Problems related to magnetic saturation of electromagnetic current transformer

 

The Hall current sensor is based on the magnetic balance Hall principle. According to the Hall effect principle, a current Ic is passed through the control current end of the Hall element, and a magnetic field with a magnetic field strength of B is applied in the normal direction of the Hall element plane. Then, an electric potential VH will be generated perpendicular to the current and magnetic field direction (i.e., between the Hall output ends). It is called the Hall potential, and its magnitude is proportional to the product of the control current Ic and the magnetic field strength B. That is, there is a formula: K is the Hall coefficient, which is determined by the material of the Hall element; Ic is the control current; B is the magnetic field strength; VH is the Hall potential.

 

 

Principle of Hall Current Sensor

 

The Hall device is a magnetoelectric conversion device made of semiconductor materials. If a control current IC is passed through the input end, when a magnetic field B passes through the magnetic sensing surface of the device, a Hall potential VH appears at the output end. The magnitude of the Hall potential VH is proportional to the product of the control current IC and the magnetic flux density B, that is: VH=KHICBsinΘ. The Hall current sensor is made according to the Hall effect principle and applies Ampere’s law, that is, a magnetic field proportional to the current is generated around the current-carrying conductor, and the Hall device is used to measure this magnetic field. Therefore, non-contact measurement of current becomes possible. The magnitude of the current in the current-carrying conductor is indirectly measured by measuring the magnitude of the Hall potential. Therefore, the current sensor undergoes an electrical-magnetic-electrical insulation isolation conversion.

 

 

How it works

1.Direct-type (open-loop) current sensor (CS series)

Working principle of Hall current sensor and its magnetic saturation problem

When the primary current IP flows through a long wire, a magnetic field will be generated around the wire. The magnitude of this magnetic field is proportional to the current flowing through the wire. The generated magnetic field is gathered in the magnetic ring, and the Hall element in the air gap of the magnetic ring is measured and amplified. The output voltage VS accurately reflects the primary current IP. The general rated output is 4V.

 

 

2.Magnetic balance (closed loop) current sensor (CSM series)

 

Working principle of Hall current sensor and its magnetic saturation problem

 

The magnetic balance current sensor is also called a compensation sensor, that is, the magnetic field generated by the primary current Ip at the magnetic ring is compensated by the magnetic field generated by a secondary coil current, and its compensation current Is accurately reflects the primary current Ip, so that the Hall device is in a working state of detecting zero magnetic flux.

 

The specific working process is: when a current passes through the main circuit, the magnetic field generated on the wire is gathered by the magnetic ring and induced to the Hall device. The generated signal output is used to drive the power tube and turn it on, thereby obtaining a compensation current Is. This current then generates a magnetic field through a multi-turn winding. The magnetic field is exactly opposite to the magnetic field generated by the measured current, thereby compensating the original magnetic field and gradually reducing the output of the Hall device. When the magnetic field generated by multiplying Ip by the number of turns is equal, Is no longer increases. At this time, the Hall device plays the role of indicating zero magnetic flux. At this time, Ip can be tested by Is. When Ip changes, the balance is destroyed, and the Hall device has a signal output, that is, the above process is repeated to achieve balance again. Any change in the measured current will destroy this balance. Once the magnetic field loses balance, the Hall device has a signal output. After power amplification, the corresponding current immediately flows through the secondary winding to compensate for the unbalanced magnetic field. The time required from magnetic field imbalance to re-balance is theoretically less than 1μs, which is a dynamic balance process. Therefore, from a macroscopic point of view, the ampere-turns of the secondary compensation current are equal to the ampere-turns of the primary measured current at any time.

 

 

 

3.Hall voltage (closed loop) sensor (VSM series)

The working principle of the Hall voltage sensor is similar to that of the closed-loop current sensor, and it also works in a magnetic balance mode. The primary voltage VP generates current through the current limiting resistor Ri, flows through the primary coil to generate a magnetic field, and gathers in the magnetic ring. The compensation current IS controlled by the output signal of the Hall element in the air gap of the magnetic ring flows through the magnetic field generated by the secondary coil for compensation. The compensation current IS accurately reflects the primary voltage VP.

 

 

 

4.AC current sensor (A-CS series)

The AC current sensor mainly measures the current of AC signal lamps. It converts the AC signal induced by the Hall effect sensor into a standard DC signal of 0-4V, 0-20mA (or 4-20mA) through AC-DC and other conversions for various systems.

