Technical Data

Factors affecting the Transformer size

1. The physical size of a transformer is mainly determined by the load VA which it is to supply. Runthala Transformers are designed to provide a long life at rated load and there is no need to derate, provided the rated ambient temperature does not exceed.
2. Where electrical isolation is not required between the input and output circuits, the use of an auto transformer can often present a useful reduction in size. For a simple step-up or step-down transformer the frame VA needed is given by:-

Frame VA =
High Voltage - Low Voltage
High Voltage
x Output VA High Voltage

It is seen that the size advantage is particularly great when the difference between input and output voltage is small.
3. The provision of a multi-tap primary requires more winding space than a single voltage primary and tappings should not be specified unless they are actually required. By way of example, if the transformer's primary winding is specified as 0-380-415-440-500-550, the VA size will be increased by about 12%. The use of series parallel windings (e.g. 110/220 volts) does not in principle carry a size penalty but will nevertheless increase costs and possibly give rise to a reduction in reliability.

4. Transformers are designed for operation in free air at ambient temperatures up to 45° centigrade. When operating in a cubicle, provision must be made for an adequate exchange of air. A flow of 3 cubic meters per minute per kW of loss is generally sufficient. If air flow is too low the loss must be reduced by using a bigger sized transformer.

Single Phase Loads on 3 Phase Systems

It is not possible, using only transformers, to connect a single load to a three phase source in such a way as to present a balanced load to the supply. Various transformer systems are used and these are listed below with performance given in terms of a 1KVA load supplied from a 415 volt three phase source. For most applications, either the single phase transformer or the open delta transformer are preferred.
Transformer Type
Line Currents
Red Yellow Blue
Transformer Size
Single phase transformer
connected line-line
2.41 2.41 0 1KVA
3 phase core with open delta windings on the outer limbs 2.78 1.39 1.39 1KVA
Scott connection utilising two single phase transformers 2.68 1.96 139 900 VA (Main TX)
700 va (Teaser)
3 phase transformer with the Le-Blanc connection 2.68 1.96 0.72 1.2 KVA

Rectifier Transformers

The specification of a transformer, used as part of a rectifier system, is a complex matter generally best left to the transformer designer. Given the rectification circuit, with it's component values, the transformer designer can make the necessary allowance for transformer "imperfections" when doing his calculations. Information generally needed is:-

a) Required DC output voltage and rated output current
b) Supply voltage and frequency
c) Source impedance of AC supply - if not normal mains
d) Rectification system-
  • • Single phase half wave
  • • Single phase push-pull
  • • Voltage doubler
  • • Single phase bridge
  • • 3 phase bridge
  • • 6 pulse single way Multiplex bridge circuits e.g. 12 pulse with inter-phase reactor
    e) Rectifier voltage drop or rectifier type
    f) Smoothing system-
  • • Capacitor input filter (uF value or allowed ripple voltage)
  • • Choke input filter (value and voltage drop)
  • • Un-smoothed-resistive load

- Capacitive load

The relationship between required transformer "VA size" and load power is dependent upon the required output voltage and output power value as well as the rectifier system being used. Some guide values for transformer VA transformer VA based on a typical 24 volt DC output are:-
  • • Single phase bridge with capacitive smoothing - 1.7x Vdc x Idc
  • • Single phase bridge with no smoothing - 1.3 x Vdc x Idc
  • • 3 phase bridge with inductive smoothing - 1.2 x Vdc x Idc