Transformer core structureThe iron core is composed of silicon steel sheets. In order to reduce eddy current, there is a certain insulation resistance between the sheets (generally only a few ohms to tens of ohms). Due to the great capacitance between the sheets, it can be regarded as a path in the alternating electric field. Therefore, only one point of grounding in the iron core can clamp the potential of the whole stack of iron cores at the ground potential.The iron core is the main part of the magnetic circuit in the transformer. It is usually made of hot-rolled or cold-rolled silicon steel sheets with high silicon content and coated with insulating paint. The iron core and the coil wound on it form a complete electromagnetic induction system. The transmission power of power transformer depends on the material and cross-sectional area of iron core.

Key points of transformer installation protection(1) An iron yoke clamp grounding lead shall be set separately. Because if the iron core touches the upper clamp, resulting in multi-point grounding fault, the grounding current only flows inside the iron core clamp, and there is no current in the iron core grounding outgoing line, which will lead to the staff mistakenly believing that the iron core has not failed; After setting, no matter where the iron core touches the clamp, it will form a loop through two external grounding leads, so that the grounding current can be correctly detected externally.(2) The iron core grounding strip is placed in the middle of the cross section of the yoke. In this way, no matter where the insulating bolt of the iron yoke strap is and where the fault grounding point is, the maximum induced voltage of the circuit is only 1 / 4 of the turn voltage, and the maximum grounding current is only about a few amps, which is much smaller than when the iron core grounding strip is placed in other positions.

(3) If it is really necessary to place the iron core grounding strip at other positions due to inconvenient on-site installation, the insulating bolt and grounding strip of the iron yoke strap shall also be placed diagonally, so as to prevent large current.Iron core protective grounding in power transformerWhen the transformer works, an uneven electric field will be generated around the winding, and the suspension induced voltage will be generated on the iron core and other metal components. When the voltage difference between the suspension potentials exceeds the insulation voltage, internal discharge or ground discharge will occur. Therefore, a reliable grounding must be made at a certain point on the transformer core to discharge the suspended induced voltage of the core and protect the transformer.

Power transformer core materialSilicon steel lamination is generally used as the core material of power transformerSilicon steel has high permeability, so it is widely used in the manufacture of motors and transformers. Because the whole ferromagnetic material will produce large eddy current when working, which is unfavorable for the transformer. To reduce the eddy current (eddy current has skin effect), it is necessary to make ferromagnetic materials into thin sheets and then stack them into the required cross-sectional area.

Advantages of electronic current transformer1) Complete isolation of high and low voltage, high safety, excellent insulation performance, no iron core, eliminating magnetic saturation and ferromagnetic resonance. The measured signal of the electromagnetic transformer is coupled with the secondary coil through the iron core, the insulation structure is complex, and its cost increases exponentially with the voltage level. The unconventional transformer transmits the high-voltage side signal to the secondary equipment through the optical fiber with good insulation performance, which greatly simplifies its insulation structure. The higher the voltage level, the more obvious the cost performance advantage.2) There are no potential flammable, explosive and other hazards due to oil filling. The insulation structure of unconventional transformer is relatively simple. Generally, oil is not used as the insulation medium, which will not cause fire, explosion and other hazards.

3) Large dynamic range, high measurement accuracy and wide frequency response range. During the normal operation of the power grid, the current flowing through the current transformer is small, but the short-circuit current is generally large, and the short-circuit current increases with the increase of the power grid capacity. Unconventional transformer has a wide dynamic range, which can meet the needs of measurement and relay protection at the same time.4) The anti electromagnetic interference performance is good, and there is no risk of open circuit and high voltage at the low-voltage side. There is only optical fiber connection between high-voltage side and low-voltage side of unconventional transformer. The signal is transmitted through optical fiber. The high-voltage circuit and secondary circuit are completely isolated electrically. The transformer has good anti electromagnetic interference ability, and there is no high-voltage risk caused by open circuit at low-voltage side.5) Strong anti-interference ability of data transmission. Electromagnetic transformer transmits analog signals. Traditionally, measurement, control and relay protection in power station transmit the electrical signals measured by electrical sensors to the control room through coaxial cable.

