Problems needing attention in application of ring transformer

1、 Power capacity of transformer: the power capacity of transformer is the main basis for determining the size of iron core. In many cases, the load of transformer is intermittent, such as the power transformer in audio equipment. At this time, the volume and weight of the transformer are much less than that of continuous operation. As shown in the figure, load section a is a smaller section for the whole section B. at this time, the working cycle of the transformer is much shorter than its thermal time constant. The rated power of the transformer can be calculated by equation (1). Where: pn&mdash& Mdash; Rated power of transformer (VA); Pl&mdash& Mdash; Transformer load power (VA); A&mdash& Mdash; Load on time; B&mdash& Mdash& Mdash; Transformer duty cycle.

2、 Voltage regulation rate voltage regulation rate is an important index to measure the load characteristics of transformer. Voltage regulation rate refers to the relative change value of output voltage U2 when the input voltage is constant and the load current rises from zero to the rated value, which is usually expressed as a percentage, as shown in formula (2): where:δ U&mdash& Mdash; Voltage regulation rate; U20&mdash& Mdash; No load output voltage (V); U2&mdash& Mdash; Output voltage at rated load of transformer (V).

3、 Due to the iron loss and copper loss of the transformer, the output power Po is always less than the input power PI. The efficiency of the transformer increases significantly with the increase of the capacity. The efficiency of the transformer with a capacity of more than 300VA can be as high as 95% under the rated load.

4、 When autotransformer only requires step-up or step-down, but does not require primary and secondary winding isolation, it is appropriate to use autotransformer. Autotransformer has the advantages of small volume, low cost, large transmission power and so on. Because the primary winding of autotransformer wound with ring iron core does not need insulation, it is very convenient to process, smaller volume, weight and lower cost. It should be noted that the common terminal (COM) of the primary and secondary windings of the autotransformer should be connected to the neutral line, so as to be safe. The autotransformer circuit is shown in Figure 5, and its rated power PAH is calculated according to formula (4). Pah=pao (uh － UL)/uh (VA) (4), where: pao&mdash& Mdash; Autotransformer output power (VA); Uh&mdash& Mdash; High voltage winding voltage (V); Ul&mdash& Mdash; Low voltage winding voltage (V).

5、 The temperature rise characteristic curve of the ring transformer with temperature rise problem is shown in Figure 6. It can be seen from Figure 6 that the temperature rise of the ring transformer is relatively low. For the standard series, even if the overload is 120%, the temperature rise will not exceed 70 ℃. The temperature rise of transformer is determined by the iron loss and copper loss. For laminated transformer, the two parts are basically equal. However, since the ring transformer is wound with high-quality cold-rolled silicon steel sheets, and is equipped with good annealing process, its core loss is only (10-20)% of the total loss, so the temperature rise is mainly determined by the winding copper loss. A reasonable design is that the power consumption of the primary and secondary windings should be basically balanced. The temperature rise is also closely related to the heat dissipation area. Because the temperature rise of the iron core of the ring transformer is low, the winding is wound evenly on the whole iron core, and the heat dissipation area and heat dissipation conditions are good, so a lower temperature rise can be obtained.

6、 Closing current Generally, the transformer will generate a large closing impulse current when closing, while the ring transformer will generate a larger closing current due to its lack of air gap and high permeability. Ring transformers below 300VA can be protected by general fuses, but in order to prevent the closing current from burning the fuse, the current of the fuse selected should be 8~10 times greater than the primary current of the transformer. For ring transformers above 300VA, slow speed fuses or temperature fuses should be considered for protection. Sometimes, in order to reduce the impulse current, the B value of transformer magnetic flux density can be taken lower.    

7、 Transformer and rectifier circuit Most of the ring transformers used as power supply are connected to the rectifier circuit. The most commonly used rectifier circuit and transformer secondary voltage U2, secondary current I2 and DC voltage Ud DC current Id are listed in Table 3 for reference in design timing.