Stepper Motors

Stepper Motors

A stepper motor is a special type of synchronous motor which is designed to rotate

a specific number of degrees for every electric pulse received by its control

unit. Typical steps are 7.5 or 15° per pulse. these motors are used in many control

systems, since the position of a shaft or other piece of machinery can be controlled

precisely with them.

A simple stepper motor and its associated control unit are shown in Figure

1. To understand the operation of the stepper motor, examine Figure2.

This figure shows a two-pole three-phase stator with a permanent-magnet rotor. If

a dc voltage is applied to phase a of the stator and no voltage is applied to phases

band c, then a torque will be induced in the rotor which causes it to line up with

the stator magnetic field B_s, as shown in Figure 2b.

Now assume that phase a is turned off and that a negative dc voltage is applied

to phase c. The new stator magnetic field is rotated 60° with respect to the

previous magnetic field, and the rotor of the motor follows it around. By continuing

this pattern, it is possible to construct a table showing the rotor position as a

function of the voltage applied to the stator of the motor. If the voltage produced

by the control unit changes with each input pulse in the order shown in Table

1, then the stepper motor will advance by 60° with each input pulse.

It is easy to build a stepper motor with finer step size by increasing the number

of poles on the motor. From equation the number of mechanical degrees

corresponding to a given number of electrical degrees is

Since each step in Table 1 corresponds to 60 electrical degrees, the number of

mechanical degrees moved per step decreases with increasing numbers of poles.

For example, if the stepper motor has eight poles, then the mechanical angle of the

motor's shaft will change by 15° per step.

the speed of a stepper motor can be related to the number of pulses into its

control unit per unit time by using previous Equation. this Equation gives the

mechanical angle of a stepper motor as a function of the electrical angle. If both

sides of this equation are differentiated with respect to time, then we have a relationship between the electrical and mechanical rotational speeds of the motor:

Since there are six input pulses per electrical revolution, the relationship between

the speed of the motor in revolutions per minute and the number of pulses per

minute becomes

Where n_pulses is the number of pulses per minute.

There are two basic types of stepper motors, differing only in rotor construction:

permanent-magnet type and reluctance type. The permanent-magnet

type of stepper motor has a permanent-magnet rotor, while the reluctance-type

stepper motor has a ferromagnetic rotor which is not a permanent magnet. (The

rotor of the reluctance motor described previously in this section is the reluctance

type.) In general, the permanent-magnet stepper motor can produce more torque than the reluctance type, since the permanent-magnet stepper motor has torque

from both the permanent rotor magnetic field and reluctance effects.