ENERGY POWER SUPPLY TECHNOLOGY - THE E.HOUSE TECHNOLOGY FOR DISTRIBUTION SYSTEM,

Energy house for Power Supply - http://howelectrical.blogspot.com

Make the process of power distribution more efficient, reliable and safe with energy house power supply as new technology in Electrical power engineering and field. In many cities, infrastructures, buildings automation and industrial plants. They can be linked to industrial and building automation, and are rounded out by comprehensive support throughout the entire life-cycle. What are the benefits of Energy house for power supply solution? there are three main benefits as given below.

Benefits

There are three main benefits of energy houses for power supply solution.

1. Cost Effective
2. Fast to install
3. Flexible

Cost Effective

The installation of an Energy house spare you from dealing with planning, controlling and complex civil works, as well as with crafts on site and construction risks.

Fast to Install

An energy house for power supply arrives at your site ready for plug commission and play. this really speeds up your lead time.

Flexible

You can choose from several energy house types according to your application requirements, equip them with exactly the products you need.

Plug and Play Power Distribution

The Energy house can also be used for plant balancing of fossil and renewable energy, as reliable power supply for critical processes, for grid coupling, as well as for the grid connection of electrical energy storage systems. Energy house are optimal approach to install electrical power and control equipment for a fast and reliable power supply. An energy house is a pre-fabricated electrical building, fully equipped. Actually industries needs a reliable and efficient power supply as well as flexible solutions that can be adapted to individual requirements. Energy house for power supply are fast and very easy way to install, can be used as an interim solution. They are easy to upgrade, and used available space optimally. This makes them the most suitable option for a broad range of application.

    The Energy house offer more; one stop solution, consistency, safety, flexibility, cost effciency, reliability and advanced technology.

Read More

TERTIARY WINDING OF TRANSFORMER - THREE PHASE TRANSFORMER

Electrical Transformer is a static device which transform electrical energy from one electrical circuit to another electrical circuit without any direct connection. It is also used for electrical power transmission (Step-Up) and distribution the electrical power (Step-Down) and Special purposes. Commonly we know that there are two windings in transformer, The primary winding and the secondary winding. The primary winding is used for input (not fixed) and the secondary winding is used for the output whether it depend upon the situation and requirement. Now there is a question asked an engineer from our Facebook group (Facebook-Group) that what is Tertiary winding in the three phase transformer, the advantages and disadvantages of tertiary winding.

Working Principle Of Transformer

The working principle of transformer is totally depends upon the Faraday's Law of mutual induction.
Faraday's Law of Mutual Induction is:

"Rate of change of flux linkage with respect to time is directly proportional to the induced EMF in a conductor or a coil."

As you all know well about the transformer working, I will try not to discuss here again. but the main constructional parts of the transformer I share below.

Main Constructional Parts of Transformer

There are following main part of transformer in construction.

• Primary Winding
• Secondary Winding
• Magnetic Core of transformer

Now What is Primary winding? The Primary winding of transformer which produce magnetic flux when it is connected to electrical source. And What is the secondary winding? The Secondary winding of transformer is the flux, produce by primary winding, passes through the core will link with the secondary winding. This is also wound on the core of transformer and gives the desired output of the transformer. And the magnetic core of transformer is the magnetic flux produced by the primary winding that will pass through with low reluctance path linked with secondary winding and create a close magnetic circuit in the transformer.

tertiary winding how electrical blog

Tertiary Winding of Three Phase Transformer

As we already discuss, there are two windings in the transformer. There is another additional winding we used named "Tertiary Winding". This winding is used in the high rating transformer for the purposes below mentioned.  Because of this third winding, the transformer with tertiary winding is also known as three winding transformer.

Advantages of Tertiary Winding in three phase transformer

There are following advantages of tertiary phase winding.

1. Tertiary Winding reduces the unbalancing in the primary due to unbalancing in three phase load.
2. Tertiary Winding redistributes the flow of fault current.
3. Sometime Tertiary Winding is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer.
4. As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.

Rating of Tertiary Winding.

Rating of tertiary winding depends upon its use. If it has to supply additional load, its winding cross - section and design philosophy is decided as per load, and three phase dead short circuit on its terminal with power flow from both sides of HV & MV. In case it is to be provided for stabilizing purpose only, its cross - section and design has to be decided from thermal and mechanical consideration for the short duration fault currents during various fault conditions single line to ground fault being the most onerous.

Read More

MCB, MCCB, ACB AND VCB DIFFERENCE AND CHARACTERISTICS - CIRCUIT BREAKERS

Circuit Breakers of differents poles.

