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Tuesday 21 February 2017

Why transformers rated output expressed in kVA and not in Kw

February 21, 2017 Transformer , 0 Comments
Why transformers rated output expressed in kVA and not in Kw
Transformer rating is in kVA not in kw
The manufacturer of transformers fixed a name plate on the transformer on which are recorded the rated output, the rated
voltages, the rated frequency etc. of a particular transformer.
A typical name plate rating of a single phase transformer is as follows : 20 kVA, 3300/220 V, 50Hz. Here 20 kVA is the rated output at the secondary terminals.
Note that the rated output is expressed in kilo-volt-amperes (kVA) rather than in kilowatts (kw). This is due to the fact that rated transformer output is limited by heating and hence by the losses in the transformer.
These losses depend on transformer voltage (core loss) and current and are almost unaffected  by the load power factor.
Consequently the transformer rated output is expressed in kVA and not in Kw.
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Monday 20 February 2017

Why motor protection is necessary? And types of fuses used in motor protection

February 20, 2017 electrical machine , protection 0 Comments
Why motor protection is necessary? And types of fuses used in motor protection
explanation about protection provided to induction and other types motor are given below:
In order to avoid unexpected breakdowns, costly repairs and subsequent losses due to motor downtime, it is important that the motor is fitted with some sort of protective device. Generally speaking,

Motor protection can be divided into the following 3 levels:

• External protection against short circuit in the whole installation. External protection device is normally different types of fuses or short circuit relays. This kind of protection device is compulsory and legal and placed under safety regulations.

• External protection against overload of specific equipment; i.e. to avoid overload of
pump motor and thereby prevent damage
and breakdown of the motor. This type of
protection reacts on current.

• Built-in motor protection with thermal
overload protection to avoid damage and
breakdown of motor. The built-in protector
always require an external circuit breaker
while some built-in motor protection types
even require an overload relay list of the most common fault conditions where motor damage can be avoided by some sort of motor protection:
• Problems with the power supply quality:
– Overvoltage
– Undervoltage
– Imbalanced voltages/currents
– Frequency variation
• Installation, supply & motor failures
• Slowly developing temperature rise:
– Insufficient cooling
– High ambient temperature
– High altitude operation
– High liquid temperature
– Too high viscosity of the pumping liquid
– Frequent starts
– Too big load inertia
– (not common for pumps)
• Quickly developing temperature rises:
– Locked rotor
– Phase breakage

To protect a circuit against overloads and short circuits, a circuit protective device must determine when one of these fault conditions occurs.It must then automatically disconnect the circuit from the power source. A fuse is the simplest device for accomplishing these two functions.

Normally fuses are built together by means of
a safety switch, which can switch off the circuit.

In this post, we will discuss three
types of fuses as per their function and to where they are used: Fusible safety switch, “quick-acting” fuse and “time-lag” fuse

Fusible safety switch:
A fusible safety switch is a safety switch, which is combined with a fuse in a single enclosure. The switch manually opens and closes the circuit, while the fuse protect against overcurrent protection.Switches are generally used in connection with
service when it is necessary to cut off the current, or in connection with fault situations.The safety switch is a switch, which is placed 
in a separate enclosure. The enclosure protects personnel against accidental exposure to electrical connections and against exposure to weather conditions. Some safety switches come with a built-in function for fuses, and some safety switches come without built-in fuses, containing only a switch.
The overcurrent protection device (fuse) has to
recognise the difference between overcurrent
and short circuit. Slight overcurrents for 

example, can be allowed to continue for a short period of time. But as the current magnitude increases, the protection device has to react quickly. It is important to interrupt short circuits immediately.
The fusible disconnect switch is an example of a device which is used for overcurrent protection.Properly sized fuses in the switch open the circuit when an overcurrent condition occurs.

Quick-acting:
"Quick-acting” fuses Nontime-delay fuses provide excellent short circuit protection. However, brief overloads, such as motor starting currents, may cause problems for this kind of fuse. Therefore, nontime-delay fuses are best used in circuits, which are not  subject to large transient currents. Normally, nontime-delay fuses hold some 500% of their rated current for one-fourth of a second. After this time, the current-carrying element melts, and opens the fuse.Thus, in motor circuits, where the starting current often exceeds 500% of the fuse’s rated current, nontime-delay fuses are not recommended.

