Inductive Coupling of Power Converter`s – EMC

Inductive Coupling of Power Converter`s – EMC
Acta Polytechnica Hungarica
Vol. 6, No. 2, 2009
Inductive Coupling of Power Converter’s – EMC
Irena Kováčová, Dobroslav Kováč
Faculty of Electrical Engineering and Informatics, Technical University of
Košice, Letná 9, 042 00 Košice, Slovak Republik
[email protected], [email protected]
Abstract: The paper presents a computer analysis of inductive coupling of the
electromagnetic compatibility (EMC) problem. Its focus is on power electronics and
electrical drives and tests performed by a numerical computer simulation that can disclose
suite surprising findings about EMC.
Keywords: converters, inductance, electromagnetic compatibility, power electronics
1
Introduction
Importance of electromagnetic compatibility (EMC) of all electrical products has
been rapidly growing during the last decade. The environment is increasingly
polluted by electromagnetic energy. The interference impact on the surroundings
is being doubled every three years and covers a large frequency range.
Equipment disturbances and errors have become more serious as a consequence of
the growth of the electronic circuit complexity. According to new technical
legislation and also economic consequences, the EMC concept of all products
must be strictly observed [1], [2]. It must start with the specification of the
equipment performance and end with the equipment installation procedures.
2
EMC and Environmental Waste
We all know the environmental pollution problems caused by solid, liquid and
gaseous wastes. We are aware of most of these pollutants through our senses.
Due to the increasing life standard, contamination of our environment by the
electromagnetic energy is constantly increasing too. Since human beings have no
organs for perception of such contamination, they cannot perceive it. The great
producers of such waste are electronic systems developed by man and meant to be
– 41 –
I. Kováčová et al.
Inductive Coupling of Power Converter’s – EMC
effective within these electromagnetic surroundings producing, of course,
electromagnetic waste in turn [3].
On one side, interferences are deliberately or involuntarily produced. The place of
their origin is called interference source. On the other side, devices may be
hindered in their function by such interferences. Those objects are called
interference objects.
The possible interfaces between sources and objects are shown in Fig. 1. There
are four basic types of coupling that can realize these interfaces.
Interference
source
Electricity
consumers
NEMP, EPM
Atmospheric events
Cosmic noise
Galvanic
Capacitive
Inductive
Electromagnetic spark
Coupling
Interference
object
Domestic appliances
Industrial electronics
Automotive electronics
Military electronics
Figure 1
Interference diagram
3
EMC – The Interference Mechanism
The interference mechanism can be described in a simplified form as follows. The
interference source can be for instance a power semiconductor converter or motor.
Interference is produced in the interference source getting into electronics in
undesirable ways and is due to various effects distorting signals [4], [5].
Transmission can be direct, for example by galvanic coupling between
interference source and interference sink. Interference can be spread through air
or via ducts, or coupled inductively or capacitively into signal lines.
Development of power semiconductor elements has caused vehement evolution of
the power electronics branch in the last ten years. To investigate the converter
functionality, it was necessary first to theoretically analyze and then practically
verify its assumed activity. Now, we can eliminate the laborious theoretical
analysis by a numerical computer simulation, which can disclose quite surprising
findings about EMC.
– 42 –
Acta Polytechnica Hungarica
4
Vol. 6, No. 2, 2009
Inductive Coupling
Inductive coupling is typical for two and more galvanically separated electric
loops at the moment when the smaller one is driven by a time variable- current
creating the corresponding, time-variable magnetic field [6], [7]. In such case their
mutual intercircuit effect is expressed as a function of the slope of the current
increase or decrease, circuit environmental magnetic property as well as circuit
geometric dimensions.
To predict the intercircuit inductive coupling, our focus will be on two electric
loops l1 and l2 with currents i1 and i2. We will try to determine the effect of loop l1
on loop l2, Fig. 2.
Ψ1 = N.Φ1
l2
dl1i
l1
N=1
rij
dl12
dl1mdl11
i1
i2
dl2k
dl21
dl22
dl2j
Figure 2
Investigated loops
According to the Maxwell’s equation for a quasi-stationary magnetic field:
rot E = −
∂B
∂t
(1)
and following its integral form: ∫ rot E dS = − ∫ ∂B dS = − ∂ ∫ B dS
S
S
∂t
∂t
(2)
S
and after applying the Stoke’s theorem, we obtain the equation for the induced
voltage:
ui 2 = − N
∂φ1
∂ψ 1
∂i
=−
= −M 1
∂t
∂t
∂t
(3)
where M is the coefficient of the mutual inductance. For the magnetic flux Ψ1
definition the equation.