MCB (Miniature circuit breaker)

• The characteristics of miniature circuit breaker are below,
• MCB rated current is not more than 100 A. means the current limit (or current rating) is maximum 100A).
• Trip characteristics are normally meant not adjustable.
•  MCB operation is thermal based or thermal-magnetic.

MCCB (Moulded case circuit breaker)

• The characteristics of Moulded case circuit breaker are below,
• The current rating of MCCB is from 101 A to 1000 A.
•  The Current of a trip (switch off the circuit) may be adjustable, means current rating we can adjust in MCCB.
• MCCB operates in thermal or thermal-magnetic operation.

ACB (AIR Circuit Breaker)

• The characteristics of ACB (Air Circuit Breaker) are below.
• The current rating of ACB (Air circuit breaker) is from 1001 A to 10000 A.
• Trip characteristics of Air Circuit Breaker (ACB) often fully adjustable including configurable trip thresholds and delays.
• ACB (Air Circuit Breaker) often used in Main Electrical Panels (usually in medium voltage MV or high voltage electrical panels HV). ACB (Air circuit breaker) also usually used for main power distribution in a large industrial plant, where the breakers are arranged in drawn-out enclosures for ease of maintenance.

VCB (Vacuum Circuit Breaker)

• Some important characteristics of Vacuum circuit breaker are below,
• The VCB (Vacuum circuit breaker) current rating is up to 3000 Amperes.
• The main characteristics of vacuum circuit breaker are, it interrupts the arc in a vacuum bottle.
• These can be applied at up to 35 thousand volts.

Is there any other you know, Share with us from below comment box.

Read More

WHAT IS DIELECTRIC - ELECTRICAL TECHNOLOGY BLOG - HOW ELECTRICAL WORKS.

A dielectric is an electrical insulator that can be polarized by an applied electric field.

When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization.

 Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field which reduces the overall field within the dielectric itself.

While the term "insulator" implies low electrical conduction, "dielectric" is typically used to describe materials with a high polarizability. The latter is expressed by a number called the dielectric constant.

The term insulator is generally used to indicate electrical obstruction while the term dielectric is used to indicate the energy storing capacity of the material (by means of polarization).

If the space between the plates of a capacitor is filled with an Dielectric, the capacitance of the capacitor will change compared to the situation in which there is vacuum between the plates.

The change in the capacitance is caused by a change in the electric field between the plates. The electric field between the capacitor plates will induce dipole moments in the material between the plates. These induced dipole moments will reduce the electric field in the region between the plates. A material in which the induced dipole moment is linearly proportional to the applied electric field is called a linear dielectric.

For linear dielectric:

Where K is called the dielectric constant. Since the final electric field E can never exceed the free electric field E_free, the dielectric constant k must be larger than 1.

The potential difference across a capacitor is proportional to the electric field between the plates.

Since the presence of a dielectric reduces the strength of the electric field, it will also reduce the potential difference between the capacitor plates (if the total charge on the plates is kept constant):

The capacitance C of a system with a dielectric is inversely proportional to the potential difference between the plates, and is related to the capacitance C_free of a capacitor with no dielectric in the following manner.

Since k is larger than 1, the capacitance of a capacitor can be significantly increased by filling the space between the capacitor plates with a dielectric with a large k.

The electric field between the two capacitor plates is the vector sum of the fields generated by the charges on the capacitor and the field generated by the surface charges on the surface of the dielectric.

Read More

NUCLEAR POWER PROCESS - NUCLEAR FISSION PROCESS - RADIOACTIVE DECAY

Radioactive decay, also known as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses energy in the form of radiations. A material that spontaneously emits this kind of radiation - which includes the emission of energetic alpha particles, beta particles, and gamma rays - is considered radioactive. When unstable nuclei decompose in nature, the process is referred to as natural radioactivity or spontaneous radioactivity. When the unstable nuclei are prepared in the laboratory, the decomposition is called induced radioactivity.

Fission is a splitting of something into two parts.

In nuclear physics and nuclear chemistry, nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei).

The fission process often produces free neutrons and photons (in the form of gamma rays), and releasing a very large amount of energy. When a nucleus fissions, it splits into several smaller fragments. These fragments, or fission products, are about equal to half the original mass. Two or three neutrons are also emitted. The sum of the masses of these fragments is less than the original mass. This 'missing' mass has been converted into energy according to Einstein's equation:

                                                E = mc²

Fission can occur when a nucleus of a heavy atom captures a neutron, or it can happen spontaneously.

The Fission Process

A neutron travels at high speed towards a uranium-235 nucleus. A neutron travels towards a uranium-235 nucleus. The neutron strikes the nucleus which then captures the neutron. The nucleus changes from being uranium-235 to uranium-236 as it has captured a neutron. The uranium-236 nucleus formed is very unstable. It transforms into an elongated shape for a short time.

