The induction coil is a device for getting a high voltage from a low one. It was at one time used for X-ray tubes, and is nowadays used in car radios. It consist of a core of iron wires, around which is wrapped a coil of about a hundred turns of thick insulated wire, called the primary. Around the primary is wound the secondary coil, which has many thousands of turns of fine insulated wire.
The primary is connected to a battery of accumulators, via a make-and-break M, which works in the same way as the contact-breaker of an electric bell: it switches the current on and off many times a second, thus varying the magnetic flux.
When the primary current Ip is switched on, the rise of its magnetic field induces an e.m.f. Es in the secondary. A similar e.m.f., but in the opposite sense, is induced in the secondary when the primary current is switched off, by the collapse of the magnetic field. The secondary e.m.f.s are determined by the number of turns in the secondary coil, and by the rate of change of the magnetic flux through the iron core. Because of the great number of secondary turns, the secondary e.m.f. may be high and of the order of thousand of volt.
In practice, an induction coil such as we have described consisting simply of primary, secondary, and contact – breaker – would not give high secondary e.m.f.s. For, at the make of the primary current, the current would rise slowly, because of the self- inductance of the primary winding.
The rate of change of flux linked with the secondary would therefore be small, and the secondary e.m.f. low. And at the break of the primary current a spark would pass between the contacts of the make-and-break. The spark would allow primary current to continue to flow, and the primary current would fall slowly. At the instant of break, before the spark began, the primary current would be falling rapidly and the secondary e.m.f. would be high; but the e.m.f. would remain high for only a very short time: as soon as the spark passed the secondary e.m.f. would fall to a value about as low as at make.
Nothing can be done about the low secondary e.m.f. at make. But the secondary e.m.f. at break can be made high, by preventing sparking at the contact-breaker. To prevent sparking, a capacitor is connected across the contact.
Inductors are essential components used in circuits. They lower current ripple, maintain performance and boost signal transmission.
When the current is flowing in the primary coil, it induces a voltage across the spark gap, which jumps from the primary to the secondary. This voltage is known as the secondary voltage.
Working Principle of the Induction Coil
When current flows through a coil of wire, it creates a magnetic field around the core. This magnetic field couples with the secondary winding of the coil, and when the primary current is interrupted, the magnetic field collapses rapidly, creating a high voltage pulse across the secondary terminals by electromagnetic induction. This voltage, which can be thousands of volts, is enough to produce an electric spark across the air gap between the primary and secondary coil.
The induced voltage in the primary coil depends on the number of turns of the coil, the type of metal used to wind the coil and the core’s relative magnetic permeability (defined as m0 divided by the permeability of a vacuum). A high frequency (high speed) coil requires a larger number of windings to achieve a higher value of inductance than a low frequency coil with the same number of turns.
In order for the induction coil to operate, the current fed into the primary winding has to be interrupted on a continuous basis. This is done by a device called an interrupter. The interrupter consists of a vibrating mechanical contact that connects and breaks the current to the primary winding. Whenever the magnetic field produced by the primary coil changes, the interrupter causes the iron armature to move in and out of a contact gap that is connected to a spring. This movement of the armature causes the spring to open the contacts that are connected to the power source. Then the contacts close and the cycle begins again.
The core of an induction coil is usually made of an iron-based material with a relatively high relative magnetic permeability, which can be measured by the ratio of its magnetic permeability to that of vacuum. The core is surrounded by the secondary coil, which consists of many turns of thin insulated copper wire. The ends of the secondary coil are connected to an iron hammer that is mounted against the core and connected via a platinum contact point P1 to another contact point P2 on the iron hammer. A parallel plate capacitor of about 1 micro farad capacitance is placed across the automatic make and break arrangement to reduce the losses caused by eddy currents.
Design of the Induction Coil
Inductors are designed to meet specific application requirements and boost performance. They are often essential for power circuits and to block radio frequency interference. The primary design consideration is the core material, as this determines the inductance and the current that can be generated. Core materials can range from simple air to ferrite and iron.
The higher the magnetic permeability of the core, the greater the inductance of the coil. The permeability of the core can be determined from the magnetic properties of the material, or it can be measured using a simple experiment: if a magnet is moved across an inductor, the current changes. The induced voltage is proportional to the number of turns of the coil and the change in the magnetic field. The inductance is also dependent on the temperature and magnetic characteristics of the core material.
