Electromagnetic Interference-Classification

Electromagnetic Interference can be classified into two broad categories
Narrow band and Broadband
Narrow band interference originates from devices and frequencies used for communications such as radio, television and cell phones.  The term narrowband implies that the interference is mostly in one particular frequency or a narrow set of frequencies.  This type of interference can be easily filtered out. 
Broadband Interference originates from sources such as electric power transmission lines.  This radiation is also called unintentional radiation as the radiation is not deliberate by incidental to the power transmission.  Broadband Electromagnetic interference occupies a large part of the electromagnetic spectrum.  It does not have a limited frequency range like narrowband interference.  This kind of EMI is mostly caused by electric phenomena such as corona, switching transients, power line radiation . 
Fluorescent lights, brush arcing in motors and contact chattering in relays can also cause broadband interference in electronic devices located close by.
Electromagnetic interference can also be classified into Conducted interference and Radiated interference.
In conducted interference, the interference is transmitted through physical contact (conduction) between the devices.  An example may be noise in a television when an electric appliance in the house is switched on.
Radiated interference occurs when the interference radiates out of the conductors into space and couples with other devices.  For instance, the radiation from a power line can get coupled to a telephone line running alongside.
Another method of classification of Electromagnetic interference is based on the duration. 
Continuous interference and Impulse Noise
Continuous interference comes from sources such as power devices or from natural sources.  This interference is of long duration.
Impulse Noise is of short duration.  This is caused usually by switching actions. 

Faraday Cages

A Faraday Cage is a a cage or an enclosure made of conductive material.   The Faraday cage is used to protect a device or a region from external electric fields.  The Faraday cage is named after Sir Michael Faraday who invented them in 1836.

Faraday proved that no electric field can exist inside a room made of conductive material. 

The Faraday cage can block static and non-static electric field.  The charge passes through the conductive surface of the cage.

When an airplane or a car is struck by lightning, it acts as a Faraday's cage protecting the passengers from the lightning. 

Faraday cage are used in laboratories to protect sensitive instruments. 

Applications

Electric conductors used in instrumentation are protected from external interference by the use of a Shield.  The shield serves to protect the conductors from electromagnetic couplings.

Electromagnetic Interference (EMI)

EMI or Electromagnetic interference is a term which is commonly used  to describe interference in electric circuits by an external source.

There are many different sources for Electromagnetic Interference.  EMI is generally classified based on its source into two broad categories.

• Man-Made EMI which originates from other man made sources such as power lines, electric appliances and circuits.  The switching of electric circuits can cause interference.  Microwave ovens can also cause EMI
• Naturally occurring EMI which refers to interference from natural sources such as Lightning Discharges, Solar phenomena, etc.

EMI causes disturbance to electronic systems such as radio and television and wireless communication systems.  Analog communication systems based on amplitude modulation (AM) are particularly susceptible to EMI interference as they have low selectivity.

Protecting Devices against EMI

1. Electric and Electronic Devices are usually protected against EMI by enclosing them in a metallic enclosure. 
2. If possible, move known sources of EMI such as microwave ovens, cell phones away from sensitive electronic circuits.
3. Ensure that the device is properly grounded. 
4. To protect a large area such as a room from Electromagnetic Interference, a Faraday cage may be constructed.  A Faraday Cage is an enclosure made of conductive material or a mesh.

High Temperature Pressure Sensors-An Overview

High Temperature Pressure Sensors are used in applications where pressure has to be measured at high temperatures.  Ordinary pressure sensors cannot operate at such high temperatures.  These sensors work on the piezo resistive principle. 

The usual construction is a diaphragm which is filled with oil.  The pressure of the media to be measured is thus conducted to the oil.   The oil is carried via a capillary tube to the measuring chamber.   The temperature of the oil drops in the capillary.  The piezo resistive element then measures the pressure of the oil which is the pressure of the media.

Thus the pressure sensor is able to measure pressure at elevated temperature. 

Pressure Transducers using the piezo resistive principle can measure pressure at temperatures up to 300 degrees Celsius. 

Pressure Sensors designed for operation at very high temperatures use interferometer based sensors.

Aneroid Barometer

Aneroid Barometer are a portable form of barometer.  The aneroid barometer does not use a barometric liquid like mercury.  Instead, of the liquid it has a corrugated metal sheet which deforms depending on the shape of the machine. 

The Aneroid Barometer gets its name from the shape of the Aneroid Capsule inside the instrument.  The Aneroid Capsule is an evacuated circular corrugated disc. 

The corrugated disc deforms under air pressure.  A mechanism of gears and linkages is used to convey the deflection to a pointer across a dial.

The Aneroid Barometer is an extremely handy instrument.  It can be used in any orientation. 

A modified form of the Aneroid Barometer is used as the Altimeter to indicate the Altitude ( See Article).

The Aneroid Barometer is sensitive to temperature.  The metallic components used in its construction expand and contract with the temperature.  Hence, to compensate for the variation due to the temperature, a bimetallic strip is used.

