How to Connect Transformers? Introduction of Transformer Connection Method

Transformer is a device that uses the principle of electromagnetic induction to change AC voltage. Its main components are primary coil, secondary coil and core. The main functions are: voltage conversion, current conversion, impedance conversion, isolation, voltage stabilization (magnetic saturation transformer) and so on. It can be divided into power transformer and special transformer (electric furnace transformer, rectifier transformer, power frequency test transformer, voltage regulator, mine transformer, audio transformer, medium frequency transformer, high frequency transformer, impact transformer, instrument transformer, electronic transformer, reactor, transformer, etc.).

The working principle of transformer:

The transformer consists of iron core (or magnetic core) and coil. The coil has two or more windings. The winding connected with the power supply is called primary coil, and the other windings are called secondary coil. It can change AC voltage, current and impedance. The simplest core transformer consists of a core made of soft magnetic material and two coils with different turns on the core, as shown in the figure below.

The role of the core is to strengthen the magnetic coupling between the two coils. In order to reduce the eddy current and hysteresis loss in iron, the core is made of coated silicon steel sheet laminated; there is no electrical connection between the two coils, and the coils are made of insulated copper (or aluminum) wires. One coil connected to AC power supply is called primary coil (or primary coil), and the other coil connected to electrical appliances is called secondary coil (or secondary coil). The actual transformer is very complex. There are inevitable copper loss (coil resistance heating), iron loss (core heating) and magnetic leakage (air-closed magnetic induction line), etc. In order to simplify the discussion, only ideal transformer is introduced here. The conditions for an ideal transformer to be established are: ignoring flux leakage, ignoring the resistance of the primary and secondary coils, ignoring the loss of the core, and ignoring the no-load current (the current in the original coil of the secondary coil open circuit). For example, the power transformer is close to the ideal transformer when it runs at full load (the rated output power of the secondary coil). Transformers are static electrical appliances based on the principle of electromagnetic induction.

When the original coil of the transformer is connected to the AC power supply, the alternating magnetic flux is generated in the core, which is commonly expressed in_. The original and secondary coils have the same, and is also a simple harmonic function, expressed as =_msin_t. According to Faraday's law of electromagnetic induction, the induced electromotive force in the primary and secondary coils is e1=-N1d/dt and e2=-N2d/dt. In the formula, N1 and N2 are the turns of the primary and secondary coils. From the graph, we can see that U1=-e1, U2=e2 (the physical quantity of the original coil is represented by the lower corner label 1, and the physical quantity of the secondary coil is represented by the lower corner label 2). Its complex effective value is U1=-E1=jN1_, U2=E2=-jN2_, so that k=N1/N2, the ratio of transformer. U1/U2=-N1/N2=-k, i.e. the ratio of the effective voltage of the primary and secondary coils of the transformer, is equal to its turn ratio, and the phase difference of the voltage of the primary and secondary coils is pi.

It is concluded that when the no-load current of U1/U2=N1/N2 can be neglected, there is I1/I2=-N2/N1, that is, the effective value of the current of the primary and secondary coils is inversely proportional to the number of turns, and the phase difference is pi.

Then I1/I2=N2/N1 can be obtained.

The power of original and secondary coils of ideal transformer is equal to P1=P2. The ideal transformer itself has no power loss. The actual transformer always has loss, and its efficiency is_=P2/P1. The efficiency of power transformer is very high, up to 90%.

Transformer wiring method:

There are two common connection methods for transformer windings, namely "triangular connection" and "star connection". In the connection group of transformer, "D" means triangular connection, "Y n" means star-shaped connection with neutral line, Y means star-shaped connection, n means with neutral line, and "11" means that the line voltage of secondary side of transformer Uab lags behind that of primary side line voltage UAB330 degrees (or 30 degrees ahead).

The connection group of transformer is represented by uppercase letters indicating the connection mode of primary side (or primary side) and lowercase letters indicating the connection mode of secondary side (or secondary side). Y (or y) is a star connection, D (or d) is a triangle connection. The number is expressed by clock, which is used to express the phase relationship between the primary and secondary side line voltage. The primary side line voltage phasor is used as a minute needle, fixed at the position of 12 o'clock, and the secondary side line voltage phasor is used as a time needle. "Yn, d11", 11 of which means that when the primary side line voltage phasor is used as a minute needle to point at 12 o'clock, the secondary side line voltage phasor is at 11 o'clock. That is to say, the line voltage Uab on the secondary side lags behind the line voltage UAB330 degrees (or 30 degrees ahead) on the primary side.

When the two windings of transformer are combined, four kinds of connection groups are formed: "Y, y", "D, y", "Y, d" and "D, d". Our country only adopts "Y, y" and "Y, d". Since there are two kinds of connection with and without neutral line, no sign representation is added without neutral line, and the letter N is added after the letter Y to indicate with neutral line. N denotes that the neutral point has a lead-out line. Yn0 connection group, UAB and UAB coincide, time, minute needle refers to 12. "12" is expressed as "0" in the new connection group. At present, there are two main connection methods of transformer used by transformer manufacturers: triangular connection and star connection.

Two wiring methods wiring schematic diagram

The triangle connection method is to connect the power supply or load of each phase in turn, and draw out each connected point as three phase lines of three-phase electricity. There is no neutral point or neutral line in triangle connection, so only three-phase three-wire system is needed. After adding ground wire, it becomes three-phase four-wire system and triangle connection three-phase electricity. The line voltage is equal to the phase voltage and the line voltage is equal to the phase voltage.