1 chapter 1 magnetic circuit. 2 it is the path which is followed by magnetic flux. it is basically...
TRANSCRIPT
1
Chapter 1MAGNETIC CIRCUIT
2
It is the path which is followed by magnetic flux.
It is basically ferromagnetic with coil wound around them
MAGNETIC CIRCUIT
Why MAGNETIC CIRCUIT?
It is an important component
in the design of electrical machines.
4
Examples of Magnetic Circuits
Simple magnetic circuit
Magnetic circuit with air gap
Simple machine
8
It is required to understand the concepts of magnetic circuits
The object of today lecture
By making Analogy between Electric circuit and magnetic circuit
Analogy between Electric circuit and magnetic circuit
10
Definitions Related to Electromagnetic Field(Unit is Weber (Wb)) = Magnetic Flux
is the number of flux lines crossing a surface area.
B (Unit is Tesla (T)) = Magnetic Flux Density
Is the number of flux lines per unit area = /A
H (Unit is Amp/m) = Magnetic Field Intensity =
B
Permeability
It is the degree of magnetization of a material to allow magnetic flux to pass through it.
It is analogous to conductivity in an electrical circuit
relative permeability.
µ permeability of a material
For ferromagnetic materials
For non-ferromagnetic materials
0 r
o = Permeability of air = 4*10-7 H/m
Magnetic Reluctance•It is the property of a material which
opposes the creation of magnetic flux in it
• It is analogous to •resistance in an electrical circuit•The reluctance of a material is given
by
Magneto Motive Force (mmf)FIt is the external force required to set
up the magnetic flux lines within the magnetic material.
The magneto motive force F is equal to the product of the number of turns around the core and the current through the turns of wire.
Magnetic Field Intensity (H)It is The magneto motive force per unit length
magnetic field intensity (H) produce a magnetic flux density B (Tesla).
a magnetic flux density is given by:
15
Magnetization Curves
•In magnetic circuit calculations,
Magnetic Circuit Calculations
• it is required to determine
• the excitation mmf (F) needed • to
establish• a desired flux or• flux density at a given
point.
The magnetic circuit for the toroidal coil can be analyzed to obtain an expression for flux.
Magneto motive force F is
Where the reluctance is
`
and the magnetic flux is
Magnetic Circuit Calculations
Magnetic Circuit Calculationsobtain an expression for flux for the shown magnetic circuit
Effect of air gap on a magnetic circuitobtain an expression for flux for the shown magnetic circuit
20
•Increase the reluctance. •Greater values of ampere-turn •are required to obtain the same• value of B for circuit without air gap•linearize magnetic circuits•i.e. no saturation
Summary of Effect of air gap on a magnetic circuit
21
Air gap is practically an unavoidable part of any magnetic circuitThe B-H loop of a magnetic circuit is affected by the presence of air gap.so greater values of H are required to obtain the same value of B as compared with magnetically materials.
Effect of air gap on Magnetization Curves
22
As a result the B-H loop gets slanted,
Effects of air gaps on Magnetization Curves
Example 1
Find magneto motive force (mmf) F in a coil, if
the number of turns is 100, and I=2 A.
Find the reluctance if the flux
produced is 100mWb.
Find the permeability if l=50cm and A=0.5 m2
Find the flux density B
Find the magnetic force H
Example 2:Given : i=1 A, N=100, lc=40 cm, A= 100 cm2
r =5000 Calculate : F, H, B, and
100. iNF
2504.0
100
cl
FH
0157.001.057.1. AB
385.636601.05000104
4.07
Alc
In the shown Magnetic circuit relative permeability of the core material is 6000, its rectangular cross section is 2 cm by 3 cm. The coil has 500 turns. Find the current needed to establish a flux density in the gap of Bgap=0.25 T.
Magnetic Circuits (Example 3)
The current needed to establish a flux density in the gap of Bgap can be calculated as follow:
Magnetic Circuits (Solution Example 3)
where
Medium length of the magnetic path in the core is lcore=4*6-0.5=23.5cm, and the cross section area is Acore= 2cm*3cm = 6*10-4 m2
the core permeability is
Magnetic Circuits (Solution Example 3)
Am
Wbrcore
370 1054.71046000
The core reluctance is
the gap area is computed by adding the gap length to each dimension of cross-section:
thus the gap reluctance is:
Continue Solution Example 3
Wb
A
A
l
corecore
corecore
443
2
10195.51061054.7
105.23R
241075.85.035.02 mcmcmcmcmAgap
Wb
A
A
l
gap
gapgap
647
2
0
10547.41075.8104
105.0R
Total reluctance is
based on the given flux density B in the gap, the flux is
thus magneto motive force is
thus the coil current must be
Continue Solution Example 3.
Wb
Acoregap
6106.4RRR
WbAB gapgap44 10188.21075.825.0
AF 100610188.2106.4R 46
AN
Fi 012.2
500
1006
30
•AC Excitation will increase core losses•It is important for the engineer to understand •Why the core losses increase ?•Core losses are important in• determining heating, •temperature rise, •rating and efficiency.
Magnetic Circuits with AC Excitation
1-Hysteresis Losses: hysteresis loss is proportional to the loop area (shaded).
CORE LOSSES (iron losses) are
To minimize hysteresis loss use materials with thin hysteresis(Silicon steel)
2-Eddy Current Losses:Eddy currents are created
when a conductor experiences changes in the magnetic field.
CORE LOSSES (iron losses) are
These induced currents cause Eddy Current Losses. These losses can be reduced by using thin sheets of laminations of the magnetic material.
CORE LOSSES (iron losses) are
34
Thus, Iron Losses in Magnetic Circuit are:
a)Hysteresis losses
b)Eddy Current Losses
The iron loss is the sum of these two losses