Double winding transformer

Transformers are used in almost all electronic products. The principle is simple, but depending on the application (different applications), the winding process of the transformer will have different requirements. The functions of the transformer mainly include: voltage conversion; impedance transformation; isolation; voltage regulation (magnetic saturation transformer); protection of personal safety, etc., commonly used core shapes of transformers generally have E-type and C-type iron cores.

Introduction

The most basic type of transformer consists of two sets of coils wound with wires and they are inductively coupled together. When an alternating current (having a certain known frequency) flows into one of the sets of coils, an alternating voltage having the same frequency is induced in the other set of coils, and the magnitude of the induced voltage depends on the two coil couplings and the magnetic cross-links. The extent of it.

Generally speaking, the coil connected to the AC power source is a "primary coil"; the voltage across the coil is called "primary voltage". The induced voltage in the secondary coil may be greater or less than the primary voltage, which is determined by the "turns ratio" between the primary coil and the secondary coil. Therefore, the transformer is divided into two types: boost and step-down transformers.

Most transformers have a fixed core with a primary and secondary coil wound around it. Due to the high magnetic permeability of the iron, most of the magnetic flux is confined to the core, so that the two sets of coils can thereby obtain a relatively high degree of magnetic coupling. In some transformers, the coil and the iron core are tightly coupled, and the ratio of primary to secondary voltage is almost the same as the coil turns ratio of the two. Therefore, the turns ratio of the transformer can generally be used as a reference for transformer boost or buck. Due to the function of boosting and stepping down, the transformer has become an important accessory of modern power system. Increasing the transmission voltage makes the long-distance transmission of electricity more economical. As for the step-down transformer, it makes the power application more diversified. It can be said that if there is no transformer, modern industry cannot achieve the status quo of development.

In addition to the small size of the electronic transformer, there is no clear dividing line between the power transformer and the electronic transformer. Generally speaking, the power supply of the 60 Hz power network is very large, and it may cover a large capacity in half a continent. The power limitation of an electronic device is usually limited by the ability to rectify, amplify, and other components of the system, some of which are amplified power, but it is still in the range of small power compared to the power generation capability of the power system.

Various electronic equipment is commonly used in transformers. The reason is: providing various voltage levels to ensure normal operation of the system; providing electrical isolation between different potential operating parts in the system; providing high impedance to alternating current, but providing low impedance to direct current; Maintain or modify the waveform and frequency response at different potentials. One of the important concepts of "impedance", that is, one of the electronic characteristics, is a device that pre-sets a device when the impedance of the circuit component changes from one level to another. -transformer.

For electronic devices, weight and space are often an effort to pursue, and efficiency, safety and reliability are important considerations. In addition to being able to occupy a significant percentage of weight and space in a system, on the other hand, in terms of reliability, it is also a factor in the measurement factor. Because of the above differences with other applications, power transformers are not suitable for use in electronic circuits.

Method of operation

Power transformer inspection should meet the following requirements

1 Daily inspections should be conducted at least once a day, and night inspections should be done at least once a week.

2 The number of inspections should be increased in the following cases:

1) Within the first 72 hours of commissioning or overhauling and transformation.

2) When there is a sudden change in weather (such as thunderstorms, strong winds, heavy fog, heavy snow, hail, cold waves, etc.).

3) High temperature season and peak load period.

4) When the transformer is overloaded.

3 The daily inspection of the transformer should include the following:

1) The oil temperature should be normal, there should be no oil leakage or oil leakage, and the oil level of the oil storage tank should correspond to the temperature.

2) The oil level of the casing should be normal, and there should be no cracks, no serious oil, no discharge marks and other abnormal phenomena on the outside of the casing.

3) The transformer sound response is normal.

4) The temperature of the touch of each part of the radiator should be similar, and the heat sink attachment should work normally.

5) The moisture absorber should be intact and the adsorbent should be dry.

6) There should be no signs of heat generation in lead connectors, cables, and bus bars.

7) Pressure release, safety air passage and explosion-proof membrane should be intact.

8) The tap position and power indication of the tap changer should be normal.

9) There should be no gas in the gas relay.

10) Each control box and secondary terminal box should be closed and not damp.

