1.0 Testing Line Frequency Transformers
Transformers appear in almost every electrical and electronic product
that the world produces providing the world with an enormous need for
transformers.
Testing transformers and wound components is essential before final
assembly into product. This filters out failures ahead of time, avoids
costly re working, reduces manufacturing costs and improves overall
reliability.
Transformer testing thus requires:
Fast effective quality controlled manufacturing methods.
100% testing securing zero rejects sent to the customer
Laminate transformers are mostly used as line frequency, low frequency
and low/high voltage step-up, step-down transformers. Two coils are wound
over a core such that they are magnetically coupled. The two coils are
known as the primary and the secondary.
The core material tends to be constructed from thin sheets of a soft
magnetic material (approx. 0.35mm thick), usually made of 4% silicon
steel, called laminations, these are insulated from each other by varnish.
These thin sheets reduce eddy currents by increasing the resistance to the
flow of such currents. Eddy currents are one of the elements associated
with overall core losses.
Core loss is the sum of hysteresis and eddy current loss in a magnetic
core. Hysteresis is the energy used up by changing the magnetic state of
the core during each cycle and eddy currents are currents induced in the
core by time varying fluxes.
The core is partially assembled prior to the windings being inserted and
once inserted the remaining laminate sheets are then interleaved to avoid
all of the joints coming into one place, the joints are then staggered
similar to laying bricks.
Laminate transformers are used in most low frequency applications usually
between 50Hz and 400Hz. The primary tends to have a high inductance this
allows low frequency use with minimal core losses. Laminate transformers
provide the following: -
- High voltage step-up.
- Low voltage step-down.
- High current output.
- Isolation.
For the purpose of this document we will concentrate on a step-down
laminate transformers. By designing the number of turns in the primary and
secondary windings, any desired step-up or step-down transformer can be
realized.
The coupling between the primary and secondary must be tight in a power
transformer in order to reduce the leakage reactance, otherwise the drop
in reactance will be considerable and will vary with secondary voltage and
current.
Therefore laminate transformers are wound with concentric windings (the
primary and secondary are wound with half the turns onto the core limb,
one o ver the other (to give a close coupling) with intervening
insulation.
Voltech transformer testers combine virtually all tests into one box
resulting in fast test time avoiding re configuration for each test.
2.0 Critical Transformer Tests for Line Frequency Transformers
Test Parameter
|
Critical for
|
Tester essentials
|
Magnetizing current (MAGI) |
Check transformer has been assembled properly, with
the
appropriate number of turns, the right grade of magnetic
material for the core, and the correct air gap if required. |
Check primary turns and correct
core material properly assembled
|
Resistance (R) |
Check for the correct wire and
good solder termination |
DCR is the direct current (DC) resistance
offered by an inductor due to the resistance of
the winding. Expressed in ohms or milliohms
maximum.
 |
Wattage (WATT) |
Core loss measurement to
confirm that the transformer
has been assembled properly |
Measured power is the power dissipated by
eddy current and hysteresis effects in the core
and is known as the core loss
|
Hi-pot (HPAC) |
Ensures that the windings are positioned
correctly with the correct materials to provide
the required level of safety isolation. |
Measures and controls the applied voltage
throughout the complete duration of the test.
The AT3600 applies a voltage between two
groups of windings (or core) with the windings
in each group being shorted together.
|
SURGE (SURG) |
Check shorted turns. Ensures
that the insulation material
around the copper wire
(usually lacquer) has not been
damaged during manufacture |
A high energy impulse is discharged into a
winding. The transformer is characterized
by the area under the waveform, measured in
voltseconds.
|
Insulation Resistance (IR) |
check the integrity of the
insulation between separate
windings, or between a
windings, or between a winding
and a screen. |
Tester applies a dc voltage between
two groups of windings with the windings
in each group being shorted together.
|
MAGX, VOCX, WATX,
STRX |
Extend test range with AC
interface |

|
3.0 Transformer Basics
Primary turns, Np
An alternating voltage, Vin applied to the primary creates an alternating
current Iin in the primary winding.
The current produces an alternating magnetic flux in the core.
The alternating magnetic flux generates a voltage, Vout, in the secondary
For sine-waves, the flux density, B = Vin / ( 4.44 N A f) where
N = Number of turns
A is the cross sectional area of the core
f is frequency.
Since for a given transformer, B, A and f are constant: -
Transformers
Step up or step down ac voltage
Step up or step down ac current
Because there is no electrical connection between the primary and
secondary windings they provide isolation from one electrical circuit to
another.
It is these unique properties of transformers that make them so widely
used in all kinds of electrical / electronic equipment.
4.0 Transformer cores
Core power losses comprise, the hysteresis losses from magnetising and
de-magnetising the core through the BH loop.
PLUS Eddy Current Losses: -
Cross Section of: Ferrite, Laminated, Solid Core
In a solid core, current can circulate inside the core material
generating I2R (resistive) losses.
Iron cores are usually laminated to restrict the current path and reduce
this effect.
Ferrite cores have high resistance and low eddy current losses.
5.0 Transformer Equivalent Circuit

