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.