The disadvantages of the Aneroid barometer are

• It is less precise than a mercury barometer
• It requires frequent calibration

Barometers -An Overview

A barometer is an instrument which measures atmospheric pressure.  The barometer was invented by the Italian scientist, Evangelista Torricelli. 

The barometer in its basic form consists of a bowl of mercury and a tube which is sealed at one end.  The tube is filled with mercury.  It is then inverted over the bowl.  The tube is made to stand vertically over the bowl with the help of a stand.

The level of mercury in the tube now falls in the tube.  This is the indication of the atmospheric pressure.  The column of mercury in the tube is supported by the atmospheric pressure.  The level of mercury in the tube is dependant on the atmospheric pressure.

The standard atmospheric pressure is 76 cms of mercury.

one bar of pressure = 75.0061561303 cms of mercury

Technically, the barometer can also be made using water.  However, a very long tube may be required which is not practical.

The standard atmospheric pressure can support a column of water 10 metre high.

The mercury barometer is not a portable instrument.  Hence, it has been replaced by the Aneroid Barometer.

See Article on Aneroid Barometer

What is the unit of Vacuum ?

The unit of Vacuum is Torr.  Torr is named after Evangelista Toricelli, the inventor of the barometer. 

One Torr is equal to 1/760 of a standard atmosphere. 

One Torr is also defined as the pressure necessary to support a column of mercury which is 1 mm high at 0�C.  One Torr is equal to 1333.2 microbars.

What is an Altimeter ?

An Altimeter is a device which indicates the altitude.  Altimeters are widely used in aviation, in mountaineering and in skydiving sports. 

Altimeters work on the principle that the atmospheric pressure drops with altitude.  As the height above sea level increases, the pressure drops.  The Altimeter is basically a pressure measuring device which is calibrated in terms of distance above sea level. 

Altimeters are among the most common instruments in aviation.  They are also used by mountaineers to calculate their altitude. 

Hand held Altimeters are worn by skydivers. 

What is the difference between absolute pressure and gauge pressure

Absolute Pressure is the pressure measured with vacuum as the reference.  Gauge pressure is measured with the ambient air pressure as the reference.

Most pressure measurements in the industry are gauge pressure measurements and the gauges and sensors used are adjusted for the atmospheric pressure. 

Absolute pressure measurements are used for measuring vacuum pressures and in altimeters.

Gauge pressure is also known as relative pressure.

U tube Manometer - Pressure Measurement

A U tube manometer is used to measure the differential pressure or the pressure difference between two points.  The U tube manometer is a simple and inexpensive instrument.  It can be easily made in the field by using a tube and wooden board.

The U tube manometer consists of a tube bent in a U shape.  The tube is made of transparent plastic.  Glass tubes bent in a U shape can also be used. 

The U tube is fixed against a wooded board.  A scale in centimeter is placed between the two limbs of the U tube to measure the levels.    The vessel is then filled with a liquid usually water.  Mercury is also used in certain situations.

One end of the U tube is connected to the inlet of an equipment while the other end is placed on the outlet. 

If the pressure at both ends is equal, the water level in the two limbs will be equal.  If the pressure at one end is more than the other, one limb will show a lower level to the other limb.

The difference between the levels (in cms) is noted.

The difference in pressure can be calculated from the following formula

Differential Pressure = Difference in levels x Density of the Liquid x Acceleration due to gravity

Inclined U tube Manometer

The Inclined U tube manometer is used to measure very low values of differential pressure.  Inclining the U tube by a certain angle increases the accuracy.  The difference in height increases due to the inclination and enables more accurate reading.

The formula for Differential Pressure then changes to

Differential Pressure = Difference in levels x Density of the Liquid x Acceleration due to gravity x Sin ?

where ? is the angle of inclination

The Units of Pressure

Pressure is an important parameter in instrumentation. 

The unit of pressure commonly used in the industry is the bar.

One bar equals 100000 Pascal.  One pascal equals one Newton per sq. meter.

One bar is also equal to 1.01 kg/cm ² .

Sometimes, differential pressure is measured in mm of water column,

The formula for converting pressure from mm.h2o to bar is as follows

Pressure in bar = 9.806648601 � 10^-5 � Pressure in H₂o

Differential Pressure Measurement

Differential Pressure measurement is an important of any instrumentation system.  Differential Pressure is the difference in pressure between two points.  If the pressure in one point is 10 kg/cm²    and the pressure at another point is 15 kg/cm². The differential pressure will be 5 kg/cm²  .

Differential Pressure is measured across equipment which have both an inlet and an outlet.  Filters, coolers, heaters are equipment across which the differential pressure is measured. 

A simple method of measuring the differential pressure is by using a U-tube manometer which consists of a U tube .  One end of the U tube is connected to the inlet of the equipment while the other end is connected to the outlet.  The differential pressure is measured by measuring the difference in levels between the two limbs of the U-tube.

Special sensors which measure the differential pressure are also available.