The uranium-236 nucleus formed is very unstable. It transforms into an elongated shape for a short time. The uranium-236 nucleus formed is very unstable. It transforms into an elongated shape for a short time. It then splits into 2 fission fragments and releases neutrons. It then splits into 2 fission fragments and releases neutrons.

Nuclear Chain Reactions:

A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus, in turn, produces neutrons, and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons). 

Image from University of Florida

U²³⁵ + n → fission + 2 or 3 n + 200 MeV

If each neutron releases two more neutrons, then the number of fissions doubles each generation. In that case, in 10 generations there are 1,024 fissions and in 80 generations about 6 x 10 ²³ fissions.

1 MeV (mega electron volts) = 1.609 x 10 ^-13 joules

Uranium-235 combines with a neutron to form an unstable Uranium-236, which quickly splits into barium-144 and krypton-89 plus three neutrons in the process of nuclear fission.

Image from University of Dehli

Energy from Fission

Both the fission fragments and neutrons travel at high speed.  The kinetic energy of the products of fission are far greater than that of the bombarding neutron and target atom.

E_K before fission << E_K after fission

Energy is being released as a result of the fission reaction. The energy released can be calculated using the equation:

                                    E = mc²

Where:

E = energy released (J)

m = mass difference (kg)

c = speed of light in a vacuum (3 x 10⁸ ms^-1)

The energy released from this fission reaction does not seem a lot. This is because it is produced from the fission of a single nucleus. Large amounts of energy are released when a large number of nuclei undergo fission reactions. Each uranium-235 atom has a mass of 3.9014 x 10^-25 kg.

The total number of atoms in 1 kg of uranium-235 can be found as follows: No. of atoms in 1 kg of uranium-235 = 1/3.9014 x 10^-25.  No. of atoms in 1 kg of uranium-235 = 2.56 x 10²⁴ atoms

If one uranium-235 atom undergoes a fission reaction and releases 2.385 x 10^-11 J of energy, then the amount of energy released by 1 kg of uranium-235 can be calculated as follows:

total energy = energy per fission x number of atoms

total energy = 2.385 x 10^-11 x 2.56 x 10²⁴

total energy = 6.1056 x 10¹³ J

Critical mass:

Although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. If the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.

At the point where the chain reaction can become self-sustaining, this is referred to as critical mass.

In an atomic bomb, a mass of fissile material greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes. To maintain a sustained controlled nuclear reaction, for every 2 or 3 neutrons released, only one must be allowed to strike another uranium nucleus.

If this ratio is less than one then the reaction will die out; if it is greater than one it will grow uncontrolled (an atomic explosion). A neutron absorbing element must be present to control the amount of free neutrons in the reaction space.

Most reactors are controlled by means of control rods that are made of a strongly neutron-absorbent material such as boron or cadmium.

The nuclear force (or nucleon–nucleon interaction or residual strong force) is the force between two or more nucleons. It is responsible for binding of protons and neutrons into atomic nuclei. The energy released by such binding causes the masses of nuclei to be less than the total mass of the protons and neutrons which form them; this is the energy used in nuclear power and nuclear weapons. The force is powerfully attractive between nucleons at distances of about 1 femtometer (fm) between their centers, but rapidly decreases to insignificance at distances beyond about 2.5 fm. At very short distances less than 0.7 fm, it becomes repulsive, and is responsible for the physical size of nuclei, since the nucleons can come no closer than the force allows. So, if you feel anything important missed from here so you can comment below and mention How Electrical.

Read More

WHY WE USE AC MOTOR INSTEAD OF DC MOTOR - APPLICATIONS OF AC MOTORS AND APPLICATIONS OF DC MOTORS

For electrical technology, Motor is known as the main unit of electrical technology. because of motor use in all industries with all machines and usually all the places normally. the Electrical motor is the main sub-sub-branch of electrical engineering because it is not easy to learn all concepts about electrical motor technology. There are many types and kinds of electrical motors like,

1. AC motors
2. DC motors

and there are also differents kinds in AC motors and DC motors as synchronous motors, induction motors, explosion proof, low voltage and motor with permanent magnet etc.

Where We Use AC Motors:

The alternating current electric motor (AC motors) are ideal for most applications linear, for fans, pumps, compressors, mills, machine tools, boilers, robots, generators and in many other products categories.
As regarding the choice of the type of electric motor for a given application, this is influenced by different factors, starting from the costs of purchase and operation, the yield, the efficiency, the periodic maintenance.

