Saturday, July 15, 2017
Thursday, March 30, 2017
ELECTRIC INDUCTION FURNACE
ELECTRIC INDUCTION FURNACE
The electric induction furnace is a type of melting furnace that uses electric currents to melt metal. Induction furnaces are ideal for melting and alloying a wide variety of metals with minimum melt losses, however, little refining of the metal is possible.
PRINCIPLE OF INDUCTION FURNACE
The principle of induction furnace is the Induction heating
INDUCTION HEATING:
Induction heating is a form of non-contact heating for conductive materials.
The principle of induction heating is mainly based on two well-known physical phenomena:
1. Electromagnetic induction
2. The Joule effect
1) ELECTROMAGNETIC INDUCTION
The energy transfer to the object to be heated occurs by means of electromagnetic induction. Any electrically conductive material placed in a variable magnetic field is the site of induced electric currents, called eddy currents, which will eventually lead to joule heating.
2) JOULE HEATING
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat.
The heat produced is proportional to the square of the current multiplied by the electrical resistance of the wire.
Induction heating relies on the unique characteristics of radio frequency (RF) energy - that portion of the electromagnetic spectrum below infrared and microwave energy. Since heat is transferred to the product via electromagnetic waves, the part never comes into direct contact with any flame, the inductor itself does not get hot and there is no product contamination.
Induction heating is a rapid, clean, non-polluting heating.
The induction coil is cool to the touch; the heat that builds up in the coil is constantly cooled with circulating water.
FEATURES OF INDUCTION FURNACE
An electric induction furnace requires an electric coil to produce the charge. This heating coil is eventually replaced.
The crucible in which the metal is placed is made of stronger materials that can resist the required heat, and the electric coil itself cooled by a water system so that it does not overheat or melt.
The induction furnace can range in size, from a small furnace used for very precise alloys only about a kilogram in weight to a much larger furnaces made to mass produce clean metal for many different applications.
The advantage of the induction furnace is a clean, energy-efficient and well-controllable melting process compared to most other means of metal melting.
foundries use this type of furnace and now also more iron foundries are replacing cupolas with induction furnaces to melt cast iron, as the former emit lots of dust and other pollutants.
Induction furnace capacities range from less than one kilogram to one hundred tonnes capacity, and are used to melt iron and steel, copper, aluminium, and precious metals.
The one major drawback to induction furnace usage in a foundry is the lack of refining capacity; charge materials must be clean of oxidation products and of a known composition, and some alloying elements may be lost due to oxidation (and must be re-added to the melt).
CONSTRUCTION OF INDUCTION FURNACE
There are many different designs for the electric induction furnace, but they all center around a basic idea.
The electrical coil is placed around or inside of the crucible, which holds the metal to be melted. Often this crucible is divided into two different parts. The lower section holds the melt in its purest form, the metal as the manufacturers desire it, while the higher section is used to remove the slag, or the contaminants that rise to the surface of the melt.
Crucibles may also be equipped with strong lids to lessen how much air has access to the melting metal until it is poured out, making a purer melt.
TYPES OF INDUCTION FURNACE
There are two main types of induction furnace: careless and channel.
Coreless induction furnaces
The heart of the coreless induction furnace is the coil, which consists of a hollow section of heavy duty, high conductivity copper tubing which is wound into a helical coil.
Coil shape is contained within a steel shell.
To protect it from overheating, the coil is water-cooled, the water being recirculated and cooled in a cooling tower.
The crucible is formed by ramming a granular refractory between the coil and a hollow internal.
The coreless induction furnace is commonly used to melt all grades of steels and irons as well as many non-ferrous alloys. The furnace is ideal for remelting and alloying because of the high degree of control over temperature and chemistry while the induction current provides good circulation of the melt.
Channel induction furnaces
The channel induction furnace consists of a refractory lined steel shell which contains the molten metal. Attached to the steel shell and connected by a throat is an induction unit which forms the melting component of the furnace.
The induction unit consists of an iron core in the form of a ring around which a primary induction coil is wound.
