For a long time, the current transformer as a standard device in instruments and equipment has been used to measure precision current. Even in the harsh environment and high temperature conditions, the device is very accurate, easy to use and reliable.
In applications such as switching power supply, motor current load detection, lighting and instrumentation, CT transformers are generally used as control, circuit protection and monitor devices. With the increasing number of current transformer in stock, how to choose a suitable current transformer needs to consider many factors. This paper introduces a simple selection method, which is very helpful for selecting appropriate high cost performance devices in many applications. Although the off the shelf devices are cheap and desirable immediately, there are some functional limitations in use. Some applications may require special products or even need to be completely customized.
First of all, the selection of current transformer must define and verify a number of indicators, such as size, frequency, function and range of sampling current. The accuracy and efficiency of this method depend on these parameters. In addition to the possible compromise on the accuracy of CT transformers, if the current of current transformers used exceeds the rated current specifications specified by the manufacturer, the operating temperature of current transformers will continue to rise and cannot be controlled, resulting in circuit failure.
In addition, if the rating of a current transformer is much higher than its "sampling current", the size of the device will inevitably be large, which is too expensive for its application. Generally speaking, it is a wise choice to select a current transformer whose rating is about 30% higher than the maximum expected value of its "sampling current".
The common current transformer turns ratio range from 1:10 to 1:1000. The higher the turn ratio (r = nsec / NPRI), the higher the resolution of current measurement.
However, it should be noted that too high turn ratio will lead to the increase of distributed capacitance and leakage inductance, which will reduce the accuracy and performance of current transformer at high frequency (caused by self resonance). However, if the turn ratio is too low (low inductance), the output signal may be distorted or "dropped" (the single-stage input signal must be skewed), resulting in the instability of the control circuit and inaccurate measurement results.
Induction coefficient and exciting current
The secondary induction coefficient of current transformer determines the fidelity of output signal. The value of induction coefficient is inversely proportional to the exciting current, which is commonly known as "induced current".
In order to ensure the maximum fault-tolerant performance of current transformer, the amplitude of exciting current should be several times smaller than that of sampling current. For most applications such as switching power supply, it is ideal to take 10% of the sampling current as the maximum excitation current. For example, if a circuit must guarantee a maximum loss of 10% for 1-20a sampling current at 100kHz, then the maximum excitation current must be set to 100mA (i.e. 10% of the minimum sampling current value).
The sampling current of 1A will produce 10% error and 20A sampling current will produce 0.5% error. If the excitation current is not indicated in the data book provided by the manufacturer, it can be calculated by the following formula:
e = CLdI/dt
|dI/dt| = e/L
Where e is the device output voltage (in V), l is the inductance (in H), and | di / dt | is the ratio of excitation current to time (in a / s).
Output voltage and load resistance
The output voltage (VO) should be set as low as possible to reduce the insertion loss. Assuming that the optimal secondary output voltage of a circuit is 0.5V and the output current is 20a, the current transformer with 1:100 turns ratio will produce about 200mA secondary current.
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