Where We Use DC Motors:

A DC motor is an electric motor that runs on direct current power. In any electric motor, the operation is dependent upon simple electromagnetism. A current carrying conductor generates a magnetic field, when this is then placed in an external magnetic field, it will encounter a force proportional to the current in the conductor and to the strength of the external magnetic field Resources and Information. is a device which converts electrical energy to mechanical energy. It works on the fact that a current carrying conductor placed in a magnetic field experiences a force which causes it to rotate with respect to its original position. Practical DC Motor consists of field windings to provide the magnetic flux and armature which acts as the conductor.

Applications of DC Motors

Series Motors

The series DC motors are used where high starting torque is required, and variations in speed are possible. For example – the series motors are used in Traction system, Cranes, air compressors.

Shunt Motors

The shunt motors are used where constant speed is required and starting conditions are not severe. The various applications of DC shunt motor are in Lathe Machines, Centrifugal Pumps, Fans, Blowers, Conveyors, Lifts, Weaving Machine, Spinning machines, etc.

Compound Motors

The compound motors are used where higher starting torque and fairly constant speed is required. The examples of usage of compound motors are in Presses, Shears, Conveyors, Elevators, Rolling Mills, Heavy Planners, etc.

Read More

ELECTRICAL POWER | ENERGY METER DISCUSSIONS

Theory: - 

Energy meter is an instrument which measures electrical energy. It is also known as watt-hour (Wh) meter. It is an integrating device. There are several types of energy meters single phase induction type energy meter are very commonly used to measure electrical energy consumed in domestic and commercial installation. Electrical energy is measured in kilo watt-hours (kWh) by this energy meter. 

Construction: - 

A single phase induction type energy meter consists of driving system, moving system, braking system and registering system. Each of the systems is briefly explained below. 

Driving system: -

This system of the energy meter consists of two silicon steel laminated electromagnets. M1 & M2 as shown in fig.1The electromagnet M1 is called the series magnet and the electromagnet M2 is called the shunt magnet. The series magnet M1 carries a coil consisting of a few turns of thick wire. This coil is called the current coil (CC) and it is connected in series with the circuit. The load current flows through this coil. The shunt magnet M2 carries a coil consisting many turns of thin wire. This coil is called the voltage coil (VC) and is connected across the supply it consist of current proportional to the supply voltage. Short circuited copper bands are provided on the lower part of the central limb of the shunt magnet.

By adjusting the position of these loops the shunt magnet flux can be made to lag behind the supply voltage exactly 90° . These copper bands are called power factor compensator (PFC). A copper shading band is provided on each outer limb of the shunt magnet (fc1 &fc2) these band provides frictional compensation.

Moving system: -

The moving system consists of a thin aluminium disc mounted on a spindle and is placed in the air gap between the series and the shunt magnets. It cuts the flux of both the magnet forces are produced by the fluxes of each of the magnets with the eddy current induced in the disc by the flux of the other magnets. Both these forces act on the disc. These two forces constitute a deflecting torque.

Braking system: -

The braking system consists of a permanent magnet called brake magnet. It is placed near the edge of the disc as the disc rotates in the field of brake magnet eddy current are induced in it. These eddies current react with the flux and exert a torque. This torque acts in direction so that it opposes the motion of disc. The braking torque is proportional to the speed of the disc.

Registering system: -

The disc spindle is connected to a counting mechanism this mechanism records a number which is proportional to the number of revolutions of the disc the counter is calibrated to indicate the energy consumed directly in kilo watts-hour (kWh)

Read More

WHAT IS COHESIVE DEVICES? DISCUSSIONS

Coherence in writing means achieving a consistent relationship among parts. Cohesive devices show the logical relationships between the various parts of an essay as well as between sentences and paragraphs.

Cohesive devices include: transitional words and expressions,  paragraph hooks

cohesive devices are like bridges between parts of your paper

They are cues that help the reader to interpret ideas in the way that you, as a writer, want them to understand cohesive devices help you carry over a thought from one sentence to another, from one idea to another, or from one paragraph to another with words or phrases.

cohesive devices link your sentences and paragraphs together smoothly so that there are no abrupt jumps or breaks between ideas.

Cohesive words and phrases  are used to link sentences and paragraphs, to show which direction your thought patterns are going, to help the reader accurately follow your train of thought.

They signal the relationships among the various parts of your subject.

Types Of Cohesive Devices

There are several types of cohesive devices, and each category leads your reader to make certain connections or assumptions about the areas you are connecting.

Some lead your reader forward and imply the "building" of an idea or thought,

while others make your reader compare ideas or draw conclusions from the preceding thoughts.