The differential pressure should be as low as possible.  If the differential pressure across any equipment increases, it may indicate choking of the inlet. 

The differential pressure can also be used to measure the flow through a pipe.

Eye Bolts in Motors-An Overview

An Eye bolt is an important component of the motor.  It is used in lifting the machine.  The eye bolt consists of a loop at one end and a threaded end at the other.  The threaded end is screwed into the motor body. 

When the motor or alternator is to be lifted, a sling is connected to the eye bolt and the machine is lifted using a crane.  It is important to note the capacity of the eye bolt. Every Eye bolt has a WLL ( Working Load Limit).  If this is exceeded the eye bolt will fail. 

This can result in injury to crew members or even death.  Check the capacity of the eye bolt before lifting.

The eye bolt is intended only for vertical lifting.  Angular lifting (lifting in an angle) will cause the bolt to fail quickly.  Angular eye bolts will have the safe Working load limit.  As the angle of lifting increases, the WLL decreases.  Thus, while the WLL will be maximum at vertical, it decreases as the angle increases.  The angle of lift should be calculated and the safe Working Load Limit determined.

There are special types of eyebolts which can withstand angular load (up to a certain degree, usually 45 degrees.  Check the angle with the manual).

These are known as shouldered eye bolts.  Check that the machine has shouldered eye bolts before lifting the machine. 

Foundation Bolts in Machines

Foundation Bolts or Anchor Bolts are used to attach electric machines to their foundations.  Anchor bolts is a bolt which is grounded in the foundation.  A nut fastens a hole in the machine bottom to the anchor bolt.  Anchor bolts are sometimes connected to epoxy grout in the concrete.  At most installations, the Anchor bolt is fitted in the concrete when it is poured. 

Different manufacturers have their own proprietary designs. 

Anchor bolts should be sized properly for their yield strength. Incorrect dimensioning and fitment can cause problems during the machine operation such as high vibration, misalignment.

Shunt in Galvanometer

A shunt is a resistance which is used in electric circuits to divert or to limit the current.  A galvanometer measures the current passing through it. 

If the shunt is used in series with the galvanometer, the galvanometer can be used to measure voltage.  If the shunt is used in parallel with the galvanometer, the galvanometer can be used to measure current and works as an ammeter.

 

What is Reluctance in a magnetic circuit ?

Reluctance is the resistance to the flow of magnetic flux in a magnetic circuit.  Its unit is Henry^-1.  It is often compared to resistance in an electric circuit. However, unlike a resistance where energy is dissipated as heat.  The reluctance does not cause a dissipation of energy.
Reluctance is denoted by a capital 'R'.  The R being cursive to distinguish it from resistance.

It is also denoted as the ratio between the magnetomotive force and the magnetic flux.

Reluctance = Magnetomotive force / Magnetic flux

This relation is also known as Hopkinson's law and is analogous to ohm's law.

Materials which can be magnetised easily such as iron and steel have low reluctance while materials which cannot be magnetised have high reluctance.  The opposite of reluctance is permeability.

Electromagnetic Torque, Synchronous Torque and Reluctance Torque in Electrical machines

Electromagnetic Torque is the torque produced in induction machines.  It is the product of time varying electromagnetic field due to time varying voltage  or motion of the rotor with the stator as the reference.

Synchronous Torque is the torque obtained due to the interaction of a time varying field based in the stator and a stationary field in the windings and the field poles of the rotor as in a synchronous machine. 

Reluctance Torque is the torque seen in machines such as the reluctance motor. Here, the torque as the rotor shifts to align its minimum reluctance path with the rotating air gap which varies with time.

Jewel Bearings

Jewel Bearings are used in instruments to support the spindle which carries the pointer.  The jewel bearing is made from a  synthetic sapphire or ruby.  It consists of a pivot hole in which the spindle rotates. 

Jewel bearings are also used in watches.  They are chosen over metallic bearings as they have a very low coefficient of friction. 

The bearings are made from synthetic sapphire or ruby which have no impurities or flaws.  The advantages of jewel bearings are their long life, low friction and dimensional accuracy.

Phosphor Bronze-An overview

Phosphor bronze is an alloy of Copper, Tin ( 3.5 to 10%) and Phosphorous (1%).  Phosphor bronze is used in applications which require resistance to fatigue, corrosion and wear.  They are used in springs, bolts, etc. 

In Electrical Engineering, phosphor bronze is used in springs in instruments such as galvanometers and other instruments.   The current is guided to the coil through the phosphor bronze rings.  Phosphor Bronze has low resistivity. 

The phosphor bronze is used as a wound spring which is used to provide the restraining torque in the instrument. 

Another important characteristic of phosphor bronze is that it is non magnetic. So it does not interfere with the instrument's functioning.

Phosphor bronze is also used for contact springs in Electric motors.  When plated with silver, the conductivity of phosphorous increases further.

What is a Galvanometer ? How does it work ?

