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Electrical Machines
EE41 Electromechanical Energy Conversion
Prepared by:
Engr
...
Magnetic Field
β’ Ampereβs Law:
π» β ππ = πΌ πππ‘
Magnetic Circuits Fundamentals
β’ Magnetomotive Force, ο
β’ Magnetic Flux, Ο
β’ Reluctance, ο
β’ Magnetic Field Density, B
β’ Magnetic Field Intensity, H
Magnetic Circuits Fundamentals
ο= Ο ο = NI
ο=
π
ππ΄
π = π π π0
where:
π0 = 4Ο x (10^-7)
Magnetic Circuits Fundamentals
π
π΅ = = ππ»
π΄
Therefore,
π΅
ππΌ
π»= =
π
π
Magnetic Circuits Fundamentals
β’ The figure shows a simple magnetic
circuit
...
Assume that ΞΌr = 1000, N
= 500, and i=100mA
...
Magnetic Circuits Fundamentals
β’ Three sides of a ferromagnetic core
are of uniform width, while the
fourth side is somewhat thinner (as
shown on the right)
...
Assuming a
relative permeability of 2500, how
much flux will be produced by a 1-A
input current?
Magnetic Circuits Fundamentals
β’ A ferromagnetic core whose mean
path length of 40cm is shown in the
right
...
cm
...
Assume that fringing in the
air gap increases the effective crosssectional area of the gap by 5%
...
5T in the air gap
...
So, they operate
near the knee of the magnetization
curve
...
Energy Losses
β’ Hysteresis Loss
- Hysteresis is the dependence on the
preceding flux history and the
resulting failure to retrace flux paths
...
Hysteresis Losses
β’ When mmf is removed, the magnetic field in each domain will try to
revert to its random state
Hysteresis Losses
β’ However, not all magnetic field domainβs would revert to its random
state due to the lack of energy required to disturb the alignment
...
This extra energy is known as Hysteresis loss
...
- Area enclosed in the hysteresis loop
formed by applying an AC current to
the core is directly proportional to
the energy lost in a given AC cycle
...
ππ
π πππ = βπ
ππ‘
- In addition to the desirable effect of inducing an emf in the coil, the flux
induces undesirable voltages in the core
...
Energy Losses
β’ Eddy Current Loss
- To minimize the amount of energy lost, the ferromagnetic material is broken
down into small strips or laminations with very thin layers of insulating
material inserted between each pair of the laminations
...