φ1 = ∫ A2 dl2
(4)
l2
– 43 –
I. Kováčová et al.
Inductive Coupling of Power Converter’s – EMC
Is valid where A2 is the vector of the magnetic field potential created by the
current i1. We can calculate the value of this vector by the following equation.
μ i1 dl1
4π ∫l r12
A2 =
(5)
1
After substituting the last equation with the equation valid for the magnetic flux
φ1, the next relation is obtained:
⎡μ i
1
4
π
l2 ⎢
⎣
φ1 = ∫ ⎢
d l1 ⎤
μi
∫l r12 ⎥⎥ dl2 = 4π1
1
⎦
⎛μi
∂⎜ 1
⎜ 4π
and then, ui 2 = − ⎝
∫∫
l1 l2
∂t
∫∫
l1 l2
d l1 d l 2
r12
⎛ μ
dl1 dl2 ⎞⎟
⎜
⎜ 4π
r12 ⎟⎠
=−⎝
(6)
∫∫
l1 l2
dl1 dl2 ⎞⎟
∂i1
r12 ⎟⎠
∂i
= −M 1 .
∂t
∂t
(7)
For the practical use, it is more advantageous to express the induced voltage in the
form of a differential [8], [9].
ui = −
di
dt
m
k
∑∑
i =1 j =1
μ dl1i .dl2 j . cos γ dlij
4π
rij
(8)
Ψ1 =N.Φ1
l1
N=1
i1
B1
A
A1 2 rij
dl21
B2
dl11
dl11
γ
l2
i2
dl21
Figure 3
Geometric dimensions of the investigated loops
If we know the geometrical dimensions of the investigated loops Fig. 3 and want
to determine their mutual inductive coupling then we can use the next relation (9),
(10), (11) for the induced voltage. It is based on the 3D Cartesian coordinate
system.
ui = −
di m k μ D+F +G
∑∑
dt i=1 j=1 4π O2 +P2 +S2
(9)
– 44 –
Acta Polytechnica Hungarica
Vol. 6, No. 2, 2009
where, D = ( A x 2 i − A x 1 i ).( B x 2 j − B x 1 j ) ,
(10)
F = ( A y 2 i − A y 1 i ).( B y 2 j − B y 1 j ) ,
G = ( A z 2 i − A z 1 i ).( B z 2 j − B z 1 j ) ,
⎛
Bx 2 j − Bx1 j
O = ⎜ Bx1 j +
⎜
2
⎝
⎞ ⎛
⎟ − ⎜ A + A x 2i − A x1i
⎟ ⎜⎝ x1i
2
⎠
⎞
⎟,
⎟
⎠
⎛
B y 2 j − B y1 j
P = ⎜ B y1 j +
⎜
2
⎝
⎞ ⎛
A − A y1i
⎟ − ⎜ A + y 2i
⎟ ⎜ y1i
2
⎠ ⎝
⎛
⎞
B − B z1 j
⎟ , S = ⎜ B + z2 j
z1 j
⎜
⎟
2
⎝
⎠
(11)
⎞ ⎛
⎟ − ⎜ A + A z 2i − A z1i
⎟ ⎜⎝ z1i
2
⎠
⎞
⎟
⎟
⎠
For a global solution of the inductive coupling part of the EMC problem inside the
overall electric power system [10], [11], it is necessary to analyze the circuit
globally paying due regard to the mutual intercircuit inductance coupling. The
result is the following integral-differential system of equations.
u cc 1 = Rc1 .i1 + Lc1 .
k
di1
1
+
i
.
dt
+
u ij
∑
1
dt
C c1 ∫
j =1
.
j ≠1
.
.
u cck = Rck .ik + Lck .
di k
1
+
dt C ck
(12)
k
∫ i .dt + ∑ u
k
j =1
j≠k
(13)
ij
For this purpose it is very suitable to explore the existing simulation programs
such as for instance the PSPICE program utilized worldwide [12], [13], [14].
In the next part, we will try to determine the effect of the one-quadrant impulse
converter on the sensing circuit as it shown in Fig. 4. The circuit dimensions are
a = 0.2 m, b = 0.3 m, c = 0.1 m, d = 0.05 m, e = 0.005 m. The radius of the copper
wires is R = 0.0006 m and the relative permitivity of the circuit environment is
μr = 0.991.
M
RG
Q
uG
ND
a
e
c
d
V
Figure 4
Investigated circuit
– 45 –
+
M
Ucc
b
I. Kováčová et al.
Inductive Coupling of Power Converter’s – EMC
The inductance of the first loop is given as (14) and of the second as (15).
