Induction heating which is an efficient process of heating metals to improve their mechanical properties like ductility,tensile strength etc.,is now improved by means of connecting a frequency supply using resonant inverters. This process makes use of impedance matching inductor,dc blocking capacitor,bi-directional switches(MOSFET's) and a resonating tank circuit. THe working proncple involved here is same as that of transformer i.e., "MUTUAL INDUCTANCE".
REQUIREMENTS OF INDUCTION HEATING
POWER CONTROL METHODS
ADVANTAGES AND DISADVANTAGES
IT IS AN ELECTRICAL
PROCESS USED FOR HEATING
IT USES HIGH FREQUENCY
IT WORKS ON THE PRINCIPLE OF
NEED OF HIGH FREQUENCY
HYSTERSIS LOSSES IS DIRECTLY
PROPORTIONAL TO FREQUENCY
SHOULD BE ELECTRICALLY CONDUCTIVE
EDDY CURRENT LOSSES IS
DIRECTLY PROPORTIONAL TO
SQUARE OF FREQUENCY
EDDY CURRENT LOSSES
HENCE DUE TO ALL ABOVE FACTORS LARGE AMOUNT OF
HEAT IS PRODUCED
A SOURCE OF HIGH FREQUENCY
A WORK COIL TO GENERATE THE
ALTERNATING MAGNETIC FIELD
AN ELECTRICAL CONDUCTIVE
MATERIAL WHICH IS TO BE HEATED
The alternating magnetic field induces a current
flow in the conductive work piece.
The arrangement of the work coil and the work piece
can be thought of as an electrical transformer.
The work coil is like the primary where electrical
energy is fed in, and the work piece is like a single turn
secondary that is short-circuited.
This causes tremendous currents to flow through
the work piece. These are known as eddy currents.
In addition to this, the high frequency used in
induction heating applications gives rise to a
phenomenon called skin effect.
This skin effect forces the alternating current to
flow in a thin layer towards the surface of the
The skin effect increases the effective resistance of
the metal to the passage of the large current.
Therefore it greatly increases the heating effect
caused by the current induced in the workpiece.
BY VARYING DC LINK VOLTAGE
BY VARYING DUTY RATIO OF DEVICES
IN THE INVERTER
BY VARYING THE OPERATING
FREQUENCY OF INVERTER
BY USING IMPEDANCE MATCHING
The power processed by the inverter can be
decreased by reducing the supply voltage to
This can be done by running the inverter from
a variable voltage DC supply such as a
controlled rectifier using thyristors to vary the
DC supply voltage derived from the mains
Varying the DC link voltage allows full control
of the power from 0% to 100%.
The power processed by the inverter can be decreased
by reducing the on-time of the switches in the inverter.
Power is only sourced to the work coil in the time that
the devices are switched on.
The load current is then left to freewheel through the
devices body diodes during the deadtime when both
devices are turned off.
Varying the duty ratio of the switches also allows full
control of the power from 0% to 100%.
However, a significant drawback of this method is the
commutation of heavy currents between active devices
and their free-wheel diodes.
For this reason duty ratio control is not usually used in
high power induction heating inverters.
The power supplied by the inverter to the work
coil can be reduced by detuning the inverter from
the natural resonant frequency of the tank circuit
incorporating the work coil.
As the operating frequency of the inverter is
moved away from the resonant frquency of the
tank circuit, there is less resonant rise in the tank
circuit, and the current in the work coil diminishes.
Therefore less circulating current is induced into
the workpiece and the heating effect is reduced.
IT DOES NOT CONTAMINATE THE
MATERIAL BEING HEATED
IT IS AN EFFECTIVE METHOD OF HEATING
HEAT CONTROL IS EASY
WE CAN CONTROL HEAT FROM 0% TO 100%
NON –CONDUCTIVE MATERIALS CANNOT BE
INITIAL COST IS MORE
ADAPTABILITY TO POOR
DIFFICULT TO GUARANTEE THE QUALITY OF SOME
OF THE COMPLEX SHAPES OF WORK PIECE
INTERCHANGEABILITY OF INDUCTION COIL IS POOR
But obviously , the advantages outweighed
Therefore, the induction heating is a better
choice of metalworking for replacing coal
heating, oil heating, gas heating, electric
cooker, electric oven heating and other heating
In induction cooking an induction coil in the
cook-top heats the iron base of cookware.
Copper-bottomed pans, aluminium pans and
other non-ferrous pans are generally unsuitable.
The heat induced in the base is transferred to
the food via (metal surface) conduction
Benefits of induction cookers include
efficiency, safety (the induction cook-top is not
heated itself) and speed.
Both permanently installed and portable
induction cookers are available.
Induction heating is used in cap sealing of containers in the food
and pharmaceutical industries.
A layer of aluminum foil is placed over the bottle or jar
opening and heating by induction to fuse it to the container.
This provides a tamper-resistant seal, since altering the
contents requires breaking the foil
ALSO USED IN
Induction heating is used in plastic injection molding
machines. Induction heating improves energy efficiency for
injection and extrusion processes.
Heat is directly generated in the barrel of the
machine, reducing warm-up time and energy
The induction coil can be placed outside thermal
insulation, so it operates at low temperature and has a long
The frequency used ranges from 30 kHz down to 5
kHz, decreasing for thicker barrels.
The reduction in cost of inverter equipment has made
induction heating increasingly popular.
Induction heating can also be applied to molds, offering
more even mold temperature and improved product