Understanding Core Magnetization In Current Transformers—Avoiding Saturation And Dangerous Output Voltage

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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Author & Publication:

Gregory Mirsky, Vitesco Technologies, A Spinoff Of Continental Automotive Systems, Deer Park, Ill., How2Power Today, May 15 2020

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