16 transformer basic knowledge

1. What is a transformer?

 

In the AC circuit, the device that will increase or decrease the voltage is called the transformer. The transformer can convert any numerical voltage into the voltage value that we need with the same frequency, so as to meet the transmission, distribution and use requirements of electric energy. For example, the electricity from the power plant has a low voltage grade, so the voltage must be raised before it can be transmitted to the distant power consumption area. The power consumption area must be reduced to an appropriate voltage grade to supply power equipment and daily electrical equipment for use.

 

2. How does the transformer change the voltage?

 

Transformers are made from electromagnetic induction. It consists of an iron core made of silicon steel sheet (or silicon steel sheet) and two groups of coils around the iron core. The iron core and the coil are insulated from each other without any electrical connection, as shown in the figure. We will transformer and power side of the coil connected to the primary coil (or called the primary side), the transformer and electrical equipment connected to the coil called secondary coil (or secondary side). When the primary coil of the transformer is connected to the AC supply, the magnetic field lines in the iron core change. As the secondary coil is wound around the same core, the magnetic force line cuts the secondary coil, and the induced electromotive force must be generated on the secondary coil, so that the voltage appears at both ends of the coil. As 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 proved by theory that the voltage ratio between primary coil and secondary coil is related to the ratio of turns between primary coil and secondary coil, which can be expressed as follows: primary coil voltage/secondary coil voltage = primary coil turns/secondary coil turns, indicating that the more turns, the higher the voltage will be. Therefore, it can be seen that the secondary coil is less than the primary coil, which is the step-down transformer. On the contrary, it is a booster transformer.

 

3. What are the types of transformer designs?

 

There are single-phase and three-phase transformers according to the number of phases. According to the use of power transformers, special power transformers, voltage regulating transformers, measuring transformers (voltage transformers, current transformers), small power transformers (for small power equipment), safety transformers, according to the structure can be divided into core and shell type two. The coil has double winding and multi winding, autotransformer, according to the cooling method can be divided into oil immersed and air cooled.

 

4. What parts are the transformer components made up of?

 

Transformer components are mainly composed of iron core, coil, in addition to the oil tank, oil pillow, insulation sleeve and tap beginning, etc.

5. What is the use of transformer oil?

 

The role of transformer oil is :(1) insulation. (2) heat dissipation. (3) the elimination of arc.

 

 

 

6. What is an auto-transformer?

 

The autotransformer has only one set of coils, and the secondary coils are tapped out from the primary coils. Its electric energy transmission, in addition to electromagnetic induction transmission, also has the transmission of electricity. The number of silicon steel sheet and copper wire in this kind of transformer is less than that in the general transformer, and it is often used for regulating voltage.

 

7. How is the voltage regulator regulated?

 

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

 

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

 

When the transformer is running with load, the change of the secondary coil current will cause the corresponding change of the primary coil current. According to the principle of magnetic potential balance, the current of primary secondary coil is inversely proportional to the number of coil turns. The side with more turns has a smaller current, while the side with fewer turns has a larger current, which can be expressed in the following formula: primary coil current/secondary coil current = number of secondary coil turns/number of primary coil turns. 9. What is the voltage change rate of the transformer?

 

The voltage change rate of voltage regulator is one of the main performance indexes of transformer. When the transformer supplies power to the load, the voltage at the load end of the transformer is bound to drop. Comparing the decreased voltage value with the rated voltage value, the percentage is the rate of voltage change, which can be expressed by the formula. Voltage change rate = [(secondary rated voltage - load terminal voltage)/ secondary rated voltage] ×100%. Usually power transformers, when connected to the rated load, the voltage change rate is 4 ~ 6%.

 

10, how to ensure that the transformer has a rated voltage output?

 

The voltage is too high or too low will affect the normal operation and service life of the transformer, so the voltage must be regulated. The way to regulate the pressure is to lead out a few taps in the primary coil, connected to the tap beginning, tap beginning by turning the contact to change the number of turns of the coil. The desired voltage rating can be obtained by simply turning the tap position. It should be noted that voltage regulation should normally be carried out after cutting off the load connected to the transformer.

 

11. What kind of small transformers are commonly used? Where is it used?

 

Small transformer refers to the capacity of 1 kVA or less single-phase transformer, mostly used for electrical equipment control power transformers, electronic power transformers and safety lighting power transformers.

