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18-02-2011, 10:12 AM

.ppt   POWER ELECTRONICS.234.ppt (Size: 1,014 KB / Downloads: 93)

 Power electronics is the applications of solid-state electronics for the control and conversion of electric power. Power electronics combine power, electronics, and control. Power electronics are based primarily on the switching of power semiconductor devices.
 Switch mode power supplies (SMPS)
 Uninterruptible power supply (UPS) systems
 Photo-voltaic and fuel-cell power conversion systems
 Rectifier supplies for electrochemical processes
 Heating and lighting, including high frequency illumination control
 Induction heating
 DC and AC servo drives
 High efficiency industrial/commercial drives
 Electric vehicle applications
 Electric traction
 Flexible AC transmission systems (FACTS)
 Induction heating is a process which is used to bond, harden or soften metals or other conductive materials. For many modern manufacturing processes, induction heating offers an attractive combination of speed, consistency and control. The material to be heated is known as work piece and the coil wound around it is known as work coil.
 When a high frequency ac voltage is applied across the work coil, a magnetizing current flows through it. this will generate flux in the work piece and induce voltage into the work piece. since the work piece is closed onto itself, eddy current flows into it. the work piece will be heated up due to finite resistance offered by the work piece to the flow of the eddy current. The heat loss in the work piece is normally confined to surface of the work piece (skin-effect).
Maxwell Equation Electro magnetism fundamental Laws
rot H =j +∂D/∂t
rot E= -∂B/∂t ;
Induction Heating Setup

 The efficiency of an induction heating system for a specific application depends on several factors: the characteristics of the part itself, the design of the induction coil, the capacity of the power supply, and the degree of temperature change required for the application.
The Characteristics of the Part
First, induction heating works directly only with conductive materials, normally metals. Plastics and other non-conductive materials can often be heated indirectly by first heating a conductive metal susceptor which transfers heat to the non-conductive material.
It is easier to heat magnetic materials. In addition to the heat induced by eddy currents, magnetic materials also produce heat through what is called the hysteresis effect. During the induction heating process, magnetics naturally offer resistance to the rapidly alternating electrical fields, and this causes enough friction to provide a secondary source of heat. This effect ceases to occur at temperatures above the "Curie" point - the temperature at which a magnetic material loses its magnetic properties. The relative resistance of magnetic materials is rated on a “permeability” scale of 100 to 500; while non-magnetics have a permeability of 1, magnetic materials can have a permeability as high
as 500.
With conductive materials, about 80% of the heating effect occurs on the surface or "skin" of the part; the heating intensity diminishes as the distance from the surface increases. So small or thin parts generally heat more quickly than large thick parts, especially if the larger parts need to be heated all the way through.
Research has shown a relationship between the heating depth of penetration and the frequency of the alternating current. Frequencies of 100 to 400 kHz produce relatively high-energy heat, ideal for quickly heating small parts or the surface/skin of larger parts. For deep, penetrating heat, longer heating cycles at 5 to 30 kHz has been shown to be most effective.
Steel – along with carbon, tin and tungsten – has high electrical resistivity. Because these metals strongly resist the current flow, heat builds up quickly. Low resistivity metals such as copper, brass and aluminum take longer to heat. Resistivity increases with temperature, so a very hot piece of steel will be more receptive to induction heating than a cold piece.
VOLTAGE-SOURCE SERIES RESONANT INVERTER- Here the output current is nearly sinusoidal at the switching freq slightly below the resonance. The power is controlled by a variable switching frequency control
CURRENT SOURCE PARALLEL RESONANT INVERTER –Here the output current source parallel resonant inverter, the output current is nearly sinusoidal at the switching freq slightly above the resonance
The circulating currents in the metal pan on the top of the induction coil directly heats the pan
process is used to soften metal for improved ductility and machinability, as well as to relieve residual stress. In contrast to hardening, annealing involves a much slower heating step followed by gradual cooling of the metal

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