Understanding Core Magnetization In Current Transformers—Avoiding Saturation And Dangerous Output Voltage
by Gregory Mirsky, Vitesco Technologies, A Spinoff Of Continental Automotive Systems, Deer Park, Ill., How2Power Today, May 15 2020
Focus:
Used as ac current sensors and power handling devices in current-fed power supplies,
current transformers (CTs) often operate in circuits where the primary current has a
dc component. This dc adds magnetic flux to the core, leaving less headroom for the
ac magnetizing. Therefore the CT design or selection should be based on the maximum
or peak operating flux density of the core. This article derives equations for 1) the
magnetic flux density of the primary winding of a CT 2) the max value of primary
current that the CT can support without saturating and 3) an equation for the
secondary voltage that includes magnetic flux density, which can be used in
evaluating whether the dc primary current is excessively contributing to the magnetic
flux density of the core. Another issue with CTs is that secondary voltage can reach
dangerously high values under no load. This article derives an expression for the
secondary voltage in a CT under no-load.
What you’ll learn:
- How to avoid saturation in current transformers resulting from dc in the primary current
- How to determine the output voltage of a current transformer under no load when driven by a rectangular shaped primary current
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