A galvanometer is a very basic device which measures the current passing through it.  It consists of a coil placed in a magnetic field.  When a current passes through the coil, it experiences a deflection proportional to the charge through it.  If a spring is used to restrain the coil, the deflection can be used to measure the current or a voltage.

The galvanometer is a simple instrument.  The principle of the galvanometer is used in ammeters as well as voltmeters. 

The galvanometer can be used as an ammeter by connecting a shunt in parallel to the coil.  If the shunt is connected in series with the galvanometer coil, the galvanometer becomes a voltmeter.

See also Mirror Galvanometer

What is a Mirror Galvanometer ?

A galvanometer is a sensitive device which deflects when a current passes through it.  The principle of the galvanometer is used in the construction of voltmeters and ammeters. 

The galvanometer consists of a coil which is placed in a magnetic field.  The coil deflects when a current is passed through it.  The coil drives a pointer across a dial which indicates the value of the current or the voltage.  However, when very small currents are to be measured, the current passing through the galvanometer will not be able to drive the pointer against the spring tension.  The pointer may not even deflect for small currents. 

Hence, to measure small currents a mirror galvanometer is used. 

The mirror galvanometer consists of a coil which is suspended by means of silken threads.  A small mirror is attached to the coil.  The light from a source is made to fall on the mirror.  The reflected light will fall on a screen.  When the coil deflects, the mirror deflects with it.  The light which is reflected also moves in the screen.  The distance of the screen determines the sensitivity of the galvanometer.  As the distance increases, the deflection of the reflected light even for a small current will be large.  Thus, even low values can be measured with high precision.  Even microamperes can be measured by the deflection galvanometer. 

The mirror galvanometer is used in engineering, seismology, communication and in any application where high sensitivity is required in measurements.

What is a Pyrometer ?

A Pyrometer is an instrument which measures very high temperatures.  Objects at very high temperatures cannot be measured by contact type thermometers.  Hence, non contact thermometers are used.  Today, the most widely used pyrometer is the infrared thermometer.  The infrared thermometer measures the wavelength of the radiation emitted by a hot body.  As the temperature increases, the wavelength increases

The infrared thermometer has a lens which focuses the thermal radiation from the hot object on to a detector which converts the radiation into an electric signal.

Another type of pyrometer which was popular before the advent of the infrared thermometers was the disappearing pyrometer.  In the disappearing pyrometer, a filament in the device is heated by an electric current.  As it gets heated up, it glows hot.  The instrument is pointed at the hot object whose temperature is to be measured such that the filament and the object are in the same line. 

The filament is now heated so that the filament disappears in front of the hot object.  This happens when the hot filament and object are have the same color.  This will happen when both are at the same temperature.  Thus the temperature of the filament is now the temperature of the hot object.

The pyrometer has a small dial which indicates the temperature of the hot object.  It does this by measuring the current flowing into the filament. 

(See article on Infrared Thermometer)

What are Temperature Baths ? Where are they Used ?

Temperature baths are instruments used to calibrate temperature sensors, switches and gauges.  The temperature bath consists of a hot bath and a controller.  The controller is used to set the temperature of the bath. 

The sensor or gauge to be calibrated is inserted in the bath and the temperature at which it is to be calibrated is set.  Once the bath reaches the set temperature, the reading on the gauge is read.  If it indicates the set temperature of the bath, the gauge is accurate.  Sensors can also be similarly calibrated by comparing the bath temperature with the temperature indicated in the scanner or the PLC.

Temperature Baths are available as dry baths and wet baths.  In dry bath, the sensor dimensions the size of the bath and the sensor perfectly fits the bath. 

For sensors with irregular stem sizes, wet bath with a semi-solid paste which facilitates heat transfer is used. 

Modern Temperature baths have accurate temperature controls and can produce reference temperatures with resolution of .1 degrees celsius.

They are also compact and portable.

Loop Resistance in Current Signal Circuits

Current Signal have become extremely popular in recent decades.  Current signals have an inherent resistance to noise.  Current signals can also travel over long distances as compared to voltage signals.  Loop resistance is an important factor in current signals. 

In current signals, the sensor varies its resistance depending on the parameter to be measured.  This varies the current.  The maximum current the source (usually the recording or measuring instrument) can drive is depending on the total resistance of the circuit.  The total resistance of the current signal circuit depends on the resistance of the sensor and the wire resistance.

Each channel of the analog measuring or recording instrument (PLC or any recorder) has a maximum resistance across which it can drive the resistance. Hence, it is necessary to ensure that the total resistance does not exceed this resistance.

The wire resistance can be measured by shorting the wires at one end and measuring the resistance from the other end using a multimeter.  This resistance has to be added to the sensor resistance (usually mentioned in the datasheet of the sensor) to give the total resistance.

Sometimes, over the course of many years, a new meter may be connected in series to the circuit.  This may increase the loop resistance and the value will be affected.  Hence, whenever any new instrument is added to the current signal circuit, the resistance needs to be recalculated. 

