What type of protective device is used for the protection of an alternator against overheating of its (a) Rotor (b) Stator ?

Winding RTD Temperature Sensors which sends temperature reading into ECM module of the generator and thus it saves winding from overheating by cutting the load in time to save the winding work 

simply use thermal fuse it will save your cost and if you want to automate this than use RTD and place a microcontroller with it to protect your motor from. Using a microcontroller you can automatically switch off and again switch it on according to the desired conditions. If you want more detailed answer or solution than you can inbox me.

galvanize steel, prevents from rousting and heat.