L1 = Le1 + Li1 =
μ0b a − R μ0 a b − R μ.2.(a + b)
ln
+
ln
+
= 1.294 μH
π
π
R
R
8π
(14)
L2 = Le2 + Li2 =
μ0c d − R μ0d c − R μ.2.(c + d)
ln
+
ln
+
= 0.294μH
R
R
8π
π
π
(15)
The mutual inductance M calculated from the above mentioned equation is
M = 477.4 nH. The magnetic coupling coefficient k is given as:
k=
M
= 0.774 .
L1 + L2
(16)
Now we can use the PSPICE simulation program for solving the inductive
coupling problem between the two circuits [15], [16]. Parameters of the circuit
simulation are RZ = 11.66 Ω, LZ = 400 μH, R = 10 Ω, RG = 100 Ω and UCC = 70V.
The schematic connection is shown in Fig. 5. The IGBT transistor Q was switched
on at the frequency 10 kHz and the switch on/off ratio was 0.5.
LZ
RZ
RG
iC
ND
+
-
Q uCE
uG
L1
Ucc
M
L2
R
ui
Figure 5
Simulation circuit
Simulation results are shown in Fig. 6. Results obtained by measurements are
shown in Figs. 7 and 8 and switching details in Figs. 9 and 10, where damping
transient resonant state created by parasitic transistor capacitances, parasitic
conductor inductance and transistor internal resistance causes undulation of the
waves of current iC and voltage uCE.
– 46 –
Acta Polytechnica Hungarica
Vol. 6, No. 2, 2009
ic
Uip2
ui
Uip1
uce
ic
ui
Uip1
Uip2
Uip3
Figure 6
Simulation results
Uip1
0
-ui
5V/d
uCE
50V/d
Uip3
Uip2
0
ic
2A/d
ic
2A/d
0
0
50V
10mV
50V
20μs
10mV
20μs
SAVE
Figure 7
Figure 8
Measured voltage uCE and current iC
Measured voltage -ui and current iC
uCE
50V/d
uCE
50V/d
0
0
ic
2A/d
ic
2A/d
0
0
50V
10mV
200ns
SAVE
50V
10mV
100ns
SAVE
Figure 9
Figure 10
Switching on voltage uCE and current iC
Switching off voltage uCE and current iC
– 47 –
I. Kováčová et al.
Inductive Coupling of Power Converter’s – EMC
A comparison of the simulated and measured results shows that peaks of transistor
current ic have the same values, that is 8.4 A, in both cases. The same values, that
is 4.4 A, have both the simulated and measured transistor current at the moment
when transistor is switched off. There is a small difference only between the
simulated and measured curves of the transistor voltage uCE. The overvoltage
generated at the transistor switching off reaches the value of 150 V for the
simulated result. However, the corresponding overvoltage has only the value of
130 V for the measured result. Peaks of the simulated and measured induced
voltages have the same values of Ui1 = -2.2V, Ui2 = 5.02V, Ui1 = 2.1V. This
means that such method is acceptable for inductive coupling investigation of the
EMC problem.
To improve the obtained results, the numerical solution of the magnetic field by
finite element method program was also used [17]. The result of such analysis is
shown in Fig. 11.
Figure 11
The finite element simulation method of the magnetic field
From the “integral result” data window it is seen, that the value of the magnetic
flux inside the sensing circuit is 3.317x10-9 Wb. Based on the basic program
property allowing semi-real 3D space simulation with the third dimension equal
only to the basic unit of the depth (1 mm), we multiplied the obtained value of the
magnetic flux by the value of the sensing circuit depth c = 100 mm. The total
magnetic flux was then 331.7x10-9 Wb. This flux was excited by the peak circuit
current 8.4 A, the rising time of which was 140 ns. On the basis of the above
equations, the first peak of the induced voltage can be calculated as:
U ip1 =
ΔΦ1 0 − 331.7 x 10 −9 − 331.7 x 10 −9
=
=
= −2.369 V .
Δt1
140 x 10 −9
140 x 10 −9
– 48 –
(17)
Acta Polytechnica Hungarica
Vol. 6, No. 2, 2009
Similarly, it is possible to calculate the rest of the peaks of the induced voltage ui:
U ip2 =
ΔΦ2 331.7 x 10−9 − 55.3 x 10−9 276.4 x 10−9
=
=
= 5.025 V
Δt 2
55 x 10−9
55 x 10−9
U ip3 =
Δ Φ 3 173 . 7 x 10 − 9 − 0 173 . 7 x 10 − 9
=
=
= 2 . 171 V .
Δ t3
80 x 10 − 9
80 x 10 − 9
(18)
(19)
The results obtained by the finite element numerical simulation method are again
confirming the correctness of the above mentioned methods.