6) Small size and light weight. Unconventional transformer has no iron core, and its weight is much smaller than that of electromagnetic transformer with the same voltage level.Application of electronic current transformerElectronic current transformers usually have higher bandwidth and are suitable for fundamental and harmonic measurement of currents with large harmonic content. In order to accurately measure power, voltage and current combined electronic transformer can also be used, because the combined electronic transformer can better control the phase difference of voltage and current signal and improve the accuracy of power measurement.

1 Output current waveform of two-level frequency converterThe voltage waveform and current waveform are close to sine wave, the voltage waveform is PWM wave, and the higher burr in the figure is overshoot voltage. Shortening the cable and adding DV / dt filter can reduce or eliminate overshoot voltage.PWM wave has less low-order harmonics, and the harmonics are mainly concentrated near the integer multiple of carrier frequency.

The fundamental frequency of the frequency converter is recorded as FS; The carrier frequency of the frequency converter is recorded as FC;Then, n = FC / Fs is the carrier frequency ratio.For SPWM modulated three-phase frequency converter, when n is an integer, it does not contain harmonics below n-2.

Assuming FS = 50z and FC = 2KHz, n = 40, and the output of the frequency converter does not contain harmonics below 38 times.And the harmonic frequency is KFC MFs, where KS = 1, 2, 3, 4, 5, 6, 7 k1=12457. Generally speaking, the load of the frequency converter is the motor, and the motor is the inductive load, which can better suppress high-order harmonics. Therefore, theoretically, the higher the carrier frequency ratio of the frequency converter, the less the harmonic content of the output current of the frequency converter.

2 Input current and voltage waveform of six pulse rectifier frequency converterThe input current waveform of six pulse rectifier frequency converter contains large harmonics, which are mainly 6K 1 times, K is an integer, and K 1. Similarly, the harmonic of the input current of 12 pulse rectifier is mainly 12K 1, K is an integer, and K 1.Assuming that the frequency converter is not p-pulse rectified, the input current of the frequency converter does not contain harmonics below P-1. The harmonics are mainly PK 1, K is an integer, and K 1.

3 Transformer no-load input waveformThe above figure shows the no-load input waveform of single-phase transformer. The current waveform in the figure has large distortion. According to its shape characteristics, it is generally called "spike wave". It can be seen from the spectrum on the right that the harmonics of spike wave are mainly odd harmonics. The higher the harmonic order, the smaller the harmonic content.4 Superimposed frequency wave of stator thermal test by superimposed frequency method

The waveform above contains two frequency components, 50Hz and 40Hz respectively. 50Hz is called the main power supply and 40Hz is called the auxiliary power supply. In the voltage waveform, the amplitude of the auxiliary power supply is about 25% of that of the main power supply.Due to the low impedance of 40Hz auxiliary power supply, the relative content in the current waveform increases and the waveform fluctuation increases.

Switching power transformer is a power transformer with switch tube. In the circuit, in addition to the voltage conversion function of ordinary transformer, it also has the functions of insulation isolation and power transmission. It is generally used in switching power supply and other occasions involving high-frequency circuits.Function and classification of switching power transformer

The switching power supply transformer and the switch tube form a self-excited (or other excited) intermittent oscillator to modulate the input DC voltage into a high-frequency pulse voltage. It plays the role of energy transfer and conversion. In the flyback circuit, when the switch is on, the transformer converts the electric energy into magnetic energy for storage, and releases it when the switch is off. In the forward circuit, when the switch is on, the input voltage is directly supplied to the load and the energy is stored in the energy storage inductor. When the switch is cut off, the energy storage inductor is used for continuous flow to the load.

