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Electromagnetic Induction & Inductors
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Revision of Electromagnetic Induction and Inductors
(Much of this material has come from Electrical & Electronic Principles & Technology by John Bird)
Magnetic Field
A permanent magnet is a piece of ferromagnetic material and if freely suspended will align in
a north and south direction. The north seeking end being called the north pole and the south the
south pole. The area around the magnet is called the magnetic field and its existence can easily
be demonstrated using iron filings and paper. See below.
Magnetic Flux and Flux Density
Magnetic flux is the amount of magnetic field (or number of lines of force) produced by a
magnet. The magnetic flux is given the symbol ϕ and its unit is the weber (Wb).
If the magnet has a cross-sectional area A, then it is possible to define a flux density (B), where
B =ϕ
A. The unit of magnetic flux density is the tesla (T) and 1T = 1 Wb/m2.
Electromagnets
These usually consist of an iron core with a current carrying coil wound around it and have a
similar flux as a permanent magnet. It was found by Faraday that the flux in an electromagnet
is directly proportional to the current (I), the number of turns (N) and the length of the magnetic
path (l), This means that for a given electromagnet the value of IN
l is constant and is called its
magnetising force and denoted H. Therefore, we can write
Magnetising force = H =IN
l (Ampere turns/metre)
The length of the magnetic circuit is difficult to establish because the flux will flow through
the surrounding air as well as the core. However, if the coil is wound on a toroid as shown
below, the flux will be entirely within the core and the length of the circumference.
Now if the coil is wound on a core which has no magnetic properties we find that the ratio B
H is
12.5 x 10-7. It is given the symbol µo and called the absolute permeability of free space.
If we use a magnetic material for the core, often iron, the ratio is dramatically changed and a
relative permeability µr is used to correct the values.
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B
H= μoμr
Where μoμr = μ and is called absolute permeability
Introduction to Electromagnetic Induction
Electromagnetic induction is the production of a potential difference across a conductor when
it is exposed to a varying magnetic field. It is generally considered a discovery attributed to
Faraday.
When a conductor is moved across a magnetic field such that it cuts the lines of force (flux),
an e.m.f. is produced in the conductor. If the conductor forms part of a closed circuit, an electric
current will flow. This is known as electromotive induction.
This can be demonstrated with a magnet and a coil connected to a galvanometer (sensitive
ammeter) as shown below.
An increase in the speed of movement, a stronger magnet, or an increase in the number of turns,
all increase the amount of current flow.
Faraday’s Laws
1) An induced e.m.f. is set up whenever a magnetic field linking the circuit changes.
2) The magnitude of the induced e.m.f. is proportional to the rate of change of magnetic
flux linking the circuit.
Lenz’s Law
The direction of an induced e.m.f. is always such that it tends to set up a current opposing the
motion or the change of flux responsible for inducing that e.m.f.
An alternative method to Lenz’s law for determining relative directions is given using
Fleming’s Right Hand Rule. See below.
Magnet moved at constant speed will
produce a reading as shown.
When magnet moved in other direction,
direction of current is reversed
Alternatively, could hold magnet stationary
and move the coli.
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In a generator, conductors forming an electric circuit are made to move through a magnetic
field. By Faraday’s law an e.m.f. is induced in the conductor and thus a source of e.m.f. is
produced.
In the diagram below and e.m.f. will be induced by the movement of the conductor between
the magnets.
Induced e.m.f is given by E = Blν volts
Where B = flux density in teslas
l = length of conductor in the magnet in metres
ν = velocity of conductor in m/s
If the conductor moves at an angle θ to magmetic field instead of 90o as shown above, then
E = Blν sin θ volts.
Consider now a rotating loop in a magnetic field as shown below.
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In the diagram, the direction of current flow has come from Fleming RH rule and the total e.m.f
is now E = 2NBlν sin θ volts.
Inductance
This is the name given to the property of a circuit whereby there is an e.m.f induced into the
circuit by a change in flux linking produced by a current change.
When the e.m.f. is induced in the same circuit as that in which the current is changing, the
property is called self-inductance and is denoted L. When the e.m.f. is induced in a circuit by
a change of flux due to current changing in an adjacent circuit, the property is called mutual
inductance and denoted M. The unit of inductance is the henry (H)
A circuit has an inductance of one henry when an e.m.f. of one volt is induced in it by a current
changing at a rate of one ampere per second.