11) The appearance of the dry-type transformer should be free of dirt.

12) The transformer room is not leaking, the doors, windows and lighting should be intact, the ventilation is good, and the temperature is normal.

13) The transformer casing and components should be kept clean.

working principle

principle

When a sinusoidal alternating voltage U1 is applied across the primary coil, there is an alternating current I1 in the conductor and an alternating magnetic flux Ñ„1 which forms a closed magnetic circuit along the core through the primary and secondary coils. The mutual inductance potential U2 is induced in the secondary coil, and Ñ„1 also induces a self-induced potential E1 on the primary coil. The direction of E1 is opposite to the applied voltage U1 and the amplitude is similar, thereby limiting the size of I1. In order to maintain the existence of magnetic flux Ñ„1, it is necessary to have a certain power consumption, and the transformer itself has a certain loss. Although the secondary is not connected to the load at this time, there is still a certain current in the primary coil. This current is called "no-load current." ".

If the secondary is connected to the load, the secondary coil generates a current I2, and thus the magnetic flux Ñ„2, the direction of Ñ„2 is opposite to Ñ„1, which acts to cancel each other out, so that the total magnetic flux in the core is reduced, thereby The primary self-inductance voltage E1 is reduced, and as a result, I1 is increased, and it is seen that the primary current is closely related to the secondary load. When the secondary load current increases, I1 increases, Ñ„1 also increases, and the Ñ„1 increase partially complements the portion of the magnetic flux that is offset by Ñ„2 to keep the total magnetic flux in the core constant. If the loss of the transformer is not taken into account, it can be considered that the power consumed by an ideal transformer secondary load is also the primary power drawn from the power supply. The transformer can change the secondary voltage by changing the number of turns of the secondary coil as needed, but cannot change the power that allows the load to be consumed.

Repair and maintenance

loss

When the primary winding of the transformer is energized, the magnetic flux generated by the coil flows in the core. Since the core itself is also a conductor, an electric potential is induced in a plane perpendicular to the magnetic field line. This potential forms a closed loop on the cross section of the core. The current is generated as if it is a vortex, so it is called "eddy current". This "vortex" increases the loss of the transformer and increases the temperature rise of the transformer's core heating transformer. The loss caused by the "eddy current" is called "iron loss". In addition, a large number of copper wires are required to wind the transformer. These copper wires have resistance. When the current flows, the resistor consumes a certain amount of power. This loss is often consumed as heat. We call this loss "copper loss". . Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss.

Since the transformer has iron loss and copper loss, its output power is always less than the input power. For this reason, we introduce an efficiency parameter to describe this, η = output power / input power.

material

To wind a transformer we must have a certain understanding of the materials related to the transformer, so I will introduce this knowledge here.

1, core material

The core material used in the transformer mainly consists of iron sheets, low silicon wafers, and high silicon wafers. The addition of silicon to the steel sheet can reduce the electrical conductivity of the steel sheet and increase the resistivity, which can reduce the eddy current and reduce the loss. We usually refer to the silicon-added steel sheet as silicon steel sheet. The quality of the silicon steel sheet used for the quality of the transformer has a great relationship. The quality of the silicon steel sheet is usually expressed by the magnetic flux density B. The B value of the black iron sheet is generally 6000-8000, low silicon wafer is 9000-11000, high silicon wafer is 12000-16000.

2, the material usually used to wind the transformer

Enameled wire, yarn wrapped wire, wire wrapped wire, the most commonly used enameled wire. For the requirements of the wire, the conductive property is good, the insulating paint layer has sufficient heat resistance, and has a certain corrosion resistance. In general, it is best to use the QZ model of high-strength polyester enamel.

3. Insulation material

In the wound transformer, the isolation between the coil frame layers and the isolation between the windings are all made of insulating materials. The general transformer frame material can be made of phenolic paperboard, and the interlayer can be separated by polyester film or telephone paper. Yellow wax cloth can be used for isolation.

4, impregnating materials

After the transformer is wound, the last process is to dip the insulating varnish, which can enhance the mechanical strength of the transformer, improve the insulation performance and prolong the service life. Under normal circumstances, cresol varnish can be used as the impregnating material.