This assembly forms a simple transformer in which the molten metal loops comprises the secondary component.
The heat generated within the loop causes the metal to circulate into the main well of the furnace.
The circulation of the molten metal effects a useful stirring action in the melt.
Channel induction furnaces are commonly used for melting low melting point alloys and or as a holding and super heating unit for higher melting point alloys such as cast iron.
ADVANTAGES OF INDUCTION FURNACE:
Induction furnaces offer certain advantages over other furnace systems. They include:
Higher Yield. The absence of combustion sources reduces oxidation losses that can be significant in production economics.
Faster Startup. Full power from the power supply is available, instantaneously, thus reducing the time to reach working temperature. Cold charge-to-tap times of one to two hours are common.
Flexibility. No molten metal is necessary to start medium frequency coreless induction melting equipment. This facilitates repeated cold starting and frequent alloy changes.
Natural Stirring. Medium frequency units can give a strong stirring action resulting in a homogeneous melt.
Cleaner Melting. No by-products of combustion means a cleaner melting environment and no associated products of combustion pollution control systems.
Compact Installation. High melting rates can be obtained from small furnaces.
Reduced Refractory. The compact size in relation to melting rate means induction furnaces require much less refractory than fuel-fired units
Better Working Environment. Induction furnaces are much quieter than gas furnaces, arc furnaces, or cupolas. No combustion gas is present and waste heat is minimized.
Energy Conservation. Overall energy efficiency in induction melting ranges from 55 to 75 percent, and is significantly better than combustion processes.
The electric induction furnace is a type of melting furnace that uses electric currents to melt metal. Induction furnaces are ideal for melting and alloying a wide variety of metals with minimum melt losses, however, little refining of the metal is possible.
PRINCIPLE OF INDUCTION FURNACE
The principle of induction furnace is the Induction heating
INDUCTION HEATING:
Induction heating is a form of non-contact heating for conductive materials.
The principle of induction heating is mainly based on two well-known physical phenomena:
1. Electromagnetic induction
2. The Joule effect
1) ELECTROMAGNETIC INDUCTION
The energy transfer to the object to be heated occurs by means of electromagnetic induction. Any electrically conductive material placed in a variable magnetic field is the site of induced electric currents, called eddy currents, which will eventually lead to joule heating.
2) JOULE HEATING
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat.
The heat produced is proportional to the square of the current multiplied by the electrical resistance of the wire.
Induction heating relies on the unique characteristics of radio frequency (RF) energy - that portion of the electromagnetic spectrum below infrared and microwave energy. Since heat is transferred to the product via electromagnetic waves, the part never comes into direct contact with any flame, the inductor itself does not get hot and there is no product contamination.
Induction heating is a rapid, clean, non-polluting heating.
The induction coil is cool to the touch; the heat that builds up in the coil is constantly cooled with circulating water.
FEATURES OF INDUCTION FURNACE
An electric induction furnace requires an electric coil to produce the charge. This heating coil is eventually replaced.
The crucible in which the metal is placed is made of stronger materials that can resist the required heat, and the electric coil itself cooled by a water system so that it does not overheat or melt.
The induction furnace can range in size, from a small furnace used for very precise alloys only about a kilogram in weight to a much larger furnaces made to mass produce clean metal for many different applications.
The advantage of the induction furnace is a clean, energy-efficient and well-controllable melting process compared to most other means of metal melting.
foundries use this type of furnace and now also more iron foundries are replacing cupolas with induction furnaces to melt cast iron, as the former emit lots of dust and other pollutants.
Induction furnace capacities range from less than one kilogram to one hundred tonnes capacity, and are used to melt iron and steel, copper, aluminium, and precious metals.
The one major drawback to induction furnace usage in a foundry is the lack of refining capacity; charge materials must be clean of oxidation products and of a known composition, and some alloying elements may be lost due to oxidation (and must be re-added to the melt).
CONSTRUCTION OF INDUCTION FURNACE
There are many different designs for the electric induction furnace, but they all center around a basic idea.