Therefore, the induced e.m.f in a coil of N turns is E = −Ndϕ
dt volts, where dϕ is the change in
flux in Webers and dt is the time taken for the flux to change is seconds (dϕ
dt is rate of change
of flux).
Alternatively, the induced e.m.f in a coil of inductance L henry’s is E = −LdI
dt, where dI is the
change in current in amperes and dt is the time taken for the current to change in seconds (dI
dt is
rate of change of current).
The minus sign in each equation reminds us of its direction (given by Lenz’s Law)
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Inductors
A component called an inductor is used in an electrical circuit when the property of inductance
is required. Any plain coil of wire is an inductor, although more often than not the coil will be
wound on a magnetic core.
Factors affecting the inductance of an inductor include:
The number of turns of wire – the more, the higher the inductance
The coils cross-sectional area – the greater, the higher the inductance
The presence of a magnetic core – this increases the concentration of the magnetic field
and hence the inductance is increases
The way the turns are arranged – a short, thick wire will have higher inductance that a
long, thin one.
Energy Stored in and Inductor
Inductors have the ability to store energy (W) in the magnetic field and this is given by
W =1
2LI2 joules
Where L is inductance measures in henry’s (H) and I is the current in amperes
Inductance of a Coil
The proofs are not given here, but the inductance of a coil may be calculated a number of ways,
and the relevant equations are given below.
L =Nϕ
I where N is the number of turns, ϕ is the flux linkage and I is the current.
We also have
L =N2
S where N is the number of turns and S is a property of a coil called reluctance and is
given by the equation:
S =l
µoµrA where µo is absolute permeability of free space and has a value of 12.5 x 10-7 H/m,
µr is the relative permeability, and l is the magnets length in m.
Worked Example 1
A flux of 25mWb links with a 1500 turn coil when a current of 3A passes through the coil.
Calculate (a) the inductance of the coil, (b) the energy stored in the magnetic field and (c) the
average e.m.f. induced if the current falls to zero in 150ms.
(a) Inductance L =Nϕ
I=
1500×25×10−3
3= 12.5H
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(b) Energy stored W =1
2LI2 =
12.5×32
2= 56.25J
(c) Induced e.m.f. E = −LdI
dt= −12.5 (
3−0
150×10−3) = −250v
Or we could use E = −Ndϕ
dt= −1500 (
25×10−3
150×10−3) = 250v
Worked Example 2
A 750 turn coil of inductance 3H carries a current of 2A. Determine the flux linking the coil
and the e.m.f. induced in the coil then the current collapses to zero in 20ms.
We have that L =Nϕ
I, rearranging we obtain ϕ =
LI
N=
3×2
750= 8 × 10−3Wb = 8mWb
Also E = −LdI
dt= −3 (
2−0
20×10−3) = −300v
We could also have used E = −Ndϕ
dt
Worked Example 3
A silicone ring is wound with 800 turns, the ring having a mean diameter of 120mm and a
cross-sectional area of 400mm2. If when carrying a current of 0.5A the relative permeability is
found to be 3000, determine (a) the self-inductance of the coil, (b) the induced e.m.f. if the
current is reduced to zero in 80ms.
(a) Inductance L =N2
S where S =
l
µoµrA=
π×120×10−3
12.5×10−7×3000×400×10−6 = 2.51 × 105A/Wb
Therefore, the self-inductance is L =N2
S=
8002
2.51×105 = 2.55H
(b) Induced e.m.f. E = −LdI
dt= −2.55 (
0.5−0
80×10−3 = 15.93v)
Quick Recap on Relevant Equations
Equation Units
E = −Ndϕ
dt
Volts (v)
E = −LdI
dt
Volts (v)
W =1
2LI2
Joules (J)
L =Nϕ
I
Henry (H)
L =N2
S
Henry (H)
S =l
μoμrA
1
Henry (H−1or A/Wb)
𝜇𝑜 = 12.5 × 10−7 Henry/metre (H/m)
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Tutorials Problems
1) A flux of 30mWb links with a 1200 turn coil when a current of 5A is passed through
the coil. Determine (a) the inductance of the coil, (b) the energy stored in the magnetic
field, and (c) the average e.m.f. induced is the current is reduced to zero in 0.20s.
(7.2H, 90J, 180v)
2) A coil of 2500 turns has a flux of 10mWb linking with it when carrying a current of
2A. Calculate the coil inductance and e.m.f. induced in the coil when the current
collapses to zero in 20ms.
