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Current transformer problems and how the sensor works

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Current transformer problems and how the sensor works

Date of release:2018-06-08 Author:Kangpusen Technology Co., Ltd. Click:

I. Review the magnetic saturation problem of electromagnetic current transformer


1, magnetic saturation phenomenon


The so-called magnetic saturation means that after the magnetic flux density in the electromagnetic current transformer core is greater than the saturation magnetic flux density, the magnetic flux density is no longer increased by the increase of the primary current.


2, the cause of magnetic saturation


The magnetic flux density is an alternating variable. When magnetic saturation does not occur, the maximum value of the magnetic flux density of the transformer core is: Bm=E2/(4.44*f*N2*S)


In the formula, E2 is the secondary winding induced electromotive force, which is approximately equal to the secondary winding output voltage. N2 is the number of turns of the secondary winding, and S is the cross-sectional area of the core. For fixed transformers, N2 and S are constant values.


Therefore, the core flux density is proportional to the secondary voltage and inversely proportional to the current frequency.


The secondary voltage is determined by the secondary current and the secondary load. It can be seen that the magnetic saturation of the electromagnetic current transformer has the following reasons:


A, the primary current is too large, greater than the rated current;


B, the secondary load is too large, greater than the rated secondary load;


C, the current frequency is too low, lower than the rated frequency.


3. Magnetic saturation hazard


After the current transformer is magnetically saturated, the primary current is no longer proportional to the secondary current, and the current transformer cannot perform normal measurement or protection, causing a safety accident. In addition, in the magnetic saturation state, the magnetic flux density in the iron core is large, the eddy current loss and the hysteresis loss are large, the iron core is heated, and the transformer is easily damaged.


Second, the Hall current sensor works


The Hall current sensor is divided into an open-loop Hall current sensor and a closed-loop Hall current sensor according to the working principle.


1. Working principle of open-loop Hall current sensor


Open-loop Hall current sensors are also called: direct-release Hall current sensors, direct-check Hall current sensors, and so on.


As shown in Fig. 1, the open-loop Hall current sensor is composed of a magnetic core, a Hall element, and an amplifying circuit. The core has an open air gap and the Hall element is placed in the air gap. When the primary conductor flows a current, a magnetic field whose magnetic field strength is proportional to the magnitude of the current is generated around the conductor, the magnetic core concentrates the magnetic lines of force to the air gap, and the Hall element outputs a voltage signal proportional to the magnetic induction intensity at the air gap, and amplifies The circuit amplifies and outputs the signal. This type of sensor usually outputs a voltage signal of about ±10V. Some sensors also convert to a current signal output in order to enhance electromagnetic compatibility.


Current Transformer


Figure 1 Open-loop Hall current sensor works


2, closed-loop Hall current sensor working principle


Closed-loop Hall current sensors are also called: zero-flux Hall current sensors, zero-flux transformers, magnetic balance Hall current sensors, and so on.


As shown in FIG. 2, the closed-loop Hall current sensor includes a magnetic core, a Hall element, an amplifying circuit, and a secondary side compensation winding. Compared with the open-loop Hall current sensor, the closed-loop Hall current sensor has more secondary compensation windings, which is the secondary compensation winding, which greatly improves the performance of the closed-loop Hall current sensor.


Current Transformer


Figure 2 Closed-loop Hall current sensor works


The amplifying circuit receives the output of the Hall element and amplifies the current signal to the secondary side compensation winding. The magnetic field generated by the secondary side compensation winding in the magnetic core is equal in size and opposite in direction to the magnetic field generated by the primary current. The primary magnetic field forms a negative feedback closed-loop control circuit.


If the secondary current is too small, the generated magnetic field is not enough to cancel the primary magnetic field, and the amplifying circuit will output a larger current. Conversely, the output current of the amplifying circuit is reduced, thereby maintaining the magnetic field balance at the air gap.


If the primary current changes, the magnetic field balance at the air gap is destroyed, and the negative feedback closed-loop control circuit also adjusts the secondary output circuit to rebalance the magnetic field.


Macroscopically, zero flux is maintained at the air gap to maintain magnetic balance, which is also the reason for the name of zero flux transformer and magnetic balance Hall current sensor.