 

12. What are the losses of the transformer in operation? How to reduce the loss?

 

The loss in transformer operation includes two parts; (1) It is caused by the iron core. When the coil is energized, the magnetic force lines are alternating, causing eddy current and hysteresis loss in the iron core, which is collectively referred to as the iron loss. (2) is caused by the resistance of the coil itself, when the primary coil and secondary coil of the transformer have current through, it is necessary to produce power loss, this loss is called copper loss.

 

The sum of iron loss and copper loss is transformer loss, which is related to transformer capacity, voltage and equipment utilization rate. Therefore, in the selection of transformers, should try to make the equipment capacity and the actual use of the same, in order to improve the utilization of equipment, pay attention not to make the transformer light load operation.

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

 

The nameplate of the transformer indicates the performance, technical specifications and occasions of use of the transformer to meet the user's selection. The main technical data usually paid attention to in selection are:

 

(1) kilovolt-ampere of rated capacity. That is, the output capacity of the transformer at rated state. Such as single-phase transformer rated capacity =U line ×I line; Three-phase transformer capacity =U line ×I line.

 

(2) Rated voltage volts. Indicate the end voltage of the primary coil and the end voltage of the secondary coil (when no load is connected) respectively. Pay attention to the terminal voltage of the three-phase transformer.

 

(3) Rated current ampere number. Refers to the line current I line value of the primary coil and secondary coil allowed to pass through for a long time under the condition of rated capacity and allowable temperature rise.

 

(4) Voltage ratio. Ratio of the primary coil voltage rating to the secondary coil voltage rating.

 

(5) Wiring mode. Single-phase transformer only has a group of high and low voltage coils, only for single-phase use, three-phase transformer has Y/△ type. In addition to the above technical data, there are transformer rated frequency, phase number, temperature rise, transformer impedance percentage, etc.

 

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

 

First to investigate local electricity power supply voltage, the user's actual electricity load and local conditions, and then refer to the technical data by selecting transformer nameplate marking, generally from transformer capacity, voltage, current and environmental conditions into account, the capacity selection should be based on user capacity of electrical equipment, properties and use of time to determine the required load, in order to choose the transformer capacity.

 

In normal operation, the power load should be about 75 ~ 90% of the rated capacity of the transformer. When the actual load borne by the transformer is measured to be less than 50% during operation, the small-capacity transformer should be replaced. If it is greater than the rated capacity of the transformer, the large-capacity transformer should be replaced immediately. At the same time, the primary coil voltage value of the transformer is determined according to the line power supply in the selection of the transformer. The voltage value of the secondary coil is selected according to the electrical equipment. It is best to choose the low-voltage three-phase four-wire power supply. This can provide power and lighting power at the same time.

 

For the selection of current to pay attention to the load in the motor starting can meet the requirements of the motor (because the motor starting current is 4 ~ 7 times larger than the sinking operation).

15. Why can't the transformer run over load?

 

Overload operation means that the transformer runs beyond the current value specified on the nameplate. Overload is divided into normal overload and accident overload. The former refers to the increase in electricity consumption caused by the normal power supply. It often makes the transformer temperature rise, promotes the aging of transformer insulation and reduces the service life, so the transformer is not allowed to overload. Under special circumstances, the overloaded operation of the transformer in a short period of time shall not exceed 30% of the rated load (winter), and shall not exceed 15% in summer. For the latter, the requirements of accident overload and allowable time are shown in the table below.

16. What kinds of tests should be done for the transformer in operation?

 

In order to ensure the normal operation of the transformer, the following tests should be carried out frequently;

 

(1) Temperature test. Transformer operation is not normal, the temperature is very important. The code stipulates that the upper oil temperature must not exceed 85C(i.e., temperature rise of 55C). General transformers are equipped with special temperature measuring device.

 

(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 really bear must be measured in the transformer operation. Measurement work is usually carried out in each season peak period of electricity consumption, with clamp ammeter direct measurement. The current value should be 70 ~ 80% of the rated current of the transformer. If it exceeds the rated current, the load should be adjusted immediately.

 

(3) voltage measurement. The regulation requires that the range of voltage variation should be within ±5% of the rated voltage. If it exceeds this range, the tap should be used to adjust the voltage to reach the specified range. In general, voltmeters are used to measure the voltage at the secondary end of the coil and the voltage at the end of the user.

(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. Measurement should try to make the transformer stop running, the use of the meter to determine the transformer insulation resistance value, the resistance is not less than 70% of the previous measured value, the selection of the meter, the low voltage coil can be used 500 volts voltage level.