5
Electric Motor Interference Simulation
The following interference simulation investigation was done by electric motor,
which also can be representing the interference source. The transmission of
interference can be direct via galvanic coupling or can be propagated through the
air or via ducts with electromagnetic field.
Electric motors can be divided by course of rotor design and course of magnetic
flux in regard to motor axle into two series:
1 Standard motors with radial magnetic flux.
2 Disc motors with axial magnetic flux.
For the drives, which needs to have a great dynamics, is possible to choice the
motor from the group of disc motors. As consequence of the fact that magnetic
flux in disc motor have axial direction, the stray flux to the surroundings of such
motors is generally greater than stray flux of classic motor with cylindrical rotor.
In due to requirement of extension of motor torque these motors have generally
double-sided stator (on both of them side discoid curl). The air gap is also divided
on two sections, the dimensions of which have important influence on the total
stray flux in the area of air gap.
By improving of all electromagnetic construction motor parameters we will obtain
the motor with the higher power. But such motor will be also representing the
source with higher interference. Because these motors are used as a part of the
more complicated equipments with serious electronic control, so their backward
influence on control unit must be as smallest as possible and such a way it will be
possible to avoid of EMC equipment troubles. Study of EMC of such electrical
drives has more and more bigger importance for praxis.
EMC investigation is possible to do either by measures of real motor parameters
or easier and more economically by computer simulation [18], [19]. The task is
then to create real computer simulation model of interference source [20], [21]. In
– 49 –
I. Kováčová et al.
Inductive Coupling of Power Converter’s – EMC
our case such source will represent the disc motor. Simplified electromagnetic,
two dimensional model of such disc motor was developed. Magnetic field
investigation was done by student version of Qfield program. The simplified
simulation model of disc motor with enquired magnetic field is shown on Fig. 12.
Φr
Φh
Φc
Figure 12
Magnetic field calculation
The EMC quality of designed disc motor we can state on the basis of difference of
two magnetic fluxes. First magnetic flux Φc is representing total magnetic flux
produced by motor. Its value is depending on the motor power. The second (main)
magnetic flux Φh represents the part of the total magnetic flux, which is closed
only through magnetic circuit of disc motor. By the difference of the total and
main magnetic fluxes we can obtain stray magnetic flux Φr, which is closed
outside of the motor in surrounding space. This magnetic flux represents the real
source of interference.
The motor frame material and its dimensions have important influence on motor
stray field. According this fact the simulation analysis was done for stating of
magnetic flux dependencies on motor frame materials and its dimensions. Results
are presented in Tables 1 and 2. Table 2 presents results for aluminum frame.
Table 1
Magnetic fluxes inside motor frame materials
Motor frame size (L) is 3.8 mm
Material
Φc [Wb]
Al
0.026627
Steel1511
0.03283
Steel1211
0.033323
Steel St3
0.03381
Φh [Wb]
0.025885
0.025088
0.025081
0.025082
– 50 –
Φr [%]
2.87
30.86
32.86
34.78
Acta Polytechnica Hungarica
Vol. 6, No. 2, 2009
Table 2
Magnetic fluxes for different material thicknesses
L [mm]
3.8
1.6
Φc [Wb]
0.026627
0.026156
Φh [Wb]
0.025885
0.025099
Φr [%]
2.87
4.21
In following we will suppose only change of the current density (J) in motor
excitation coil. Results of magnetic field analyses under such conditions are
presented in Table 3.
Table 3
Magnetic fluxes for different current densities
J[A.m-2.106]
2.67
4.67
6.00
Φc [Wb]
0.014945
0.026156
0.033626
Φh [Wb]
0.014342
0.025099
0.03226
Φr [%]
4.20
4.21
4.23
Conclusions
Although the investigated example seems to be not very realistic so we must take
into consideration also the following four conclusions. The first one is that the
presented example is very simple for demonstration of EMC consequences. The
second one is that the real dimensions of the converter circuits are obviously in
millimeters and so the circuit area is smaller in comparison with demonstration
example, but we should take into consideration also the opposite influence
resulting from the shorter switching times. Such a way the realistic final result
could be that the induced voltages will reach values of few tenths of millivolts in
real equipments. The third conclusion is that induced voltage in real equipment is
created as sum of more circuit interactions and so it can reach also range of the
volts. The fourth one is that also low-level induced voltage can have the important
impact on properly work of some circuits (for example microprocessors, etc.).
However the main contribution of performed analyses consist in derivation of
equations by which any inductive coupling EMC problem is possible simply
analyse by computer numerical method.
Acknowledgement
The paper has been prepared by the support of Slovak grant projects VEGA No.
1/4174/07, 1/0660/08, KEGA 3/5227/07, 3/6388/08, 3/6386/08.
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