Fig. 2,012.—Method of determining core loss. Connect voltmeter and wattmeter as shown in the illustration to the low tension side of the transformer. By means of a variable voltage transformer bring the applied voltage to the point for which the transformer is designed. The wattmeter indicates directly the core loss, which includes a very small loss due to the current in the copper.
Cautions.—1. Make sure of the voltage and frequency. The manufacturers' tabulated statements refer to a definite voltage and frequency and these have a decided influence upon the core loss. 2. The high tension circuit must remain open during the test.
The voltage between any main wire and the neutral will be 57 per cent. of the voltage between any two main wires. For general distribution this system is desirable, requiring less copper and greater flexibility than other systems.
Three phase 200 volt motors may be supplied from the main wires and 115 volt lamps connected between each of the three main wires and the neutral; if the lamp load be very nearly balanced the current flowing in the neutral wire will be very small, as in the case of the ordinary three wire direct current system.
How to Test Transformers.—The troubles incident to gas or water service have their parallels in electric power distribution.
Fig. 2,013.—Method of determining copper loss. Connect ammeter and wattmeter to high tension side of transformer short circuit secondary leads, as shown in the illustration, and by means of a variable voltage, adjust current to the full load value for which the transformer is intended. The wattmeter reading shows the copper loss at full load. The full load primary current of any transformer is found from the following equation.
- full load current = full load watts ÷ primary volts
EXAMPLE: To find proper full load current on a five kw. 2,200 volt transformer, divide 5,000 watts by 2,200 volts, the full load current will then be 2.27 amperes. A slight variation in primary current greatly increases or decreases the copper loss.