The electrical coil is placed around or inside of the crucible, which holds the metal to be melted. Often this crucible is divided into two different parts. The lower section holds the melt in its purest form, the metal as the manufacturers desire it, while the higher section is used to remove the slag, or the contaminants that rise to the surface of the melt.
Crucibles may also be equipped with strong lids to lessen how much air has access to the melting metal until it is poured out, making a purer melt.
TYPES OF INDUCTION FURNACE
There are two main types of induction furnace: careless and channel.
Coreless induction furnaces
The heart of the coreless induction furnace is the coil, which consists of a hollow section of heavy duty, high conductivity copper tubing which is wound into a helical coil.
Coil shape is contained within a steel shell.
To protect it from overheating, the coil is water-cooled, the water being recirculated and cooled in a cooling tower.
The crucible is formed by ramming a granular refractory between the coil and a hollow internal.
The coreless induction furnace is commonly used to melt all grades of steels and irons as well as many non-ferrous alloys. The furnace is ideal for remelting and alloying because of the high degree of control over temperature and chemistry while the induction current provides good circulation of the melt.
Channel induction furnaces
The channel induction furnace consists of a refractory lined steel shell which contains the molten metal. Attached to the steel shell and connected by a throat is an induction unit which forms the melting component of the furnace.
The induction unit consists of an iron core in the form of a ring around which a primary induction coil is wound.
This assembly forms a simple transformer in which the molten metal loops comprises the secondary component.
The heat generated within the loop causes the metal to circulate into the main well of the furnace.
The circulation of the molten metal effects a useful stirring action in the melt.
Channel induction furnaces are commonly used for melting low melting point alloys and or as a holding and super heating unit for higher melting point alloys such as cast iron.
ADVANTAGES OF INDUCTION FURNACE:
Induction furnaces offer certain advantages over other furnace systems. They include:
Higher Yield. The absence of combustion sources reduces oxidation losses that can be significant in production economics.
Faster Startup. Full power from the power supply is available, instantaneously, thus reducing the time to reach working temperature. Cold charge-to-tap times of one to two hours are common.
Flexibility. No molten metal is necessary to start medium frequency coreless induction melting equipment. This facilitates repeated cold starting and frequent alloy changes.
Natural Stirring. Medium frequency units can give a strong stirring action resulting in a homogeneous melt.
Cleaner Melting. No by-products of combustion means a cleaner melting environment and no associated products of combustion pollution control systems.
Compact Installation. High melting rates can be obtained from small furnaces.
Reduced Refractory. The compact size in relation to melting rate means induction furnaces require much less refractory than fuel-fired units
Better Working Environment. Induction furnaces are much quieter than gas furnaces, arc furnaces, or cupolas. No combustion gas is present and waste heat is minimized.
Energy Conservation. Overall energy efficiency in induction melting ranges from 55 to 75 percent, and is significantly better than combustion processes.
Wednesday, March 29, 2017
Winding Resistance
Winding Resistance is an important measurement in electrical machines. Winding resistance tells us about the condition of the winding. Any fault in the winding such as an open circuit or an inter-turn short circuit will be reflected in the winding resistance value. Besides, winding resistance is used to measure I2R losses in the winding.
The machine to be tested is disconnected from the lines and de-energized. The measurement are usually taken phase-to-phase. The three readings should be within 1% of the average value.
Winding resistance can change with temperature. The measurement are usually taken at the cold temperature known as the cold resistance. The transformer or the motor is allowed to cool for a few hours and the temperature taken.
Based on the measurement taken at a particular temperature, the resistance at any other temperature may be calculated from the following formula
Where
Rs= Resistance value to be calculated at a specific temperature
Rm= Resistance valued measured
Tm= Temperature at which the resistance was measured
Ts= Temperature at which the resistance is to be calculated
Tk= Winding Material Constant ( 234.5 °C for copper or 225 °C for aluminum)
The winding can store a huge amount of electromagnetic energy when a current is passed through them during measurement. When the test current is stopped, there may be a voltage kickback from the winding. The test equipment should be able to absorb the voltage kick and safely discharge it.
