A power transformer is connected to a power supply system. The side receiving the power from the power grid is often called the power supply side winding, also known as the secondary winding. The side where the output energy is connected to the electrical load is called the load side winding, also known as the secondary winding. In order to explain the working principle of the transformer, the following physical quantities are first introduced:

E e - transformer primary side voltage rating;

Transformer secondary side rated voltage value;

E -- primary lateral induction potential;

- secondary side induction potential;

I - current in the primary side winding;

- secondary side load current;

- primary winding turns;

- secondary side winding number of turns;

Φ м - the main magnetic flux in core maximum;

I -- excitation current of transformer;

Z - transformer primary side impedance;

Z - transformer secondary side impedance;

Po -- transformer no-load loss;

Ps с - transformer short-circuit loss;

Integral -- power frequency;

S ? - power transformer.

For a three-winding transformer, the physical quantity measured in the first time of the transformer is annotated with symbol 1 (footnote in the lower right corner), the second side is annotated with 2, and the low-voltage side is annotated with 3, or it is indicated by lowercase letters.

The working principle of single-phase transformer

A power transformer is an electrical device that transforms voltage and transmits electrical power. Its primary side is connected with the power supply, and the power supply voltage is added to receive the electric energy in the power network. The secondary side is the output end, which is connected with the electrical equipment to supply the electric energy received from the power supply to the electrical load. The relationship between the circuit and the magnetic circuit.

When the primary side winding is connected to the power supply, under the action of rated voltage, the primary side winding has an alternating current with an power frequency of 50Hz, and the alternating current establishes a sine wave alternating flux with the same frequency, forming a magnetic circuit in the iron core and passing through the primary and secondary side windings of the transformer at the same time. When the voltage remains the same, the magnetic flux in the core also remains the same, and this magnetic flux is the main magnetic flux. When transformer secondary open-circuit, the transformer is in no-load condition, the current flows through is by a winding no-load current, no-load current transformer primary winding and the product of the number of turns N ?, namely I ? N ? is the main magnetic potential side of the transformer.

Changes according to the principle of electromagnetic induction, magnetic flux through a coil, can produce inductive electromotive force, due to the magnetic flux through the set on the same core at the same time in the two groups of winding, therefore, in the transformer primary winding of an induction electric potential E ?, on both ends of the secondary winding produce a induction electric potential E ?, if transformer winding is connected to the load at a time, there will be in the load flow load current (I) ?, such a transformer is put in the power supply for electric power to the load, output power, this is the basic working principle of the transformer.

The load working state of the transformer

If the transformer secondary circuit is connected, under the effect of secondary winding terminal voltage, the load current is the load current of the transformer, the load current depends on the secondary load impedance Z. Transformers with load current, can make magnetic flux change in the transformer core, I ? and load current transformer secondary winding is the product of the number of turns N ? secondary side magnetic potential, the magnetic potential is the role of magnetic flux in the core of demagnetization, it decreases the main magnetic flux, so that in the primary winding induction potential also reduced accordingly, the power supply voltage U ? and induced potential E ? have difference, in under the action of the difference voltage, make of the primary winding current ? increase, I at this time in the primary winding current is not only the exciting current, And increased the part to balance the second degaussing magnetic potential of current I ? (secondary load current conversion to the transformer primary side current value) to maintain the primary magnetic flux in the core remains the same, the secondary load current, the greater the current I ? is, the greater the ? I also is a current, the greater the therefore, transformer through electromagnetic induction power supply of electrical energy transferred to the load. General transformer no-load current I ? accounts for only a 2% to 10% of rated current, if you ignore the excitation current, transformer magnetic potential relation are just: I ? N ? = I ? N ?, I ? contains ? and I/I ?.

So the ratio of the transformer's primary current to its secondary current is equal to the ratio of the number of secondary turns to the number of primary turns, which is the inverse of the transformer ratio K.

Short circuit test of transformer

Another working condition of the transformer is the short circuit working condition of the transformer. When the rated voltage on the primary side of the transformer is connected and the load impedance on the secondary side is very small (close to zero), it is the short-circuit working state of the transformer. For example, the welding machine and arc furnace transformer are designed according to the short-circuit working state. If the power transformer in the normal load state of operation, the secondary load impedance suddenly becomes very small or even equal to zero, this state is called the transformer of the sudden short circuit, it is an accident. The sudden short circuit of the transformer will generate a much larger short circuit current than the normal rated current in the primary and secondary windings. The thermal and electrodynamic effects of the short circuit current can make the transformer generate more heat than the allowable value, and cause the transformer to be burnt down and damaged by strong electric power. Therefore, it is necessary to connect a variety of protective devices, such as fuse, relay protection and temperature protection, to protect the transformer from damage due to short circuit. During short circuit test, power meter measurement by numerical called copper loss, the data is usually reduced to 75 ℃ when the numerical, marking on the transformer nameplate.

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