Humidity Sensors-An Overview

Humidity sensors are used measure humidity.  These sensors are used in process industries where the humidity of the atmosphere or a closed surface is a critical parameter.  It is also widely used in medical instruments in respiratory devices.    Humidity sensors have a special polymer which absorbs the water vapour.  The absorption depends on the availability of water vapour in the surround air.  This, in turn, is related to the humidity of the atmosphere.

Thus, the sensor is able to measure the humidity. 

Humidity sensors are classified into two broad types

Capacitive Sensors and Resistive Sensors

In capacitive sensors the polymer which absorbs the moisture is sandwiched between two metallic plates.  When the humidity increases, the capacitance increases.

In resistance based, humidity sensors, a thin film of metal is printed on a board in a zig zag pattern.  A film made of the moisture-absorbing polymer is applied over the board.  When the moisture increases, the resistivity of the board decreases.  This is measured by the sensor.

Temperature Switches

Temperature switches are used in instrumentation to generate a signal when a particular temperature has been reached.  The switch closes or opens giving a signal.  This signal can be used to stop an equipment in case of high temperature protection or start a compressor in a refrigeration system.

Temperature switches operate in a variety of ways from simple bimetallic strips to thermistor driven electronic switches.  In some switches, wax pellets which expand at high temperature can be used. 

The output of a temperature switch can be configured to be either normally closed (NC) or normally open (NO).

Temperature Switches need to be periodically calibrated in a Temperature bath to ensure set point accuracy.

Thermostats-An Overview

A Thermostat is a device which maintains the process temperature at a specified set point.  Thermostats are used in room heating, in refrigeration, in almost all process industries and in heating systems.

A thermostat can be a simple mechanical set-up which contains a sensing elements which opens or closes a valve.

Other thermostats receive the temperature from a sensor and give a command to an actuator or a valve to open or close depending on the input signal. 

These thermostats receive the temperature input from a thermocouple, thermistor or an RTD. 

Modern digital thermostats can be programmed and have the temperature values and the set points on touchscreen.  The set points and the response of the thermostats can be programmed. 

The thermostat is different from a temperature switch in that the temperature switch does not control the temperature.  Two temperature switches can be made to function like a thermostat and control a heater.  One controlling the upper limit to switch off the heater and other the lower limit to switch on the heater.  This, however, may not be accurate.

Gas Filled Temperature Gauges

Gas Filled Temperature gauges work on the bourdon tube principle.  The bourdon tube is a flattened tube made of a material such as phosphor bronze.  This tube is filled with gas.  The tube is connected to the stem of the gauge. 

The gas filled temperature gauge can have a capillary tube with the help of which the gauge can be located away from the sensing point. 

When the temperature of the the medium rises, the gas in the stem and the bourdon tube expands due to the temperature.  The pressure inside the tube rises and the bourdon tube unwinds.  This movement is communicated to the pointer by means of a rack and pinion. 

The pointer moves across a graduated scale which indicates the temperature.

Gas filled temperature gauges are used in all types of industries such as petrochemical industries, thermal and hydro power stations, chemical industries and iron and steel industries.

What are the types of bearings used in Motors ?

Roller bearings and ball bearings are two of the most common types of bearings.  Roller bearings have high load bearing capacity and are used in large motors with heavy rotors.  Smaller sized motors use ball bearings.

In case of very large motors, shell bearings are used.  Grease is used as the lubricant in case of roller and ball bearings.

For Shell bearings, lube oil is used as the lubricant.

The bearings are usually designed to withstand radial loads.  However, in some applications such as in the use of gears and belts, the motor may also be subjected to axial loads.  In such cases, bearings such as angular ball bearings which can withstand axial loads can be used.

Soft foot in Electrical equipment

Soft Foot in rotating electrical equipment such as motors and generators is  condition in which all the four feet are not in equal contact with the surface.  This leads to a distortion of the frame once the bolts in the feet are tightened.  This, in turn, affects the shaft alignment.

Soft foot can also result in vibration when the machine rotates. 

Soft foot can be caused by

• manufacturing defects
• improper shim fitment
• presence of foreign material such as dirt or trash

Soft Foot can be minimized by selecting proper shims and ensuring the the surface of the foot does not have any foreign materials.

http://thealignmentblog.com/blog/2010/06/04/soft-foot-what-it-is-and-how-to-minimize-it/

Vertically Mounted Motors

Vertically Mounted Motors are used for applications such as pumps and blowers.  These motors are similar to their horizontal counterparts, except that they have few differences in the design.  The most obvious example is in their ability to withstand axial loads.

Horizontally mounted motors do not have to withstand axial loads.  Hence, roller bearings or ball bearings can be used.  In vertically mounted motors, angular ball bearings are usually used to help the motor withstand axial thrust. 

In case of higher thrust loads, special thrust bearings will be used.

Grease in Electric Motors

Grease is the most widely used lubricant in electric motors.  Grease is used in motors with ball and roller bearings. 
The function of grease is

• to minimize friction and wear
• To prevent corrosion and
• to prevent the entry of foreign objects which can contaminate the bearing.

