Measuring Principle of Current Sensors

Measuring Principle of AC Zero Flux Method (Winding Detection Type) Current Sensor

The AC zero-flux method improves the low-frequency band characteristics of the CT method by means of a current transformer (winding detection type).

Measurement principle:

● In order to eliminate the magnetic flux Φ generated inside the core caused by the AC current flowing through the measuring conductor (primary side), a secondary current corresponding to the turns ratio of the feedback winding on the secondary side flows.

● The magnetic flux in the low-frequency band is not canceled completely, and magnetic flux remains.

The detection winding detects this incompletely canceled flux, and the secondary current flows through the AMP circuit to cancel the flux Φ.

The secondary current flows through the shunt resistor and generates a voltage across the shunt resistor.

This voltage is output in proportion to the current flowing through the measurement conductor.

Characteristics compared to other methods:

● Since the negative feedback operation eliminates the magnetic flux inside the core, it is not affected by the B-H characteristics of the core and has excellent linearity.

Low phase error at low frequencies, making it suitable for power measurement.

Low insertion impedance due to small operating flux level

● Operates as a CT in the high-frequency band, enabling broadband measurement.

Measurement is limited to AC current because the windings are detected, so DC current cannot be measured.

AC/DC Zero Flux Method Current Transformer (Hall Element Detection Type) Measurement can be started from DC current by using the CT method in conjunction with a Hall Element.

Measurement principle:

● In order to eliminate the magnetic flux Φ generated inside the core caused by the AC current flowing through the measuring conductor (primary side), a secondary current corresponding to the turns ratio of the feedback winding on the secondary side flows.

● In the low-frequency band from DC, the magnetic flux is not canceled completely, and there is a residual magnetic flux.

This incompletely canceled magnetic flux is detected by a Hall element, and a secondary current flows to cancel the magnetic flux Φ through the AMP circuit.

The secondary current flows through the shunt resistor and generates a voltage across the shunt resistor.

This voltage is output in proportion to the current flowing through the measuring conductor.

Features compared to other methods:

● Excellent linearity and high accuracy even at low levels.

● High S/N ratio is realized in a wide frequency band.

● Operates as a CT in the high-frequency band, so wide frequency can be realized.

● Detection by Hall elements enables detection of magnetism from DC, so it is possible to measure from DC current.