The operation of a hydro generator may be observed by five characteristics:
 Opencircuit characteristic:
U = f(I_{f}), with I = 0
Fig. 1. Opencircuit characteristic
 Shortcircuit characteristic:
I_{sc} = f(I_{f}), with U = 0
Fig. 2. Shortcircuit characteristic
 Saturation characteristic:
U = f(I_{f}), with I = const. and φ = const. Idle running characteristic is a particular case of saturation characteristic.
Fig. 3. Idle running characteristic
 Voltage/current characteristic:
U = f(I), with I_{f} = const. and φ = const.
Fig. 4. Voltagecurrent characteristic
 Vcurve characteristic:
I = f(I_{f}), with P = m·U·I·cosφ = const.
Fig. 5. Vcurve characteristic
In order to be able to determine all the above characteristics, we need to know both the idle running magnetic characteristic Φ=f(θ_{f}), and the onload magnetic characteristic Φ=f(θ_{r}).
Using the opencircuit and shortcircuit characteristic, we are able to determine the field current and the onload voltage drop.