Switching power transformer is divided into single excitation switching power transformer and double excitation switching power transformer. The working principle and structure of the two switching power transformers are not the same. The input voltage of the single excitation switching power supply transformer is a unipolar pulse, and it is also divided into positive flyback voltage output; The input voltage of double excitation switching power supply transformer is bipolar pulse, which is generally bipolar pulse voltage output.Principle of switching power transformer

For switching power supply, the working principle of switching transformer is different from that of ordinary transformer. The positive and negative half cycle waveforms of the input AC voltage or current of the ordinary transformer are symmetrical, and the input voltage and current waveforms are generally continuous. Within a cycle, the average value of the input voltage and current is equal to 0, which is the basic feature of the working principle of the ordinary transformer; Switching transformers generally work in the switching state, and their input voltage or current is generally not continuous, but intermittent. Most of the average value of input voltage or current in a cycle is not equal to 0. Therefore, switching transformers are also called pulse transformers, which is the biggest difference between switching transformers and ordinary transformers in working principle.

PWM (pulse width modulation) controls the switch tube to turn on the rectified DC voltage, so that the high-frequency current flows into the primary side of the high-frequency transformer of the switching power supply, and the induced current is generated at the secondary side of the transformer. After rectification, the required voltage or multi-channel voltage can be obtained.Test details of switching power transformer1. Check whether there are obvious abnormalities by observing the appearance of the transformer: such as whether the coil lead is broken, desoldered, whether the insulating material has scorched traces, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.

2. Insulation test: use multimeter R & times; 10K gear shall measure the resistance between iron core and primary, primary and secondary, iron core and secondary, electrostatic shielding layer and secondary and secondary windings respectively. The pointer of multimeter shall point to the infinity position. Otherwise, the insulation performance of the transformer is poor.3. Coil on-off detection: place the multimeter in R & times; Gear 1: in the test, if the resistance value of a winding is infinite, it indicates that the winding has open circuit fault.4. Discrimination of primary and secondary coils: the primary pin and secondary pin of power transformer are generally led out from both sides, and the primary winding is mostly marked with 220V, while the secondary winding is marked with rated voltage, such as 15V, 24V, 35V, etc. Then identify according to these marks.

5. Detection of no-load current: A. open all secondary windings by direct measurement method, and place the multimeter in AC current gear (500mA, connected in series into the primary winding. When the plug of the primary winding is plugged into 220V AC mains power, the multimeter indicates the no-load current value. This value shall not be greater than 10% 20% of the full load current of the transformer. Generally, the normal no-load current of the power transformer of common electronic equipment shall be about 100mA. If it exceeds too much, it indicates that the transformer has short-circuit fault. B. the indirect measurement method is used in voltage transformation A 10 / 5W resistor is connected in series in the primary winding of the power transformer, and the secondary is still no-load. Turn the multimeter to the AC voltage gear. After power on, use two probes to measure the voltage drop u at both ends of the resistor R, and then use Ohm's law to calculate the no-load current I empty, i.e. I empty = u / R. connect the primary stage of the power transformer to 220V mains power, and use the AC voltage of the multimeter to measure the no-load voltage of each winding in turn (U21, u22, U23 and u24) shall meet the required values, and the allowable error range is generally: high voltage winding 10%, low voltage winding 5%, and the voltage difference between two groups of symmetrical windings with center tap shall be 2%.

6. Test the temperature range of power transformer: generally, the allowable temperature rise of low-power power transformer is 40 50 . If the insulating material used is of good quality, the allowable temperature rise can be improved.7. Detect and judge the same name end of each winding: when using the power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be used in series. When using the power transformer by series method, the same name end of each winding in series must be connected correctly without mistake. Otherwise, the transformer cannot work normally.

