The efficiency of the Electric Motor is about 92 %.
The efficiency is lower for smaller size motors. The efficiency for smaller motors can drop to around 80%
The losses in the motor are the iron losses due to the magnetic field, the copper losses due to the current, the mechanical losses (friction and windage losses) and the stray losses such as the harmonic losses.
Ceramic Fuses are used in locations where the temperature is high. In such locations, glass fuses cannot be used. Ceramic fuses come in a wide range of sizes. Some ceramic fuses are filled with 
sand to absorb the heat and to extinguish the arc.
Ceramic Fuses can also interrupt high currents as compared to glass fuses.
See Glass Fuses
Glass fuse are fuses of low current rating. These fuses are widely used in electronics, toys and in domestic appliances. The glass fuse consists of a hollow glass tube with two metal caps. The fuse element is connected between the two end caps. 
The transparent glass body makes the fuse element clearly visible from the outside. This makes it easy for the fuse to be checked.
The downside of the glass fuse is that it has low breaking capacity. It is also sensitive to temperature. Hence, it cannot be used in high temperature applications.
HRC fuses are used in circuits where the current is high. HRC stands for High Rupturing Capacity.
The HRC fuse consists of a porcelain body containing silica sand. The fuse element is enclosed in a porcelain body. The HRC fuse differs from a normal fuse in that it contains silica sand.
When high currents are interrupted, a powerful arc occurs. This arc needs to be quenched. The HRC fuse does this by using the silica sand. The high temperature generated by the arc causes the silica sand to fuse.
This absorbs the heat energy of the arc and the arc is extinguished.
The HRC fuse has a tin alloy in the fuse section. This enables precise operation at a specified overcurrent value.
The advantages of the HRC fuse are quick action, ability to interrupt high currents and reliable operation.
Hermetically Sealed Transformers are transformers which are airtight i.e. they do not allow the air from the atmosphere to enter the transformer. These transformers are totally filled with oil. Preventing the entry 
of air protects the oil from oxidation and consequent deterioration.
Since the transformer is totally sealed, water cannot enter the transformer. This protects the insulation from moisture.
The transformer has low maintenance as there is no need for testing the oil or the insulation for a period of 10 years. This reduces the life cycle cost of the Transformer.
Since, there is no ingress of water or air into the transformer, the aging of the transformer insulation and oil are considerably reduced.
Hermetically sealed Transformers are used applications where it is difficult to conduct maintenance such as Offshore Platforms. Hermetically sealed transformer are more expensive to purchase. However, since they have lower maintenance costs and downtime, the life cycle cost of these transformer is cost effective as compared to conventional transformers.
Transformers generate tremendous amounts of heat. This heat can be cooled by air in case of small transformers. In large transformers, more substantial cooling media are needed. Mineral oil is widely as a cooling 
Silicone Transformer Oil has high flash point. Hence, it can be placed closer to buildings as the risk of fire is reduced. Silicone fluid is also self extinguishing. The heat and smoke produced in the event of a fire is minimal. It is not hazardous and environmentally friendly. It can be recycled. Its base is polydimethylsiloxane is a chemically inert material. It does not form sludge and down not break down over its lifecycle. The Transformer has a longer life with reduced maintenance.
The downside is that Silicon Transformer Oils is not biodegradable.
Useful Links
http://www.siliconerecycling.com/silicone-oil-usage-for-transformers/
Capacitor duty contactors are special contactors which are used to switch on capacitors and capacitor banks. When a capacitor is switched, it initially appears as a short circuit. In a capacity duty contactor, this current is reduced by means of a series resistors and special circuits. 
Ordinary contactors will get damaged over repeated switching of the capacitors. Hence, only capacitor duty contactors should be used for switching on capacitors. The series resistors and contacts inside the unit can be replaced.
These special contactors limit the inrush current and thus reduce the stress inside the capacitor itself. This tends to increase the capacitor lifespan.
What are the causes for the failure of capacitors?
What are Bleeder Resistors in Capacitors
Capacitor Protection Relays - An Overview
What is the difference between a condenser and a capacitor ?
