Lesson 1/Learning Event 2
This generator type differs from the previously described Lundell type, in that the rotor contains no
windings. Excitation is induced in the rotor poles by stationary field coils located at the ends of the
rotor. This results in elimination of slip rings and rotating windings. Further advantages can be
obtained by casting a nonmagnetic material around the pole fingers, thus producing a smooth rotor
with low-windage losses and high-speed capability. An inherent design requirement of this
stationary field arrangement is the inclusion of an auxiliary air gap in the magnetic circuit. This
requires greater field current for excitation.
A Lundell inductor has several advantages. There are no contamination problems or slip ring wear,
and the unit is inherently explosion proof. The rotor can be solid and permanently balanced. All
windings are stationary and readily accessible for cooling. The low-rotor mass reduces bearing loads
and permits rapid acceleration. The bearing center-to-center distance is minimized by the
elimination of slip rings and this, combined with a large shaft diameter, permits high-speed
operation. The field windings are simple, bobbin-wound coils permitting short mean turn length.
The only disadvantage is that extra air gaps in the magnetic circuit require increased excitation