8. Comprehensive detection and discrimination of short-circuit fault of power transformer: the main symptoms of short-circuit fault of power transformer are serious heating and abnormal output voltage of secondary winding. Generally, the more inter turn short-circuit points in the coil, the greater the short-circuit current, and the more serious the heating of transformer. The simple method to detect and judge whether there is short-circuit fault of power transformer is to measure no-load power Current. For the transformer with short-circuit fault, its no-load current value will be much greater than 10% of the full load current. When the short-circuit is serious, the transformer will heat rapidly within tens of seconds after no-load power on, and it will feel hot to touch the iron core by hand. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.

What is the difference between switching power supply and transformer in use?Switching power supply: the switching power supply can stably turn the voltage within a certain range into a very accurate low voltage or high voltage (for example, 110v-250 input, the output voltage can be stably controlled at the required voltage positive and negative difference of 0.5V).Transformer: the output voltage of the transformer changes with the input voltage, that is, the input voltage increases, the output voltage increases, the input voltage decreases, and the output voltage decreases.

To sum up, it can be concluded that the switching power supply first changes the AC into DC, and then changes the DC into AC with higher frequency through the power switch tube. The voltage conversion through the high-frequency power transformer not only improves the efficiency, but also greatly reduces the volume after the frequency is high, and saves the copper and iron loss. Because it is controlled by the power switch tube, the switch tube is conductive at low current The power on time is short and the output voltage can be maintained. When the load is large, the switch tube works continuously to maintain the output voltage. Therefore, the output voltage of the switching power supply is stable and can be selected as high-precision instruments such as LED display screen.

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For a long time, as a standard device in instruments and equipment, current transformer has been used to measure precision current. Even in harsh environment and high temperature conditions, this device is very accurate, convenient and reliable.In applications such as switching power supply, motor current load detection, lighting and instruments, current transformer is generally used as control, circuit protection and monitoring devices. With the increasing stock of current transformer, how to select a suitable current transformer needs to consider many factors. This paper introduces a simple selection method, which is very helpful to select suitable cost-effective devices in many applications. Although spot devices are cheap and desirable, they have some functional limitations in use. Some applications may require special products or even complete customization.Many factors should be considered in the selection of current transformer, such as size, frequency, function and current range.

Input currentFirstly, the selection of current transformer must clarify and verify many indicators, such as size, frequency, function and the range of sampling current. Its accuracy and efficiency actually depend on these parameters. In addition to the possible compromise on the accuracy of the current transformer, if the current of the current transformer exceeds the rated current specification specified by the manufacturer, its operating temperature 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 this device will inevitably be large, which is too expensive for its application. Generally speaking, it is a wise choice that the rated value of the selected current transformer is about 30% higher than the maximum expected value of its "sampling current".

Turns ratioThe turns ratio of common current transformer ranges 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 of current transformer and the working performance at high frequency (caused by self resonance). However, if the turn ratio is too low (low induction coefficient), the output signal may be distorted or "decreased" (the single-stage input signal must be skewed), resulting in unstable control circuit and inaccurate measurement results.

Induction coefficient and excitation currentThe secondary inductance of current transformer determines the fidelity of output signal. The value of the induction coefficient is inversely proportional to the excitation current, which is commonly known as "induced current".In order to ensure the maximum fault-tolerant performance of current transformer, the excitation current should be several times smaller than the amplitude 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 a sampling current of 1 20A at 100kHz, the maximum value of the excitation current must be set to 100mA (i.e. 10% of the minimum sampling current value).

The sampling current of 1A will produce an error of 10%, and the sampling current of 20a will produce an error of 0.5%. 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 (unit: V), l is the induction coefficient (unit: H), and|di / dt|is the ratio of excitation current to time (unit: A / s).Output voltage and load resistanceThe output voltage (VO) should be set as low as possible to reduce the intervention loss. Assuming that the optimal secondary output voltage of a circuit is 0.5V and the output current is 20a, the current transformer with a turn ratio of 1:100 will produce a secondary current of about 200mA. The load resistance should be: RO = VO / is = 0.5 / 0.2 = 2.5 .