Current Probes, Sensors | Non-contact voltage probe,CAN sensors Principle of Current Sensors

This section describes the principles behind current sensor operation. You should choose a current sensor to suit the application in which you plan to use it, based on an understanding of the differences among the various methods of measuring currents, including the Hall element method, the Rogowski method, and the zero-flux method.

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01. Current sensor measurement principles

Classification of Current Sensors by Operating Principle


 

Measurement principles: CT current sensors

 Characteristics
•    No power source required (for current sensing function)
•    Affordable
•    Dedicated to AC (DC not supported)
•    Popular with clamp meters used for low energy management in buildings

Measurement Principle
•    A magnetic flux (Φ) is induced in the magnetic core due to the flow of the alternating current (AC) being measured. A secondary magnetic flux (Φ’) is induced in the secondary coil (N) as a reaction to this primary flux in an effort to cancel it out. A secondary AC current is also induced in proportion to the secondary magnetic flux (Φ’).
•    This secondary current flows through the shunt resistor and voltage difference occurs between both sides of the resistor. This voltage is proportional to the current flowing through the measured conductor.

Hioki CT Method (AC only) Sensors

CT7126, CT7131, CT7136, CT7116, 9694, 9660, 9661, 9669, 9675, 9657-10, 9661-01, 9695-02, 9695-03, 9010-50, 9132-50, 9018-50, 9650, 9651, 9298, 9291

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Measurement principles: Hall element current sensors

 Characteristics
•    Measure DC to AC (<10 kHz)
•    Affordable
•    Lacks precision due to the linearity of the Hall element and the B-H characteristics of the magnetic core
•    Not suited for long-term measurement due to drifting caused by humidity and change over time which is a characteristic caused by the Hall element

Measurement Principle
•    When the measured current (principle current) passes through the magnetic core’s aperture, a magnetic flux is induced in the core. As this magnetic flux flows through the Hall element, a voltage generates in proportion to the magnetic flux. This voltage induction is known as the Hall effect.
•    Since the voltage induced by the Hall effect is small, it is boosted with an amplifier before being output.
•    The output voltage which is proportional to the measured current allows for current measurement.    

Hioki Hall Element (AC/DC) Sensors
CT7631, CT7636, CT7642, CT7731, CT7736, CT7742

*The above products feature improved drift and precision.
*See individual product pages for more detailed specifications. 

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Measurement principle: Rogowski coil current sensors

 Characteristics
•    Large currents can be measured because the coreless structure eliminates any magnetic saturation
•    Magnetic loss allows for:

•    No heat generation
•    No saturation
•    No hysteresis
•    Flexible and slim due to the air-core coil
•    Small insertion impedance
•    Affordable
•    Dedicated to AC (DC not supported)
•    Not recommended for high precision measurement because of high susceptibility to noise

Measurement Principle
•    A voltage is induced in the air-core coil by interlinking the magnetic field produced by the AC current flowing in the conductor being measured (the primary side of the circuit) and the air-core coil.
•    This induced voltage is then output as the time derivative (di/dt) of the measured current, and an output signal proportional to the constant current is obtained by passing it through an integrator.

Hioki Rogowski Coil Method (AC only) Sensors
CT7046, CT7045, CT7044, CT9667-01, CT9667-02, CT9667-03

*The above products feature improved noise resistance.
*See individual product pages for more detailed specifications. 

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Measurement principles: AC zero-flux (winding detection type) current sensors

 Characteristics
•    Since sensor operation depends on canceling out the magnetic flux in the magnetic core, AC zero-flux sensors have excellent linearity and are not affected by the magnetic core’s B-H magnetic characteristics.
•    Well-suited for use in high accuracy power measurement since they are characterized by small phase error, even at low frequencies.
•    Since AC zero-flux sensors operate using the CT of the secondary feedback winding in the high-frequency region, and utilize an amplifier for the low-frequency region, a broad frequency bandwidth is supported.
•    Dedicated to AC (DC not supported)

Measurement Principle
•    In the zero-flux method, in order to cancel out the magnetic flux (Φ) produced inside the magnetic core by the AC current flowing in the conductor being measured, a secondary current flows to the secondary side of the feedback winding inducing a secondary magnetic flux (Φ’).
•    However, in the low-frequency regions, the magnetic flux (Φ-Φ’) cannot be cancelled and thus remains in the magnetic circuit.
•    The detecting coil detects this remaining magnetic flux (Φ-Φ’). Then, a secondary feedback current is added through an amplifier circuit so as to cancel out the magnetic flux (Φ-Φ’) in the low Hz regions.
•    This secondary current flows to the shunt resistor, producing a voltage across its terminals.
•    The voltage is identified as proportional to the current flowing in the conductor being measured, giving us the true current level.

