In order to determine armature copper losses of alternator short circuit the armature of alternator and drive it at synchronous speed with enough excitation to cause full-load current to circulate. At this point, note the power required.
Now, open circuit the armature and measure iron and friction losses as mentioned below:
Iron and friction losses of alternator are measured by coupling alternator to suitably calibrated DC Motor and driving the motor at synchronous speed with normal excitation. The iron and friction losses equal the output of motor in watts.