“Time-lag” fuses:
This kind of fuse provides both overload and short-circuit protection. Typically, they allow up to 5 times the rated current for up to 10 seconds and for shorter periods even higher currents. Usually, this is sufficient to allow a motor to start without opening the fuse. On the other hand, if an overload condition occurs and persists for a longer period of time, the fuse will eventually open.

Reference:Motor Book Grundfos


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Different types of dc link in direct current transmission

February 20, 2017 current , 0 Comments

DIFFERENT TYPE OF DC LINKS:

How many types of dc links? 

Direct-current links are classified as follows
  1. Mono polar dc link
  2. Bipolar dc link
  3. Homo polar dc link

1) Mono polar dc link: The monopolar link has one conductor usually of negative polarity, and ground or sea return.
Mono polar dc link
Fig :Mono polar dc link

2) Bipolar dc link: The bipolar link has two conductors one positive, the other is negative. Each terminal has  two converters of equal rated voltages in series on the dc side.  The midpoint (junction between converter) are  grounded at one or both ends. neutrals are grounded he two poles can operate independently .  Normally they operate at equal current;  then there is no ground current.  In the event of a fault on one conductor,  the other conductor with ground return can carry up to half of the rated load. The rated voltage of a bipolar link is expressed as 100kv or pronounced plus and minus 100 kV

Bipolar dc link
Fig: Bipolar dc link

3)The homopolar link: the homopolar link has two or more conductors all having the same polarity,  usually negative,  and always operates with ground return. 
homopolar link
Fig: homopolar link
In the event of a fault on one conductor,the entire converter is available for connection to the remaining conductor or conductors,  which,  having some overload capability,  can carry more than half of the rated power,  and perhaps the whole rated power,  at the expense of increased line loss.  In a bipolar scheme reconnection of the whole converter to one pole of the line is more complicated and is usually not feasible becausc graded insulation.  In this respect a homopolar of line is preferable to a bipolar line in cases where continual ground current is not deemed objectionable

Reference:direct current transmission by kimbark
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Tuesday 14 February 2017

Definations of different electrical terms voltage,current,resistance

February 14, 2017 current , 0 Comments
ELECTRICAL TERMINOLOGY

Different electrical terms are defined  below 
a.Conductor
b. Insulator
c. Resistor
d. Voltage
e. current flow
f.  Direct current (DC)
g. Alternating current (AC)


a. what is good Conductor of electricity: 

Conductors are materials with electrons that are loosely bound to their atoms, or materials that permit free motion of a large number of electrons. Atoms with only one valence electron, such as copper, silver, and gold, are examples of good conductors. Most metals are good conductors.
 
b.Insulators:
Insulators, or nonconductors, are materials with electrons that are tightly bound to their atoms
and require large amounts of energy to free them from the influence of the nucleus. The atoms
of good insulators have their valence shells filled with eight electrons, which means they are
more than half filled. Any energy applied to such an atom will be distributed among a relatively
large number of electrons. Examples of insulators are rubber, plastics, glass, and dry wood.

c. What is Resistor:
resistors
 
Resistors are made of materials that conduct electricity, but offer opposition to current flow.
These types of materials are also called semiconductors because they are neither good conductors nor good insulators. Semiconductors have more than one or two electrons in their valence shells,but less than seven or eight. Examples of semiconductors are carbon, silicon, germanium, tin, and lead. Each has four valence electrons.

d. what is Voltage or potential difference:
The basic unit of measure for potential difference is the volt (symbol V)  and because the volt
unit is used, potential difference is called voltage. An object’s electrical charge is determined
by the number of electrons that the object has gained or lost. Because such a large number of
electrons move, a unit called the "coulomb" is used to indicate the charge. One coulomb is equal to 6.28 x 1018 (billion, billion) electrons. . 
A volt is defined as a difference of potential causing one coulomb of current to do one joule of work. 
A volt is also defined as that amount of force required to force one ampere of current through one ohm of resistance.

e. what is Current and how it flow :
The density of the atoms in copper wire is such that the valence orbits of the individual atoms
overlap, causing the electrons to move easily from one atom to the next. Free electrons can drift
from one orbit to another in a random direction. When a potential difference is applied, the
direction of their movement is controlled. The strength of the potential difference applied at each end of the wire determines how many electrons change from a random motion to a more
directional path through the wire. The movement or flow of these electrons is called electron
current flow or just current. To produce current, the electrons must be moved by a potential difference. The symbol for current is (I). The basic measurement for current is the ampere (A). 

