The control transformer is a small dry-type transformer. Generally, the control voltage should not be above 220V. Because with the increasing of the working voltage, the capacitive load current between the control wire increases. If the extended distance of the control grid is longer, the nomarly opened contact line of application will make the contactor connected by mistake and the normally closed line will make the contactor cannnot be opened.
With the increase of the voltage, the cross-section of conductor which the contactor and relay coil use minishes, weakening the mechanical strength and overload capacity of the conductor, increasing the failure risks. Overload capacity plays an important role because the air gap of the contactor magnetic system getting bigger as time goes on or any external foreign matter invading the air gap will make the current keep rising.
With the increase of the voltage, the creepage current which is also called the resistive current, will increase too. Because of the similar appearance and specification of the contactor and relay, some fults caused by excessive charging current usually happens especially in the occasion of contamination hazard.
Now we return to the subject, when should we use control transformers?
1. The working voltage of the power is above 220V or the other voltages of the power are not suitable for the purpose of controlling, and a control transformer is required.
2. Due to some special reasons, the demand for the control voltage is different from the power voltage especially in the use of the small voltage.
3. Power supply without the neutral wire.
4. Control power with completely insulation against the ground as the work required.
5. When the each voltage to ground of the incoming and outcoming line in the control circuit is half as the voltage between the phase lines.(Grounding the secondary winding centre point of the control transformer.)
6. While in the demand of reducing the capacity of short circuit. (reducing the risk of control contact fusion welding)
7. In a grid where a control loop needs to be connected between two phase lines.Especially in the network of star connection scheme, different control voltages appear when the phase line is connected to the neutral line.