Electrician: Basic knowledge of 16 transformers, the old master said that 12 is already very good! (III)

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

The loss in the operation of the transformer includes two parts;(1) is caused by the iron core, when the coil is energized, because the magnetic lines of force are alternating, causing eddy current and hysteresis loss in the iron core, which is collectively referred to as iron loss. (2) It is caused by the resistance of the coil itself. When the primary coil and the secondary coil of the transformer have current flow, power loss will occur. 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. 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 rate of equipment, pay attention not to make the transformer light load operation. 13. What is the nameplate of the transformer? What are the main technical data on the nameplate?

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

(1) KVA of rated capacity. That is, the output capacity of the transformer in the rated state. Such as single-phase transformer rated capacity = U line x I line; Three-phase transformer capacity = U line x I line.

(2) Rated voltage volts. Indicate the terminal voltage of the primary coil and the terminal voltage of the secondary coil (when not connected to the load). Note that the terminal voltage of the three-phase transformer refers to the line voltage U line value.

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

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

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

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

First of all, we should investigate the power supply voltage of the power consumption place, the actual power consumption load of the user and the local conditions, and then select one by one according to the technical data marked on the transformer nameplate. Generally, we should consider the transformer capacity, voltage, current and environmental conditions comprehensively. The capacity selection should determine the required load according to the capacity, nature and use time of the user's power equipment, so as to select the transformer capacity.

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

The choice of current should pay attention to the load when the motor starts to meet the requirements of the motor (because the motor starting current is 4 to 7 times larger than the sinking operation). 15. Why can't the transformer be overloaded?

Overload operation refers to the operation of the transformer exceeds the current value specified on the nameplate. Overload is divided into normal overload and accident overload two, the former refers to the normal power supply situation, the user power consumption increase caused by, it often makes the transformer temperature rise, promote transformer insulation aging, reduce the service life, so do not allow the transformer overload operation. Under special circumstances, the overload 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. 16. What kinds of tests should the transformer do during operation?

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

(1) Temperature test. Whether the running state of the transformer is normal or not, the temperature is very important. The regulations stipulate that the upper oil temperature shall not exceed 85C (that is, the temperature rise of 55C). General transformers are equipped with a special temperature measuring device.

(2) load determination. In order to improve the utilization rate of the transformer and reduce the loss of power, the power supply capacity of the transformer must be measured during the operation of the transformer. The measurement is usually carried out during the peak period of electricity consumption in each season and is directly measured by a clamp-on ammeter. The current value should be 70 ~ 80% of the rated current of the transformer, which indicates overload and should be adjusted immediately.

(3) voltage measurement. The procedure requires that the voltage variation range should be within ± 5% of the rated voltage. If this range is exceeded, a tap shall be used to adjust the voltage to the specified range. Generally, the voltage meter is used to measure the terminal voltage of the secondary coil and the terminal voltage of the terminal user respectively. (4) Determination of insulation resistance. In order to keep the transformer in normal operation, the insulation resistance must be measured to prevent insulation aging and accidents. When determining, try to stop the transformer, and measure the insulation resistance value of the transformer with a shaking table. The measured resistance is required to be not less than 70% of the previously measured value. When selecting a shaking table, the low-voltage coil can be of 500V voltage level.