YOKOGAWA wt3000, Руководство пользователя, Страница: 357 / 384

App-25
IM 760301-01E
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App App
Index
Appendix
Effects of Stray Capacitance
The effects of stray capacitance on the measurement accuracy can be minimized by
connecting the current input terminal of the instrument to the side that is close to the
earth potential of the power source (SOURCE).
The internal structure of the instrument is as follows:
The voltage and current measurement circuits are each enclosed in shielded cases. These
shielded cases are placed inside the outer case. The shielded case of the voltage measurement
circuit is connected to the
± terminal of the voltage input terminal and the shielded case of the
current measurement circuit is connected to the ± terminal of the current input terminal.
Because the outer case is insulated from the shield case, stray capacitance Cs exists. Cs is
approximately 100 pF. The current generated by this stray capacitance, Cs, will cause errors.
U
±
I
±
Cs
Cs
Shielded case of the voltage
measurement circuit
Outer case
Grounding
Shielded case of the current
measurement circuit
For example, we will consider the case when one side of the power source and the outer
case are grounded.
In this case, two current flows can be considered, load current i
L
and the current that flows
through the stray capacitance i
Cs
. i
L
flows through the current measurement circuit, then through
the load, and returns to the power source (shown with a dotted line). i
Cs
flows through the current
measurement source, through the stray capacitance, and then through the earth ground of the
outer case, and returns to the power source (shown with a dot-dash line).
Therefore, the current measurement circuit ends up measuring the sum of i
L
and i
Cs
even
though it wants to measure only i
L
. i
Cs
is the amount of error in this case. If the voltage
applied to Cs is V
Cs
(common mode voltage), then i
Cs
can be found using the following
equation. Because the phase of i
Cs
is ahead of the voltage by 90 ° the effects of i
Cs
on the
measurement accuracy increases as the power factor gets smaller.
i
Cs
= V
Cs
× 2 π f × Cs
SOURCE
LOAD
Cs
i
L
i
L
i
Cs
±
±
I
U
i
Cs
i
L
When measuring high frequencies as in this instrument, this error, i
Cs
, cannot be ignored.
By connecting the current input terminal of the instrument to the side that is close to the
earth potential of the power source (SOURCE), V
Cs
becomes approximately zero and
very little i
Cs
flows.
Appendix 5 For Making Accurate Measurements