Stepper motor basics

AN4158

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DocID023635 Rev 2

The torque applied to the stator at each step change is directly proportional to the stator

magnetic field, which is proportional to the current flowing into the phase and the sine of the

displacement angle between the rotor and the target step electrical positions. This relation is

described by Equation 1.

Equation 1

When a step change is applied, the magnetic field is rotated by 90° in a clockwise or

counterclockwise direction. As a consequence, the torque applied to the motor shaft

reaches its maximum value (according to the phase current) because the displacement

angle

is about 90° (see Figure 2). During the approach of the rotor to the

target position, the torque decreases until it is equal to that of the load. The displacement

angle corresponding to the load torque applied to the shaft (

motor position at the end of motion.

Table 2. Phase driving sequence for a clockwise rotation of a 200-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

Positive

Zero

1

5

Zero

Positive

2

6

……

…

…

Negative

Zero

3

199

Zero

Negative

4

200



Stat



Rotor

–



sin





Stat



Rotor

–