Control –Power Transformers
Control-Power transformers are used to supply control circuits. Their application is laid down in DIN VDE 0570/2-2 which recommends the use of control-power transformers in control circuits with more than five electromagnetic operating coils. A power supply with control transformers offers a number of benefits, for example simple matching to different mains voltages. The short-circuit currents in the control network remain at levels which are supportable for the equipment in use. In the voltage test specified in VDE 0570/2-2 the equipment in the control circuit is not subjected to the test voltage. Voltages which are different from the control voltage (e.g. 24 V) can be produced via tapping points on the control-power transformer. Voltage peaks occurring in the distribution network do not have any effect on the equipment in the control circuit. The control circuits can be operated earthed or unearthed regardless of the type of networks. Control-power transformers are manufactured with separate primary and secondary windings in accordance with VDE 0550 part 3.
In the SET series the clearances and creepage distances as well as the test voltage conform to VDE 0570/2-2. Suitable measures must be taken by the user to protect against overloading and short-circuit.
In order to match the voltage our control-power transformers are equipped on the primary side with +5% and -5% taps. Control-power transformers must be connected downstream of the main switch in the particular installation, preferably between two phase conductors and suitable measures must be taken to protect against short-circuiting. Voltages up to max. 250 V, preferably 230 V, are possible at the output of the control-power transformers. The main factors dictating the size are the rated power and the short-time rating. The values specified for the short-time rating are the values of the output power at cos. phi 0.5 at which the output voltage can permissibly drop by 5% compared with the nominal value.