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Multi-point Zero Flux Technology Principle

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Update time : 2021-09-22 10:25:59

Multi-point zero flux technology control system includes excitation module, excitation flux closed-loop control module and multi-flux closed-loop control module.
 

As shown in Fig. 1, the current to be measured Id in the circuit to be tested generates DC magnetic flux, AC magnetic flux and high frequency magnetic flux. The excitation oscillator outputs an alternating voltage signal of a preset frequency to the excitation unit to excite the excitation unit to generate excitation magnetic flux. The excitation magnetic flux detects the DC magnetic flux generated by the current Id and outputs a DC magnetic flux signal corresponding to the DC magnetic flux.
 

The excitation magnetic flux closed-loop control module detects the excitation magnetic flux generated by the excitation unit and generates an excitation compensation magnetic flux around the excitation magnetic flux. The excitation compensation magnetic flux and the excitation magnetic flux are superimposed on each other, and the superimposed excitation superimposed magnetic flux passes through the magnetic flux in the vertical plane of the magnetic induction line is zero, and the zero-flux closed-loop control of the excitation magnetic flux is realized. The multi-flux closed-loop control module detects the DC magnetic flux, the AC magnetic flux and the high-frequency magnetic flux generated by the current Id, and a multi-flux compensating magnetic flux is generated around the circuit to be tested. The multi-flux compensating magnetic flux and the magnetic flux generated by the current to be measured Id are superimposed on each other, and the magnetic flux of the superimposed multi-flux superimposed magnetic flux passes through a plane perpendicular to the magnetic induction line thereof is zero, and the zero-flux closed loop control of DC magnetic flux, AC magnetic flux and high frequency magnetic flux is realized.

 

 

Fig. 1. Control system schematic of multi-point zero flux technology


The specific implementation of the multi-point zero flux technology is shown in Fig. 2. The DC flux closed-loop control consists of a DC bias detection resistor R1, a proportional winding N6, a DC bias detection module, a proportional integral amplifier, and a power amplifier. The DC bias magnetic flux reacted by the DC bias detecting resistor R1 is compensated by proportional winding N6 to form a DC flux closed loop control, so that the DC bias magnetic flux is zero.

The excitation magnetic flux closed-loop control is composed of an excitation compensation core T3, an excitation compensation winding N3, an excitation detection winding N5, an excitation flux detection module, and a proportional integral amplifier. The winding N3 is separately wrapped around the iron core T3, and the winding N5 surrounds both iron cores T1 and T3.
 

The excitation generating system is composed of an exciting magnet core T1, an exciting winding N1, a DC bias detecting resistor R1, and an exciting oscillator. The winding N1 surrounds the core T1. The excitation generating system drives the excitation winding N1 by emitting a certain frequency square wave voltage through the excitation oscillator. The output frequency of the excitation oscillator is determined by the designer according to different application requirements. The excitation compensation winding N3 forms an excitation magnetic flux closed-loop control by compensating the induced magnetic flux on the excitation detection winding N5, so that the magnetic flux on the excitation detection winding N5 is zero. The induced magnetic flux on the excitation detecting winding N5 is generated by the sum zero of the alternating magnetic fluxes generated by the exciting windings N1, N2.
 

The magnetic flux compensated by the excitation compensation winding N3 is equal to the magnitude of the magnetic flux generated by the excitation winding N1, while the direction is opposite, that is, I1W1 = -I3W3. The entire excitation system presents zero magnetic flux. I1 and I3 are currents of the windings N1 and N3 respectively, W1 and W3 are the turns of the windings N1 and N3 respectively.
 

The AC magnetic flux closed-loop control is composed of an AC magnetic flux detecting core T4, an AC magnetic flux detecting winding N4, a proportional winding N6, AC magnetic flux detecting module, a proportional integral amplifier and a power amplifier. The winding N4 and N6 surround T1, T2, T3 and T4 four iron cores at the same time. The proportional winding N6 forms an AC magnetic flux closed loop control by compensating the induced magnetic flux on the AC magnetic flux detecting winding N4, so that the magnetic flux on the AC magnetic flux detecting winding N5 is zero.
 

The principle of zero-flux compensation by the large current flowing through the transducer is to eliminate the induced magnetic flux generated on the AC magnetic flux detecting winding N4 and the DC bias magnetic flux reflected by the DC bias detecting resistor R1 by using the power amplifier output compensating current. The magnetic potential generated by the primary winding Nd is completely balanced with the magnetic potential generated by the secondary side proportional winding N6, that is, IdWd=I6W6, and the number of ampere turns of the primary side and the secondary side is balanced.
 

The working principle of the zero-flux compensation of the internal excitation flux is to use the proportional integral amplifier output compensation current to eliminate the induced magnetic flux generated on the excitation detection winding N5, so that the sum of the magnetic potentials generated by the excitation winding N1 and the magnetic potential generated by the excitation compensation winding N3 are completely balanced, that is, I1W1 = -I3W3. The ampere-turns of the excitation coil and the excitation compensation coil are balanced.
 

When the measured DC current Id ≠ 0, the DC bias detector output correction voltage URE. URE is amplified by proportional integration, and the secondary compensation current I6 is output through the power amplifier, so that the number of ampere-turns in the core is completely balanced, that is, IdWd =I6W6. With the larger Id, the compensation current generated by the power amplifier is larger, and vice versa. In fact, due to the limited gain and zero drift of the amplifier, the magnetic potentials of the primary and secondary sides cannot be fully balanced. In order to restore the balance of the ampoule number, a system with negative feedback is needed. Here, the AC flux closed-loop control achieves this purpose. All changes in the magnetic flux will generate an induced voltage in the AC flux detecting winding N4. After PI regulation, the induced voltage is driven at the inverting input of the power amplifier, thereby changing the compensation current I6 of the power amplifier output, so that the magnetic potential generated by the primary and secondary windings is completely balanced. 
 

By measuring the DC voltage signal formed by the compensation current on the load resistor RL, the magnitude of the primary side current signal can be calculated proportionally to achieve the purpose of high precision DC current measurement.
 

Excessive excitation of the current will saturate the output power of the power amplifier if the power amplifier exceeds the output capability. If over-excitation is a transient characteristic, such as a pulse, etc., N6 will work normally. When the pure DC flux caused by over-excitation is saturated, it is sensed by the detector and signals the control system to indicate an excess of current in the DC measurement unit. If the current is over-currented for a long time, the proportional winding N6 or the load resistor RL may be burnt. When the excitation drops to the normal working range, the transducer immediately returns to normal.
 

In order to balance the ampere-turning number of the excitation system, a system with negative feedback is required. The excitation flux closed-loop control achieves this purpose, and the unbalanced magnetic flux of the exciting winding N1 generates an induced voltage in the excitation detecting winding N5. The excitation flux detector outputs a correction voltage URI, and the URI is proportionally amplified to drive the excitation compensation winding N3 and output the excitation compensation current I3, so that the magnetic potential generated by the excitation winding and the excitation compensation winding are completely balanced, that is, I1W1 = -I3W3. Therefore, the excitation portion of the transducer presents zero magnetic flux to the outside and does not cause any interference to the magnetic modulator, the proportional winding, and the detection winding.

Fig. 2. Specific schematic of multi-point zero flux technology

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