Drying of Electrolytic Capacitors
Capacitance Drift in Capacitors
Pressure Relief Vent in Electrolytic Capacitors
Capacitors and Power Factor Correction
What are polarised and non polarised capacitors
What is the difference between AC and DC capacitors ?
Low Smoke Cables are cables which produce low smoke when they burn. They are used in closed areas where ventilation is less. When a cable burns as a result of a short circuit or high current, the smoke emitted
by the insulation is extremely dense.
In emergency situations, this density smoke can reduce visibility and can impede evacuation. The dense smoke also contains harmful chemicals which may affect breathing.Hence, many installations stipulate low smoke cables.
Halogens are a group of elements. Fluorine, Bromine, Chlorine and Iodine are the common halogens. When a material such as PVC burns, it releases halogen gas. Chlorine gas, for instance, forms hydrochloric acid on contact with moisture. This can be harmful to health. Zero Halogen cables are made of materials which will not produce any halogen on burning.
| Solid Conductors | Stranded Conductors |
| Solid Conductors are rigid and cannot for application which require frequent bending such as the cable in a robotic arm. They will fail due to fatigue if used in such applications | Stranded conductors can be used for flexible applications |
| They are smaller in diameter for a given current density | Larger in diameter for a given current density |
| Less prone to corrosion | More prone to corrosion due to entry of corrosion due to capillary action |
| Cheaper to produce | More expensive to Produce |
 |  |
Converter Transformers are equipment which convert the voltage and the frequency of the system voltage. Converter Transformers are used in applications such as Variable Frequency Drives, Electric Traction, copper refining and even in HVDC Transmission.
Converter Transformers have a converter unit on the primary or secondary side. The voltage is stepped up or down in the transformer and then converted into the desired frequency in the converter circuit. Alternatively, the frequency can be changed in the converter prior to stepping the voltage up or down.
Converters are used to couple two AC power systems asynchronously.
Converter Transformers are designed to withstand high harmonic content.
Automotive Cables are special cables used for wiring in Automobiles. Automotive Cables are used to Electric and Electronic circuits in the automobile. Automobile Cables are designed to withstand heat, vibration, 
oil and fumes. For this, they are made of a specially designed polymer. They are also designed to withstand a wide range of temperature.
There are different types of automobile cables such as solid conductor and shielded cables which are used depending on the circuit such as power or instrumentation.
Automotive Cables are classified into single core, twisted and sheathed cables. The temperature classes in automotive cables are specified for 3000 hrs instead of the 20 000 hrs used in other fields of the industry.
Instrumentation Cables are used in industry to connect monitoring equipment and sensors. They are also used in the field of telecommunication. They are used in process industries such as cement, steel, etc. 
Process industries need to monitor thousands of parameters in real-time to ensure optimum functioning of the process. These cables are designed to be flexible and easy to install. They are also to be protected against interference.
Instrumentation Cables have a conductor which is insulated. Above the insulation there is a shield.
Some instrumentation cables are shielded. The shield is enclosed in a sheath which is made of PVC or HDPE.
Solar Cables are special type of cables suited for the photo-voltaic industry. These cables are designed to withstand sunlight and tough 
outdoor conditions for many years. These cables are made with special material which can withstand ultraviolet radiation from the sun. They can also withstand a wide variation in temperature (-450C to + 1200C) and are ozone resistant.
Besides, they are also resistant to chemicals such as acids and alkalis.
Heat Resistant cables are used in applications where there is a risk of fire or where the equipment has to function at high temperatures for a specific period of time.
Examples are Thermal power stations, nuclear installations, Airports, Railways.
Heat Resistant cables are made with special fire-resistant insulation. They also have an outer heat barrier which prevents the heat from reaching the inner areas of the conductor.
Heat Resistant cables can work at high temperatures such as 660, 750 or 950 degrees.
When the insulation of a normal cable burns, hazardous halogen gases are released. Fire Resistant cables are made with a special compound which emits very less smoke and does not emit hazardous gas.
