Voltech AT5600 User Manual page 114

C H A P T E R 7 – T E S T S
Measurement Conditions
To measure capacitance, the tester applies an AC voltage between the 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.
The test voltage can be in the range of 1mV to 5V at a frequency of 20Hz to
3MHz.
The table below gives the recommended test conditions for different values of
capacitance:
Capacitance range
1 pF
10 pF
100 pF
1 nF
10 nF
When choosing the test conditions, the following potential problems should be
considered:
Current levels
For larger capacitance, particularly at higher frequencies, the current flowing
during the measurement can be extremely high, and similarly the measured
current could also be exceedingly small for small capacitance 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-linearity in the stray capacitance of transformers is not a problem,
and therefore capacitance is measured with as large a voltage as possible.
AT5600 User Manual 98-119 issue 30
A N D T E S T C O N D I T I O N S
10 pF
→
100 pF
→
1 nF
→
10 nF
→
100 nF
→
The Test Conditions for Capacitance Measurement
Preferred test signal
Frequency Voltage
100 KHz
100 KHz
10 KHz
1 KHz
100 Hz
5 V
5 V
5 V
5 V
5 V
Page 114