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Name

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
Expand 2 Available Tests2 Available Tests
Collapse 3 Examples of Different Transformer Types3 Examples of Different Transformer Types
3.0 Constructing A Test Program
3.1 Using The Editor Software To Create Test Programs
3.2 Testing Line Frequency Transformer
3.3 Testing Miniature Line Frequency Bobbin Transformers
3.4 Testing Switched-Mode Power Supply Transformers
3.5 Pulse Transformer
3.6 Testing Audio Transformers
3.7 Current Transformers
3.8 DC Chokes

3.7 Current Transformers


Part Number CT1127

When current passes through the primary of a current transformer, this current induces a magnetic field in the core.

This magnetic field generates a current in the secondary, which flows through the secondary load (burden).

Voltage appears across the secondary winding proportional to both the secondary current and the value of the load resistance

.

The level of magnetic flux in the core depends on the secondary voltage, and the core should be checked at the highest working voltage that appears across the secondary.

Measuring the magnetizing current that flows into the secondary when the secondary is energized at this voltage provides confirmation that the current transformer will operate at its rated burden or load.


To determine the correct energization voltage for the secondary :-

1) Calculate secondary current at maximum rated primary current


2) Calculate rms voltage across secondary at this secondary current using maximum burden or maximum specified load resistance.

Recommended Measurements for Current Transformers

Test

Why?

Secondary Winding Resistance and
Primary Winding Resistance if wound
primary
To ensure correct wire size has been
used. Checks that transformer is properly
installed in fixture.
Magnetizing Current Checks for correct core material. Ensures
that core is operating at correct point on
B-H curve
Turns Ratio Checks that CT ratio is correct
Insulation Resistance or Hi-Pot Checks isolation primary to secondary,
particularly with wound primaries

Fixture

This current transformer is one of a range with ratios as high as 10,000:1.

The fixture was therefore constructed to incorporate a test primary consisting of a loop of 10 turns of insulated wire with a miniature 10 pin circular connector that could fit through that center of the current transformer.


With only a one turn primary, the primary voltage measured during a turns ratio test will be very small on high ratio transformers. A 10 turn primary increases this voltage and achieves much better accuracy.

Note: F is a foil wrapped around the primary for testing the insulation around the secondary of the current transformer.


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