DocID023635 Rev 2

5/34

AN4158

Stepper motor basics

34

1

Stepper motor basics

A stepper motor is a particular kind of synchronous electrical motor designed to reach and

keep an angular position. Each position is obtained as a steady-state of the rotor according

to one of the four possible directions of the magnetic field generated by a pair of windings

(called phases) on the stator. The distance between the next two angular positions is called

step and the number of steps composing a full mechanical rotation is a key characteristic of

this kind of motor. A simplified representation of a stepper motor is shown in Figure 1.

Figure 1. Basic two-phase bipolar stepper motor

Forcing a current in one phase at a time, the four positions shown in Figure 1 can be

reached by the rotor. When the driving sequence listed in Table 1 is generated, the rotor

performs a complete rotation in a clockwise direction. According to the step definition, the

model in Figure 1 is a 4-step motor.

Increasing the complexity of the stator and rotor design, the number of available positions

for each driving condition can be increased obtaining a high number of steps per round. The

driving sequence making the motor rotate is the same, but a full rotation of the magnetic

field of the stator (electrical cycle) does not correspond to a full rotation of the rotor

(mechanical rotation). Table 2 shows the driving sequence of a 200-step stepper motor.

Table 1. Phase driving sequence for a clockwise rotation of a 4-steps per round

stepper motor

Phase A current Phase B current

Stator magnetic field position

Rotor position

Positive

Zero

1

1

Zero

Positive

2

2

Negative

Zero

3

3

Zero

Negative

4

4

Phase A

Rotor

Phase B

Stator phases

NS

1

4

3

2

AM15103v1