Before, meanwhile, later, soon, at last, earlier, thereafter, afterward, by that time, from then on, first, next, now, presently, shortly, immediately, finally

Likewise, similarly, once again, once more

But, yet, however, although, whereas, though, even so, nonetheless, still, on the other hand, on the contrary As a result, consequently, therefore, hence, for this reason

I knew my dieting had gotten out of hand, but when I could actually see the movement of my heart beating beneath my clothes, I knew that I was in trouble. At first, the family doctor reassured my parents that my rapid weight loss was a “temporary phase among teenage girls.” However, when I, at fourteen years old and five feet tall, weighed in at 63 pounds, my doctor…

Transition words are audience cues that help the reader shift from one paragraph to the next.

These connections between paragraphs help the reader see the relationships of the various parts.

Transition words or phrases at the beginning of a new paragraph—such as first, second, next, another, finally, on the other hand, however—show the reader where the essay is going next.

In addition to transition words, writers often tie paragraphs together by repeating a key word or idea from a previous paragraph in the opening sentence of the next paragraph.

This “hooks” the paragraphs together, creating for the reader a logical flow of thought.

Read More

INDUCTION MOTORS QUESTIONS ANSWERS

1.                  Explain construction and working principle of a 3-phase indcuiton motor.

2.                  Explain the terms slip, slip frequency, wound rotor and cage rotor.

3.                  Draw and explain the torque-slip & torque-speed charactoristics of a typical 3-phase induction motor. Mark the starting torque and maximum troque on the daigram so drawn.

4.                  Explain  the terms air-fap power P_g, inernal mechanical power developed and shaft power P_sh. How  are these terms related with each others. Hence show that :

P_g :Rotor ohmic loss : P_m =1 :s (1-s)

5.                  A 3-phase induction motor runs at 1140 rpm at full load when supplied with power from a 60 Hz, 3-phase line calculate                                       

                        (i)         No of poles.

                        (ii)        Slip at full load.

(iii)             Frequency of rotor voltage.

(iv)             Speed of rotor field with respect to rotor.

(v)               Speed of rotor field with respect to stator and stator field.

(vi)             Speed of rotor at a slip of 10% and rotor frequency at this speed.

(vii)           If the applied voltage per phase is 230 V, find th0 rotor induced emf at stand still and at 10% slip, with stator to rotor turn ration of 1: 0.5

(Ans: - (i) 1200 rpm, (ii) 5% (iii) 3 Hz (iv) 60 rpm (v) 0 rpm (vi) 1080 rpm, (vii) 11.5 V)

6.                  A 6-pole, 3-phase indcution motor develops 30 hp including 2 hp mechanical losses at a speed of 950 rpm on 550 V, 50 Hz mains. The power factor is 0.88 lagging. Calculate (i) slip (ii) torot copper loss (iii) total input if stator losses are 2 kW (iv) efficiecny (v) line current. (Ans : - (i) 0.05, (ii) 1161 Watts, (iii) 20594 Watts, (iv) 81.64% (v) 30 A)

7.                  Explain construction and principle of operation of 1-phase induction motor.

8.                  Why single phase induciton moto is not self starting ?

9.                  What are the different methods of statring a single phase induction motors and what are the diffenet types of 1-phase induction motors ? 

Read More

DC MACHINES AND SYNCHRONOUS MACHINES - QUESTIONS FOR PRACTICES

1. Explain construction and working principle of dc machines with the help of a neat diagram. Drive an emf equation for the same. What is back emf?
2. What are the different efficiency associated with dc generator and dc motors explain?
3. Drive an expression for torque in case of dc machine.
4. A d.c. generator has an armature emf of 100 V when the useful flux per pole is 20 mWb, and the speed is 800 rpm. Calculate the generated emf (i) with the same flux and a speed of 1000 rpm, (ii) with a flux per pole of 24 mWb and a speed of 90 rpm. (Ans.:- (i) 125 V, (ii) 135 V)
5. An 8 pole dc generator has 500 armature conductors and a useful flux of 0.05 Wb. What will be the emf generated, if it lap connected and runs at 1200 rpm? What must be the speed at which it is to be driven to produce the same emf, if it is wave wound? (Ans.:- 500V, 300 rpm)
6. A 4 pole dc shunt generator with lap connected armature has field and armature resistance of 80 Ω and 0.1 Ω respectively. It supplies power to 50 lamps rated for 100 volts, 60 watts each. Calculate the total armature current and the generated emf by allowing a contact drop of 1 V per brush. (Ans.:- 26 A, 104.6 V)
7.  A d.c. shunt machine connected to 230 V supply has resistance of armature as 0.115 Ω and of field winding as 115 Ω. Find the ratio of the speed as a generator to the speed as a motor with the line current in each case being 100 A. (Ans.:- 1.1052:1)
8. A d.c. shunt motor draws 10 A at no load from 230 V mains and runs at 1500 rpm. At full load, armature current is 100 A and speed is 1470 rpm. Armature resistance of the motor is 0.1 Ω and field current negligible. Find (a) back emf at no – load and full load (b) speed at which armature should be run to make it deliver 100 A at 220 V as a generator. Assume same flux as with motor operation at full load of 10 A. (Ans.:- (a) 229 V , 220 V (b) 1536.8 rpm)
9. A 200 V dc series motor runs at 500 rpm when taking a current of 25 A. The resistance of the armature is 0.5 Ω and that of the field is 0.3 Ω. If the current remains constant, calculate the resistance necessary to reduce the speed to 250 rpm. (Ans.:- 3.6 Ω)
10. A d.c. series motor has an armature resistance of 0.12 Ω and field resistance of 0.08 Ω. The supply voltage across the motor terminals is 230 V. Determine the back emf and power developed by the motor when line current drawn by motor is 30 A. (Ans.:- 224 V, 6.72 kW)

