density in the air gap is 0.87 wb/m2 and the ampere conductors per metre length of

armature periphery are 31000; The ratio of pole arc to pole pitch is 0.67 and the efficiency is 91

percent. Assume that the ratio of length of core to pole pitch = 0.75.

5. A rectangular field coil of a dc machine is to produce an mmf of 7500 ampere turns when

dissipating 220 watts at a, temperature of 60°C The inner dimensions of the coil are: length = 0.24

metre. Width == 0.1 metre. Height of the coil = 0.15 metre. The heat dissipation is 30 w/m2/oC

from the outer surface neglecting the top and bottom surfaces of the coil. The temperature of the

ambient air is 20°C. Compute the thickness of the coil. Resistivity of copper is 0.02 Ω/m and mm2 .

6. A 250 kW, 500 volt, 600 rpm. d.c. generator is built with an armature diameter of 0.75 m and a

core length of 0.3 m. The lap connected armature has 720 conductors. Using the data obtained

from this machine, determine the armature diameter core length, number of armature slots,

armature conductors and commutator segments for 350 kW, 440 volt, 720 rpm. 6 pole d.c.

generator. Assume a square pole face with ratio of pole are to pole pitch equal to 0.66.The 'full

load efficiency is 0.91 and the internal voltage drop is 4 percent of rated voltage.The diameter of

commutator is 0.7 of armature diameter. The pitch of commutator segments should not be less than

4mm. The voltage between adjacent segments should not exceed 15 V at no load.

7. Explain the design procedure for the shunt field winding of DC machine.

8. Compute suitable dimensions of armature core of a d.c. generator which is rated 50 kW. P = 4, N =

600 rpm. Full load terminal voltage is 220 volts. Maximum gap flux density is 0.83 Wb/ m2 and

specific electric loading is 30,000. ampere conductors/metre. Full load armature voltage drop is 3

percent of rated terminal voltage. Field current is 1 percent of full load current Ratio of pole arc to

pole pitch is 0.,67 pole face is a square.

9. A 4 pole 50 HP de shunt motor operates with rated voltages of 480 volts at rated speed of 600 rpm.

It has wave wound armature with 770 conductors. The leakage factor of the poles is 1.2 . The poles

are of circular cross section. The flux density in the poles is 1.5 Wb/ m2. Compute diameter of

each pole.

UNIT –III

1. Determine the dimensions of core and yoke for a 200 KVA, 50 Hz single phase core type

transformer. A cruciform core is used with distance between adjacent limbs equal to 1.6 times the

width of core laminations. Assume voltage per turn of 14 volts, maximum flux density of 1.1

wb/m2, window space face of 0.32, current density of 3 A/mm2 and stacking factor equal to 0.9. the

net iron area is 0.56 d2 wher d is diameter of circumscribing circle. Width of the large-stamping is

0.85d.

2. A 250 KVA, 6600/400 volts, 3 phase core type transformer has a total loss of 4800 watts at full

load. The transformer tank is 1.25 metre in height and 1 m x 0.5 m in plan. Design a suitable

scheme of tubes if the average temperature rise is to be limited to 35 oC. The diameter of each tube

is 50 mm 'and the tubes are spaced 75 mm from each other due to radiation and convection is

respectively 6 and. 6.5 W/m2 oC. Assume that convection is improved by 35 percent due to

provision of tubes.

3. A three A three phase 50 Hz core type transformer has the following data:

Width of H.V. winding = 25mm

Width of L.V. winding = 16 mm

Height of the coils =0.5 metre

H.V. winding turns = 830.

Width of duct between HV and LV windings. = 15mm, Complete leakage reactance of the

transformer referred to H.V. side. The transformer ratings are 300KVA, 6600/400 volts,

Delta/Star.

4. Determine the main dimensions of the core of a 5 KVA, 11000/1400 volts, 50 Hz, single phase

core type distribution transformer having the following data: The net conductor area in the window

is 0.6 times the net cross sectional area of iron in the core. The core is of square cross section,

maximum flux density is 1 wb/m2. Current density is 1.4 A/mm2. Window space' factor is 0.2.

Height of the window is 3 times its width.

5. Derive output equation of a three phase transformer.

6. State different methods of cooling the transformers and explain each method with relevant

diagrams. State merits and limitations of each method.

7. Calculate approximate overall dimensions for a 200 KVA, 6600/440 V, 50 Hz, 3 phase core type

transformer. The following data may be assumed: emf per turn = 10 V, Maximum-flux density =

1/3 Wb/m2 , currentdensity = 2.5 A/mm2, window space factor = 0.3, overall height = overall

width, stacking factor = 0.9. Use a 3 stepped core. For a three stepped core, width of largest

stamping = 0.9 d and net iron area == 0.6 d2 where d is a diameter of circumscribing circle.

8. A 250 KVA 6600/400, 3 phase core type transformer has a total loss of 4800 W at full load. The

transformer tank is l.25 m in height and 1m x 0.5m in plan. Design a suitable scheme for tubes if

the average temperature rise is to be limited to 35 oC. The diameter of tubes is 50mm and are

spaced 75 mm from each other. The average height of tubes is1.05m.

9. Calculate the main dimensions of core of 100 KVA, 2000/400 Volts, 50 Hz, single phase shell

type transformer. Voltage per turn = 10 volts. Peak flux density in the core is 1.1 Wb/ m2. Window

space factor is 0.33. Ratio of core depth to width of central limb = 2.5. Ratio of window height to

window width = 3.0 current density in the winding is 2 A/mm2, Stacking factor = 0.9.

