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Under what circumstances should you use a control transformer?

First, the three-phase transformer power supply operating voltage is greater than 220V, or other power supply voltage is not applicable and control purposes.
Second, for special reasons, the required voltage is different from the supply voltage, especially when using a small voltage for control purposes.
Third, the machine transformer without neutral power supply.
Fourth, due to work needs, must be completely insulated to the ground control power.
Fifth, when the ground voltage on the control circuit in and out of the line is half the voltage between the phases (the transformer secondary winding center is grounded).
Sixth, in order to reduce the short-circuit capacity (reduce the risk of controlling the welding of the contact) in a grid with high short-circuit current (industrial grid).
Seventh, the need to connect between the two lines connected to the control circuit of the grid, especially in the star point of the displacement of the network, the phase line and the neutral connection will appear when the different control voltage.