Heat Resistant cables are specified in terms of the temperature they can withstand for a specific time such as 15, 30, 60, 90 and 120 minutes of operation.
Transformer oil regeneration refers to the treatment of old transformer oil. Oil Gets contaminated due to the entry of moisture and the formation of sludge. The contaminated oil in the transformer Regenerated. Regeneration involved degasification, dehydration and filtration. The old transformer oil is regenerated by passing it through columns containing fuller's earth. Fuller's earth is a type of clay which removes the impurities in oil when it is passed through it.
The impurities are thus removed without the use of any chemical. The oil which is purified can be reused in the tank. After a certain number of times of cleaning, the Fuller's Earth can be reactivated.
The oil is drawn from the transformer, purified and sent back to the tank. Thus the transformer need not be taken offline.
Acidity in Transformer oil is measured by the TAN (the Total acidity number). Acids cause the breakdown of oil which results in sludge
Thus, measuring the acidity of Transformer is an important parameter. The acidity is expressed in Total Acidity Number (TAN) which is the milligrams of Potassium Hydroxide (KOH) required to neutralize the acid present in one gram of transformer oil.
Oils which have a high value of TAN will have to be treated.
Useful Links
http://www.machinerylubrication.com/Read/1052/acid-number-test
Sludge in Transformer is formed as the oil breaks down. Sludge attacks the cellulose of the transformer windings causing it to deteriorate. It reduces the oil circulation inside the transformer. 
Sludge also forms a layer on the transformer winding and reduces the heat transfer. These lead to higher transformer winding temperatures.
Sludge thus has to be removed. The transformer oil can be replaced or filtered.
See Also: Transformer oil Deterioration
Medical Isolation Transformers are transformer used in medical facilities to power medical equipment. These transformers are designed to provide isolated AC power with noise suppression and are designed to withstand AC transients.
Noise can disturb sensitive medical equipment and interfere with measurements. Medical Transformers are designed with withstand short-circuits and are explosion proof.
They are also designed to have very low leakage currents. Excess leakage currents can cause unwanted and sometimes dangerous physiological effects in the patient.
Medical Isolation Transformer come in a box shaped enclosure to which the equipment can be connected.
Transformers are designed to withstand a very high hipot voltage.
See Also Isolation Transformers
Traction Transformers are used in locomotives to step down the voltage from the overhead power lines. These transformers are similar to power transformers. However, they can be designed to function at different frequencies from 16 2/3 Hz to 50 Hz. The size of the core will vary depending on the frequency the transformer is designed for. At lower frequencies, a higher cross section of the core is required.
Cooling is carried out by different means such as through the use of silicone or mineral oil.
These transformers have a heater winding if they are operated at low temperatures. Traction Transformers will also have multiple traction windings. They may also have auxiliary windings for other circuits.
Traction Transformers are generally roof mounted. Some locomotives have transformers below the chassis as well.
(image courtesy : Siemens)
Unbalanced loading on Generators occurs when the loading on the three phases is not equal. This occurs when the generators are used to power single phase loads such as domestic lighting. Some three phase loads such as induction furnaces and drives can also create unbalanced loading.
When unbalanced loading occurs, the negative sequence component increases. Referred as I2 , this component rotates in a direction opposite to the rotating magnetic field of the alternator. For the alternator rotor, this component rotates at double the frequency of the alternator. This induces heat in the rotor of the alternator.
Every alternator has a limit for unbalanced loading or the negative sequence limit.
This should not be exceeded. The load should be studied prior to selection of the alternator. Negative sequence relays also help trip the alternators if the unbalance limit is exceeded.
Zero Sequence current refers to the unbalanced current flowing in a Generator during an earth fault. It is denoted by Io . The zero sequence current does not flow under normal conditions.
The zero Sequence current is also known as the dc component of the fault current. It is represented by three vectors in parallel.
The Zero sequence current can be detected by adding the vectors of the three phase currents. As long as there is no zero sequence current, the vector sum of the phase currents is zero. When there is an earth fault, the vector sum of the phase currents is no longer zero. This indicates the presence of the zero sequence current.