One ampere of current is defined as the movement of one coulomb of charge past any given point of a conductor during one second of time.

If a copper wire is placed between two charged objects that have a potential difference, all of the negatively-charged free electrons will feel a force pushing them from the negative charge to the positive charge.
 
Potential Difference Across a Conductor Causes a Current to Flow
Potential Difference Across a Conductor Causes a Current to Flow

The direction of electron flow, shown in Figure, is from the negative (-) side of the battery,
through the wire, and back to the positive (+) side of the battery. The direction of electron flow
is from a point of negative potential to a point of positive potential. The solid arrow shown in
Figure 10 indicates the direction of electron flow. As electrons vacate their atoms during electron current flow, positively charged atoms (holes) result. The flow of electrons in one direction causes a flow of positive charges. The direction of the positive charges is in the opposite direction of the electron flow. This flow of positive charges is known as conventional current and is shown in Figure as a dashed arrow. All of the electrical effects of electron flow from negative to positive, or from a higher potential to a lower potential, are the same as those that would be created by a flow of positive charges in the opposite direction.

Generally, electric current flow can be classified as one of two general types: Direct Current
(DC) or Alternating Current (AC). A direct current flows continuously in the same direction.
An alternating current periodically reverses direction. We will be studying DC and AC current
in more detail later in this text. An example of DC current is that current obtained from a
battery. An example of AC current is common household current.

Reference:DOE FUNDAMENTALS HANDBOOK ELECTRICAL SCIENCE
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Tuesday 7 February 2017

What is capacitor? different types of capacitors

February 07, 2017 method to improve power factors , 0 Comments

CAPACITORS:
A capacitor consists of two metal plates, separated by an insulating layer called the dielectric.
 It has the ability of storing a quantity of electricity as an excess of electrons on one plate and a deficiency on the other.The p.d. which may be maintained across the plates of a capacitor is determined by the type and thickness of the dielectric medium. Capacitor manufacturers usually indicate the maximum safe working voltage for their products.Capacitors are classified by the type of dielectric material used in their construction. Figure shows the general construction and appearance of some capacitor types to be found in installation work.


1) Air-dielectric capacitors:
variable capacitor Air-dielectric capacitors
Fig:air dielectric capacitor

Air-dielectric capacitors are usually constructed of multiple aluminium vanes of which one section moves to make the capacitance variable. They are often used for radio tuning circuits.Mica-dielectric capacitors Mica-dielectric capacitors are constructed of thin aluminium foils separated by a layer of mica. They are expensive, but this dielectric is very stable and has low dielectric loss. They are often used in high-frequency electronic circuits.

2) Paper-dielectric capacitors:
Paper-dielectric capacitors construction
Paper-dielectric capacitors
 
Paper-dielectric capacitors usually consist of thin aluminum foils separated by a layer of waxed paper. This paper–foil sandwich is rolled into a cylinder and usually contained in a metal cylinder. These capacitors are used in fluorescent lighting fittings and motor circuits.

3) Electrolytic capacitors:
Electrolytic capacitors
photo credit:123rf
 
The construction of these is similar to that of the paper-dielectric capacitors, but the dielectric material in this case is an oxide skin formed electrolytically by the manufacturers. Since the oxide skin is very thin, a large capacitance is achieved for a small physical size,but if a voltage of the wrong polarity is applied, the oxide skin is damaged and the gas inside the sealed
container explodes. For this reason electrolytic capacitors must be connected to the correct voltage polarity.
They are used where a large capacitance is required from a small physical size and where the terminal voltage never reverses polarity
  
4) Ceramic capacitors:
Ceramic capacitors construction 103
Ceramic capacitors

 A ceramic capacitor is a non-polarized fixed capacitor made out of two or more alternating layers of ceramic and metal in which the ceramic material acts as the dielectric and the metal acts as the electrodes. The ceramic material is a mixture of finely ground granules of paraelectric or ferroelectric materials, modified by mixed oxides that are necessary to achieve the capacitor's desired characteristics.
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Working principle of full wave bridge rectifier and digram

Full wave bridge rectifier - Rectifier is a circuit which convert alternating voltage into the direct voltage. There are three types o...

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