This document discusses losses and efficiency in AC machines like alternators. It lists the main types of losses as copper losses, magnetic losses, and mechanical losses. It then provides examples of efficiency calculations for various alternators operating at different loads and power factors. Several problems are also presented for calculating alternator efficiencies based on given parameters like output power, resistance, losses, and operating power factor.

1. AC machines
NEE 423
Engr. Jessica Laine M. Tumbaga

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3. Alternator losses and efficiency
The various losses occurring in a generator can be
subdivided as follows:
1. Copper Losses
a. Armature copper loss
b. Field copper loss
c. Brush contact loss
2. Magnetic Losses
a. Hysteresis loss
b. Eddy current loss
3. Mechanical Losses
a. Friction loss
b. Air-friction or windage loss
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4. Losses of an alternator
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5. Power stages
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𝜂 =
𝑃𝑂
𝑃𝑖𝑛
𝑥100%
Pgen = Eg*Ia Total losses = ML + EL
Po = Vt*IL Pgen = Po + EL
Pin = Po + TL Pcu = 3Ia
2Ra (if 3Ø)
= Pgen + ML PF= EF*IF

6. Problems
1. An alternator is rated 25kVA, has a total loss of 2000W when it
delivers rated kVA at 0.76 pf lagging. Calculate its efficiency.
Ans. 90.48%
2. A 3Ø 25kVA, 220V alternator delivers rated kVA at a pf of 0.84.
The effective resistance between armature winding terminals is
0.18Ω. The field takes 9.3A at 115V. If friction and windage loss
is 400W and the core loss is 610W, compute for its efficiency.
Ans. 86.41%
3. A 2MVA, 2.3kV 3Ø Y-connected alternator operates at rated
kVA at a power factor of 80%. The DC armature winding
resistance between terminals is 0.08Ω. The field takes 70A at
125V from an exciter equipment. Friction and windage loss is
20kW, iron losses 36kW and stray load losses are 2kW.
Calculate the efficiency of the alternator. Assume the effective
armature winding resistance is 120% of the DC resistance.
Ans. 93.9%
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7. Problems
4. A 1500kVA, 3Ø, 60Hz, 6.9kV, wye-connected alternator has a
field resistance of 0.45Ω and an armature resistance of 0.05Ω
per phase. The field current at full-load 0.8 pf is 200A. The
stray power losses amount to 42kW. Calculate the efficiency of
the alternator at full-load, 0.8 pf lagging.
5. Determine the efficiency of 1500kVA, 2300V, 3Ø, Y-connected
alternator, which operates at rated output with a power factor
of 80%. The DC armature resistance at 70°C between terminals
is 0.08Ω. The field takes 70A at a 120V from the exciter
equipment. Friction and windage loss are 15kW, iron loss is
35kW and stray load losses is 1.5kW. Assume the effective
armature winding resistance is 1.5 times the DC value.
6. The DC armature resistance between the terminals of a
750kVA, 4,400-volt, Y-connected, 3Ø alternator is 0.9Ω at 25°C.
Calculate the winding copper loss at full load.
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8. Problems
7. A 6000kVA, 6,600-volt, Y-connected, 2-pole, 60Hz turbine
driven alternator. At rated load with unity power factor, the
armature loss is 1.5% of output. The field excitation voltage is
240V; the field current at unity power factor and 0.8 power
factor are 120 and 140A, respectively; resistance of field
winding at 75°C is 1.5Ω; friction and windage loss is 75kW.
Excluding field rheostat loss, determine the efficiency of the
alternator at rated load and at a) unity power factor and b) 0.8
power factor if core loss at unity and 0.8 power factor are 60
and 65kW, respectively.
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9. James Thurber
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~The End~