Grease, thus, has a sealing effect.  Grease is a semi-solid lubricant.  It is composed of a base oil, additives and a thickener.  The base oil can be synthetic or natural.  Synthetic oils are used in applications with high temperatures and longer regreasing intervals. 

The function of the thickener is to prevent the base oil from leaking.  Thickeners are usually metallic soaps.  Additives include oxidation and corrosion inhibitors, anti-wear agents.

The grease in motors will have to be replaced over time.  The regreasing intervals are based on the bearing manufacturers' recommendations.

Infrared Thermometer

Infrared Thermometers also known as contact less thermometers are devices which measure the temperature of any object without physically coming in contact with them.  Infrared Thermometers are ideal to measure temperatures of very hot objects or objects in motion.

These instruments work by measuring the wavelength of radiation emitted by an hot object.  All objects above zero kelvin emit radiation.  The wavelength of this radiation is dependent on the temperature.   The wavelength of the emitted radiation increases with the temperature.  By measuring the wavelength of the radiation, the infrared thermometer calculates the temperature. 

The infrared thermometer consists of a lens which focuses the radiation on to a detector which measures the wavelength of the radiation.  The temperature is then calculated and displayed on the screen.

Infrared Thermometers have become popular in recent years.  They are used to verify other instruments as well.

Infrared Thermometers can also be used in medicine where they can be used to monitor the temperature of infectious patients without physical contact.

Noise in Electric Motors

Noise in Motors is a nuisance and a waste of energy.  The more efficient a motor, the lesser the noise it produces.   However, noise is also an important parameter for diagnosing the condition of a motor.  However, a careful analysis of the noise of a motor can given an idea of the condition of the different components of a motor.

Noise analysis can tell about the condition of the bearings.  Misalignment of the motor coupling can also given a distinct noise signal

A choked filter for cooling will produce a distinct noise caused by the altered airflow. 

Powerful tools such as FFT (Fast Fourier Transforms) help analyze specific frequencies and localize abnormalities, if any. 

Technicians and Engineers should be trained to detect abnormality in sound.  Noise Analysis of motors in an industry can be carried out at periodic schedules in consultation with the motor manufacturer.

Bimetal Thermometer-An Overview

A bimetal is a strip of metal obtained by cladding two metals which have different temperature coefficients of expansion.  The bimetal, bends in different directions, which results in deflection of the bimetallic strip.  If the bimetallic strip is coiled, the strip will uncoil when heat is applied.  Bimetallic coils can also be helically wound.

In a bimetallic Thermometer, the bimetallic strip is helically wound and placed inside them stem.  A long shaft is connected to the bimetallic helix and the dial which moves across the scale.

When the temperature rises, the helix unwinds and the shaft rotates moving the dial across the pointer.  The Bimetallic Thermometer is a simple device.  It is also accurate.  The downside of the bimetallic thermometer is that they are not accurate at low temperatures as there is not much of a difference in the expansion of the two metals at low temperatures.

Thermo Electric Generator

The Thermoelectric generator is a device which works on the Peltier Effect.  The Peltier Effect states that if a difference in temperature is maintained between a junction of two different metals, an emf will be generated.

This principle is used in the Thermoelectric Generator which is also known as the Peltier Generator.  The Peltier Generator converts heat directly to electricity.  The efficiency of the Peltier Generator is low (less between 5 and 8 per cent).  Therefore, it cannot be used for commercial generation.  Efforts are on to find more efficient materials which can improve the efficiency.

The advantages of the thermoelectric generator are that it has no moving parts and can deliver a long, service-free life.  It is simple in construction.

The Thermoelectric generator is used in long pipelines for cathodic protection.  It can create the impressed voltage for the cathodic protection from the heat obtained locally.  There are thermoelectric generators available which can be powered by propane burners.  This can be used in remote locations, especially in the higher altitudes where solar radiation is less or in forested areas where solar panels may not work.

Automotive Thermoelectric Generation is an emerging technology which seeks to improve the efficiency of automobiles by converting the waste heat of the engines into useful electricity.

Thermowells for sensors and instruments

Thermowell is the protective sheath into which the stem of an instrument or a sensor used for temperature measurement is inserted. 

The following are the functions of the thermowell

1) The thermowell serves to protect the instrument or the sensor from the medium whose temperature or pressure is to be measured.  For instance, if a temperature sensor is used to measure the temperature of an acidic solution, it can get corroded by the acidic solution.  Hence, the sensor is usually enclosed in a thermowell which is made of material that can resist the corrosion.

2) The thermowell ensures proper heat transfer between the medium and sensor.  In media which are fast flowing, the heat transfer between the media and the sensor is vital for accurate measurements.  The thermowell is designed in such a manner that it captures the required heat from the media and transfer it to the sensor.  The transfer is done through a heat conducting paste in the thermowell.

3)The thermowell also enables easy replacement of the sensor.  If the thermowell did not exist, the pipeline would have to be drained of its media, before the sensor is removed.  Otherwise, there would be a leakage.  The thermowell makes replacement easy.

