Introduction:
The electromagnetic system is an essential element of all rotating electrical machinery and electromechanical device and static device like the transformer. Electromechanical energy conversion takes place via the medium of a magnetic field or electrical field, but most practical converter use magnetic field as the coupling medium between electrical and mechanical system. In transformer, the energy convert from one electrical circuit to anther electrical circuit via the medium of magnetic field as the coupling medium between one electrical circuit to another electrical circuit. This is due to fact that the energy storing capacity of magnetic field is much greater then that of the electrical field.
Magnetic Circuit:
F = MMF= NI (ampere-turns) or (ATs)
where reluctance in magnetic circuit is same as resistance in electrical circuit.The electromagnetic system is an essential element of all rotating electrical machinery and electromechanical device and static device like the transformer. Electromechanical energy conversion takes place via the medium of a magnetic field or electrical field, but most practical converter use magnetic field as the coupling medium between electrical and mechanical system. In transformer, the energy convert from one electrical circuit to anther electrical circuit via the medium of magnetic field as the coupling medium between one electrical circuit to another electrical circuit. This is due to fact that the energy storing capacity of magnetic field is much greater then that of the electrical field.
Magnetic Circuit:
- The complete closed path followed by the lines of flux is called a magnetic circuit. In low power electrical machines, magnetic field be produced by permanent magnetic. But in high-power electrical machinery and transformer, coupling magnetic field is produced by electric current.
- In a magnetic circuit, the magnetic flux is due to the presence of a magnetomotive force same as in an electric circuit, the current is due to the presence of a electromotive force.
- The mmf is created by a current flowing though one or more turns.
F = MMF= NI (ampere-turns) or (ATs)
- The magnetic flux ɸ may be defined as the magnetomotive force per unit reluctance. ɸ= MMF /Reluctance.
- It means the oppostition offered by the magnetic flux is called reluctance.
where,
l = length of the magnetic path
A=area of cross-section normal to flux path, m2
μ=µ0 µr = permeability of the magnetic material
µr = relative permeability of the magnetic material
µ0 =permeability of the free space=4π 10−7 (H/m)
Here the concept of permeability can be understand in easy way with following examples.
Suppose a current I carrying conductor in a free space .
According to the right hand grip rule, around the current carrying conductor a magnetic flux path is generated. Actually right hand grip rule stated that grip the conductor with thumb pointing in the direction of conductor current then four fingers give the direction of magnetic flux created by the current.
leakage flux:
In ideal magnetic circuits, all the flux produced by an exciting coil is confined to the desired magnetic path of negligible reluctance. But in practical magnetic circuits, a small amount of flux does follow a path through the surrounding air. Therefore, leakage flux may be defined as that flux which does not follow the intended path in a magnetic circuit. Leakage flux does exist in all practical ferromagnetic device. Its effect on the analysis of electrical machinery is carried out by replacing it by an equivalent leakage reactance.
Fringing:
At an air-gap in a magnetic core, the flux fringes out into neighboring air path as shown in above figure .Longer the air gap, more is the flux fringing. The effect of fringing flux is to increase the effective cross-sectional area of the air gap. As a result, flux density in the air gap is not uniform and average flux density gets reduced.
→ B = ɸ /A
If area of air gap increases then total area of core with consideration of air gap increase. Then average flux density gets reduced.
Induced EMF:
Faraday's law of electromagnetic induction states that an e.m.f is induced in a coil when the magnetic flux linking this coil change with time.
e= - Ndɸ / dt
where,
e= e.m.f. induced in volts
N= Number of turns in the volts
Ѱ= Nɸ = Flux linkages with the coil, wb-turns
t= time in seconds
Here mines(-) sign shows that induced current opposes very cause of production. This theory is called Lenz's law. According to this law: the induced current develops a flux which always opposes the change responsible for inducing this current.
leakage flux:
In ideal magnetic circuits, all the flux produced by an exciting coil is confined to the desired magnetic path of negligible reluctance. But in practical magnetic circuits, a small amount of flux does follow a path through the surrounding air. Therefore, leakage flux may be defined as that flux which does not follow the intended path in a magnetic circuit. Leakage flux does exist in all practical ferromagnetic device. Its effect on the analysis of electrical machinery is carried out by replacing it by an equivalent leakage reactance.
Fringing:
At an air-gap in a magnetic core, the flux fringes out into neighboring air path as shown in above figure .Longer the air gap, more is the flux fringing. The effect of fringing flux is to increase the effective cross-sectional area of the air gap. As a result, flux density in the air gap is not uniform and average flux density gets reduced.
→ B = ɸ /A
If area of air gap increases then total area of core with consideration of air gap increase. Then average flux density gets reduced.
Induced EMF:
Faraday's law of electromagnetic induction states that an e.m.f is induced in a coil when the magnetic flux linking this coil change with time.
e= - Ndɸ / dt
where,
e= e.m.f. induced in volts
N= Number of turns in the volts
Ѱ= Nɸ = Flux linkages with the coil, wb-turns
t= time in seconds
Here mines(-) sign shows that induced current opposes very cause of production. This theory is called Lenz's law. According to this law: the induced current develops a flux which always opposes the change responsible for inducing this current.
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