A thermodynamic prime mover drives the rotor, which generates voltage in each phase of the three identical stator coils. The three phase voltages are equal in magnitude but 120° out of phase in time (or in ωt, to be precise). The stator coils are usually connected in three-phase Y. That way, the conductors in the stator slots need to have insulation to the ground for only 1/√3 = 0.577 or 57.7% of the line voltage; hence, more conductor can be packed in the slots. In the conventional generator, the dc excitation field current comes from a small separate exciter via slip rings and carbon brushes.