A transformer is a device that converts alternating voltage, current, and impedance. When an alternating current is passed through the primary coil, an alternating magnetic flux is generated in the iron core (or core) to induce a voltage (or current) in the secondary coil. The transformer consists of a core (or core) and a coil. The coil has two or more windings. The winding connected to the power supply is called the primary coil, and the other winding is called the secondary coil.
First, the production principle of the transformer:
In the generator, whether the coil motion passes through the fixed coil through the magnetic field or the magnetic field, the potential can be induced in the coil. In both cases, the value of the magnetic flux is constant, but the number of magnetic fluxes intersecting the coil is Change, this is the principle of mutual induction. A transformer is a device that uses electromagnetic mutual induction to transform voltage, current, and impedance.
Second, classification
Classified by cooling method: dry (self-cooling) transformer, oil immersed (self-cooling) transformer, fluoride (evaporative cooling) transformer. Classified according to the moisture-proof method: open transformer, potted transformer, sealed transformer.
According to the core or coil structure classification: core transformer (insert core, C core, ferrite core), shell transformer (insert core, C core, ferrite core), Ring transformer, metal foil transformer.
Classified by power phase number: single-phase transformer, three-phase transformer, multi-phase transformer.
Classified by use: power transformer, voltage regulator, audio transformer, intermediate frequency transformer, high frequency transformer, pulse transformer.
Third, the power transformer's characteristic parameters
The working frequency transformer core loss has a great relationship with frequency, so it should be designed and used according to the frequency of use. This frequency is called the working frequency.
Rated power At a specified frequency and voltage, the transformer can operate for a long period of time without exceeding the output power of the specified temperature rise.
The rated voltage refers to the voltage that is allowed to be applied to the coil of the transformer and must not be greater than the specified value during operation.
The voltage ratio refers to the ratio of the primary voltage to the secondary voltage of the transformer, and has the difference between the no-load voltage ratio and the load-to-voltage ratio.
When the no-load current transformer is open secondary, there is still a certain current in the primary. This part of the current is called no-load current. The no-load current consists of magnetizing current (generating magnetic flux) and iron loss current (caused by core loss). For a 50 Hz power transformer, the no-load current is substantially equal to the magnetizing current.
No-load loss refers to the measured power loss at the primary when the transformer is open secondary. The main loss is the core loss, followed by the loss (copper loss) of the no-load current on the primary coil copper resistance, which is small.
Efficiency refers to the percentage of the ratio of secondary power P2 to primary power P1. Generally, the higher the rated power of the transformer, the higher the efficiency.
The insulation resistance represents the insulation performance between the coils of the transformer and between the coils and the iron core. The level of insulation resistance is related to the properties of the insulation used, the temperature and the degree of humidity.
Fourth, the technical parameters of the low frequency transformer
There are corresponding technical requirements for different types of transformers, which can be expressed by corresponding technical parameters. For example, the main technical parameters of the power transformer are: rated power, rated voltage and voltage ratio, rated frequency, operating temperature level, temperature rise, voltage regulation, insulation performance and moisture resistance. The main technical parameters for general low-frequency transformers are: transformation ratio, frequency characteristics, nonlinear distortion, magnetic shielding and electrostatic shielding, and efficiency.
Voltage ratio: The number of turns of the two sets of transformers is N1 and N2, N1 is the primary and N2 is the secondary. An alternating voltage is applied to the primary coil, and an induced electromotive force is generated across the secondary coil. When N2>N1, its induced electromotive force is higher than the voltage applied by the primary. This transformer is called a step-up transformer: when N2 is called voltage ratio (turn ratio). When n<1, then N1>N2, V1>V2, the transformer is a step-down transformer. The reverse is the step-up transformer.
Transformer efficiency: At rated power, the ratio of the output power of the transformer to the input power is called the efficiency of the transformer, ie η is the efficiency of the transformer; P1 is the input power and P2 is the output power.
When the output power P2 of the transformer is equal to the input power P1, the efficiency η is equal to 100% and the transformer will not produce any loss. But in fact this kind of transformer is not available. Losses are always generated when the transformer transmits electrical energy. This loss mainly includes copper loss and iron loss. Copper loss is the loss caused by the resistance of the transformer coil. When current is generated by the coil resistance, a portion of the electrical energy is converted into thermal energy and lost. Since the coil is generally wound by an insulated copper wire, it is called a copper loss.
The iron loss of the transformer includes two aspects. One is the hysteresis loss. When the alternating current passes through the transformer, the direction and magnitude of the magnetic field lines passing through the silicon steel sheet of the transformer change, so that the internal molecules of the silicon steel sheet rub against each other, releasing heat energy, thereby losing a part of the electric energy, which is the hysteresis loss. .
The other is eddy current loss when the transformer is working. The magnetic core has a magnetic flux passing through it, and an induced current is generated on a plane perpendicular to the magnetic line. Since this current forms a circulating current in a closed loop and is spiraled, it is called an eddy current. The presence of eddy currents causes the core to heat up, consuming energy, and this loss is called eddy current loss.
The efficiency of the transformer is closely related to the power level of the transformer. Generally, the higher the power, the smaller the ratio of loss to output power, and the higher the efficiency. Conversely, the lower the power, the lower the efficiency.