Why are gauge type instruments filled with glycerine ?

Gauge type instruments are filled with glycerine.  Glycerine serves three functions

It acts as a vibration damper. 

Gauge type instruments can be fitted on machines and pipes where there can be high vibration.  These vibrations can cause the intricate components in the gauge to fall apart. Glycerine, being a highly viscous liquid, serves to dampen these vibration.

It acts as coolant

It also prevents the external heat from reaching the instruments and affecting their accuracy.

It acts as a lubricant

Glycerine is a good lubricant.  The gauge instruments have a number of mechanical parts such as the rack and pinion, springs and pointer.  These need lubrication to function properly. 

Heat Conducting Paste for Temperature Sensors and Instruments

Heat Conducting Paste is used in temperature measuring instruments which are mounted on a thermowell, a protective metal enclosure (See Article on Thermowells).  The heat conducting paste ensures good heat conducting between the thermowell and the sensor.  This is important as poor conducting can result in inaccurate readings. 

The heat conducting pastes are designed to withstand very high temperatures without solidifying. The pastes can bond with metal, ceramics, glass and a range of other materials. These pastes also possess excellent electrical insulation properties.

Heat Sinks in Electronics-An Overview

Heat Sinks are very important components in the field of electronics.  Heat sinks have enabled the designed of smaller electronic components and devices as they help remove heat more efficiently.  Heat in electronics is produced due to the flow of current.  As components become smaller and smaller, large amounts of heat have to be removed from smaller surfaces.  Natural methods of ventilation and cooling will not be effective in such situations. 

Hence, Heat sinks are required.  Heat sinks are metal objects designed to have large surface area.  They are cooled naturally or by means of forced air (using a fan).

The object from which the heat is to be removed such as a transistor or an integrated circuit is connected to a heat sink.  The connection is done by a special heat conducting paste. 

The heat flows from the component to the heat sink which dissipates the heat into the surrounding air.  In some other applications, special Peltier cooling plates are used to remove the heat.

Heat sinks are rated as �C/W -  The increase in temperature per watt of Energy.

Thermal Paste in Electronics-An Overview

Thermal Paste is used to bond an electronic component to a heat sink.  The thermal paste is designed to conduct heat effectively.  It is also an electrical insulator. The thermal paste has excellent adhesive properties.  When a two surfaces such as a CPU and the heat sink are brought into contact, there are imperfections.  These imperfections trap air bubbles and increase thermal resistance. 

When a thermal paste is used, it ensures that these imperfections are filled and heat transfer is proper.

Thermal pastes are usually made of a silicone base and zinc oxide, silver or ceramic.

Once applied, the thermal compound solidifies.  The thermal paste can be removed later if required by using a cotton swab dipped in isopropyl alcohol.

Thermal paste is also known as Thermal grease.

The conductivity of the thermal matrix is indicated in watts/metre. kelvin

Thermal Pads for CPU cooling

The CPU in a computer generates an enormous amount of heat.  This heat is dissipated by using a heat sink.  For the heat to be transferred between the CPU and the heat sink heat conducting pastes known as thermal paste or thermal grease are used.

Thermal Pads can also be used for CPU cooling.  Thermal Pads have a compound already applied to them.  The thermal pads are fitted between the CPU and the Heat sink.  When the CPU is started and heat is produced, the compounds in the thermal pads undergo a phase change.  This ensures proper heat transfer.

Winding and Bearing Temperature measurement in Motors

Temperature is an important parameter in motor operation.  Temperature can indicate any abnormality before any serious damage occurs.  The  two temperatures which are monitored at the bearing temperatures and the winding temperature. 

Winding temperature is measured by temperature sensors which are attached to the windings in the stator slots at the time of manufacture.  These temperature sensors are not always accessible from outside.  The sensors (usually RTDs) can be connected to any external monitoring device.

Abnormal winding temperature in a motor winding can indicate overloading or poor ventilation.  Poor ventilation can be due to a filter which has choked.  High ambient temperature can also be a reason for high winding temperature.

Abnormal winding temperature can cause damage to the insulation eventually leading to failure. 

Bearing Temperature

Bearing Temperature in motors is measured by temperature sensors which are machined in the bearing housing such that the tip of the sensor comes in contact with the bearing.  These sensors are also connected to the temperature monitoring equipment.

High bearing Temperature can be caused by improper alignment with the load, inadequate lubrication and even high ambient temperature.

In some motors, temperature gauges or industrial thermometers which indicate temperature are provided.  Readings from these instruments can be periodically taken and monitored.

In smaller motors, no temperature monitoring provision is given.  If any abnormality is suspected, non-contact thermometers can be used to measure the temperature.

What are the advantages and disadvantages of the induction motor ?

The induction motor is a sturdy and reliable equipment.  It has revolutionized industry and has made electromechanical conversion more efficient.  Induction motors constitute more than 60% of loads in the industry.