A simple inductor with an air core is a coil of insulated wire wrapped around a cylindrical core. The core can be made from a variety of materials, but it is important to use a material with high magnetic permeability. Using a core made of a metal with low permeability will result in a large loss of energy, as the magnetic field generated by the inductor will be weak. A core can be made of a ceramic or ferrite, and it is best to use a ceramic core for high-frequency applications.
If the conductive coil is wound around a core, a secondary coil can be connected to it. A current is induced in the secondary coil by the same process that creates a magnetic field in the primary. This current is the source of the alternating current that drives a device that uses the inductor.
Induction coils are used in many applications including power supplies and industrial machinery. They are also used to heat objects. The induced current in the secondary coil can be used to melt a material such as metal, which allows it to be shaped and formed. The induced current can also be used to generate a spark or pulse.
Applications of Induction Coil
The induction coil is used in many applications mainly because of its ability to heat a conductive work piece without direct contact. This avoids thermal inertia losses which can be significant when heating objects with resistive elements. This method also reduces the time needed for heating, compared to traditional resistance heating. Induction heating can also be used on reactive metals and other difficult to heat materials, as well as biological tissues for medical applications.
The core material used in an induction coil is important, because it affects the inductance of the coil. The higher the magnetic permeability of the core, the greater the inductance. The inductance is also affected by the number of turns in the coil and its diameter. The coil’s windings are insulated from each other to prevent electrical leakage between them. The inductor may be designed with a circular, donut-shaped or rod-shaped core, depending on the application requirements. The type of inductor chosen should be compatible with the circuit design to achieve the desired performance, such as to boost power or remove noise.
When the current in the primary coil is turned on, the magnetic field of the core progressively increases to its maximum value and becomes stable. At the same time, the induced electromotive force in the secondary coil grows until it is equal to the applied current. Then, when the current in the primary coil is switched off, the magnetic field collapses rapidly to its minimum value.
A voltage of tens of thousands of volts is developed in the secondary coil during this process, but it lasts only a short time. The voltage can be reduced by connecting a quenching capacitor of 0.5 to 15 F across the interrupter break.
Another way to reduce the voltage spike is to use an alternating current in the primary coil. However, this is less efficient and produces more heat loss in the core.
Safety of the Induction Coil
The operation of induction coils involves the use of lethal voltages. This is why the doors of their cabinets and panels are fitted with locks to prevent unauthorized access to their contents. It is essential that the key to these locks is held only by a designated member of staff, who has been delegated this responsibility. It is also important that all maintenance and rectification tasks are carried out while the power supply and coil are switched off at the mains.
Typically, an induction coil is built with an iron core and has primary and secondary windings of insulated wire. When the primary is energized, it creates a magnetic field that induces eddy currents in the workpiece to be heated. These eddy currents generate heat, which in turn causes the workpiece to melt. The electromagnetic induction theory behind this process is the same as that behind wireless charging.
To reduce energy losses, the coil’s iron core is made of a bundle of parallel, iron wires wrapped around each other and coated with shellac to insulate them electrically. In order to avoid arcing between the coil and the workpiece, the individual turns of the wire in the primary are separated by air gaps. The coil is then encased in a metal shell, often tin or lead.
A series of switches called interrupters are used to control the flow of current through the coil. These switches are connected to the coil via brass support straps and wires which are attached to two rosewood electro-shock handles. The device is mounted on a walnut base and measures 6-5/8″ x 4-1/2″.
Coils can be damaged by excessive heating due to insufficient cooling, mechanical stress, poor design or careless workmanship. This can lead to a breakdown of the insulation, which then exposes the copper conductors to the high voltages of the coil windings. Other damage can be caused by overheating and the formation of air gaps within the core material, or by contamination such as rust or slag from the workpiece.
Coils should be regularly inspected and maintained by an experienced engineer. This will involve checking that all guards, safety interlock circuits and earth/ground circuits are correctly fitted and functional. The engineer should also ensure that the induction coil is fully switched off and plugged out at the mains before any maintenance or rectification work is undertaken.