Electromagnetic Induction Definition – Electromagnetic Induction is a process in where a conductor is put in a particular position and magnetic field are varying with respect to the conductor. This produces a Voltage or EMF (Electromotive Force) across the electrical conductor.
In other words, Electromagnetic Induction is the process of using magnetic fields to produce voltage, and in a closed circuit, a current.
When a DC current passes through a conductor a magnetizing force and a static magnetic field is developed around it.
If the wire is then wound into a coil, the magnetic field is greatly intensified producing a static magnetic field around itself forming the shape of a bar magnet giving a distinct North and South pole.
Now we disconnect the electrical current from the coil and instead of core we placed a magnet inside the core of the coil of wire.
Then by either moving the wire or changing the magnetic field we can induce a voltage and current within the coil and this process is known as Electromagnetic Induction.
The amount of induced voltage / induced current (emf) in to the coil is determined by following three factors,
Number of turns in the coil – By increasing the amount of individual conductors cutting through the magnetic field, the amount of induced emf produced will be the sum of all the individual loops of the coil, so if there are 20 turns in the coil there will be 20 times more induced emf than in one piece of wire.
Relative motion between the coil and the magnet – If the same coil of wire passed through the same magnetic field but its speed or velocity is increased, the wire will cut the lines of flux at a faster rate so more induced emf would be produced.
Magnetic field Strength – If the same coil of wire is moved at the same speed through a stronger magnetic field, there will be more emf produced because there are more lines of force to cut.
If we were able to move the magnet in the diagram on left, in and out of the coil at a constant speed and distance without stopping we would generate a continuously induced voltage that would alternate between one positive polarity and a negative polarity producing an alternating or AC output voltage and this is the basic principle of how an electrical generator works similar to those used in dynamos and car alternators.
In small generators such as a bicycle dynamo, a small permanent magnet is rotated by the wheel of the bicycle inside a fixed coil.
Alternatively, an electromagnet powered by a fixed DC voltage can be made to rotate inside a fixed coil, such as in large power generators producing in both cases an alternating current.
The simple dynamo type generator here consists of a permanent magnet which rotates around a central shaft with a coil of wire placed next to this rotating magnetic field.
As the magnet spins, the magnetic field around the top and bottom of the coil constantly changes between a north and a south pole.
This rotational movement of the magnetic field results in an alternating emf being induced into the coil as defined by Faraday’s law and Lenz’s law of electromagnetic induction.
Faraday’s law of electromagnetic induction is whenever a conductor is placed in a varying magnetic field, EMF induces between conductor terminals and this emf is called an induced emf and if the conductor is a closed circuit then the induced current flows through it. It is stated in Electromagnetic Induction Definition in the first paragraph of this article.
Further, Faraday’s law of induction states the magnitude of the induced EMF is equal to the rate of change of flux linkages.
Faraday’s Law tells us that inducing a voltage into a conductor can be done by either passing it through a magnetic field, or by moving the magnetic field past the conductor and that if this conductor is part of a closed circuit, an electric current will flow.
This voltage is called an induced emf as it has been induced into the conductor by a changing magnetic field due to electromagnetic induction with the negative sign in Faraday’s law telling us the direction of the induced current (or polarity of the induced emf).
But, a changing magnetic flux produces a varying current through the coil which itself will produce its own magnetic field as we saw in the Electromagnets tutorial.
This self-induced emf opposes the change that is causing it and the faster the rate of change of current the greater is the opposing emf.
This self-induced emf will, by Lenz’s law oppose the change in current in the coil and because of its direction this self-induced emf is generally called a back-emf.
Lenz’s Law states that: ” the direction of an induced emf is such that it will always opposes the change that is causing it”.
In other words, an induced current will always OPPOSE the motion or change which started the induced current in the first place and this idea is found in the analysis of Inductance.
Likewise, if the magnetic flux is decreased then the induced emf will oppose this decrease by generating and induced magnetic flux that adds to the original flux.
Lenz’s law is one of the basic laws in electromagnetic induction for determining the direction of flow of induced currents and is related to the law of conservation of energy.
According to the law of conservation of energy which states that the total amount of energy in the universe will always remain constant as energy can not be created nor destroyed. Lenz’s law is derived from Michael Faraday’s law of induction.
One final comment about Lenz’s Law regarding electromagnetic induction. We now know that when a relative motion exists between a conductor and a magnetic field, an emf is induced within the conductor.
But, the conductor may not actually be part of the coils electrical circuit, but may be the coils iron core or some other metallic part of the system, for example, a transformer.
The induced emf within this metallic part of the system causes a circulating current to flow around it and this type of core current is known as an Eddy Current.
Eddy currents (also called Foucault’s currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday’s law of induction.
Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor.
The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material.
When graphed, these circular currents within a piece of metal look vaguely like eddies or whirlpools in a liquid.
By Lenz’s law, an eddy current creates a magnetic field that opposes the change in the magnetic field that created it, and thus eddy currents react back on the source of the magnetic field.
For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to eddy currents induced in the surface by the moving magnetic field.
This effect is employed in eddy current brakes which are used to stop rotating power tools quickly when they are turned off. The current flowing through the resistance of the conductor also dissipates energy as heat in the material.
Thus eddy currents are a cause of energy loss in alternating current (AC) inductors, transformers, electric motors and generators, and other AC machinery, requiring special construction such as laminated magnetic cores or ferrite cores to minimize them.
Eddy currents are also used to heat objects in induction heating furnaces and equipment, and to detect cracks and flaws in metal parts using eddy-current testing instruments.
Eddy currents generated by electromagnetic induction circulate around the coils core or any connecting metallic components inside the magnetic field. Because, for the magnetic flux they are acting like a single loop of wire.
Eddy currents do not contribute anything towards the usefulness of the system but instead they oppose the flow of the induced current by acting like a negative force generating resistive heating and power loss within the core.
However, there are electromagnetic induction furnace applications in which only eddy currents are used to heat and melt ferromagnetic metals.
The changing magnetic flux in the iron core of a transformer above will induce an emf, not only in the primary and secondary windings, but also in the iron core.
The iron core is a good conductor, so the currents induced in a solid iron core will be large. Furthermore, the eddy currents flow in a direction which, by Lenz’s law, acts to weaken the flux created by the primary coil.
Consequently, the current in the primary coil required to produce a given B field is increased, so the hysteresis curves are fatter along the H axis.
Eddy current and hysteresis losses cannot be eliminated completely, but they can be greatly reduced.
Instead of having a solid iron core as the magnetic core material of the transformer or coil, the magnetic path is “laminated”.
These laminations are very thin strips of insulated (usually with varnish) metal joined together to produce a solid core.
The laminations increase the resistance of the iron-core thereby increasing the overall resistance to the flow of the eddy currents, so the induced eddy current power-loss in the core is reduced, and it is for this reason why the magnetic iron circuit of transformers and electrical machines are all laminated.