Informational Encyclopedia

Molding materials commonly used in the foundry shop are molding sand, sand binders and sand additives.Quartz and other silica rocks are the source of silica sand which are commonly used for molding. The binders added to the sand for holding the sand grains. 

These binders must be clay-type, organic and non organic . The additives such as sea coal, wood flour and silica flour are used to build some extra features.

The molding sand must contain the following properties.

1: Porosity or Permeability

It is very important to permits the steam and other gases to pass through the sand mold. The porosity of sand depends on the grain size, shape , moisture and clay content in the molding sand. If the sand is too fine then its porosity will be low.

2: Plasticity

The quality of sand should be plasticize so the sand flows its all portions to the molding box or flask. The sand must be sufficient plasticizer to produce a good mold.

3: Adhesiveness

It is the property of sand due to which it adhere or cling to the sides of the molding box.

4: Cohesiveness

Property of sand must be cohesiveness by which the sand grains stick together during ramming. It is defined as the strength of the molding sand.

5: Refractoriness

Property of sand by which the sand enables it to resist under high temperature of the molten metal without breaking down or fusing.

For the use of light an power, the system of distribution of electric energy the field of application of A.C. motors are widely used. Due to rapid use of motors the manufacturers have tried over the last few decades, to perfect various types of ac motors suitable for all classes of industrial drives and for both single and three phases a.c. supply.

This has been rise to bewildering multiplicity of types whose proper classification often offers considerable difficulty. Different a.c. motors may, however be classified and divided into various groups and as following as below.

1: Due to their principle of operation

A: Synchronous motors

(i) Plain  (ii) super

B: Asynchronous motors

a: Induction motors

   (i) Squirrel cage ( single / double)   (ii) Slip ring (external resistance) 

b: commutator motors

    (i) series (single phase / universal)        (ii) compensated ( conductively/inductively)        

   (iii) Shunt ( simple / compensated)   (iv) Repulsion (straight / compensated)          (v) repulsion-start induction      (vi) repulsion induction

2: Due to the type of current

(a) single phase        (b) three phase

3: Due to the speed

(a) constant speed     (b) variable speed    (c) adjustable speed

4: Due to Structural features

(a) open        (b) enclosed     (c) semi-enclosed      (d) ventilated     (e) Pipe-ventilated    (f) riveted frame eye 

It is defined as the mass per unit volume of a liquid at the standard temperature and pressure.Its symbols is  ρ  (rho) and its unit is kg/m3.It is also termed as mass density. So we can say

                   density = mass/volume

                      ρ = m/V

Specific Weight

Specific weight or weight density  is the weight per unit volume of liquid at a standard temperature and pressure. It is denoted by w and its unit is KN/ m³  or N/ m³ . So mathematically 

                                   w =  ρ .g

Example : specific weight for water w = 9.81 KN/m³

Specific Volume

It is defined as the volume per unit mass of the liquid. It is denoted by v so mathematically

                               v = V/m

Specific Gravity

Ratio of the specific weight of a liquid to the specific weight of the pure water at the standard temperature( 4 degree Centigrade). It has no unit.

Viscosity

Property of liquid which offers resistance  to the movement of one layer of liquid over anoher adjacent layer of liquid. Viscosity of the liquid is becuase of cohesion and interation between the particles. Viscosity is also known as absolute viscosity or dynamic viscosity.

kinematic viscosity

It can be defined as the ratio of dynamic viscosity to the density of liquid.

Compressibility

Property of a liquid by virtue of which liquids undergo a change in volume with the change in pressure. It is reciprocal of bulk modulus of elasticity.

Surface tension

property of liquid which enables it to resist tensile stress. 

It is denoted by sigma and expressed in N/m.

A phenomenon of rise or fall of a liquid surface in a 

small vertical tube held in a liquid relative to general level of the liquid.  

  

Generator Principle

An electrical generator is a machine which converts mechanical energy or power into electrical energy (or Power). The energy conversion is based on the principle of production of dynamically induced e.m.f.

Whenever a conductor cuts magnetic flux,dynamically induced e.m.f. is produced in it according to Faraday's Laws of Electromagnetic induction. This emf causes a current to flow if the conductor circuit is closed. So there are two basic parts of an electrical generator are

1: a magnetic field            2 : a conductor or conductors which can so move as to cut the flux.

Construction of simple loop generator.

A single turn rectangular copper coil ABCD rotating about its own axis in a magnetic field provided by either permanent magnet or electromagnets. The both ends of the coil is fastened to two slip-rings a and b which are insulated from each other and from the main shaft.

Two collecting bushes of carbon or copper press against the slip-rings. Their function is to receive the current induced from the coil and to pass on it to external load resistance R. The rotating coil can be called "armature" 'and the magnets are as "field magnets".

Working of simple loop generator

Imagine the coil is to be revolving in clock-wise direction.As the coil assumes successive position in the field, the flux connected with the charge. Hence the emf is produced in it which proportional to the rate of change of flux linkage. When the plane of the coil is at the right angles to lines of flux i.e when it is in position,l,then flux connected with the coil is maximum but rate of charge of flux linkage is down. It is due to in position , the coil side AB and CD do not cut the flux, rather they slide along them.(they move in parallel).

 Hence there is no emf produced in the coil. If we consider no emf in the coil at the starting position, the angle of rotation will be calculated from the position. As the coil continues revolving more , the rate of change in the flux linkage increases till the position 3 is reached where angle is 90 degrees. Here the coil plane is horizontal and parallel to the lines of flux. As seen the flux linked with the coil is minimum but rate of change of flux linkage is high. Hence, the maximum emf is induced in the coil when in this position.Next quarter from 90 to 180 degrees the flux linked gradually increases but rate of change of flux decreases. From 180 to 360 degrees revolution  the variation in the magnitude of emf are similar to those in the first half revolution.For making the flow of current unidirectional in the external circuit i, the slip rings are replaced by split-rings.The split rings are conducting cylinder typed material which is used to cut two segments insulated from each other by a thin sheet of mica or other insulated material.