Hioki Zero Flux Method (AC only) Sensors

9272-10, 9272-05

*See individual product pages for more detailed specifications. 

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Measurement principles: AC/DC zero-flux (Hall element detection type) current sensors

 Characteristics
•    Operates by canceling out the magnetic flux in the magnetic core, giving it excellent linearity unaffected by the magnetic core’s B-H magnetic characteristics.
•    Since the probes operate using the CT of the secondary feedback winding in the high-frequency region, and utilize an amplifier for the low-frequency region, a broad frequency bandwidth is supported with a high S/N.
•    Measure DC to AC
•    Due to lack of excitation current noise, overall noise is extremely low.

Measurement Principle
•    In the zero-flux method, in order to cancel out the magnetic flux (Φ) produced inside the magnetic core by the AC current flowing in the conductor being measured, a secondary current flows to the secondary side of the feedback winding inducing a secondary magnetic flux (Φ’).
•    However, in the low-frequency regions resulting from DC currents, the magnetic flux (Φ-Φ’) cannot be cancelled and thus remains in the circuit.
•    The Hall element detects this remaining magnetic flux (Φ-Φ’). Then, a secondary feedback current is induced through an amplifier circuit so as to cancel out the magnetic flux (Φ-Φ’) in the low Hz regions.
•    This secondary current flows to the shunt resistor, producing a voltage across its terminals.
•    The voltage is identified as proportional to the current flowing in the conductor being measured, giving us the true current level.

Hioki Zero Flux Method Hall Element Detection (AC/DC) Sensors
3273-50, 3274, 3275, 3276, CT6700, CT6701, CT6710, CT6711

*See individual product pages for more detailed specifications. 

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Measurement principles: AC/DC zero-flux (fluxgate detection type) current sensors

 Characteristics
•    Operates by canceling out the magnetic flux in the magnetic core, giving it excellent linearity unaffected by the magnetic core’s B-H magnetic characteristics.
•    Since the fluxgate exhibits an extremely small offset drift across a broad temperature range thanks to its operating principle, it can achieve exceptionally accurate and stable measurements, making this type of current sensor ideal for pairing with high accuracy power meters for uncompromising precision
•    Since the frequency and harmonics of the exciting current itself become sources of noise, current sensors using a fluxgate exhibit slightly more noise than one using a Hall element.
•    Since the probes operate using the CT of the secondary feedback winding in the high-frequency region, and utilize an amplifier for the DC and low-frequency region, a broad frequency bandwidth is supported with a high S/N ratio.

Measurement Principle
•    In the zero-flux method, in order to cancel out the magnetic flux (Φ) produced inside the magnetic core by the AC current flowing in the conductor being measured, a secondary current flows to the secondary side of the feedback winding inducing a secondary magnetic flux (Φ’).
•    However, in the low-frequency regions resulting from DC currents, the magnetic flux (Φ-Φ’) cannot be cancelled and thus remains in the circuit.
•    The fluxgate detects this remaining magnetic flux (Φ-Φ’). Then, a secondary feedback current is induced through an amplifier circuit so as to cancel out the magnetic flux (Φ-Φ’) in the low Hz regions.
•    This secondary current flows to the shunt resistor, producing a voltage across its terminals.  The voltage is identified as proportional to the current flowing in the conductor being measured, giving us the true current level.

Hioki Zero Flux Method Fluxgate Detection (AC/DC) Sensors
3273-50, 3274, 3275, 3276, CT6700, CT6701, CT6710, CT6711, CT6841, CT6843, CT6844, CT6845, CT6846, CT6862, CT6863, CT6865, 9709, CT6904, CT6875, CT6876, CT6877

*See individual product pages for more detailed specifications. 

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What is the Zero Flux Method?

 ※Explanation below refers to detection when a Hall element is used

①A magnetic flux (Φ) is produced inside the magnetic core by the current flowing in the conductor being measured.

②A secondary current flows to the secondary side of the feedback winding in order to cancel out the magnetic flux produced inside the magnetic core. (The magnetic flux of the feedback winding is marked as Φ’)

③However, in the low-frequency regions resulting from DC currents, the magnetic flux cannot be cancelled and thus remains in the magnetic circuit. (The remaining magnetic flux in the magnetic core comes out to Φ-Φ’)

④The Hall element detects this remaining magnetic flux(Φ-Φ’). Then, a secondary feedback current is added through an amplifier circuit so as to cancel out the magnetic flux (Φ-Φ’).

⑤The secondary current then flows through the coil and into the shunt resistor (r). This current is a sum of the current from the CT effect (current generated by the coil) and the amplifier (feedback current from Hall element detection). This produces a voltage across its terminals. The output voltage is proportional to the measured current.




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