L9942 stall detection methods

AN3327

14/22

DocID18343 Rev 3

3.2.2

BEMF in a stalled motor

Comparing Figure 9 and Figure 10 with a stalled rotor waveform (Figure 11) illustrates the

difference between a stalled and a running motor. Again these figures are done using a full

stepped motor to better illustrate the BEMF during these conditions.

Figure 11. Motor in hard stall

Figure 11 shows virtually no back EMF during the non driven intervals. This is understood,

as BEMF is a direct result of rotor movement. If there is some movement in the stalled rotor

then the BEMF becomes visible (Figure 12). Figure 12 illustrates the irregular nature of the

BEMF in a "loosely" stalled motor.

Figure 12. Two instances where the motor is in stall but allowed to "vibrate"

Comparing these waveforms with the previous running waveforms we can see that there is

some overlap. Of course Figure 12 shows the behavior of a full step mode driven motor.