About group dynamics

ppt

SOLAR COOKER

SOLAR COOKERS

One of the Applications of solar energy:

      The solar vessel consists of an outer box made of either fiber glass or aluminum sheet. Blackened aluminum, double glass lid, a reflector, insulator and cooking pots. Blackened aluminum tray is fixed inside the box and sides are covered with an insulating material to prevent heat losses. A reflecting mirror is provided on the box cover increases the solar energy input. Metallic cooking parts are painted black on outer side, food to be cooked is placed in cooking parts &cooker is kept facing the sun to cook the food. An electric heater may also be used as a generator to serve as a back up during non sun shine hours.

 Rating of power diode can be increased by :

 Rating of power diode can be increased by introducing a drift layer in the diode between p and n sides. Where p and n side are heavily doped and drift layer (n-) is lightly doped. During reverse bias condition because of this doping difference more depletion layer is formed. Because p+ (holes) are heavily doped and can form small space of depletion layer, while during n-  side due to lightly doped it forms a free region. So that depletion region increases. So depletion region is more in n- side. Any  signal device can be converted to high power device.  By using same method, first we should identify the region  which blocks  voltages and then add a lightly doped layer on one of it’s side. (i.e., a junction which forms between heavily and lightly doped layers can block the high voltages. So by increase in the width of depletion region the drift layer increases, by increase in drift layer rating of power semi conductors are increased.

MACHINE ARMATURE

ADVANTAGES OF STATIONARY ARMATURE OVER ROTATING ARMATURE:

The various advantages of rotating field (i.e field flux) are as follows:

1)A.c. is used everywhere, the generation level of a.c. voltage may be higher as 11 KV to 33 KV or even more. This gets induced in the armature. For stationary armature large space can be provided to accommodate large number of conductors and the insulation's.

2) It is always better to protect high voltage winding from the centrifugal forces caused due to the rotation. So high voltage armature is generally kept stationary. This avoids the interaction of mechanical and electrical stresses.

3) It is very easy to collect larger currents at very high voltage if the armature winding is stationary so that any slip rings and brushes can not come in to picture. The voltage required to be supplied to the field is very low ( 230 V d.c.) and hence can be easily supplied with the help of slip ring and brush assembly by keeping it rotating.

4) The problem of sparking at the slip rings can be avoided by keeping field rotating which is low voltage circuit and high voltage armature as stationary.

5) Due to low voltage level on the field side (i.e., rotor side) the insulation required is less and hence field system has very low inertia. Due to less winding heat liberated will be there and his can be cooled quickly when compared to armature winding in rotor side.

6) If field is rotating, to excite it  external d.c. supply two slip rings are enough. Once each for positive and negative terminals. As against this, in three phase rotating armature the minimum number of slip rings required are three and can not be easily insulated due to high voltage levels.

7) The ventilation arrangement for high voltage side can be improved if it is kept stationary.

       Due to all these reasons the most of the alternators in practice are using rotating field type (stationary armature winding) of arrangement. For small voltage rating alternators rotating armature arranged may be used.

                                          REPLY IF ANY SUGGESTIONS

CLASSIFICATION OF POWER DIODES:

All Power diodes are classified on the basis of reverse recovery time. Reverse recovery time means time taken to come to off state even after it is turned off. This is because of the minority charge carriers in the diode. Normally during the forward bias condition the holes and electrons move to either side after crossing the depletion region and then conduction takes places. Then some of the minority charge carriers (i.e., electrons) are trapped in the both regions and also in depletion region also. During off state even though we off the diode due to the minority charge carrier’s conduction takes place for some time. This time is called reverse recovery time. These are power diodes which are used for general purposes. Reverse recovery time is (few 10’s to micro seconds). And these are of low frequency. Another one is FAST RECOVERY DIODES in which reverse recovery time is about (30-500 nano sec). Last one is SCHOTKY DIODE in which reverse recovery time is about (< 30 nano sec). In this there is no minority charge carriers flow in reverse biased condition.

HOW SOLAR CELL IS MADE?

The main component for making of solar cell is silica (that is sand). It is heated to 1500⁰C and this forms as a gel. Then this 1500⁰C gel is brought to 0⁰C immediately, so then it becomes hard.  Then we can get glass like material and that to  transparency. By going in to brief  silica is converted to metallurgical grade silicon by coke reduction and then it is dissolved in HCL and then it is send to distillation in which any salts are present that may be removed. Then we get high purity trichlorosilicane. Then it is of poly crystalline silica and it is heated to 1500⁰C we get silicon crystal and these are solids and are made in to pieces by using diamonds and we get silicon pieces. So that’s how solar cell’s are made. There are 36 cells per panel generating of 270 W nearly.

Interesting doubt about ceiling fan

Does ceiling fan rotate even in anti clock wise direction?

Ceiling fans are usually used in every house mainly for cooling the rooms. These fans acts as a blowers and sends the air downwards. The blades of fans are usually at an angle to send air down and are connected to motor inside directly. When we reverse the direction of the fan by changing the winding of the motor it will rotates in opposite direction and acts as an sucker which sucks the air downward and sends to top, usually this is  used in winter  to eject the air in room.  This is about fan with out any reverse direction switches. There is also reverse button switches to run in anti clock wise direction.