II. Low t*r loss type, Fig. 155 Exciting current 4 per cent. 4 per cent. Primary resistance loss 1 per cent. 0 . 5 per cent. Secondary resistance loss Core loss 1 per cent. 1 per cent. 0 . 5 per cent. 2 per cent. For convenience, exciting current and losses are frequently given in per cent, of the full-load output of the transformer. The curves correspond to non-inductive load. The core loss comprises hysteresis, which varies with the 1.6 power of the induced voltage and eddies proportional to the square of induced voltage. Hence, within the narrow range of variation of the induced voltage between no load and full load of a constant poten- tial transformer, the core loss can be approximated as propor- tional to the 1.7 power of the induced voltage. The induced voltage at non-inductive load equals impressed voltage minus primary ir, when neglecting the inductive drop, which is permis- sible at non-inductive load. As the induced voltage thus de- creases proportional to primary ir, the core loss decreases pro- portional to 1.7 times the primary ir. Thus, with the primary tV equal to 1 per cent, at full load, the induced voltage has 282 ELEMENTS OF ELECTRICAL ENGINEERING decreased 1 per cent, and the core loss 1.7 per cent, at full load, and correspondingly at other loads. As seen, I and II have the same full-load efficiency, but II is more efficient at overload, I at partial load. EFFICIENCY and LOSSES of Low Corcloss Transformer 1% Iron Loss 2% i2r Loss .9 1.0 1.1 1.2 1.3 1.4 1.5 .1 .2 .3 .4 .5 .6 .7 .8 FIG. 154. — Efficiency and losses of low core loss transformer. 114. In transformers for lighting and general distribution {usually with 2300 volt primary and 2 X 115 volt secondary) the transformer is generally heavily loaded only for a short time during the day, partly loaded for a moderate time, and prac- tically unloaded for most of the time. Thus load curves of such a transformer would be: ALTERNATING-CURRENT TRANSFORMER 283 A. Lighting and power B. Lighting only 2 hours at IK load. 2 hours at IK load. 2 hours at % load. 2 hours at % load. 6 hours at Y2 load. 20 hours of ]/ clL /2 G load. 14 hours at Ho load. EFFICIENCY and LOSSES of High Coreloss Transformer; 2 % Iron Loss 1% t2rLoss I V V A \ \ % \ % 100 > Loss 5.0 90 \ ,^~- \ *j 4.5 80/ /* \ s. / / 4.0 / 5 X. ^X x^ 3.5 ^> ^ ^^ ^ x^ 3.0 -^o^ *\* se»^ •^^ 2^, — — — — = =r _ — — - ^. — • ^ • Ire nLos ses S / ^ x 2.0 1.5 ^^ <; 1.0 j2 V-O^ ^ .5 u_i— Lc ad^- ->- .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0 1.1 1.2 1.3 1.4 1.5 FIG. 155. — Efficiency and losses of low ilr loss transformer. 284 ELEMENTS OF ELECTRICAL ENGINEERING This gives for the two types of transformers: