In the operation of power transformation and distribution, transformers are indispensable. It is very necessary to be familiar with and master the basic knowledge of transformers. The basic knowledge reserve of transformers is an essential skill for every electrician!

1. What is a transformer?

In AC circuits, the equipment that increases or decreases the voltage is called a transformer. The transformer can convert any voltage value into the voltage value we need with the same frequency to meet the transmission, distribution and use requirements of electric energy. For example, the electricity generated by power plants has a low voltage level. The voltage must be increased before it can be transported to distant power-consuming areas. The power-consuming areas must reduce the voltage to an appropriate voltage level to supply power equipment and daily electricity. Equipment usage.

2. How does a transformer convert voltage?

Transformers are made based on electromagnetic induction. It consists of an iron core made of silicon steel sheets (or silicon steel sheets) and two sets of coils wound around the iron core. The iron core and the coils are insulated from each other and have no electrical connection, as shown in the figure. We call the coil connected to the side of the transformer and the power supply the primary coil (or primary side), and the coil connected to the transformer to the electrical equipment is called the secondary coil (or secondary side). When the primary coil of a transformer is connected to an AC power source, changing magnetic lines of force are produced in the core. Since the secondary coil is wound on the same iron core and the magnetic lines cut the secondary coil, an induced electromotive force will inevitably be generated on the secondary coil, causing a voltage to appear at both ends of the coil. Because the magnetic field lines are alternating, the voltage of the secondary coil is also alternating. And the frequency is exactly the same as the power frequency.

It has been theoretically confirmed that the voltage ratio between the primary coil and the secondary coil of the transformer is related to the turns ratio between the primary coil and the secondary coil, which can be expressed by the following formula: primary coil voltage/secondary coil voltage = primary coil turns/secondary coil turns Explain that the more turns, the higher the voltage. So it can be seen that the secondary coil is less than the primary coil, which is the step-down transformer. The opposite is a step-up transformer.

3. What types of transformer designs are there?

There are single-phase and three-phase transformers according to the number of phases. According to the purpose, they are divided into power transformers, special power transformers, voltage regulating transformers, measuring transformers (voltage transformers, current transformers), small power transformers (for small power equipment), and safety transformers. According to the structure, they are divided into core type and shell type. Two kinds. Coils have double windings and multi-windings, and autotransformers are divided into oil-immersed and air-cooled types according to cooling methods.

4. What parts are the transformer components made of?

Transformer components are mainly composed of iron cores and coils, as well as oil tanks, oil pillows, insulating bushings and tap ends.

5. What are the uses of transformer oil?

The functions of transformer oil are: (1) Insulation. (2) Heat dissipation effect. (3) Eliminate the arc effect.

6. What is an autotransformer?

The autotransformer has only one set of coils. The secondary coil is tapped from the primary coil. In addition to electromagnetic induction transmission, it also transmits electricity. This kind of transformer has more silicon steel sheets and copper wires than ordinary transformers. Few, often used to regulate voltage.

7. How does the voltage regulator regulate voltage?

The structure of the voltage regulator is the same as that of the autotransformer, except that the iron core is made into a toroidal coil and is wound around the toroidal core. The secondary coil tap uses a sliding brush contact to make the contact slide circularly along the surface of the coil to achieve smooth voltage regulation.

8. What is the current relationship between the primary coil and the secondary coil of the transformer?

When the transformer operates with a load, changes in the secondary coil current will cause corresponding changes in the primary coil current. According to the principle of magnetic potential balance, it can be deduced that the current of the primary and secondary coils is inversely proportional to the number of turns of the coil. The current on the side with more turns is smaller, and the current on the side with fewer turns is larger. It can be expressed by the following formula: Primary coil current/time Primary coil current = secondary coil turns/primary coil turns.

9. What is the voltage change rate of a transformer?

The voltage change rate of the voltage regulator is one of the main performance indicators of the transformer. When the transformer supplies power to the load, the voltage at the load end of the transformer will inevitably drop. Comparing the dropped voltage value with the rated voltage value, the percentage is the voltage change rate, which can be expressed by the formula; voltage change rate = [(secondary rated voltage -Load terminal voltage)/secondary rated voltage]×100%. When a normal power transformer is connected to a rated load, the voltage change rate is 4 to 6%.

10. How to ensure that the transformer has a rated voltage output?

If the voltage is too high or too low, it will affect the normal operation and service life of the transformer, so the voltage must be adjusted. The method of voltage regulation is to introduce several taps in the primary coil and connect them to the tap head. The tap head changes the number of turns of the coil by rotating the contacts. Just turn the tap changer position to get the required rated voltage value. It should be noted that voltage regulation should usually be performed after cutting off the load connected to the transformer.

11. What are the commonly used small transformers? In what situations is it used?

Small transformers refer to single-phase transformers with a capacity of less than 1 kVA. They are mostly used as power transformers for controlling electrical equipment, power transformers for electronic equipment, and power transformers for safety lighting.

