1 Introduction to Testing Transformers and Inductors with Constant
It is a requirement of standards such as IEC60404-2, that
measurements of core loss are made under constant current test conditions.
Ideally, current transformers will also be tested with
constant current drive.
Unlike most general-purpose transformer test equipment
the AT Series Tester has a built in, fully programmable constant current
The AT Series transformer testers from Voltech contain a
wide variety of programmable signal sources.
These are used to test multiple parameters at high speed
on parts ranging from surface-mounting ferrite inductors to steel laminate
transformers of a few kVA.
By following the guidance given in these notes, customers
may also configure the AT tester to test cores, inductors and transformers
under constant ac current conditions.
2 Features of constant current AC drive on the AT5600+AT3600
- Constant current is fully programmable for amplitude and frequency.
- Simultaneous measurement of rms or rectified mean (rms scaled)
- Current carefully ramped up and down under software control.
- Can use external ac source to extend the VA range.
- Integrates into normal test sequence with all other tests available on
an AT3600 (including Hi Pot).
3 Constant current testing with up to 2A drive
2A is the maximum available from the AT3600 or AT5600's
Greater than 2A can be delivered for testing by using an
external source (as described in a later section) or simply by using
multiple turns of the drive signal through the transformer or core under
Programming the current is simple using the Voltech AT
Editor and limits may be set as RMS or mean-scaled RMS.
Once downloaded to the AT as part of a program, the test
will be performed automatically with the constant current ramped up and
down smoothly and quickly.
Comprehensive measurement results may be stored and
analyzed later if required
4 Constant current testing with greater than 2A drive
The simplest way of testing with an apparent drive of
greater than 2 amps is simply to use more than one turn of the drive
signal through the core or transformer under test.
For transformers or cores that require greater drive, or
when the resulting drive voltage is too small for the AT to stabilize
properly, then the AC Interface accessory may be used to provide up to 10A
of constant current AC drive.
4.1 Equipment Required
- AC Interface.
- VOCX test. (MAGX is useful during program development.)
- A current-sensing resistor.
- A step-down power transformer.
4.2 Basic Principle
Constant-current AC testing uses the normal VOCX test to control the
current and make the secondary voltage measurement.
- The current to be controlled, I, is converted to a voltage, V, by
means of a current-sensing resistor, R.
- The voltage, V, is connected to the sense terminals of the AT3600,
which would normally be used to measure the energizing voltage.
- In the PC Editor, the voltage, V, is used as the energized voltage.
- When VOCX is performed, the AT3600 will trim the source, such that
the desired current is provided, I = V / R.
- The transformer secondary voltage is measured in the normal way as
the result of the VOCX test.
- The AT3600 adjusts the source voltage to maintain a constant ac
current through resistor, R.
4.3 Connection Diagram
4.4 Current-Sense Resistor
The resistor is used to convert the current flowing the in part into a
voltage for the AT3600 to measure.
- The voltage produced (V = I x R) should be in the range 10mV to 1V.
- To minimize waveform distortion, the resistor should be less or equal
to 1 ohm.
- Check the power consumed (I2R) and choose a suitably rated
4.5 Step-down Transformer
The step-down transformer is used to increase the current capability of
the AT3600, and to better match the voltage across the part to the
characteristics of the AT3600 source.
- Ratio should provide required current with good margin.
- VA rating must be greater than VA required to energize the part and
the sense resistor.
- An external ac source may be used to increase the VA rating of the
4.6 Worked Example
A laminate core is to be tested in accordance with
Under a constant ac current of up to 3A, 1V at 50Hz in
the primary test winding, the secondary voltage is measured and limits are
applied in terms of the rectified mean voltage (rms scaled).
To produce 1V at 3A,
R = 1V / 3A
R = 0.333 Ohm. Choose 0.5 Ohm standard value.
V = 3A x 0.5 Ohm
V = 1.5V
W = I x I x R
W = 4.50 Choose 5W standard part.
Resistor is 0.5 Ohm, 5W. Voltage produced will be 1.5V.
4.6.2 Step-down Transformer
Look for a stock transformer with, say, a 5A secondary
current rating. A stock transformer will usually have a 110 and/or 230V
VA rating is that of the part plus the current sense resistor.
VA = 3A x (1+1.5)V = 7.5VA
Choose a 7.5VA (or greater) transformer with a 5A low-voltage secondary.
Toroidal transformers are ideal, since they will have low losses.
Primary: 230V Secondary: 2 x 6V @ 2.5A.
The secondaries are connected in parallel to provide a
6V, 5A output.
The step-down transformer will supply the part under test
and the current-sense resistor in series. The step-down transformer's
secondary voltage, Vs, being:
Vs = 1.5V +1V = 2.5V
The AT3600 must supply 2.5 x 230:6 to the primary of the transformer.
2.5 x 230 / 6 = 95.8V
The AT3600 can supply 270V, so 95.8V is well within its capability.
It is convenient to use the MAGX test at first, since
this will confirm that the desired current is being generated.
In the previous connection diagram, the return or 'low'
end of the sense resistor has been connected to node 5, and the higher
voltage end of the resistor connected to node 3.
For optimum accuracy, it is important that the physical 'low' connection
is also designated as the 'low' terminal in the program.
4.6.5 AC INTERFACE Set Up
Next, you must set up the External Source in the Editor.
The source type is 'AT Output Transformer', and you are required to enter
the nominal turns ratio of the transformer.
Remember that the AT3600 will trim the output voltage by comparing the
desired voltage (from the program) with that measured across the
programmed sense terminals.
The ratio is used to establish a target for voltage ramp-up, when the
tester starts the test. Using the correct ratio will optimize test speed.
In this case, the ratio is modified because, when the
desired 1.5V is achieved across the resistor, the step-down transformer
must also supply the 1V across the part under test. For 1.5V programmed,
the step-down transformer delivers 2.5V.
Ratio = 230:6 x 2.5:1.5
That is, for every 1V programmed, the AT3600 must supply
63.9V to the primary of the step-down transformer.
4.6.6 MAGX Test
In the MAGX test, simply enter the required voltage, V,
across the resistor to program the constant current, I. In this case,
With an AT3600 connected to the PC Editor and the circuit
assembled as shown previously, you may now press the 'Measure' button to
run a MAGX measurement.
If the current measured is as desired, then all is well.
If not, simply adjust the program voltage to suit. If the tester should
fail to return a result, check the wiring and settings carefully.
4.6.7 VOCX Test
Now secondary connections may be made for VOCX tests.
Note that VOCX has the option to measure rms voltage or "mean sense rms
For the mean sense option, the tester measures the
rectified mean voltage and multiplies that by 1.11.
This option is offered as a means of comparing the measurements with those
made by mean sensing analog meters and calculated parameters that require
5 AC Constant Current Testing Conclusions
The versatile AT3600 transformer tester already provides
an unrivalled range of tests for checking the construction and performance
of a wide range of coils and transformers.
By following the above guidelines, users can also
seamlessly integrate the testing of cores and transformers under constant
ac current conditions into the AT3600 environment, providing high-speed
PASS / FAIL testing and accurate, detailed test results for analysis.
Adding Other Tests
To include other tests such as R, simply use extra nodes
to connect directly across the windings in the normal way.
You will have to add and extra 'dummy' winding in the
schematic and use those terminal names in the program.
Adding extra tests in this way does not interfere with
the operation of the constant current drive explained here.
Extending the Constant Current Range
To test very high VA transformers, the drive may be
extended by using an external ac source. 500VA is available from the AT's