Transformer basic principle, classification and main parameters
A transformer is a device that converts alternating current voltage, current, and impedance. When an alternating current flows in the primary coil, an alternating magnetic flux is generated in the iron core (or core), which induces a voltage (or current) in the secondary coil. The transformer consists of a core (or core) and a coil, and the coil has two or more windings, wherein the winding of the power supply is called the primary coil and the rest of the winding is called the secondary coil.
First, classification
Classified by cooling method: dry (self-cooling) transformers, oil-immersed (self-cooling) transformers, and fluoride (evaporative cooling) transformers.
According to moisture classification: open transformers, potted transformers, sealed transformers.
According to core or coil structure classification: core type transformer (insert core, C type core, ferrite core), shell type transformer (insert core, C type core, ferrite core), Toroidal transformers, metal foil transformers.
According to the number of phases of the power supply: single-phase transformers, three-phase transformers, multi-phase transformers.
Classified by purpose: power transformers, regulator transformers, audio transformers, intermediate frequency transformers, high frequency transformers, pulse transformers.
Second, the characteristics of power transformer parameters
1 operating frequency
Transformer core loss is related to the frequency, so it should be designed and used according to the frequency of use. This frequency is called the operating frequency.
2 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.
3 Rated voltage
Refers to the voltage allowed to be applied to the coil of the transformer and must not exceed the specified value during operation.
4 voltage ratio
Refers to the transformer primary voltage and secondary voltage ratio, there is no load voltage ratio and load voltage ratio difference.
5 no-load current
When the transformer secondary is open, the primary still has a certain current. This part of the current is called the no-load current. The no-load current consists of a magnetizing current (flux producing) and an iron loss current (caused by the core loss). For a 50Hz power transformer, the no-load current is substantially equal to the magnetizing current.
6 No-load loss: refers to the power loss measured at the primary when the transformer secondary is open. The main loss is the core loss, followed by the loss (copper loss) of the no-load current on the copper resistance of the primary coil. This part of the loss is very small.
7 Efficiency
Refers to the percentage of the ratio of secondary power P2 to primary power P1. In general, the higher the rated power of the transformer, the higher the efficiency.
8 Insulation resistance
Indicates the insulation performance between the transformer coils and between the coils and the iron core. The level of insulation resistance is related to the performance of the insulating material used, the temperature and the degree of humidity.
Third, the characteristics of audio transformers and high-frequency transformer parameters
1 Frequency response
Refers to the transformer secondary output voltage characteristics with the operating frequency changes.
2 pass bands
If the output voltage of the transformer at the intermediate frequency is U0, the frequency range when the output voltage (input voltage remains unchanged) falls to 0.707U0, which is called the passband B of the transformer.
3 Initial and secondary impedance ratio
The initial and secondary impedances of the transformer are connected to the appropriate impedances Ro and Ri to match the primary and secondary impedances of the transformer. The ratio of Ro and Ri is called the primary and secondary impedance ratio. In the case of impedance matching, the transformer works in the best condition and has the highest transmission efficiency.
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