If the meaning of reversing of field is the replacing of dc polarity in alternator exciting winding, it has not any effect to stator emf and generator output basically. Of course the dc polarity is important particularly in exciter winding grounding and some scheme dc protection which considered for excitation system.
For example, the scheme shown in Figure below employs both a DC driving voltage and a grounding brush. The source voltage must be connected to the field winding in additive polarity: DC source plus to field minus or vice versa. If this is not done, a blind spot will be created at the point in the winding where the field voltage is equal to, but opposite of, the DC source voltage. The sensitivity of the DC driven scheme in terms of the detectable resistance to ground is highly variable. The relay current is dependent on the fault resistance, location and the field voltage, which varies with generator load. The voltage applied to the fault increases as the fault moves from the negative terminal toward the positive terminal.
The fault resistance sensitively increases as the fault moves toward the positive terminal and with increased field voltage.