This is true that generally Alternators are grounded by resistance and motors are grounded by a reactance. The reason behind this is as follows:
In an alternator during a fault, the electric power decreases, and mechanical power is assumed to be the same. In that case, there is accelerating power develops which accelerates the rotor and makes the system unstable. But if we connect a resistance between the ground and neutral terminal then during an unbalanced fault, zero sequence current flows through this resistance and it causes power loss. This power is counted as electrical power, so we are increasing electrical power which reduces accelerating power (Accelerating Power = Mechanical Power - Electrical Power). In this way, the acceleration of the rotor is reduced, and stability is increased. So, it is done in order to improve the stability of the system. Under, a balanced condition, there is no current flow through the neutral and hence this resistance does not affect the efficiency of the system.
In the case of the motor, mostly it works in overexcited conditions at the leading power factor. So, it effectively acts as a capacitor and hence there are charging currents present in the system. Due to these charging currents whenever an LG fault occurs the phase voltage becomes equal to the line voltage and can cause Insulator Flash-over. This is also called an” Arcing Fault”. To avoid this condition, we need to cancel the capacitive charging currents and for that purpose, we use a reactance in neutral. If this reactance completely cancels the capacitive current, then it is called “Resonance Grounding” and the inductor used is called a “Peterson Coil”.
So, we use resistance grounding to improve the stability of the alternator and reactance grounding where charging currents are high.
A motor is always grounded by a reactance. Because if we use resistance grounding for the motor it will consume some electrical power (Loss) since the input power is electrical, which is not desired. Resistance grounding is used where the charging currents are low. Reactance grounding is used where the charging currents are high.