The self-inductance of power capacitors
cannot be measured by means of an LCR-bridge. There are two reasons for that:
1. The tolerance band
of any LCR bridge is too wide for a reliable measurement of the self-inductance
of power electronics capacitors.
2. Whenever a capacitor is measured, the LCR
bridge is connected with a series of a.) very large capacitance, b.) very small
inductance, and c.) series resistance (i.e. sum of all conductors in the
capacitor including terminals, wires, film coating, etc.) Both C and RS are representing such
large and dominating values that it is practically impossible to get a
reasonable reading of L. Example: The readings received by one client were in
the range of 800…1000nH and more which is completely irregular and illogical.
The above is the reason why capacitor standard IEC 61071 contains detailed
instructions in section 5.12.
as to how the self-inductance of capacitors has to be determined. This is also
an obligatory part of the type test procedure prescribed for power electronics
capacitors and required under type test clause 5.2.2.j
The standard emphasizes that this can only be
made indirectly through calculation from the resonance frequency, and that it
is essential to measure the resonance frequency by the help of a procedure
which excludes any mistakes or errors, caused by connections or auxiliary
equipment. There are two different procedures.
Procedure
1: Frequency Run Method
Measuring equipment required: frequency
generator, amplifier and high-definition circuit analyzer
By the help of the frequency generator, a
defined frequency range is checked. At the same time the voltage change is read
by a voltmeter. The frequency value at which the voltage value reaches its
minimum defines the resonance point of the capacitor.
Afterwards, the self-inductance is calculated
using the formula
Fres = 1/2pÖ(LC)
Procedure
2: Surge Discharge Method
A digital oscilloscope records the curve of
the discharge during a surge discharge of the capacitor. Afterwards, the
resonance frequency is identified by the numbers of intersection points over
time scale.
It has to be noted that in method 2, there is
an uncertainty factor which is the external inductance of the measuring
circuit.
Hence procedure 1 is the most reliable method
and therefore, we are recommending using this method only for the determination
of the self-inductance of capacitors.
Also we would like to mention that selection
of high quality measurement equipment , (Keithly for example) is key to the correct measurements.
As one can judge from the above, it is not
reasonable to measure the self-inductance of capacitors as part of the routine
test. This would require far too much manpower and time and cripple any
cost-efficient production. This is also the reason why the applicable IEC
standard 61071 does not list such measurement as part of the obligatory routine
test for power electronics capacitors.
For more information contact us at www.electronicon-se.com
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