Many alternators do not use brushes to deliver voltage to the field windings of the rotor. In this type of alternator, the field windings do not rotate with the rotor. They are held stationary while the rotor turns around them. The rotor itself retains sufficient residual magnetism to energize the stator when the truck engine is first started. Part of the voltage induced in the stator windings is then diverted to the field windings, which energizes the electromagnetic field. As the strength of the stationary field windings increases, the magnetic field of the rotor also increases and stator output reaches its specified potential.
Many current alternators use a brushless design because they produce better longevity. Examples are the Delco 33SI, 34SI, 35SI, 36SI, and 40SI alternators and the high-capacity 50DN used in many motor coaches. Examples of competing brushless alternators are the Leece-Neville 320, the Bosch T1 series, and Denso 130A. Truck brushless alternators can be rated up to 300 amperes output in 12V systems. The output ratings in current trucks are necessarily higher because of the more complex electrical and electronic circuits. The heavy-duty, oil-cooled 50DN alternator is designed to supply the electrical charging requirements of motor coaches.