The value of the vector sum in that case is the value of the zero sequence current. The zero sequence current can be measured by the zero sequence current transformer.
The zero sequence current transformer also known as a core balance current transformer can be connected to an earth fault relay which will isolate the earth fault.
Parasitic Drain in car batteries refers to the consumption of power even when the ignition key is turned off. A small quantity of power is consumed even when the car engine is switched off.
This is because power is consumed by the computer in the engine, the car alarm and device like the clock. This may not be much. But if the car is kept unused for long periods, the battery will drain.
The car may then have to be jumpstarted with another battery.
While this is normal parasitic drain, abnormal parasitic drain can cause the battery to lose power overnight. This may be due to a light that you forgot to switch off. Parasitic drain can also be due to leakage due to a ground even though a fuse is more like to blow in such a scenario.
If you suspect that there is a parasitic drain, you can measure the current of the battery and then remove the fuses one by one. This will help identify the circuit which is causing the drain.
AC and DC supplies usually do not get mixed. However, in some circuits where they are used together, there is a chance of mixing. The 
Another common cause of mixing is while using a dc bias supply to control the transistor current.
Mixing AC and DC can create ripples in the DC supply which can damage electronic cards. The mixing of supplies can also result in blown fuses or other erratic behaviour.
Optocouplers and LC Filters are some of the methods which can help ensure that the AC and DC supplies are kept separate.
Battery Testers are used in the automobile industry to test the battery. These testers work by measuring the maximum CCA ( Cold Cranking Amps) the battery can provide.
Prior to the test, a selection is made in the battery tester pertaining to the type and make of the battery.
The battery tester typically applies a load which draws about 50% of the CCA rating of the battery. It measures the maximum current supplied and also the bounce back voltage after the test). It calculates the condition of the battery using this data by applying unique algorithms which are specific to the type of battery.
The battery tester also checks the condition of the battery by measuring the conductance. It does this by injecting a low voltage ac signal with a frequency between 50 - 100 Hz. This signal is used to measure the internal resistance of the battery. Battery tester manufacturers claim that the conductance of the battery will give an indication of the health of the battery.
Self Discharge is a phenomenon in Batteries in which the charge in the battery drains without any external load. This happens when the battery is kept in storage.
The reason for self-discharge are chemical reactions within the battery. These reactions are still not properly understood.
Different types of batteries have different rates of self discharge. The discharge rate is typically high for the first month of storage and then drops to a lower rate.
The rate of self-discharge is found to be linked to the temperature. The higher the temperature, the higher the rate of discharge. Hence, batteries should be stored at lower temperatures.
A battery which has lost its charge due to self-discharge will regain its power once it is charged again.
Automotive Fuses are used in Electrical systems of Automobiles. They are rated for a voltage of 24 volts or 12 volts. They are also sometimes known as 
blade fuses as their terminals are shaped in the form of blades.
Automotive fuses have their current rating marked on the outside.
Automotive Fuses are colour coded to identify their rating.
Below is a video which describes the removal and fitment of Automotive Fuses
Specific Power in a battery refers to the maximum power the battery can deliver in relation to its size. Its unit is W/kg. Specific Power is an 
A battery with a high specific power value will deliver more power while having very less weight.
See Also
Energy Density in the amount of Energy stored in a battery in relation to its weight. This is also known as gravimetric energy density or specific Energy Density. It is specified in Wh/kg
Another type of Energy Density is the volumetric Energy Density of batteries. This is the amount of Energy stored in a battery in relation to its volume. It is specified in Wh/litre
See Also
There is no specific lifetime for a Rechargeable Battery. The lifetime would depend on the operating environment and the load the battery is connected to.
In general, rechargeable batteries are designed to provide up to 800 charge discharge cycles.
To maintain optimum life, never discharge a battery completely.
Every two months, discharge the battery and charge it to the maximum level.
