Page 689 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 675
step motor. The stator has 8 poles each with 5 teeth on each, making a total of 40 teeth
(Figure 8.47), plus the space between each pole: there are 48 spaces. Let us assume that the
rotor has 50 teeth. Using full step and half step modes, we can advance the teeth alignment
between rotor and stator by the number of rotor teeth times the number of phases times 2.
The step angle of an hybrid PM stepper motor is determined by the number of electrical
phases (N ) and the number of rotor teeth (N ),
r
ph
360 ◦
= (8.266)
step
2 ⋅ N ⋅ N
r ph
The number of steps of a PM stepper motor per revolution is the number of electrical phases
times the number of rotor teeth times two,
N = 2 ⋅ N ⋅ N (8.267)
step r ph
The switching of power transistors from one state to another from ON-to-OFF and
OFF-to-ON state instantaneously results in an instantaneous change in the magnetic field.
The motor behaves like a mass-spring system. We can take the concept of half step mode
further to ratio (smoothly change) the current in phases instead of making transitions from
full ON to full OFF states. This will result in smoother motion and electronically controlled
finer step sizes. This is the main operating principle of the so-called “micro stepping drives”
(Figure 8.50d). As a result of the smoother current switching between phases, the torque
acting on the rotor shaft between steps is smoother, and the step motion of the rotor is
less oscillatory. In addition, microstepping reduces the resonance and step loss problems
associated with step motors which are operated full step and half step current control drives.
The stator windings typically form two phases. If the step motor is to be operated
by a unipolar drive, each winding must be center tapped to ground and positive voltage
is connected to both ends (Figure 8.51a). The opposite connection can also be made: the
center tap is connected to the DC supply voltage, and the other two ends are connected to the
ground. Per winding, only one of the connections at a time is switched ON in order to control
the direction of the current and hence the generated electromagnetic pole type (north or
south). Only half of a particular winding is used at a switched ON state (Figure 8.51a). If the
step motor is to be operated by a bipolar drive, then the current direction can be controlled
by the drive and all of each winding is used at a switched ON state (Figure 8.51b,c). Some
step motors are wound with two separate windings per pole, and hence can be driven by
a unipolar or bipolar drive by appropriately terminating the winding ends. For unipolar

Stator windings Stator windings

Rotor Rotor Rotor

Stator windings

(a) (b) (c)
FIGURE 8.51: Stator winding connections of a two-phase, four winding step motor:
(a) unipolar drive configuration with center tapped connection, (b) bipolar drive configuration
with series connection, (c) bipolar drive configuration with parallel connection.