11. What are the different types of synchornous machines ? Explain the principle of operation of a 3-phase synchronus motor. Why it is not self starting.

12. Why the field winding of synchronus generators is placed on rotor.

13. Draw and explain V-curve for synchronous motor

Read More

SINGLE PHASE INDUCTION MOTOR - CONSTRUCTION OF ELECTRIC MOTOR

Actual 1-phase induction motors are,

1. Ceiling fans
2. Other single phase induction motor: 

Construction

Construction is similar as that of 3-phase squirrel induction motor

A single phase induction motor has two main parts

Stator

Similar to 3-phase induction motor

consisting of a steel frame that supports a hollow, Laminated cylindrical core core, constructed from stacked laminations (why?), having a number of evenly spaced slots, providing the space for the stator winding.

Rotor

Squirrel cage rotor is used

aluminum bus bars shorted together at the ends by two aluminum rings.

Shaft:

Should withstand maximum breaking strength

Bearing:

Roller or ball bearing

It is used in vacuum cleaner, fans, washing machine, centrifugal pump, blowers, washing machine, small toys etc.

Principle:

When stator winding is fed from a single phase supply, an alternating (pulsating) field is produced.

Due to this alternating flux an alternating EMF (current) is induced in the rotor conductors

Now when a current carrying conductor is place in magnetic filed it experiences a force.

But after each half cycle the direction of induced current is change and hence the direction of force (torque) is changed after each half cycle. So pulsating flux acting on stationary squirrel cage rotor can’t produce rotation and therefore 1-phase induction motor is not self starting.

However, if the rotor o such machine is given initial start by hand or otherwise in either direction, then motor starts rotating in that direction. Above peculiar behavior can be explained by

Double filed revolving theory

To make it self-starting, it can be temporarily converted into a two-phase motor while starting. This can be achieved by introducing an additional 'starting winding' also called as auxillary winding.

Hence, stator of a single phase motor has two windings: (i) Main winding and (ii) Starting winding (auxiliary winding). These two windings are connected in parallel across a single phase supply and are spaced 90 electrical degrees apart. Phase difference of 90 degree can be achieved by connecting a capacitor in series with the starting winding.

Hence the motor behaves like a two-phase motor and the stator produces revolving magnetic field which causes rotor to run. Once motor gathers speed, say upto 80 or 90% of its normal speed, the starting winding gets disconnected form the circuit by means of a centrifugal switch, and the motor runs only on main winding.

Type of single phase induction motors

The  single phase induction motors are made self starting by providing an additional flux by some additional means( Additional winding ). Now depending upon these additional means the single phase induction motors  are classified as:

• Resistance start induction motor
• Capacitor start induction motor
• Capacitor start capacitor run induction motor
• Permanent capacitor motor
• Shaded pole induction motor.

Read More

GENERATION OF IMPULSE VOLTAGES AND CURRENTS

IMPULSE VOLTAGE:

An impulse voltage is a unidirectional voltage which, without appreciable oscillations, rises rapidly to a maximum value and falls more or less rapidly to zero Fig. 3.1. The maximum value is called the peak value of the impulse and the impulse voltage is specified by this value. Small oscillations are tolerated, provided that their amplitude is less than 5% of the peak value of the impulse voltage. In case of oscillations in the wave shape, a mean curve should be considered.

