DocID18343 Rev 3

13/22

AN3327

L9942 stall detection methods

19

3.2.1

BEMF on a stepper motor

To more easily see the effects of torque on back EMF the following is a look at a motor

driven in full step mode. Figure 9 below illustrates an unloaded motor being driven in full

step mode. The red is the current while the purple is the voltage on the phase. The thin

black line estimates the back EMF.

Figure 9. Unloaded motor driven in full step mode

In an unloaded motor (Figure 9) the Back EMF leads the phase current. This indicates that

there is more energy provided by the rotating magnetic field, than is mandatory to keep the

rotation of the rotor. The irregular shape of the back EMF voltage indicates that there is

rotational acceleration and deceleration through a 360° cycle.

In a motor with a higher load level (Figure 10) the Back EMF signal is more aligned to the

actual driver signal. In other words the phase difference in between the cause and the effect

is smaller. If the load is increased above a certain level there may occur step loss while

driving the motor.

Figure 10. A loaded motor driven in full step mode

Systems that use stepper motors severely overdrive their motors to ensure that they never,

under all normal operating conditions, stall.