Connecting the current terminals of a multimeter in parallel to a power source is a serious mistake. It can result in minor damage (if the fuse 
Hence, always ensure that the probes are in the voltage terminals when the multimeter is being connected in parallel. Some multimeter have a blanking mechanism which closes the current probes when the selector switch it moved to a voltage measurement position.
Always check the probes and the terminals when you measure the voltage.
Memory Loss in batteries refers to the loss in battery capacity due to incomplete discharging. In certain types of batteries, particularly Nickel Cadmium batteries, the capacity of the battery is reduced if the battery is not discharged completely. 
For example, if a battery is discharged up to 25% of its capacity and then charged up to 100% two or three times, the actual capacity of the battery will be reduced by 25%.
A film forms over the unused area of the anode which prevents the battery from completely discharging.
Memory Loss can also be caused by incomplete charging.
Memory Loss can be prevented by full charging and fully discharging the battery every two months.
Cold Cranking Amps refers to the amperes that the battery can supply in a cold condition. The term originated from vehicle batteries which had to deliver the needed power to crank (start) the engine.
When the vehicle is in a cold condition, the electrolyte in the battery becomes denser.In the engine, the lubricating oils also becomes more viscous.
Thus, the vehicle draws more power than it normally would when in a cold climate. The battery should be able to deliver the high current which will be drawn by the starter motor of the engine.
Vehicle Batteries usually contain the Cold Cranking Amps. The Cold Cranking Amps is also specified by the Starter Motor which will draw the current.
Therefore, it is important that the battery is chosen such that it is able to deliver the required Cold Cranking Amps. The CCA number is an important value in the battery.
Button Cells find wide applications in small electronic devices such as watches, hearing aids and calculators. Button Cells have outputs at 1.5 volts and 3 volts. They provide very little power which is sufficient for small gadgets. 
Button Cells have a relatively long life. They are also safe enough to be used inside the human body in bio-medical devices.
They have a stable output voltage which does not drop even over long periods.
The Electrolyte used is Potassium Hydroxide (KOH) in paste form. The electrodes are Zinc (anode) and Silver (Cathode). Button cells are therefore safe for the environment.
Since silver is expensive, cost is one of the downsides of the button cell.
Coin cells are similar to button cells but they are larger in size. Coin cells are used in applications such as CMOS sets where more power is required.
The Anode is the outer container of the button cell while the cathode is the top layer of the battery.
Coin Cells can be used to power torchlights and other devices with relatively higher power consumption by connecting two or more cells in series.
Button Cells should be kept away from children who can accidentally swallow it.
Thermal Runaway in batteries is a condition in which the internal temperature of the battery increases. This causes the charging current to increase. This, in turn, causes the temperature to increase. This again leads to the charging current increasing. Thus, a vicious cycle is created.
Thermal Runaway ultimately leads to battery failure. In some cases, it can even cause explosions.
Thermal Runaway occurs mostly when the battery is in the float charging mode. The usual causes are high ambient temperature in the location where the battery is placed. This causes the battery to heat up. During charging, heat is produced inside the battery. This heat has to be dissipated. Hence, adequate ventilation should be provided to the battery.
The charging current should be monitored periodically.
VRLA batteries or Valve Regulated Lead Acid Batteries are Lead acid batteries which are completely sealed. In a normal battery, the gases which form escape into the environment. In a VRLA battery, these gases are retained and made to recombine with the electrolyte. Thus, no topping up is required. 
If the pressure inside the battery becomes excessive, valves in the battery open and release the gases into the atmosphere and relieve the pressure. Hence, the name "Valve Regulated".
Another feature of the VLRA is that the electrolyte inside the battery is not in the liquid form. The electrolyte is absorbed in an absorbent material such as silica powder or a fibre glass mat. This ensures that the electrolyte will not spill or leak out of the battery.
Thus the VRLA battery can be kept in any position unlike the flooded lead acid battery which must be kept only in the horizontal position.
VRLA batteries are classified into Gel type batteries and AGM or Absorbed Glass Mat batteries.
Deep Bar Rotors are used in induction motors to increase the torque during starting. Deep bars indicate that the bars which comprise the cage in the rotor are deeper than those in normal rotors.
