Page 557 - Mechanical Engineers' Handbook (Volume 2)

548 Servoactuators for Closed-Loop Control

The split-series field-wound motor has two windings, one for each direction of rotation.
A manual switch is usually employed to activate the appropriate winding. The two windings
of the compound motor are always excited and result in a high starting torque with good
linearity. All of the fieldwound motors are self-excited with the residual magnetism.

Permanent-Magnet Motors
Permanent-magnet (PM) motors are the most extensively used for servomotors because they
generate less heat and have higher efficiency and more compactness than field-excited mo-
tors. There are three types of PM motors with mechanical commutation: (1) iron core, (2)
surface wound, and (3) moving coil. Figure 4 shows the construction of the three types.
Details of the advantages and disadvantages of each type may be found in Ref. 5.

Figure 4 (a) Iron core; (b) surface wound.