Insulation Resistance Measurement
Insulation Resistance Measurement is an important check in the maintenance of electrical equipment such as motors, transformers. It is estimated that nearly 80% of all maintenance activities in the industry is related to checking the insulation of machines. It is therefore vital that the engineer has a fair idea of the principle behind the measurement of Insulation Resistance and the methods used. Insulation resistance is measured using a meggar.
In the normal operation of machinery, the insulation is subjected to moisture, oil, dust, electrostatic stress due to machine operation and a host of other elements. Hence, insulation ages and deteriorates. It is vital that the health of the insulation be monitored continually to avoid sudden, catastrophic failure of machines.
Principle of Insulation Resistance Measurement
The method used to measure insulation resistance is based on Ohm’s law. A high voltage is applied across the resistance; the current that flows through the insulation is measured. The ratio of voltage and current gives the resistance. The value of the insulation resistance is usually in the order of mega ohms
Instruments used in Measurement
The instrument used to measure Insulation Resistance is known as the Megger. It is similar in principle to the ohmmeter except for the fact that a higher voltage is used. The typical meggars have a test voltage of 500V, 2500V or 5000V. The Meggar has a high internal resistance hence, there it is safe to use despite the high voltage generated. The meggar has 3 terminals. Line, Earth and Guard.
The test voltage appears on the “Line” Terminal. This terminal is connected to the winding whose insulation needs to be checked. The “Earth” Terminal is connected to the ground. The “Guard” Terminal is connected to the surface of the insulation to measure the surface currents which tends to flow along the surface of the insulation.
Method of Insulation Resistance Measurement
The winding to be tested should first be isolated. The other windings of the machine which are not being tested should be connected to the ground. The voltage is applied to the winding and the reading is taken after about 60 seconds. The reading is noted. After the test is over, the winding needs to be “discharged”. This is because the insulation acts as a dielectric forming a capacitor between the winding and the earth. This can store charge and can deliver a shock if not discharged. Discharging can be done by connecting to the ground.
What should be the value of the Insulation Resistance?
The Insulation Resistance thus measured is usually in the order of mega ohms. A general rule of the thumb is that the minimum value should be greater than 1 mega ohm for every 1kV rating of the machine. Thus, for a machine rated for 11kV, the minimum acceptable value would be 11 mega ohms. Temperature has a direct impact on the value of the Insulation Resistance. The Insulation Resistance decreases with increase in temperature. Thus the values should be normalized for a standard temperature.
That is, a value measured at 20 deg. C cannot be compared with a value measured at 30 deg. C. The value at 30 deg. C needs to be corrected. A general rule of thumb is that the insulation resistance decreases by a factor of 2 for every 10 degree rise in temperature.
Hence, the value taken at 30 deg. C needs to be multiplied by a 2 to get a value corrected to 20 degrees.
How do we ensure a good value of IR?
The Insulation Resistance of a machine depends chiefly on the dryness of the windings. The entry of moisture into the windings lowers the Insulation Resistance. The ingress of moisture can be prevented by ensuring that the windings are kept dry. Special heaters known as anti-condensation heaters are provided in machines to keep them dry. It must be ensured that these heaters are kept on.
How do we improve the Insulation Resistance value?
If machines are found with low Insulation Resistance values below the permissible limits, heating the windings by connecting lamps around them is an effective method of driving moisture from the windings. If no improvement is seen even after heating, other reasons such as insulation wear or deterioration can be suspected.
Other parameters related to the health of the Insulation are the Polarization Index(PI), tan delta, hipot test, step test, etc
In the normal operation of machinery, the insulation is subjected to moisture, oil, dust, electrostatic stress due to machine operation and a host of other elements. Hence, insulation ages and deteriorates. It is vital that the health of the insulation be monitored continually to avoid sudden, catastrophic failure of machines.