(12.5H, 1.25kv)
3) An iron ring has a cross-sectional area of 500mm2 and a mean length of 300mm. It is
wound with 100 turns and its relative permeability is 1600. Calculate (a) the current
required to set up a flux of 500µWb in the coil, (b) the inductance of the system and (c)
the induced e.m.f. if the field collapses in 1ms.
(1.492A, 33.5mH, -50v)
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Current Growth and Decay in an L-R Circuit
Current Growth
A typical L-R circuit is shown below. When an inductor and resistor are connected in this way
we get a similar charging and discharging effect as with capacitors. The mathematics of this is
not considered here, however below the circuit are shown the plots of VL, VR, and I against
time during charging, together with the relevant equations.
Worked Example
The winding of an electromagnet has an inductance of 3H and a resistance of 15Ω. When it is
connected to a 120v d.c. supply, calculate (a) the steady-state value of current flowing in the
winding, (b) the time constant of the circuit, (c) the value of the induced e.m.f. after 0.1s, (d)
the time taken for the current to rise to 85% of its final value and (e) the value of the current
after 0.3s.
(a) Steady-state current I =V
R=
120
15= 8A
(b) Time constant τ =L
R=
3
15= 0.2s
(c) Induced e.m.f after 0.1s VL = Ve−t τ⁄ = 120 × e−0.1 0.2⁄ = 72.78v
(d) Time for current to rise to 85% of final value i = 0.85I and i = I(1 − e−t τ⁄ )
Decay of induced voltage
VL = Ve−t τ⁄
Growth of resistor voltage
VR = V(1 − e−t τ⁄ )
Growth of current flow
i = I(1 − e−t τ⁄ )
Where τ is the time constant and
equal to L
R
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Therefore 0.85I = I(1 − e−t/0.2)
Hence 0.85 = (1 − e−t/0.2)
Therefore e−t/0.2 = 1 − 0.85 = 0.15
or et/0.2 =1
0.15
Hence t
0.2= ln
1
0.15 → t = 0.379s
(e) Current after 0.3s i = I(1 − e−t τ⁄ ) = 8(1 − e−0.3/0.2) = 6.215A
Current Decay
When a series connected L-R circuit is connected to a d.c. supply as shown with S in position
A, a current I =V
R flows after a short time, creating a magnetic field (ϕ α I) associated with the
inductor. When S moves to position B, the current value decreases, causing a decrease in the
strength of the magnetic field. Flux linkages occur, generating voltage VL, equal to Ldi
dt. By
Lenz’s law, this voltage keeps current i flowing in the circuit, its value being limited by R.
Since V = VL + VR, 0 = VL + VR and VL = −VR. In other words, VLandVR are equal in
magnitude but opposite in direction. The current decays exponentially to zero and since VR is
proportional to the current flowing, VR decays exponentially to zero. Since VL = VR, VL also
decays exponentially to zero. The decay curves are similar to those we saw for capacitors and
the relevant equations are given below.
The equations representing the decay transients are:
Voltage decay
VL = VR = Ve−t τ⁄
Current decay
i = Ie−t τ⁄
Tutorial Problems
1) A coil having an inductance of 6H and a resistance of RΩ is connected in series with a
resistor of 10Ω to a 120v d.c. supply. The time constant of the circuit is 300ms. When
steady-state conditions have been reached, the supply is replaced instantaneously by a
short-circuit. Determine (a) the resistance of the coil, (b) the current flowing in the
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circuit one second after the shorting link has been replaced in the circuit, and (c) the
time taken for the current to fall to 10% of its initial value.
(10Ω, 0.214A, 0.691s)
2) The field windings of a 200v d.c. machine has a resistance of 20Ω and an inductance
of 500mH. Calculate (a) the time constant of the field winding, (b) the value of current
flow one time constant after being connected to the supply, and (c) the current flowing
50ms after the supply has been switched on.
(25ms, 6.32A, 8.65A)
3) An inductor has negligible resistance and an inductance of 200mH and is connected in
series with a 1kΩ resistor to a 24v d.c. supply. Determine (a) the time constant of the
circuit and the steady-state value of current flowing in the circuit, (b) the current
flowing in the circuit at a time equal to one time constant, (c) the voltage drop across
the inductor at a time equal to two time constants, and (d) the voltage drop across the
resistor after a time equal to three time constants.
(0.2s, 24mA, 15.17mA, 3.248v, 22.81v)
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