 

 

 

Working process

The open-loop Hall current sensor uses the Hall direct amplification principle, while the closed-loop Hall current sensor uses the magnetic balance principle. Therefore, the closed-loop sensor is much better than the open-loop sensor in terms of response time and accuracy. Both the open-loop and closed-loop sensors can monitor AC power. Generally, the open-loop sensor is suitable for large current monitoring, while the closed-loop sensor is suitable for small current monitoring.

 

 

The working process of the open-loop Hall sensor:

 

When the primary current (Ip) passes through a wire, a magnetic field will be generated around the wire. The magnitude of this magnetic field is proportional to the current flowing through the wire. It can be gathered through the magnetic core and induced to the Hall device to output a signal. This signal is directly output after being amplified by the signal amplifier. The signal output by the Hall device accurately reflects the output of the primary current.

 

Advantages: small package size, wide measurement range, light weight, low power loss, no insertion loss

 

 

 

Working process of closed-loop Hall current sensor:

When the magnetic flux generated by the primary current IP is concentrated in the magnetic circuit through the magnetic core, the Hall device is fixed in the air gap to detect the magnetic flux, and the multi-turn coil wound on the magnetic core outputs a reverse compensation current to offset the magnetic flux generated by the primary current (IP), so that the magnetic flux in the magnetic circuit always remains zero. The secondary compensation current generated by the Hall device and the auxiliary circuit accurately reflects the magnitude of the primary current. After processing by a special circuit, the output end of the sensor can output a current change that accurately reflects the primary current.

 

Magnetic saturation problem of Hall sensor

Many Hall current sensor manufacturers also promote the absence of magnetic saturation as an important advantage of Hall current sensors in their technical data. The absence of magnetic saturation in Hall current sensors is almost one of the main advantages of Hall current sensors that has been widely recognized since their application.

 

 

Is this true?

In fact, the Hall current sensor contains a nonlinear magnetic core, which means that the Hall current sensor will be magnetically saturated under certain circumstances!

 

 

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

 

The following figure is a schematic diagram of the typical magnetization curve of all high magnetic permeability materials:

 

Working principle of Hall current sensor and its magnetic saturation problem

In the figure, Oa’ is the starting nonlinear segment, a’a” is the linear segment, and a”a is the saturation region. As we all know, in order to obtain better measurement results, both the open-loop Hall current sensor and the electromagnetic transformer will use the section with better linearity in the magnetization curve as the working range. In other words, as long as the magnetic induction intensity exceeds a certain range in the linear region, magnetic saturation will occur.

 

Compared with electromagnetic transformers, there is only one reason for magnetic saturation in open-loop Hall current sensors, that is, the primary current is large enough.

 

The advantage of the Hall current sensor is that it will not cause magnetic saturation due to low current frequency, which is also the magnetic saturation characteristic of the open-loop Hall current sensor.

 

In comparison, the electromagnetic transformer also has an advantage, that is, when the secondary load is small enough, magnetic saturation will not occur even if the overload is large.

 

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

The magnetic saturation problem of the open-loop Hall current sensor is relatively simple. In contrast, the magnetic saturation problem of the closed-loop Hall current sensor seems incomprehensible, because when the closed-loop Hall current sensor works normally, the magnetic flux in the magnetic core is zero, and under zero magnetic flux, it will naturally not saturate.

 

 

However, this will only be under normal operating conditions!

In fact, even the magnetic saturation problem of electromagnetic current transformer or open-loop Hall current sensor occurs under abnormal working conditions such as overload, low frequency, excessive load, etc. Magnetic saturation will not occur under normal working conditions!

 

From the working principle of the closed-loop Hall current sensor, we know that zero magnetic flux is based on the premise that the magnetic field generated by the secondary compensation winding can offset the magnetic field generated by the primary conductor. So, can the closed-loop Hall current sensor maintain this zero magnetic flux under any circumstances?

 

 

Obviously not!

1.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. As long as the primary current is large enough, magnetic saturation will occur.

 

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

 

Compared with electromagnetic current transformers and open-loop Hall current sensors, magnetic saturation is less likely to occur in closed-loop Hall current sensors, but that does not mean it will not occur. Magnetic saturation may also occur if used improperly or overloaded for a long time.

 

 

 

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