Under what circumstances should the control transformers be used
Generally speaking, the control voltage should not be selected more than 220V, because of the increase of the working voltage, the capacitive load current between the control conductors increases. transformer If the extended distance of the control network is longer, the application of the common open contact line will cause the contactor to be mistakenly connected, and often closed contact line will make the contactor "open."
With the rise of the voltage, the contact and relay coils used in the wire cross-section will be reduced, thus weakening the wire mechanical strength and overload capacity, the chance of failure will increase. Overload ability plays an important role because the air gap of the contactor's magnetic system becomes larger with time, or the external foreign body invades the air gap, which will keep the current higher.
As the voltage rises, the creepage current, that is, the resistive current also increases, considering that the contact and relay dimensions are usually very compact, so especially in the case of pollution hazards, transformer will appear such as charging current too high caused by similar failures.
Under what circumstances the control transformers should be used:
First, the working voltage of the power supply is greater than 220V, transformer or the other voltage of the power supply is not applicable and controlled.
Second, for special reasons, the control voltage is required to be different from the power supply voltage, especially when using small voltages for control purposes.
Third, power supply without neutral line.
Four, because of the work need, must use completely the ground insulation control power supply.
At the end of the control circuit, the voltage of the ground voltage is half of that of the phase line (the central point of the control transformer's secondary winding is earthed).
In a power grid (industrial grid) with a higher short-circuit current, to reduce the short-circuit capacity (reducing the risk of the contact weld).
It is necessary to connect the control loop between two-phase lines in the power grid, transformer especially in the star-shaped network where the phase line and the neutral line are connected with different control voltages.