Losses in alternators are same as DC generator and they are as follows.
a. Copper losses includes following losses
Armature copper losses (Ia2 ra).
Field copper losses (Ife2 rf).
Losses in brush.
b. Stray losses are as follows
Magnetic losses (Iron loss or core loss and pole shoes loss).
Mechanical losses includes bearing friction, slip ring friction and friction due to
windage.