When an induction motor is started, the slip between the rotor and the stator is high. Thus the frequency of the rotor current is high.
This high frequency results in high reactance in the lower layers of the deep bar. Hence, most of the current flows in the surface of the rotor bars. This results in high current density and increased resistance. This resistance produces high torque during starting.
When the motor reaches its rated speed, the slip frequency drops and the reactance reduces. The current now, flows uniformly across the entire cross section of the rotor bar. The resistance in the rotor drops and the motor runs normally.
Sulfation is a phenomenon which occurs in lead acid batteries. If a lead acid battery is kept unused for long periods of time or kept in a low charge condition, the lead sulphate in the electrode solidifies and forms a crystalline layer.
This crystalline layer reduces the active surface in the electrode. This results in reduced capacity. The battery gets drained faster.
Sulfation can be classified into two stages. The reversible stage where the deposits have not solidified and can be removed and the permanent stage, where the lead sulphate crystals have solidified into a permanent layer which cannot be removed.
Desulfation involves removing this layer of sulphate deposits. This is done by charging the cell to a higher voltage. The temperature of the battery is also raised to enable the crystals to dissolve.
There are also several products in the market which claim to desulfate batteries by injecting a pulse in the shape of a specific waveform. Such claims have not been verified and are met with skepticism from some quarters.
Sulfation can be prevented by charging batteries to the optimum level and to avoid keeping batteries in the discharged condition.
Batteries have become indispensable part of modern life. They store electric power and are found in almost every electronic device we use, from laptops to cell phones to watches and car keys.
Batteries, however, are made of extremely toxic materials. Many of these materials such as lead, plastic and acid can wreak havoc on the health of people and the environment. Hence, it is necessary that every battery is properly and safely disposed off.
Never throw a battery away. The chemicals and acid in the battery can leach into the environment, polluting the land and water. They can also enter the food chain.
Batteries which are disposed can be recycled. The plastic can be reused and other materials which cannot be reused can be safely discarded.
All types of batteries can be recycled. At the recycling facility, the battery is taken apart. The plastic is recycled while the active materials of the battery such as lead, nickel are purified, melted and recycled into batteries.
Almost all battery manufacturers have facilities to recycle batteries. Many even buy back used batteries.
Some countries have laws which charge a refundable recycling deposit during the purchase of the battery. This deposit will be refunded when the battery is returned after its life. This encourages the recycling of batteries.
Never attempt to open or recycle a battery at home. The battery contains harmful chemicals such as lead and mercury which can enter the bloodstream. The acid can cause burns to the eyes and the skin.
Thermocouples are color coded for easy identification. The color code gives us details as to the type of thermocouple, the temperature range, etc.
However, different countries have different color coding. Hence, it is better to check the thermocouple color coding protocol from the thermocouple manufacturer.
Below is a link to a Color Code Table for the Thermocouple
http://www.thermibel.be/documents/graphics/thermocouples/tc-color-complete.jpg
Inrush Current in a transformer refers to the initial current rush which occurs when the transformer is first switched on. This high initial current is due to the absence of back-emf at the time of switching on.
In a transformer without load, the magnetic field is in quadrature with the voltage. That is, it lags behind the voltage at an angle of 90 degrees (electrical).
If the switching occurs when the voltage wave is at zero, the magnetic flux wave will be at its maximum and the inrush current will be maximum. If the voltage is at maximum at the instant of switching, the flux and the current will be at minimum. The high current lasts for a few milliseconds and comes its steady stage value.
Always keep the secondary open, when switching on the transformer. This will help minimize the inrush current.
For large transformers, switching on the transformer can result in high current and a consequent voltage dip in the system. This can affect the system as a whole.
Hence, special switching techniques such as the use of resistance starters or soft starters have to be employed.
Optical Current Transformers are current sensors which work on the magneto-optic effect. These sensors work by measuring the angle of rotation of a polarized beam of light.
The magneto-optic effect was discovered by Sir Michael Faraday. The Faraday Effect states that the 
rotation of the plane of polarization of a beam of light which passes through a magnetic field is proportional to the component of magnetic field in the direction of the light.