Induction motors have many advantages and a few disadvantages.

The advantages of induction motors are

1. They are robust and sturdy.  They can operate in a wide range of industrial conditions.
2. Induction motors are cheaper in cost.
3. The construction is simple.  Induction motors do not have accessories such as brushes, slip rings or commutators
4. Low Maintenance. Very little maintenance is required for induction motors.
5. It does not require any complex circuit for starting.  The three phase motor is self starting while the single phase motor can be made self-starting simply by connecting a capacitor in the auxiliary winding.
6. They can be operated in hazardous environments and even under water as they do not produce sparks unlike dc motors

 

Disadvantage of induction motor

1. Speed control in induction motors is difficult
2. At low loads, the power factor drops to very low values
3. Efficiency drops at low loads.  This is because, the low power factor causes a higher current to be drawn.  This results in higher copper losses.
4. Poor starting torque.  Induction motors have notoriously low starting torque.  Hence, they cannot be used for application such as traction and in lifting loads.  Slip ring induction motors can be made to produce good starting torque by adding resistors to the rotor windings.

Harmonic Torques in the Induction Motor

If the supply to the induction motor contains harmonics, each individual harmonic will generate a torque depending on its frequency at a specific speed.  Thus the 7th harmonic generates its torque at 1/7th of the rated speed.  The 5th harmonic produces its torque at 1/5th of the rated speed. The 11th harmonic and the 13th harmonic generate the torque at 1/11th and 1/13th of the rated speed.

The 3rd harmonic is absent in a 3 phase supply.  Harmonics whose order is greater than the 7th harmonic are usually  neglected.

Of the harmonics, the 5th harmonic and the 11th harmonic rotate in a direction opposite to the direction or rotation of the motor.  Thus the torque produced by the motor is the sum of the torque of the fundamental frequency and that of the 7th harmonic.

If the 7th harmonic is present in the supply, this can result in crawling.

See article on Crawling

Cogging in induction motors

Cogging or magnetic locking refers to the phenomenon in which the rotor of the induction motor gets magnetically locked to the stator and the motor refuses to start.  Cogging is caused by a wrong choice of rotor and stator slots.  If the number of stator slots is equal to the number of rotor slots, the reluctance is minimum when the stator and rotor teeth face each other and the motor refuses to start.  This is overcome by making the number of rotor slots unequal to the number or stator slots. 

If certain harmonics are present in the supply, the slot frequencies can coincide with the harmonic frequencies and cogging can result.

To prevent cogging, the rotors are skewed with respect to the stator such that each rotor slots is facing more than one stator slot at any given time. 

Induction Motor rotor-Construction

The rotor of the induction motor has a core which is made of electrical steel. 

The bars which constitute the squirrel cage are typically made of aluminium or copper.  The bars are placed in slots on the rotor core.  There is no need for insulation between the bars and the core as the voltage developed in the squirrel cage is very low.

The rotor bars are skewed in order to prevent magnetic locking.  Magnetic locking is also known as cogging.

Magnetic Locking can also be prevented by ensuring that the number of rotor slots is not equal to the number of stator slots. 

Peripheral Speed in Electric machines

Peripheral Speed is an important factor in design.  The peripheral speed refers to the speed on the periphery or the circumference of the rotor.  The peripheral speed is given by the product of the circumference and the speed.

The formula for peripheral speed is

Peripheral Speed =  ? x D

                             where D is the diameter of the rotor

The peripheral speed depends on the speed as well as the diameter of the rotor.  If the peripheral speed is high, the windage losses will be more.  This is an important factor in electric machine design.

Windage Losses in Electric Machines

Windage losses refers to the losses sustained by a machine due to the resistance offered by air to the rotation of the shaft.  Windage Losses occurs in electric rotating machines such as motors and generators.  Windage losses also occur in machines such as turbines and gearboxes.

When the shaft rotates, it displaces air.  This incurs an expenditure of energy which is called windage loss.

The windage loss depends on the design and shape of the rotating object.  For example, in the synchronous machine, salient pole rotors will have more windage loss than non-salient or cylindrical rotors.

In motors where fans are used for cooling, windage loss occurs in the fans as well.

The windage loss increases with the peripheral speed.  Thus a shaft with a larger diameter will have more loss as compared to a shaft with a smaller diameter.  Machines which operate at high speeds will also have higher windage losses.  Hence, these rotors are specially designed with improved aerodynamics.

Power Flow in an induction motor

In the induction motor, as in all motors, electrical energy is converted into mechanical energy. 

Some of the electrical energy supplied to the stator is lost as copper and iron loss in the stator.  The remainder of the energy is given as input power to the rotor through induction between the stator and the rotor.  The electrical power supplied to the rotor is used to produce the mechanical power in the shaft.  In the rotor, some of the power is lost as heat.  The electric power in the rotor is converted into mechanical power.

This power is available as the rotor output excluding the losses in the rotor (friction and the windage losses) 

The Power Flow Diagram of the induction motor is as follows