12. What losses occur during operation of the transformer? How to reduce losses?

The loss during the operation of the transformer includes two parts; (1) It is caused by the iron core. When the coil is energized, the magnetic lines of force are alternating, causing eddy current and hysteresis losses in the iron core. This loss is collectively called iron loss. (2) It is caused by the resistance of the coil itself. When current passes through the primary coil and secondary coil of the transformer, there will be a loss of electrical energy. This loss is called copper loss.

The sum of iron loss and copper loss is the transformer loss. These losses are related to the transformer capacity, voltage and equipment utilization. Therefore, when selecting a transformer, you should try to make the equipment capacity consistent with the actual usage to improve equipment utilization, and be careful not to operate the transformer at light load.

13. What is the nameplate of a transformer? What are the main technical data on the nameplate?

The nameplate of the transformer indicates the performance, technical specifications and usage occasions of the transformer to satisfy the users selection. The main technical data that should be paid attention to during selection are usually:

(1) Kilovolt-ampere of rated capacity. That is, the output capacity of the transformer under rated conditions. For example, the rated capacity of a single-phase transformer = U line × I line; the capacity of a three-phase transformer = U line × I line.

(2) Rated voltage volts. Mark the terminal voltage of the primary coil and the terminal voltage of the secondary coil (when no load is connected) respectively. Note that the terminal voltage of the three-phase transformer refers to the line voltage U line value.

(3) Rated current amperes. It refers to the line current I line value that the primary coil and secondary coil are allowed to pass through for a long time under the conditions of rated capacity and allowable temperature rise.

(4) Voltage ratio. Refers to the ratio of the rated voltage of the primary coil to the rated voltage of the secondary coil.

(5)Wiring method. Single-phase transformers only have one set of coils each for high and low voltage, and are only for single-phase use. Three-phase transformers have Y/Δ type. In addition to the above technical data, there are also the rated frequency, number of phases, temperature rise of the transformer, impedance percentage of the transformer, etc.

14. How to choose a transformer? How to determine the reasonable capacity of a transformer?

First of all, we need to investigate the power supply voltage of the place where electricity is used, the actual power load of the user and the conditions of the place where it is located, and then select one by one according to the technical data marked on the transformer nameplate. Generally, the capacity, voltage, current and environmental conditions of the transformer should be considered comprehensively. Among them, the capacity selection The required load should be determined according to the capacity, nature and usage time of the users electrical equipment, so as to select the transformer capacity.

During normal operation, the electrical load the transformer should bear is about 75 to 90% of the rated capacity of the transformer. When the actual load of the transformer is measured to be less than 50% during operation, the small-capacity transformer should be replaced. If the load is greater than the rated capacity of the transformer, the large-capacity transformer should be replaced immediately. At the same time, when selecting a transformer, determine the voltage value of the primary coil of the transformer based on the line power supply, and select the voltage value of the secondary coil based on the electrical equipment. It is best to choose a low-voltage three-phase four-wire power supply. This can provide both power and lighting electricity at the same time.

For the selection of current, attention should be paid to whether the load can meet the requirements of the motor when it starts (because the starting current of the motor is 4 to 7 times larger than that of sinking operation).

15. Why can’t the transformer be overloaded?

Overload operation means that the current value specified on the nameplate is exceeded when the transformer is running. Overload is divided into two types: normal overload and accident overload. The former refers to the increase in electricity consumption of users under normal power supply conditions. It often increases the temperature of the transformer, promotes the aging of the transformer insulation, and reduces the service life. Therefore, Transformers are not allowed to operate overloaded. Under special circumstances, the overload operation of the transformer in a short period of time cannot exceed 30% of the rated load (winter), and it must not exceed 15% in summer. For the latter, the accident overload and allowed time requirements are shown in the table below.

16. What kind of tests should be done on the transformer during operation?

In order to ensure that the transformer can operate normally, the following tests should be carried out frequently;

(1) Temperature test. Whether the transformer is operating normally or not and whether the temperature is high or low are very important. The regulations stipulate that the upper oil temperature shall not exceed 85C (that is, the temperature rise is 55C). Generally, transformers are equipped with special temperature measuring devices.

(2) Load measurement. In order to improve the utilization rate of the transformer and reduce the loss of electric energy, the power supply capacity that the transformer can truly bear must be measured during the operation of the transformer. The measurement work is usually carried out during the peak period of electricity consumption in each season, and is measured directly with a clamp ammeter. The current value should be 70 to 80% of the rated current of the transformer. If it exceeds, it means overload and should be adjusted immediately.

(3) Voltage measurement. Regulations require that the voltage variation range should be within ±5% of the rated voltage. If it exceeds this range, taps should be used to adjust the voltage to the specified range. Generally, a voltmeter is used to measure the secondary coil terminal voltage and the terminal voltage of the end user respectively.

(4) Insulation resistance measurement. In order to keep the transformer in normal operation, the insulation resistance must be measured to prevent insulation aging and accidents. When measuring, try to stop the transformer and use a megometer to measure the insulation resistance of the transformer. The measured resistance is required to be no less than 70% of the previously measured value. When using a megometer, the low-voltage coil can be of 500 volt voltage level.