If an impulse voltage develops without causing flash over or puncture, it is called a full impulse voltage; if flash over or puncture occur, thus causing a sudden collapse of the impulse voltage, it is called a chopped impulse voltage. A full im-pulse voltage is characterized by its peak value and its two time intervals, the wave front and wave tailtime intervals defined below:

The wave front time of an impulse wave is the time taken by the wave to reach to its maxi-mum  value starting from zero value. Usually it is difficult to identify the start and peak points of the wave and, therefore, the wave front time is specified as 1.25 times (t 2 – t 1), where  t 2  is the time for the wave to reach to its 90% of the peak value and  t 1  is the time to reach 10% of the peak value. Since ( t 2 –  t 1) represents about 80% of the wave front time, it is multiplied by 1.25 to give total wave front time. The point where the line  CB intersects the time axis is referred to be the nominal starting point of the wave.

The nominal wave tail time is measured between the nominal starting point  t 0  and the point on the wave tail where the voltage is 50% of the peak value i.e. wave fail time is expressed as (t 3 –  t 0 ).

The nominal steepness of the wave front is the average rate of  rise of voltage between the points on the wave front where the voltage is 10% and 90% of the peak value respectively.

The standard wave shape specified in BSS and ISS is a 1/50 micro sec. wave  i.e.  a wave front of 1 micro sec. and a wave tail of 50 micro sec. A tolerance of not more than  ±50% on the duration of the

wave front and 20% on the time to half value on the wave tail is allowed. The wave is completely specified as 100 kV, 1/50 micro sec. where 100 kV is the peak value of the wave.

The wave shape recommended by the American Standard Association is 1.5/40 micro sec. with permissible variations of 0.5 micro sec. on the wave front and  ±10 micro sec. on the wave tail. Here wave front time is taken as 1.67 times the time taken by the wave to rise from 30% to 90% of its peak

value and wave tail time is computed as in BSS or ISS i.e . it is given as (t 3 – t 0 ) Fig. 3.1.

Impulse Flash Over Voltage:

Whenever an impulse voltage is applied to an insulating medium of certain thickness, flash over may or may not take place. If out of a total of say ten applications of impulse voltage about 5 of them flash over then the probability of flash over with that peak voltage of the impulse voltage is 50%. Therefore, a 50 per cent impulse flash over voltage is the peak value of that impulse flash over voltage which causes flash over of the object under test for about half the number of applications of impulses. However, it is to be noted that the flash over occurs at an instant subsequent to the attainment of the peak value. The flash over also depends upon the polarity, duration of wave front and wave tails of the applied impulse voltages.

Impulse Puncture Voltage:

The impulse puncture voltage is the peak value of the impulse voltage which causes puncture of the material when puncture occurs on the wave tail and is the value of the voltage at the instant of puncture when puncture occurs on the wave front.

Impulse Ratio for Flash Over:

The impulse ratio for flash over is the ratio of impulse flash over voltage to the peak value of power frequency flash over voltage.

The impulse ratio is not a constant for any particular object, but depends upon the shape and polarity of the impulse voltage, the characteristics of which should be specified when impulse ratios are quoted.

Impulse Ratio for Puncture:

The impulse ratio for puncture is the ratio of the impulse puncture voltage to the peak value of the power frequency puncture voltage.

Read More

SERIES RESONANT CIRCUIT

The equivalent circuit of a single-stage-test transformer along with its capacitive load is shown in Fig.v2.15. Here  L 1  represents the inductance of the voltage regulator and the transformer primary,  L the exciting inductance of the transformer,  L 2  the inductance of the transformer secondary and C  the capacitance of the load. Normally inductance L is very large as compared to  L 1 and  L2  and hence its shunting effect can be neglected. Usually the load capacitance is variable and it is possible that for certain loading, resonance may occur in the circuit suddenly and the current will then only be limited by the resistance of the circuit and the voltage across the test specimen may go up as high as 20 to 40 times the desired value.

Similarly, presence of harmonics due to saturation of iron core of transformer may also result in resonance. Third harmonic frequencies have been found to be quite disastrous.

With series resonance, the resonance is controlled at fundamental frequency and hence no un-wanted resonance occurs.

The development of series resonance circuit for testing purpose has been very widely welcome by the cable industry as they faced resonance problem with test transformer while testing short lengths of cables.

With the static frequency convertor circuits the specific weight has come down to 0.5 kg/kVA.

It is to be noted that whereas the series resonant systems are quite popular for testing cables and highly loss free capacitive loads, cascaded transformers are more common in high voltage laboratories for testing equipment in  MV range and also for relatively high loads.

Read More

REACTIVE POWER COMPENSATION

As is mentioned earlier, the test transformers are used for testing the insulation of various electrical equipment's. This means the load connected to these transformers is highly capacitive. Therefore, if rated voltage is available at the output terminals of the test transformer and a test piece (capacitive load) is connected across its terminals, the voltage across the load becomes higher than the rated volt-age as the load draws leading current.

Thus, it is necessary to regulate the input voltage to the test transformer so that the voltage across the load, which is variable, depending on the test specimen, remains the rated voltage.

