Principle:
The transformer is composed of an iron core (or magnetic core) and a coil. The coil has two or more windings, among which the winding connected to the power supply is called the primary coil, and the rest The winding is called the secondary coil. It can transform AC voltage, current and impedance. The simplest core transformer consists of an iron core made of soft magnetic material and two coils with different numbers of turns wrapped around the iron core.
The function of the iron core is to strengthen the magnetic coupling between the two coils. In order to reduce the eddy current and hysteresis loss in the iron, the iron core is made of laminated painted silicon steel sheets; there is no electrical connection between the two coils, and the coil is wound by insulated copper wire (or aluminum wire). One coil connected to the AC power source is called the primary coil (or primary coil), and the other coil connected to the electrical appliance is called the secondary coil (or secondary coil).
The actual transformer is very complex, and there are inevitably copper losses (coil resistance heats up), iron losses (core heats up) and magnetic flux leakage (magnetic induction lines closed by air), etc. To simplify the discussion here Only ideal transformers are introduced. The conditions for an ideal transformer to be established are: ignore the leakage flux, ignore the resistance of the primary and secondary coils, ignore the core loss, and ignore the no-load current (the current in the primary coil when the secondary coil is open).
For example, when a power transformer is operating at full load (the secondary coil outputs rated power), it is close to the ideal transformer condition. Transformers are static electrical appliances made using the principle of electromagnetic induction. When the primary coil of the transformer is connected to the AC power supply, alternating magnetic flux is generated in the core, and the alternating magnetic flux is represented by φ.
φ in the primary and secondary coils are the same, and φ is also a simple harmonic function, and the table is φ=φmsinωt. According to Faraday's law of electromagnetic induction, the induced electromotive force in the primary and secondary coils is e1=-N1dφ/dt and e2=-N2dφ/dt. In the formula, N1 and N2 are the number of turns of the primary and secondary coils.
It can be seen from the figure that U1=-e1, U2=e2 (the physical quantity of the primary coil is represented by the subscript 1, and the physical quantity of the secondary coil is represented by the subscript 2), and its complex effective value is U1=-E1=jN1ωΦ, U2=E2=-jN2ωΦ, let k=N1/N2, which is called the transformation ratio of the transformer. From the above formula, we can get U1/U2=-N1/N2=-k, that is, the ratio of the effective value of the primary and secondary coil voltages of the transformer is equal to its turns ratio and the phase difference between the primary and secondary coil voltages is π.
Then we get: U1/U2=N1/N2
When the no-load current can be ignored, I1/I2=-N2/N1, that is, the primary and secondary coils The effective value of the current is inversely proportional to the number of turns, and the phase difference is π.
Then we can get I1/ I2=N2/N1
The power of the primary and secondary coils of the ideal transformer is equal to P1=P2. Explain that the ideal transformer itself has no power loss. Actual transformers always have losses, and their efficiency is η=P2/P1. The efficiency of power transformers is very high, reaching more than 90%.
Extended information:
Characteristic parameters of transformer:
1. Operating frequency
The core loss of transformer has a great relationship with frequency, so It should be designed and used according to the frequency of use, which is called the operating frequency.
2. Rated power
Under the specified frequency and voltage, the output power of the transformer can work for a long time without exceeding the specified temperature rise.
3. Rated voltage
refers to the voltage allowed to be applied to the coil of the transformer, which shall not exceed the specified value during operation.
4. Voltage ratio
Refers to the ratio of the primary voltage and the secondary voltage of the transformer. There is a difference between no-load voltage ratio and load voltage ratio.
5. No-load current
When the secondary of the transformer is open circuit, there is still a certain current in the primary. This part of the current is called no-load current. No-load current consists of magnetizing current (generating magnetic flux) and iron loss current (caused by core losses). For a 50Hz power transformer, the no-load current is basically equal to the magnetizing current.
6. No-load loss
Refers to the power loss measured at the primary when the secondary of the transformer is open circuit. The main loss is the core loss, followed by the loss (copper loss) caused by the no-load current on the primary coil copper resistance. 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. Generally, the greater the power rating of the transformer, the higher the efficiency.
8. Insulation resistance
Indicates the insulation performance between the coils of the transformer and between each coil and the iron core. The level of insulation resistance is related to the performance of the insulating material used, temperature and humidity.
Reference: Baidu Encyclopedia---Transformer