SURGE MODIFIER

a small shunt capacitor between the line and the earth or a series air cored induct is incorporated which by storing temporary energy in them , reduces the steepness of the surge wave front, which otherwise can be detrimental to the apparatus  and this is known as surge modifier  

CRITICAL SPEED

critical speed is the speed at which the dc shunt generator just fails to build up with no external resistance in field circuit

Excitation

An electric generator or motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnet or by field coil. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation.