7.1.16 C2 - Capacitance Match
The interwinding capacitance match test calculates the ratio between two capacitance measurements on two groups of windings. It is measured by applying a specified ac voltage between two separate windings and the voltage across and current flow between the two windings is measured to obtain a complex impedance. This is performed to the two groups in turn.
This test is suitable for switched mode power supply, audio and telecommunication transformers. It checks that the windings are installed in the correct positions on the bobbin.
When calculating capacitance match the tester performs 2 capacitance measurements. Firstly the tester applies an ac voltage between first group of windings to be tested, usually with all taps on each winding shorted together. It then measures the voltage between the windings, and the resulting current using harmonic analysis. Dividing the voltage by the current gives the inter-winding impedance, from which the capacitance may be calculated. This is then repeated for the second winding group. The capacitance match is the ratio of first to second winding group capacitances.
The test voltage can be in the range of 1mV to 5V at a frequency of 20Hz to 1MHz.
The table below gives the recommended test conditions for different values of average capacitance:
|Preferred test signal
|1pF → 10pF
10pF → 100pF
100pF → 1nF
1nF → 10nF
10nF → 100nF
The Test Conditions for Capacitance Match Measurement
When choosing the test conditions, the following potential problems should be
- CURRENT LEVELS
For larger capacitances, particularly at higher frequencies, the current flowing during the measurement can be very high, and similarly the measured current could also be very small for small capacitances at lower frequencies and voltages.
Where possible, you should use the recommended test signal levels in the table above to ensure that the currents which flow can be measured accurately.
- NON-LINEAR CAPACITANCE
Normally non-linearities in the stray capacitance of transformers are not a problem, and therefore capacitance is measured with as large a voltage as possible.
- EQUIVALENT CIRCUIT
As with inductance, capacitance is actually measured as a complex impedance, and therefore the result can be expressed in terms of either a series or a parallel equivalent circuit.
It was explained in section 1.5. of this chapter, that parallel and series equivalent inductance do not necessarily have the same values. The same is true for capacitance; parallel and series equivalents can also be different.
The tester uses a parallel equivalent circuit for capacitance measurements, and does not give you a choice of a series equivalent.
Generally, this will present no problems, as on the majority of transformers the difference between the two values is usually negligible, and can be ignored.