Page 118 - Louth Transformer Catalogue

LT DATA

AUTO TRANSFORMERS, PHYSICAL SIZES
Due to the characteristics of auto transformers their physical sizes can often give a saving in space. To determine
the equivalent size of an auto transformer against that of a double wound transformer the following formula may
be used as a guide. It should be remembered that other factors may alter the size, i.e. amperages, high winding
voltages, etc.
Voltage difference x current at highest voltage = VA

Voltage difference = The difference between the input volts and output volts
Highest voltage = The highest voltage tap should be selected (either input or output)
and the current at that tap should be determined.
DATA REQUIRED FOR QUOTATION PURPOSES

To enable a manufacturer to quote for, or make transformers to specification certain information is required. Below
is a list of essential points.
1) Primary input tappings and frequency i.e. 0/240V 50Hz
2) Outputs required in volts and amps i.e. 0/12V at 10A (120VA)
3) Type of load i.e. Resistive/inductive
4) Physical style i.e. Tags/Screw terminals/Cased
5) Size limitations
Please advise if particular attention should be given to:-
Regulation, duty cycle, ambient temperature, PCB Pin Positions, type of mounting, length of fly leads.

Remember transformers are normally rated in VA. Inductive loads produce power factors less than unity.
EFFICIENCY

Of a given transformer is given by the following formula:-
Output in watts x 100
Output in watts + total losses in watts

Expressed as a percentage
The iron loss of a transformer remains constant regardless of load. The copper loss varies with the square of the load
and is directly proportional to the load when expressed as a percentage.

RESISTANCE
The resistance of a copper winding increases at the rate of 0.4% per degree C rise in temperature.

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