How to solve the problem of large current at the moment of energization of the toroidal transformer?

First, we need to understand what surge current is. A toroidal transformer will generate a large current peak when energized, called surge current. In simple words, surge current in a transformer is caused by voltage transients. Surge currents can be divided into two types: magnetizing inrush current and excitation inrush current.  

Magnetizing inrush current: When the magnetic induction strength exceeds the saturation point, the slope decreases sharply, i.e. when the magnetic induction strength increases further, the excitation current will increase sharply, which will produce the so-called magnetizing inrush current. (A transformer is a static device made based on electromagnetic induction principles. When a toroidal transformer is energized with no load, it produces magnetizing inrush current that is 80-100 times the steady magnetizing current and 6-8 times the rated current. The surge current itself does not cause much harm to the transformer, but in some cases, it can lead to currency fluctuations. If no countermeasures are taken, it may cause the transformer to overload or differential relay protection to malfunction.)

Large-capacity toroidal transformers are prone to producing relatively large surge currents when energized, which may reach 3-8 times the rated current. This is caused by:

1. The inherent characteristics of toroidal transformers cause it. The closed magnetic circuit and the small magnetic reluctance of the toroidal core lead to this.  

2. Voltage transients cause it. When the transformer is energized at rated voltage, current, and frequency, the input enters zero phase.

3. The conversion of electrical and magnetic energy in the transformer, the phase angle of voltage when energized, and the degree of core saturation will also produce magnetizing inrush current when energized.

Large-capacity transformers (generally above 1KW) have a large core that needs to be magnetized when energized, which will cause a large current, and this magnetizing current is something that all inductive loads are prone to. Regarding the problem of large starting current of large-capacity transformers, this is a common problem of all large-capacity transformers. The transformer itself cannot solve the problem, and can only be controlled through external input terminals to realize it.

Possible methods to try:

(1) Single-phase soft starter: The function of a soft starter is to reduce the starting current of inductive loads, but most single-phase soft starters on the market are 380V three-phase soft starters, and single-phase 220V soft starters are relatively rare.

(2) Frequency converter: A frequency converter can adjust both frequency and voltage. By utilizing the voltage adjustment function of the frequency converter, soft start can be achieved.

(3) Variable frequency power supply: This can definitely achieve it, because the output voltage of a variable frequency power supply can be arbitrarily adjusted from 0-250V. When the voltage is adjusted from 0 to 220V, soft start is achieved. The price of a variable frequency power supply is higher than that of a frequency converter or soft starter.

(4) Connect an inductance coil in series. Connecting an inductance coil can reduce the starting current of the transformer, but it will also affect the normal operation of the transformer. In order not to affect normal operation, it needs to be disconnected after startup. 

The above methods either have high costs or large volumes for inverters below 20KW and cannot be practically applied. The following low-cost method realizes soft charging of transformers.  

1. Using an auxiliary winding to drive a relay with a high-power current-limiting resistor to bypass the resistor after the transformer is magnetized.   

2. Our inverter adopts this method of using a current-limiting resistor and relay to realize soft charging for the transformer input. When the inverter is turned off, the soft relay is normally open, and the AC output relay is normally open. When the city power is connected to the inverter through a current-limiting resistor to magnetize the transformer, the auxiliary winding of the transformer will generate a voltage to start the relay to bypass the current-limiting resistor to complete the startup of the inverter.