10. A 250 KVA, 6600/400 volts, three phase core type transformer has a total loss of 4800 watts at

full load. The transformer tank is 1.25 m in height and 1m x 0.5m in plan. Design a suitable

scheme for tubes if the average temperature rise is to be limited to 35°C. The diameter of tubes is

50 mm and are spaced 75 mm from each other. The average, height of tubes is 1.05 m. Specific

heat dissipation due to radiation and convection is respectively 6 and 6.5 W/ m2 / oC. Assume that

convection is improved by 35 percent due to provision of tubes.

UNIT – 4

1. Derive the output equation of a three phase induction motor.

2. State and explain factors governing the choice of ampere conductors per metre in the design of a

three phase induction motor.

3. Compute the' main dimensions of a 15 KW, three phase, 400 volts, 50 Hz, 2810 rpm squirrel cage

Im having efficiency of 88 percent and full load power factor of 0.9.Assume specific magnetic

loading equal to 0.5 Wb/m2 and specific electric loading equal to 25,000 A/m. The rotor peripheral

speed' may be approximately 20 m/sec at' synchronous speed.

4. Determined diameter and length of the stator core for a 11 kW, 400 V, 3 phase, 4- pole,1425 rpm

induction motor. Specific magnetic loading is 0.45 wb/m2 and specific electric loading is 23000

ac/m. Full load. efficiency is 0.85 and full load power factor is 0.88. The ratio of core length to

pole pitch = 1

5. A 90 kW, 500 volts, 50 Hz; three phase, 8 pole slip-ring induction motor has star connected stator

accommodating 6 conductor per slot. The number of stator slots = 63. If the slip ring voltage on

open circuit is to be about 400 volts, find the number of rotor slots and the number of conductors

in each rotor slot.

6. Write notes on:

i. Design of rotor bars and slots.

ii. Design of end rings.

7. Find the values of diameter and length of stator core of a. 7.5 kW. 220 V, 50 Hz. 4 pole. 3 phase

induction motor for best power factor.

8. Find the main dimensions of a 15 kW, three phase, 400 volts, 50 Hz, 2810 rpm squirrel cage

induction. motor having all efficiency of 88 percent and full load power factor of 0.9. Specific

magnetic loading is 0.5 Wb/ m2. Specific electric loading = 25000 A/m. Take rotor peripheral

speed 'as approximately 20 m/sec synchronous speed.

9. A 11 kW, three phase 6 pole, 50 Hz; 220 volts star connected induction motor has 54 stator slots,

each containing 9 conductors. Calculate the value of bar and end ring currents. The number of

rotor bars is 64. The machine has an efficiency of 8.6 percent and a powerfactor of 0.85. The rotor

MMF may be assumed to be 85 percent of stator MM F. Also find the bar 'and the end ring

sections if the current density is 5 A/mm2

UNIT – V

1. Compute the main dimensions of a 1000 KVA, 50 Hz, three phase, 375 rpm alternator. The

average air gap flux density is 0.55 Wb/m2 and ampere conductors per metre are 28000. Use

rectangular poles. 'Assume the ratio of arc length to pole pitch equal to 2. Maximum permissible

peripheral speed is 50 m/sec. The runaway speed is 1.8 times the synchronous speed.

2. The field coils of a salient pole alternator are wound with a single layer winding of bare copper

strip 30 mm drop, with separating insulation 0.15 mm thick. Compute thickness of the conductor,

number of turns and height of the winding to develop an mmf of 12000 ampere turns with a

potential difference of 5 volts per coil and a loss of 1200 watts/m2 of coil surface area. Mean

length of turn is 1.2 metre. Resistivity of copper is 0.021.Ω/m/mm2.

3. Calculate the .mmf required for the air gap of a salient pole synchronous machine having core

length of 0.32 metre including 4 ducts of 10 mm each; pole arc = 0.19 metre. Slot pitch = 65.4

mm, Slot opening = 5 mm. Air gap length = 5 mm. Flux per pole = 52 mwb.

4. Explain the role of digital computers in the design of electrical machine.

5. Describe the construction of Turbo – alternators with sketch.

6. Explain the design of field winding of alternator.

7. State and explain advantages of hydrogen cooling as applied to turbo alternator.

8. Draw cross sectional view of rotor slot and explain the method involved in direct hydrogen cooling

of rotor of turbo alternator.

9. Derive the output equation of a synchronous machine.

10. State and explain the salint features of Computer Aided design of electrical apparatus.

11. Determine the main dimensions of a 75000 KVA, 13.8 kV, 50 Hz, 62.5 rpm, three phase star

connected alternator. The peripheral speed of rotor should be about 40 m/sec. Assume average

gap density equal to 0.65 Wb/ m2, ampere conductors per metre equal to 40,000 and current

density =4 A/ mm2. Assume Kw = 0.955.

_______________________________

ALL THE BEST

Invention	Inventor	Year	Patent No.

Alternating Current Calculations	Charles Proteus Steinmetz	1897
Alternator	Nikola Tesla	1891	US447,921
Battery (Voltaic Pile)	Alessandro Volta	1800
Cell Phone	Martin Cooper	1973	US3,906,166
Microphone	Emile Berliner	1877	US224,573

ALL IS VEL

Sunday, 4 August 2013

CONCEPT OF SWITCH GEAR

Tuesday, 30 July 2013

INVENTION AND INVENTOR

Thursday, 25 July 2013

BASIC QUESTIONS FOR EEE

Tuesday, 23 April 2013

QUESTIONS WITH ANSWERS

Monday, 22 April 2013

QUESTION FOR CHALLENGERS

Monday, 8 April 2013

FUNCTION OF CHOKE IN TUBELIGHT

Monday, 1 April 2013

EEE LOGOS

Sunday, 31 March 2013

DESIGN OF ELECTRICAL MACHINES IMPORTANT QUETIONS