4 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
with a displacement of 90 degrees to each other; and for 
a three phase current at three points 120 degrees apart. 
7. Limit of Use.—The use of this type of generator is limited 
to low voltages and comparatively low speeds, and in cases 
where it is directly connected with a prime mover, in 
which a flywheel effect is not necessary in the revolving 
part of the generator. 
For high voltage machines, the insulation of the coils is one 
of the leading features of design. It is inherent to the 
nature of good insulation, that it cannot be subjected to 
mechanical stresses of any importanee. This difficulty is 
evidently overcome in the easiest and most reliable way, 
by making the armature stationary, as is the case in the 
other types. 
The revolving armature is only applied to low voltage and 
medium speed generators of, say, up to 100 K.W. This 
principle is also largely applied to rotary converters of 
various sizes. 
8. Inductor Type Alternator.—tThe principal feature of 
the inductor alternator is the entire absence of any re- 
volving winding and sliding contacts, as both excitation 
and armature terminals are fixed on the iron frame. A 
single stationary coil provides the necessary magneto- 
motive force for the magnetic eireuit of the machine. 
The magnet wheel of cast iron, or the cast steel spider with 
the necessary number of pole projections, is generally fitted 
with laminated pole shoes. 
The armature frame is made of cast iron of ample cross 
section, to guard against any sag due to the weight or 
magnetic pull. This frame carries the field iron stamp- 
ings, which form the magnetie path. Slots are stamped 
out on the inner periphery of the sheet iron, to take the 
armature coils. 
