VERIFICATION OF
MEASUREMENT
OFAC
ACCURRENT
CURRENT
MEASUREMENT
USING
COAXIAL
WITH
CURRENT SHUNTS
COAXIAL CURRENT SHUNTS
Bostjan Voljc1, Matjaz Lindic1, Rado Lapuh2
1
Bostjan Voljc, Mag. Matjaz Lindic
SIQ - Slovenian Institute of Quality and Metrology,
Ljubljana, Slovenia
SIQ- Metrology
- Slovenian
Institute
of
Quality
and
2 MIRS
Institute of Republic of Slovenia,
Metrology,
Slovenia
Ljubljana, Slovenia
bostjan.voljc@siq.si,
matjaz.lindic@siq.si
bostjan.voljc@siq.si,
matjaz.lindic@siq.si,
rado.lapuh@gov.si
1
Overview
1. New direct ac current
measurement method
2. Accuracy of the new method
3. Comparison of the new method
• comparison between different
shunts
• comparison to ac-dc current
transfer measurement method
4. Conclusion
2
New direct ac measurement technique
•
coaxial current shunts are suitable for ac-dc current transfer measurement and for
direct measurement of ac current
•
direct measurement of ac current: method is based on the measurement of the ac
voltage drop (Us) on resistance (Rs) of the coaxial current shunt
•
current shunts characterized for ac-dc current transfer difference, long term drift of
its dc resistance and self heating effect
•
6 coaxial current shunts covering range from 10 mA to 20 A in frequency range from
10 Hz to 30 kHz
AC current source
AC voltmeter
Current shunt resistor
3
New direct ac measurement technique
US
Im =
RS ⋅ (1 + δ drift + δ ac−dc + δ pwc )
where
is the unknown ac current,
•
Im
Us
Rs
δdrift
•
δac/dc
is correction due to ac-dc difference of the shunt (from PTB calibration
certificate),
•
δpwc
is correction due to self heating of the current shunt (from measurement of
temperature and power coefficient).
•
•
•
is measured ac voltage across the shunt,
is dc resistance of the shunt,
is correction of the dc resistance long term drift (from dc resistance
calibration history),
4
Accuracy of the new method
Shunt
100 mA
300 mA
1A
5A
10 A
20 A
10 Hz
54
54
56
57
62
70
20 Hz
44
45
47
49
55
63
40 Hz
29
29
33
37
47
54
1 kHz
29
29
33
37
47
54
Best measurement capabilities
10 kHz
29
29
34
37
47
62
5
Comparison between shunts
•
two coaxial current shunts were compared over the special current T-piece
•
additional 30 cm high current coaxial cable was used for each shunt to minimize
interference between shunts
•
two shunts compared on the nominal current of the smaller shunt
•
following combinations were compared:
AC current source
Shunt 1 Shunt 2
source
current
100 mA
300 mA
100 mA
300 mA
1A
300 mA
1A
5A
1A
Shunt 1
Shunt 2
AC voltmeter
6
Comparison to ac-dc transfer technique
•
dc+, ac and dc- measurement sequence was used
•
from the same measuring results the ac current was calculated from equations for
both methods:
•
ac-dc current transfer method
 2U ac − U dc + − U dc −


I ac = I dc ⋅ 1 +
+ δ ac − dc 
U dc + + U dc −


•
Im =
•
DC current source
direct method
U ac
RS ⋅ (1 + δ drift + δ ac −dc + δ pwc )
Current
switch
AC current source
common parameters:
•
ac-dc transfer difference (δac-dc)
•
ac voltmeter (Uac)
AC/DC voltmeter
7
Results
Shunt 1
0,1 A
0,3 A
1A
Shunt 2
0,3 A
1A
5A
source current
40 Hz
Frequency
1 kHz
5 kHz
0,1 A
0,3 A
1A
-14,0
-12,1
-0,8
-13,7
-11,2
-2,6
-14,1
-11,5
-1,4
10 kHz
-14,4
-11,5
-2,8
Difference between measurement results obtained on two different coaxial current shunts
in µA/A
Shunt
0,1 A
0,3 A
1A
5A
40 Hz
-6,6
9,4
10,6
5,9
Frequency
1 kHz
5 kHz
-6,6
10,3
10,8
7,3
-7,5
9,6
9,9
5,9
10 kHz
-8,8
8,7
8,9
4,9
Difference between the direct ac current measurement method and ac-dc transfer
difference method in µA/A
8
Conclusion
•
results shows very well controlled behavior of both shunts and measurement method
•
deviations typically less than 1/3 of the stated uncertainty
•
measuring results of the comparison prove that the direct ac current measuring
method is suitable and that the uncertainties stated are appropriate
•
uncertainty of the ac voltmeter presents the major uncertainty contribution
•
•
minimizing its uncertainty would decrease also the uncertainty of the direct acc current
measurement method
between direct ac current measurement and ac-dc transfer difference method
systematic deviations are observed
•
further investigation shall be made to identify and resolve these deviations