Thus, by measuring the rotation of the plane of polarization of light it is possible to measure the magnetic field and consequently the current producing the field.
Optical Current Transformers are also known as Fibre Optic Current Sensors (FCOS). They are widely used in substations and power systems. The advantage of these sensors is that they do not need calibration.
Fibre Optic Current Sensors are usually integrated with devices such as breakers.
High Current Transformers are used in applications which require high current such as electric furnaces and in soldering.
In these applications, a high current is passed through the material and the the heat generated melts the metal. These transformers are specially designed with thick conductors which can withstand the heat.
In resistance soldering, a high current is used to melt the solder and fuse the leads.
Split Core Current Transformers have a core which can be split into two. This makes them easy to 
fit on bus bars, cables, etc. with having to remove the conductor such as the bus bar or the cable which is tedious and time consuming.
With Split Core Current Transformers, one half of the core is placed over the de-energized conductor and the other half is bolted to it. This reduced down time and allows easy removal and replacement of the current transformers.
Split Core Current Transformers are available in many ratings and sizes.
One of the main drawbacks of the squirrel cage induction motor is that it has very low starting torque. Hence, for applications requiring high starting torques, the slip ring motor is the only choice.
However, by providing a double squirrel cage rotor to the induction motor, the starting torque of the motor can be increased.
The double squirrel cage induction motor consists of a rotor which has two cages. The outer cage consists of rotor bars with high reactance and low resistance. The inner cage, on the other hand, consists of rotor bars with low reactance and high resistance.
When the motor is started, the slip is high. As a result, the frequency of the currents induced in the rotor is high. This causes the rotor currents to flow in the outer cage. Thus the torque of the motor increases due to the high initial resistance in the outer cage.
When the motor reaches the rated speed, the slip between the stator and the rotor decreases and the frequency of the rotor currents drop. This causes the reactance of the inner cage to drop. Thus the current flows through the inner cage.
During normal operation, current flows through both the inner and the outer cages as the reactance is very low.
Thus, the double cage induction motor provides excellent speed torque characteristics during start-up.
The double cage induction motor is used in applications such as mixers and crushers in the industry where high initial torque is necessary. It is cheaper than the slip ring motor and does not require the complicated and costly starter circuits like the slip ring motor.
Variable Frequency Drives or VFDs as they are popularly known as devices which control the speed of induction motors by altering the input power supply frequency.
Induction motors rotate due to the rotating magnetic field produced by the stator winding. This induces a current in the rotor winding. The interaction of the magnetic fields of the stator and the rotor causes the motor to rotate.
Thus, the speed of the induction motor is dependent on the supply frequency.
Variable Frequency Drives are basically AC-AC converters. These converters use thyristors to alter the power frequency. This enables the precise control of the motor speed. Another feature of these drives is that no power is lost in the speed control.
The output frequency of the drives can be controlled by controlling the firing angle of the thyristors.
Variable Frequency drives also save power by operating devices such as pumps and blowers at speeds where their efficiency is maximum.
Useful Link
A thermopile is an array of thermocouples which are connected in series. Thus, the strength of the voltage signal is increased. 
Thermopiles are used along with infrared sensors. A black body absorbs the infrared radiation from the source to be measured. This heats up the thermopiles which produces an output in millivolts.
Thermopiles are used in medical applications for measuring the body temperature
Thermopiles are also used in industrial measurement applications where they are fitted on to a suface, such as that of a bearing. The temperature of the surface can be measured.
Squirrel Cage induction motors find wide applications in industries and in homes. Their rugged constructions and low maintenance makes them the motor of choice for many requirements.
Applications
However, the downside of squirrel cage motors is that they draw a very heavy current when switched on due to the absence of back-emf. Hence, they require special starters.
They also have low starting torque. Hence, they are seldom used in applications such as lifts and cranes.
Another disadvantage of the squirrel cage motor is the poor speed control. However, with the advent of Variable Frequency drives, this disadvantage has been overcome.