An Ideal Transformer has: -
- No losses.
- Perfect coupling between windings.
- Infinite open circuit impedance (No load current = 0 ).
- Infinite isolation between windings.
In reality, practical transformers show characteristics that differ from
those of an ideal transformer. Many of these characteristics can be
represented by a transformer equivalent circuit.
�Real Transformer Equivalent Circuit
Transformer equivalent circuit
Ls and Rs are used to model the effect of core losses.
R1, R2, R3 are the resistances of the windings.
Ll is the leakage inductance.
C1, C2 and C3 are the inter-winding capacitances
7.0 Essential Capability
The Voltech AT testers have the built-in capability described below.
Capability:
|
AT5600 + AT3600
|
ATi
|
20 way switching matrix |
Yes |
Yes |
PC test editor and results server |
Yes |
Yes |
Quick-change Fixture System |
Yes |
Yes |
Test fixture system |
Yes |
Yes |
Small signal tests (e.g. inductance, capacitance, turns ratio) |
Yes |
Yes |
Telecomms. tests (e.g. return loss, longitudinal balance) |
Yes |
Yes |
Insulation resistance |
7000V |
500 |
Hi-pot (AC) |
5000V |
NO |
Hi-pot (DC) |
7000V |
NO |
Magnetizing current and open circuit voltage |
270V |
NO |
Watts, Stress Watts |
25W |
NO |
Leakage Current |
2A |
NO |
To make use of this capability, the testers may be fitted with a number
of different tests such as inductance, ac resistance, turns ratio, watts
or ac hipot.
Tests are sold in packages such as Standard or Gold or may be purchased
individually and fitted by the user via firmware upgrade.
8.0 Extended Capability
External AC Power (AT5600 + AT3600) - Flexible power source for larger
transformers.
The ATs programmable internal ac source may be used to provide up to 270V
at 2A RMS from 20Hz to 1500Hz. This supply is used for measuring
magnetizing current, watts and open-circuit voltages on iron laminate
transformers.
The tests are usually made with the transformer off load or open circuit
such that transformers rated at 2kVA or more may be tested.
This internal ac source has several advantages, perhaps the most
important being the ability to ramp up the voltage and current under real
time software control to minimize inrush current and test time.
The Voltech AC Interface allows external ac sources (including simple
step-up or step-down transformers) to provide extended ac power seamlessly
within the AT3600 test environment.
With the AC Interface, the AT3600 capability may be extended up to 600V
at 10A RMS
External AC Sources that may be integrated into the AT3600 test
environment include:
- Simple step up transformers (providing up to 600V @ 0.8A)
- Simple step-down transformers (providing up to 10A @ 20V)
- Fully programmable external ac sources (providing up to 600V @ 10A).
Tests for the AT3600 and AC Interface
MAGX |
Magnetizing Current (External Source) |
50mA to 10A |
5V to 600V |
20Hz to 5kH z |
0.1% |
VOCX |
O/C Voltage (External Source) |
100μV to 650V |
1V to 600V |
20Hz to 5kH z |
0.1% |
WATX |
Wattage (External Source) |
1mW to 6kW |
5V to 600V |
20Hz to 5kH z |
0.3% |
9.0 External DC Bias - Real Saturation Test Conditions for Power
Transformers and Chokes
Power transformers and chokes that carry a high dc current are common in
power supplies and inverters. Testing these parts at their rated dc
current provides complete confidence that the parts have been correctly
wound, assembled and terminated.
The Voltech DC1000 25A dc current supply will seamlessly integrate into
the AT3600 or ATi test environment to provide up to 250A (10x DC1000 in
parallel) of smooth, programmable dc bias current with minimal effect on
the ac inductance measurement.
- 25A programmable dc current
- 250A with 10x DC1000
- Works seamlessly with AT3600 or ATi
- Unique �electronic inductor� design minimizes effect on the ac
inductance measurement
- Can be used on almost any LCR meter. Alternative for Agilent, Wayne
Kerr and Chroma types.

Tests for the DC1000 and AT3600/ AT5600 / ATi
LSBX |
Inductance with External Bias (Series) |
1nH to 1MH |
1mV to 5V |
20Hz to 3MHz |
0.5% |
LPBX |
Inductance with External Bias (Parallel) |
1nH to 1MH |
1mV to 5V |
20Hz to 3MHz |
0.5% |
ZBX |
Impedance with external bias |
1m Ω to 1MΩ |
1mV to 5V |
20Hz to 3MHz |
0.2% |