PROBLEMS 501(c) Determine the fraction of load for which
the efficiency is a maximum, and calculate
the corresponding efficiencies for the power
factors of unity, 0.8, and 0.6.
11.4.5Consider the solution of Example 11.4.1. By
means of a phasor diagram, determine the load
power factor for which the regulation is maxi-
mum (i.e., the poorest), and find the correspond-
ing regulation.
11.4.6A 10-kVA, 200:400-V, single-phase transformer
gave these test results:- Open-circuit test (LV winding supplied): 200 V,
3.2 A, 450 W - Short-circuit test (HV winding supplied): 38 V,
25 A, 600 W
Compute the efficiency when the transformer
delivers half its rated kVA at 0.85 power factor
lagging.
*11.4.7Find the percent voltage regulation and the effi-
ciency of the transformer for the following cases:
(a) Problem 11.3.1(b).
(b) Problem 11.3.2(c).
11.4.8The transformer of Problem 11.3.3 is delivering a
full load of 100 kVA at a secondary load voltage
of 230 V. Neglect the exciting current of the
transformer and determine the voltage regulation
if:
(a) The load power factor is 0.8 lagging.
(b) The load power factor is 0.8 leading.
11.4.9A 25-kVA, 2400/240-V, 60-Hz, single-phase
transformer has an equivalent series impedance
of 3. 45 +j 5. 75 referred to the primary high-
voltage side. The core loss is 120 W. When the
transformer is delivering rated kVA to a load at
rated secondary voltage and a 0.85 lagging power
factor, find the percent voltage regulation and the
efficiency of the transformer.
11.4.10A 25-kVA, 2200:220-V, 60-Hz, single-phase
transformer has an equivalent series impedance
of 3. 5 +j 4. 0 referred to the primary high-
voltage side.
(a) Determine the highest value of voltage regu-
lation for full-load output at rated secondary
terminal voltage.
(b) At what power factor does it occur?
(c) Sketch the corresponding phasor diagram.
11.4.11The following data were obtained on a 25-
kVA, 2400:240-V, 60-Hz, single-phase distribu-
tion transformer:- Open-circuit test with meters on LV side: 240
V, 3.2 A, 165 W - Short-circuit test with meters on HV side: 55 V,
10.4 A, 375 W
Compute the worst voltage regulation and the
power factor at which it occurs, when the trans-
former is delivering rated output at rated sec-
ondary terminal voltage of 240 V. Sketch the
corresponding phasor diagram.
*11.4.12The efficiency of a 400-kVA, single-phase, 60-Hz
transformer is 98.77% when delivering full-load
current at 0.8 power factor, and 99.13% with half-
rated current at unity power factor. Calculate:
(a) The iron loss.
(b) The full-load copper loss.
(c) The efficiency at 3/4 load, 0.9 power factor.
11.4.13A transformer has its maximum efficiency of
0.9800 when it delivers 15 kVA at unity power
factor. During the day it is loaded as follows:
12 hours 2 kW at power factor 0.5
6 hours 12 kW at power factor 0.8
6 hours 18 kW at power factor 0.9
Determine the all-day efficiency.
11.4.14A single-phase, 3-kVA, 220:110-V, 60-Hz trans-
former has a high-voltage winding resistance of
0.3, a low-voltage winding resistance of 0.06
, a leakage reactance of 0.8on its high-
voltage side, and a leakage reactance of 0.2
on its low-voltage side. The core loss at rated
voltage is 45 W, and the copper loss at rated
load is 100 W. Neglect the exciting current of the
transformer. Find the per-unit voltage regulation
when the transformer is supplying full load at 110
V and 0.9 lagging power factor.
11.4.15The transformer of Problem 11.3.4 operates on
the following load cycle:
12 hours full load, 0.8 power factor lagging
4 hours no load
8 hours one-half full load, unity power
factor
Compute the all-day energy efficiency.
11.4.16A 75-kVA transformer has an iron loss of 1
kW and a full-load copper loss of 1 kW. If the
transformer operates on the following load cycle,
determine the all-day efficiency: