Technical explanations

Eltra Elektromaschinen- und Transformatorenbau GmbH

Ambient temperature

The rated ambient temperature is the maximum temperature at which the transformer may be operated continously under normal conditions of use. The power output values stated in our catalog hold for ambient temperature of 40°C.

Transformers with cooling channels must be vertical. Ventilation must be free of obstruction.

For ambient temperatures higher than 40°C, the power must be reduced according to the following table.

Ambient temperature           45°C      50°C      55°C      60°C

Nominal rating as %              95          85          80          75
of unit rating


All transformers are rated, if not otherwise specified, so that the rated output is achieved at rated supply voltage, rated frequency, rated output current and rated power factor cos phi = 1. For three phase supply, the rated voltage is the phase-to-phase voltage.


All transformers listed here are suitable for use with a rated frequency of 50 – 60 Hz. On the rating plate of the standard transformer, there are two frequency  50 Hz and  60 Hz and the rated output is equal. On request, we can produce transformers with a frequency up to 1 kHz.

Time of operation

The nominal rating specification in the catalog relate to the operating mode Continuous Operation (CO). In most cases with shorter loading times, Intermittent Operation (IO), a transformer with a lower unit rating can be used.
The time of operation for transformers is the relationship between load time and duty cycle time. The duty cycle time is limited to 10 minutes. The reduction factor in the following table states the permissible reduction of the transformer in relation to continous operation.

The time of operation in % can be calculated with the following formula:
ED% = (Operating time / Duty cycle time) x   100%

ED %Factor

Rated power: 200 VA
Operating time: 6 minutes

ED = 6 / (6+4) x 100% = 60%

Type :
200 VA * 0,77 = 154 VA

The power  is: 154 VA
That`s the type: 160 VA

It should be noted that the voltage drop increases in proportion to the relationship between the nominal rating and the unit rating.

Protection type

The protection type indicated by IP xx , specifies protection against foreign bodies, contact and water ingress. The definitions of the individual characteristic numbers are stipulated in DIN  40050.


IP 00

no special protection

IP 20

  • protection against contact with fingers
  • protection against penetration by solid foreign bodies larger than 12 mm

IP 23

  • protection against contact with fingers
  • protection against penetration by solid foreign bodies larger than 12 mm
  • protection against sparkling water falling at an angle of up to 60° to the vertical

IP 54

  • complete contact protection
  • protection against dust deposits
  • protection against splash water

Installation at altitudes higher than 1000 m

The transformer, reactors and power supply units are designed for installation up to 1000 m above sea level. When installed at sites  higher  than  1000 m,  the loading must be reduced. The values for a rating are stated in the following table.

Installation at altitudes mPower reducing factor