3. The main difference between closed-loop Hall current sensor and open-loop Hall current sensor


A, the difference in bandwidth


Microscopically, the magnetic field at the air gap always changes around the zero flux. Since the magnetic field changes very little, the amplitude of the change is small, and the frequency of change can be faster. Therefore, the closed-loop Hall current sensor has a fast response time. The actual closed-loop Hall current sensor bandwidth can usually reach more than 100kHz. The open-loop Hall current sensor usually has a narrow bandwidth. For example, LEM's HAZ series open-loop Hall current sensor has a bandwidth of about 3 kHz.


B, the difference in precision


The output of the secondary side of the open-loop Hall current sensor is proportional to the magnetic induction at the air gap of the core, and the core is made of a highly magnetically permeable material. The nonlinearity and hysteresis are inherent characteristics of all highly magnetic materials. Therefore, the open-loop Hall current sensor generally has a linearity angle difference, and the primary side signal will have different secondary output during the rising and falling processes. Open-loop Hall current sensors are typically less accurate than 1%.


Because the closed-loop Hall current sensor works in the zero-flux state, the nonlinearity and hysteresis effect of the core do not affect the output, and good linearity and high precision can be obtained. Closed-loop Hall current sensors typically have an accuracy of 0.2%.


Second, the Hall current sensor magnetic saturation problem


Many Hall current sensor manufacturers also promote non-magnetic saturation as an important advantage of Hall current sensors in their technical data. The Hall current sensor does not suffer from magnetic saturation. It is one of the main advantages that Hall current sensors have been widely recognized since their application.


Is this the case?


In fact, the Hall current sensor contains a non-linear core, which has determined that the Hall current sensor will be magnetically saturated under certain conditions!


1. Magnetic saturation problem of open-loop Hall current sensor


The following figure shows a typical magnetization curve for all highly magnetically permeable materials:


Current Transformer


Figure 3 Magnetization curve of the Hall current sensor core


In the figure, Oa' is a starting nonlinear segment, a'a'' is a linear segment, and a''a is a saturated region. In order to obtain better measurement results, whether it is an open-loop Hall current sensor or an electromagnetic transformer, a segment with better linearity in the magnetization curve is used as a working interval. In other words, magnetic saturation occurs as long as the magnetic induction exceeds a certain range of the linear region.


Compared with electromagnetic transformers, open-loop Hall current sensors have only one magnetic saturation cause, that is, the primary current is large enough.


It does not cause magnetic saturation due to low current frequency, which is the advantage of the Hall current sensor and the magnetic saturation of the open-loop Hall current sensor.


In contrast, electromagnetic transformers also have the advantage that the secondary load is small enough that even if there is more overload, magnetic saturation will not occur.


2. Magnetic saturation problem of closed-loop Hall current sensor


The open-loop Hall current sensor magnetic saturation problem is simpler. In contrast, the closed-loop Hall current sensor magnetic saturation problem seems incomprehensible because the closed-loop Hall current sensor works normally, the magnetic flux in the core is zero. Under zero flux, it will not naturally saturate.


However, this can only be under normal working conditions!


In fact, even magnetic saturation problems of electromagnetic current transformers or open-loop Hall current sensors occur under abnormal operating conditions such as overload, low frequency, and excessive load. Under normal working conditions, they will not occur. Magnetic saturation!


From the working principle of the closed-loop Hall current sensor, the zero flux is established on the premise that the magnetic field generated by the secondary compensation winding can cancel the magnetic field generated by the primary conductor. So, can the zero-flux be maintained when the closed-loop Hall current sensor is under any circumstances?


Obviously not!


A. When the sensor is not powered, the secondary compensation winding does not generate current. At this time, the closed-loop Hall current sensor is equivalent to an open-loop Hall current sensor, and magnetic saturation occurs as long as the primary current is large enough.


B. Normal power supply, the primary current is too large. This is because the current that can be generated by the secondary compensation winding is limited. When the primary side current generates a larger magnetic field than the secondary side compensation winding can generate, the magnetic balance is broken, and the magnetic core passes through the magnetic field. When the current continues to increase, the magnetic field in the core also increases, and when the primary current is large enough, the closed-loop Hall current sensor enters the magnetic saturation state!


Compared with electromagnetic current transformers and open-loop Hall current sensors, the magnetic saturation phenomenon of closed-loop Hall current sensors is not easy to occur, but it does not mean that it will not occur. Magnetic saturation occurs due to improper use or long-term overload.


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Key word:CurrentTransformer

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