Principle of Insulation Resistance Measurement
The method used to measure insulation resistance is based on Ohm’s law. A high voltage is applied across the resistance; the current that flows through the insulation is measured. The ratio of voltage and current gives the resistance. The value of the insulation resistance is usually in the order of mega ohms
Instruments used in Measurement
The instrument used to measure Insulation Resistance is known as the Megger. It is similar in principle to the ohmmeter except for the fact that a higher voltage is used. The typical meggars have a test voltage of 500V, 2500V or 5000V. The Meggar has a high internal resistance hence, there it is safe to use despite the high voltage generated. The meggar has 3 terminals. Line, Earth and Guard.
The test voltage appears on the “Line” Terminal. This terminal is connected to the winding whose insulation needs to be checked. The “Earth” Terminal is connected to the ground. The “Guard” Terminal is connected to the surface of the insulation to measure the surface currents which tends to flow along the surface of the insulation.
Method of Insulation Resistance Measurement
The winding to be tested should first be isolated. The other windings of the machine which are not being tested should be connected to the ground. The voltage is applied to the winding and the reading is taken after about 60 seconds. The reading is noted. After the test is over, the winding needs to be “discharged”. This is because the insulation acts as a dielectric forming a capacitor between the winding and the earth. This can store charge and can deliver a shock if not discharged. Discharging can be done by connecting to the ground.
What should be the value of the Insulation Resistance?
The Insulation Resistance thus measured is usually in the order of mega ohms. A general rule of the thumb is that the minimum value should be greater than 1 mega ohm for every 1kV rating of the machine. Thus, for a machine rated for 11kV, the minimum acceptable value would be 11 mega ohms. Temperature has a direct impact on the value of the Insulation Resistance. The Insulation Resistance decreases with increase in temperature. Thus the values should be normalized for a standard temperature.
That is, a value measured at 20 deg. C cannot be compared with a value measured at 30 deg. C. The value at 30 deg. C needs to be corrected. A general rule of thumb is that the insulation resistance decreases by a factor of 2 for every 10 degree rise in temperature.
Hence, the value taken at 30 deg. C needs to be multiplied by a 2 to get a value corrected to 20 degrees.
How do we ensure a good value of IR?
The Insulation Resistance of a machine depends chiefly on the dryness of the windings. The entry of moisture into the windings lowers the Insulation Resistance. The ingress of moisture can be prevented by ensuring that the windings are kept dry. Special heaters known as anti-condensation heaters are provided in machines to keep them dry. It must be ensured that these heaters are kept on.
How do we improve the Insulation Resistance value?
If machines are found with low Insulation Resistance values below the permissible limits, heating the windings by connecting lamps around them is an effective method of driving moisture from the windings. If no improvement is seen even after heating, other reasons such as insulation wear or deterioration can be suspected.
Other parameters related to the health of the Insulation are the Polarization Index(PI), tan delta, hipot test, step test, etc
Measurement of Earth Resistance
Measurement of Earth Resistance is a vital part of the maintenance of any electric installation. The function of a sound earthing system is to ensure that all electric equipment are connected to the ground potential. Hence, a well-maintained earthing system ensures the proper functioning of protection systems, absorbs electrical noise and provides safety to operating personnel. The earth resistance is measured using an earth meg-gar.
“Fall of Potential” Method:
The Earth resistance is measured using the “Fall of Potential” Method. The method works by injecting a constant current between two spikes which are inserted into the ground and measuring the voltage at points between them (as shown in the figure)
The “Fall of Potential” Method is a three terminal test. The electrode whose earth resistance is to be measured is disconnected from the system or earthing grid. The earth meg-gar has a current terminal, a voltage terminal and a common terminal. The common terminal is connected to the electrode,
“Fall of Potential” Method:
The Earth resistance is measured using the “Fall of Potential” Method. The method works by injecting a constant current between two spikes which are inserted into the ground and measuring the voltage at points between them (as shown in the figure)
The “Fall of Potential” Method is a three terminal test. The electrode whose earth resistance is to be measured is disconnected from the system or earthing grid. The earth meg-gar has a current terminal, a voltage terminal and a common terminal. The common terminal is connected to the electrode,
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