Magnet Wires

A magnet wire is a wire of a small cross section which is covered with a thin layer of insulation.  Magnet wires are used extensively to construct the windings in motors, small transformers, inductors, etc. 

Magnet wires are made of copper which is electrolytically refined or aluminium. 

The small cross section of the magnet wire enables close winding in small spaces.  Magnet wires are usually made of aluminium, zinc, copper, etc.  They are provided with an insulating films made of polymers.  Different types of polymers are chosen for insulation based on the operating temperature.  

Magnet Wires are designed in both AWG and metric sizes.

The Electromagnetic Flow Measuring Principle

Lightning Masts

Lightning Masts are used to protection installations from lightning.  They are different from other forms of lightning protection in that they are made of a single long pole which is raised to a height of a few metres.

Lightning masts are made of aluminium, steel or galvanised iron. 

Masts are advantageous over shielding wires in that they are more robust.  There have been cases where shielding wires have failed due to the high current and causing the installation to be without protection. 

Masts are cheaper that shielding wires.  A mast with a pointed tip will also attract lightning more easily. 

The area around the mast which is protected is called the "cone of protection".  The area on the ground which lies within 30 degrees from the tip of the mast is called the cone of protection. 

More than one lightning mast can be used depending on the size of the area to be protected. 

Crawling in Induction Motors

The supply given to an induction motor may have harmonics present in it.  These harmonics will have their own torques in addition to the synchronous torque.  Let us consider a supply with odd harmonics.  The 3rd harmonic will be absent in 3 phase systems.  Hence, we only have to consider the 5th and 7th harmonics.  The other higher order harmonics can be neglected.

The torque produced by the 5th harmonic rotates in the opposite direction.  Thus, the forward torque is given by the sum of torques produced by the primary frequency and the 7th harmonic. 

The rotating field of the 5th will rotate at one fifth of the synchronous frequency (Ns/5).  However, the torque produced by the 5th harmonic rotates in the reverse direction.  Similarly, the 7th harmonic will rotate at one seventh of the synchronous frequency.  The torque produced by the 7th harmonic is maximum at 1/7th of the supply frequency.

When some poorly designed motors are started with load, the motors may not reach the nominal speed.  The motors will get stuck at 1/7th of the nominal speed. 

This phenomenon is known as crawling.  Crawling can be overcome by properly selecting the number of rotor bars in the rotor of the induction motor   

Potentiometers-an overview

Potentiometers find wide application in electric circuits. Potentiometers are used for controlling voltage or current in an electric circuit. A common example  for the potentiometer is the volume control knob in the radio set.

The rather strange name for the potentiometer is due to its history. In the early years of electrical technology, the potentiometer was used in circuits designed to measure voltage.

A potentiometer consists of a coil of a wire that is wound over a circular or linear support.

A metallic contact known as the wiper moves over the  coil. The wiper is usually made of graphite.

The potentiometer has three terminals. The wiper is usually the central terminal. The other two terminals belong to the coil.

Preset Potentiometers are very small potentiometers which are enclosed in the equipment. These potentiometers have a small groove to be adjusted with a screwdriver.   These Potentiometers are to be adjusted only on rare occasions when the equipment is to be calibrated. 

While the potentiometer has three terminals, in many applications only two terminals are used. In such an application, the potentiometer is used as a rheostat. When all the three terminals of the potentiometer are and the potentiometer behaves like a Voltage divider.

The linear potentiometer and the logarithmic potentiometer are  types of Specialised potentiometers.

Paper Insulation

Paper is an excellent insulating material in the dry condition.  The difficulty is in keeping it dry.  This is because paper is extremely hygroscopic (it absorbs moisture).  Hence, paper should be kept out of contact with air.  Paper insulation is, therefore, enclosed in a sheath which protects it from moisture.  One of the common materials used for the sheath is lead.  These cables are known as Paper Insulated Lead Covered (PILC) cables. 

Paper insulation is usually impregnated with mineral oil or with other special compounds.  This prevents the formation of air pockets in the paper insulation.  Paper insulation can be used upto a temperature of 80 deg. C. 



However, the downside of paper insulation is the difficult jointing process.  Special jointing procedures and equipments are needed to ensure that the insulation remains sealed.  This makes it difficult to work with.  Hence, paper insulation has gradually given way to PVC and XLPE Insulation. 

Another form of paper insulation is the pressboard.  Pressboards are formed by pressing paper sheets together to a thickness of a few mm.  An impregnating material is used for binding.  These pressboards provide mechanical support along with electrical insulation. 

Metals in Transformer Oil

The amount of metal in Transformer oil can be used as an index of the condition of different components of a transformer.  Faults which involve high energy cause deterioration of not only the insulation but also the windings and other components.  Therefore, it is not unusual to see particules of metals such as copper, zinc, tin, silver, lead, etc. 



The quantity of these metals can serve as an indication of specific components.  The levels are calculated using various types of spectroscopy in the laboratory.  The transformer windings are made of copper or aluminium while the core is made of iron.  Metals such as lead, zinc, and silver can be found in components such as bushings, tank walls, etc.

There are no specific limits for the levels.  It would be better to monitor the levels periodically and to investigate any sharp increase in the level of metals.

The Ferranti Surge Absorber

The Ferranti Surge Absorber is use to minimize the steepness of a travelling wave.  The Ferranti surge absorber is made of a coil which is enclosed in a metallic cylinder which is grounded.   The metallic Cylinder is known as the dissipator.

The construction resembles an air core transformer with a low inductance primary and a single turn secondary short which is short circuited. 

When the wave reaches the Ferranti surge absorber, it induces an emf in the dissipator.  Energy is transmitted from the coil to the dissipator and expended as heat.  This reduces the steepness of the travelling wave.

Solar Pond - An OverView

A solar pond is a device which traps the heat in sunlight and makes it available for power generation.  A solar pond consists of a reservoir of salt water. The salt water in any reservoir has an inherent salinity gradient.  That is, the water at the lower layers will have a higher concentration of salt (around 90%) as compared to water at the top layer (30% salinity).

The water in the reservoir is transparent as to allow sunlight to penetrate to the deeper layers.  When sunlight falls on the bottom of the reservoir.  It heats the bottom layers of the salt water.  When normal water is heated, it tends to rise up due to convection and the water circulates. 

However, since the pond is filled with salt water, the hot water is trapped in the lower layer of the pond as it cannot rise up due to the high density.  This heat is removed by means of a liquid which passes through a pipe which runs through the lower layers of the pond.  The fluid in this pipe is kept in circulation by means of a pump.  this heat is used to heat water in a boiler which can be used to power a turbine and a generator.