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The Voltech Handbook of Transformer Testing

Description
This article covers a wide range of transformer theory and Voltech's testing capability.
The Voltech Handbook of Transformer Testing
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Expand 1 Transformer Basics1 Transformer Basics
Collapse 2 Available Tests2 Available Tests
2.1 CTY - Continuity
2.2 R - Winding Resistance
2.3 RLS or RLP - Equivalent Series or Parallel R
2.4 LS_LP - Primary Inductance
2.5 LSB_LPB - Inductance With Bias Current
2.6 QL - Q factor
2.7 D - Dissipation Factor
2.8 LL - Leakage Inductance
2.9 C - Inter-winding Capacitance
2.10 TR - Turns Ratio and Phasing
2.11 TRL - Turns Ratio by Inductance
2.12 Z_ZB - Impedance_Impedance with Bias
2.13 R2 - DC Resistance Match
2.14 L2 - Inductance Match
2.15 C2 - Capacitance Match
2.16 GBAL - General Longitudinal Balance
2.17 LBAL - Longitudinal Balance
2.18 ILOS - Insertion Loss
2.19 RESP - Frequency Response
2.20 RLOS - Return Loss
2.21 ANGL - Impedance Phase Angle
2.22 PHAS - Inter-winding Phase Test
2.23 TRIM - Trimming Adjustment
2.24 OUT - Output To User Port
2.25 IR - Insulation Resistance
2.26 HPDC - DC HI-POT
2.27 HPAC - AC HI-POT
2.28 SURG - Surge Stress Test
2.29 STRW - Stress Watts
2.30 MAGI - Magnetizing Current
2.31 VOC - Open Circuit Voltage
2.32 WATX - Wattage (External Source)
2.33 STRX - Stress Watts (External Source)
2.34 MAGX - Magnetizing Current (Ext. Source)
2.35 VOCX - O.C. Voltage (External Source)
2.36 LVOC - Low Voltage Open Circuit
2.37 ILK - Leakage Current
2.38 LSBX - Inductance with External Bias (Series)
2.39 LPBX - Inductance with External Bias (Parallel)
2.40 ZBX - Impedance with External Bias
2.41 ACRT - AC HI-POT Ramp
2.42 DCAT - DC HI-POT Ramp
2.43 ACVB - AC Voltage Break Down
2.44 DCVB - DC Voltage Break Down
2.45 WATT - Wattage
2.0 Available Tests On The AT Series
Expand 3 Examples of Different Transformer Types3 Examples of Different Transformer Types

2.4 LS, LP - Primary Inductance

An ideal transformer, with the secondaries open-circuit, presents an infinite impedance to an AC voltage applied to the primary, the transformer acts as though it were an infinite inductor.
In practice the transformer presents a finite inductive impedance to the applied voltage given by: -
Inductive impedance (XL ) = 2πfL (ohms)
Where L is the inductance of the core (Henries) and f is the frequency of the applied voltage
The primary inductance is therefore a measure of the input impedance of the transformer. From this equation it can be seen that the smaller the inductance, the larger will be the current that will flow when the transformer is energized.

Measurement Conditions

To measure inductance, the tester applies an ac voltage across the selected winding; it then measures the voltage across and the current through the winding using harmonic analysis. The measured voltage is divided by the current to obtain a complex impedance and the inductance is calculated.
The test signal can have a frequency in the range 20 Hz to 3 MHz, and an amplitude from 1 mV to 5 V.
Generally, it is not necessary to measure the inductance at the normal operating conditions of the transformer, which could involve, for example, voltage levels of hundreds of volts.
This is because the B-H curve can normally be assumed to be linear in the operating region, and the inductance measured at a low level represents the inductance that will appear in use.
Also, it can usually be assumed that the inductance value does not vary significantly with frequency.
Therefore, although high frequencies are available with the tester, measurement frequencies above a few hundred kilohertz should be used with caution.
This is because the errors caused by the stray inductance and capacitance of your fixture may become much more significant at these frequencies. Compensation can be used to eliminate these errors.
The following table suggests suitable test conditions for different values of expected primary inductance:

Inductance range Preferred test signal
Frequency Voltage
100nH → 1uH
1uH → 10uH
10uH → 100uH
100uH → 1mH
1mH → 10mH
10mH → 100mH
100mH → 1H
1H → 10H
10H → 100H
100H → 1KH
1kH → 10KH
300KHz
100KHz
30KHz
10KHz
1KHz
100Hz
100Hz
50Hz
50Hz
50Hz
20Hz
10mV
30mV
50mV
100mV
100mV
100mV
300mV
1V
5V
5V
5V

The Test Conditions for Inductance Measurement

Wherever possible, this table should be used for all inductance tests. The inductance range should be chosen based on minimum value of inductance expected.

When choosing the test conditions, the following potential problems should be considered:

a) Current levels
The upper voltage limits should be chosen to give a maximum current level of about 50mA rms. for the lowest inductance expected. In some cases, this current may cause core saturation, and a lower voltage should be used. The minimum voltage level must be chosen so that the test current does not become so low that it cannot be sensibly measured. The lower voltage limits in the table above always give test currents higher than 3uA rms.

b) Self-Resonant Frequency
At lower frequencies, the capacitance of the windings can normally be ignored because its impedance is much higher than that of the inductance. However, at very high frequencies, this is not so, the capacitance dominates and inductance cannot be measured. The self-resonant frequency of the transformer is the change-over point between these two regions. Normally to get a good measurement of inductance, the test frequency should be less than 20% of the resonant frequency of the transformer. In general high values of inductance will have a high inter-turn capacitance and hence a low resonant frequency. Where there is a choice of test frequencies always use the lower value, to minimise any problems due to self-resonance.

c) Non-linear inductance
Normally inductance measurements should be used for transformers where the B-H characteristics are linear.
However, if inductance measurements are attempted for instance with line frequency transformers where the core material is non-linear even at low signal levels, the measured results can be highly dependent on the applied test signal.
This can be a problem when trying to compare measurements made on commercially available impedance bridges, or component testers, with measurements made using the AT. The test signal in such bridges is usually determined within the instrument, and is often at a fixed frequency and at a voltage level, which is not guaranteed to be constant for all value of inductance.
Usually, if the actual test conditions of the bridge can be determined, and the tester is then programmed to deliver the same test conditions across the inductance the results will then agree. (See also the comments below on differences caused by the choice of equivalent circuit)

d) Equivalent circuit
Inductance is always measured as part of a complex impedance, the result being expressed in terms of either a series or parallel equivalent circuit. Note that, for any given winding, the inductance values for the two circuits are not necessarily the same. This should be born in mind when specifying the test limits.
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