Page 98 - Servo Motors and Industrial Control Theory -

Chapter 5
Stepping Servo Motors

5.1 Principal Operation

A stepping motor is defined as a rotary device whose output shaft moves in discrete
steps when excited from a switched DC supply. Stepping motors are very practical
devices for converting digital pulse inputs into analogue shaft-output (or rotary)
motion as required in modern electric or electronic equipment. Each shaft revolu-
tion can be expressed in terms of a number of discrete identical steps or increments.
Each step can be triggered by a single pulse. Stepping motors can be made with a
rotor made of permanent magnets or DC energized. In the latter case, brushes must
be used to energize the rotor. The principal operation of both types are shown in
Fig. 5.1.
Figure 5.1 shows that when the phase A–A′ are energized, the rotor remains 0 at
the position shown in the figure. When the phase A–A′ are de-energized and the
phase B–B′ are energized, the rotor moves 90° (one step). The process may be re-
peated to move the rotor one more step of 90°. If the two phases are energized
simultaneously, the rotor moves 45°. By proper sequence of energizing and de-
energizing steps of 45 or 90° may be achieved.
The rotor may be designed so that magnetic poles can be achieved from external
power supply. This is shown in Fig. 5.2.
Stepping motors with rotor energized externally must have brushes to supply
power to a rotating rotor. Brushes need to be checked periodically for proper opera-
tion. For this reason, in servo control system permanent magnet rotor is preferred.
The stator of the stepping angles may be made with two, three, or four phases.
This is shown in Fig. 5.3
Figure 5.3 shows that by properly energizing and de-energizing the phases, step
angle of 60° is achieved. The rotor can be just laminated steels or permanent magnet.
If the rotor is made of permanent magnet, a larger output torque can be achieved.

R. Firoozian, Servo Motors and Industrial Control Theory, Mechanical Engineering Series, 91
DOI 10.1007/978-3-319-07275-3_5, © Springer International Publishing Switzerland 2014