Another possibility is that a variable inductor should be connected across the supply as shown in Fig. 2.13 so that the reactive power supplied by the load is absorbed by the inductor and thus the voltage across the test transformer is maintained within limits.

When the primary series is connected, for the same supply voltage, voltage per turn of primary becomes half its value when it is parallel connected and, therefore, the secondary voltage becomes ½ of the rated voltage and hence the curve starts at 50% of the rated voltage. The power of the voltage regulator is proportional to the supply voltage and, therefore, is represented by line  E in Fig. 2.14 and the maximum power at rated voltage is 33.3% of the maximum power requirement of the transformer.

All possible operating conditions of the test transformer lie within the triangular area enclosed by the line  A,  the abscissa and the 100% rated voltage line.

This area has been sub- divided into different parts,so that the permissible supply power (Here 33% of maximum transformer load) is never exceeded. The value of the highest voltage is always taken for the evaluation of the compensation arrangement. Since the impedance of the test transformer is usually large (about 20–25%), the range under 25% of the rated voltage is not considered.

Read More

MECHANISM OF BREAK DOWN OF GASES

At normal temperature and pressure, the gases are excellent insulators. The current conduction is of the order of 10–10 A/cm2. This current conduction results from the ionisation of air by the cosmic radiation and the radioactive substances present in the atmosphere and the earth. At higher fields, charged particles may gain sufficient energy between collision to cause ionisation on impact with neutral molecules.
It is known that during an elastic collision, an electron loses little energy and rapidly builds up its
kinetic energy which is supplied by an external electric field. On the other hand, during elastic collision, a large part of the kinetic energy is transformed into potential energy by ionising the molecule struck by the electron. Ionisation by electron impact under strong electric field is the most important process leading to break-down of gases.

Download this detail in a file from here.

Read More

AUTOMATIC POWER CONTROL SYSTEM

Automated control system

In this structure the controllers operating directly on individual system elements. In a generating unit these consist of prime mover controls an excitation controls.

The prime mover controls are concerned with  speed regulation and control of energy supply system variables such as boilers pressures ,temperatures , and flow. The functions of the excitation control is to regulate generators voltage and reactive power output. The desired MW outputs of the individual generating units are determined by the system generation control.

The primary purpose of the system generation control is to balance the total system generation against system load and losses so that the desired frequency and power interchange with neighboring systems (tie flows) is maintained. The transmission control include power and voltages control devices, such as static VAR compensators, synchronous condensers, switched capacitors and reactors, tap-changing transformers, phase-shifting transformers, and HVDC transmission controls.

The controls described above contribute to the satisfactory operation of the power system by maintaining system voltages and frequency and other system variables within their acceptable limits. They also have a profound effect on the dynamics performance of the power system and on its ability to cope with disturbances.

Read More

OBJECTIVES OF POWER SYSTEM - ELECTRICAL POWER SYSTEM

The objective of power system control is to maintained continuous electric supply of acceptable quality by taking suitable measures against the various disturbances that occur in the system.

These disturbances can be classified into two major heads, namely, small-scale disturbances and large scale disturbances.

Small Scale Disturbances

Small scale disturbances comprise slowly varying small magnitude changes occurring in the active and reactive demands of the system.

The small scale disturbances can be overcome by regulating controls using  governors and exciter.

Large Scale Disturbances

The large scale disturbances can only be overcome by proper planning and adopting emergency switching control.

Large scale disturbances are sudden large magnitude changes in system operating conditions such as faults on transmission network, tripping of a large generating unit or sudden connection or removal of large blocks of demand.

Objectives of Power system control

To meet continually changing load demand

Adequate “spinning” power reserve

Minimum cost of energy

Minimum environmental pollution

The “quality” of power supply

                (a) constancy of frequency

                (b) constancy of voltage; and

                    (c) level of reliability

Frequency Regulation

System frequency, must remain within its operational range

                         f _min < f(t)  <  f _max

                      49.5 Hz < f(t)  <  50.5 Hz

Voltage Regulation

Bus voltages must remain within their operational limits

                         V _min < V(t)  < V _max

                      0.95 pu < V(t)  < 1.05 pu

Quality is normally described by means of an accepted voltages profile (level and amount of flicker) and frequency (set point with a narrow band and a threshold for the time delay) of the delivered electric power.

Security is much more difficult to describe in quantitative terms. There are normally certain rules in utilities and power pools concerning power system balance, network operation etc. In order to successfully take care of care of some predefined disturbances.

Economy consists of two parts: The investment part of apparatus, control system and so on, and the running cost for the while power system.

To keep the objectives on reasonable level it is a must today to take advanced control systems into service.

Read More