Electra

Fuse

                        Fuse is nothing but a wire which melts and separate the circuit when predetermined value of current flows through it. The rating depends on the load current.

 It is added in the live circuit to avoid burning of wires when heavy current flows during short circuit. The maximum current that can be carried by the fuse without melting is rated carrying current.

The fuse element melts at the minimum current is fusing current. The ratio of fusing current to rated current is known to be fusing factor. The low voltage fuses is divided into rewirable and HRC fuses.

The rewirable fuse has one or more strands of wire that is mounted on the porcelain handle. In the HRC cartridge a ceramic body is welded with the silver current carrying elements. The special material surrounds the elements.

The operation of HRC is the melting of silver followed by the vaporization, fusing of silver vapor and finally the extinction of arc.

 The low voltage fuses are used in low distribution systems. The HRC fuses are used as a backup protection for the circuit breakers.

Speed Management of a Direct Current series motor

                There are various methods to control the speed of series motor

. 1) The variation of flux by means of field diverter in which the variable resistance is connected in parallel with the field winding. If the speed has to increase the resistance is reduced which results in the reduction of main field.

2) The variation of flux by means of armature diverter gives speeds below the normal value. It is obtained by connecting the diverter across the armature.

 3) In the Tapped field method the higher speeds results by cutting out the series turns while with the full field motor runs as its minimum speed.

4) The variable resistance in series with the motor helps in controlling the speed. If the resistance increases the voltage across motor reduces. Since the voltage is proportional to speed the speed drops. T

he demerit of this method is the wastage of power as the resistance is in series.

Learn about-Starter for DC motor

                        It is necessary for the safe operation of a machine. If we connect the motor to a supply at the time of start back emf is zero and heavy current passes to the armature which is undesirable.

 So it is essential to reduce the starting current to the safe operation. Starter is used to reduce the starting current which is nothing but a resistance which cuts out from the supply after motor gathers speed and develops back emf which regulates the speed.

 Starter consists of studs. When the starter arm points to first stud it includes the entire resistance and passes the safe value of current to armature. Armature starts to accelerate and develops back emf.

 Then the arm moves to the next stud to cut out the resistances and finally connects directly to the armature. In Series motor starter is connected in series with the armature and the field.

In other types the shunt field is connected to the stud to ensure that safe operation starter is used.

BASIC LAWS—OHMs LAW & KIRCHOFFS LAW

                              Ohm’s law gives the mathematical expression with electric current, voltage and resistance. It gives the relationship between them.

When two parameters are known with third unknown, by using this law, the third unknown can be calculated. Suppose a current I flows through a resistor with resistance of R ohm, the voltage across the resistor can be calculated by the product of the current and resistance.

V = IR This relation can be rearranged to find the current (I) and resistance (R).

I =V/R    &    R = V/I

 KIRCHOFFs LAW Kirchoff’s Current Law: In any DC circuit mode, the total current flowing towards the junction will always be equal to the total current flowing away from the junction.

In other words, the sum of currents flowing through a junction (node) will always be zero.

 Kirchoff’s Voltage Law:

 In any DC circuit, when the voltage differences across all the circuit elements including the source is summed up, the resultant is always zero.

In other words, the sum of voltage across all the circuit elements will be equal and opposite of the source voltage.

WHAT ARE THE FACTORS THAT AFFECT ALTERNATOR SIZE

                Alternators size determines the efficiency of the machine. As the power increases the efficiency of the alternator also increases.

Let us consider an example, if an alternator has got efficiency of 50% for 1kW, then the efficiency will be of 90% for 10MW alternator.

 So, for alternators in the range above 1000MW will have efficiency in the range of 99%, that’s highly efficient. Large machines have also got another advantage that as the power increases, power output per kilogram also increases.

When an alternator of 10MW rating weighs 20 Kilograms produces 50 W/kg, then an alternator of 10MW rating weighing 20000 kg will produce 500W/kg.

But the problem with the large machines is that they will have high power loss, this increases the cooling problem of the machine. Cooling alternators must be added to the machine to control the power loss and heat dissipation.

This certainly increases the cost of the total combination. Hence, the size of alternator is restricted to reduce the total cost.

Illumination terms in world

                                               Light is the best part in human’s innovation. We cannot imagine without the light. Light is nothing but an electromagnetic radiation. There are various terms in illumination.

1)      Luminous Intensity is the flux that is emitted by the source per unit solid angle.

2)      Luminous flux is the flux that is contained on unit solid angle of one candela. Its unit is lumen.

3)      Lumen hour is the amount of light delivered by a flux of one lumen in that one hour.

4)      Mean spherical candle power is the average of candle power of the source in all directions in all the planes.

5)      Mean hemispherical power candle power is defined to be the total flux emitted divided by the solid angle subtended at the point source.

6)      Reduction factor is just the ratio of the mean spherical candle power to the mean hemispherical candle power.

7)      Illumination is the ratio of the luminous flux per unit area which unit lumen per square metre.