Statistical analysis of BEMF detection

AN3327

16/22

DocID18343 Rev 3

Where:

•

Minimum detected BEMF voltage = 3.6 V

•

Maximum detected BEMF voltage = 6.6 V

•

Mean detected BEMF voltage = 4.7278 V

•

Standard deviation = 0.2007 V

Now to compare the stalled rotor BEMF with the running BEMF.

Figure 15. Combined histograms of an unloaded and stalled motor at both hot and

cold

From the graphs it can be seen that there are instances where the stalled rotor BEMF

overlaps the running BEMF values. It is simply due to rotor vibration causing BEMF to be

above zero. Statistically this overlap is minimal in that a majority of the time the BEMF is

much lower than usual.

In this example the BEMF threshold was set at something around 2 V. With that threshold a

reliable detection can take place as the vast majority of the stalled rotor BEMF

measurements were well below that level. If you look at the time it takes to effectively detect

stall for this motor we find that the stall can be detected well within what would have been

one mechanical revolution of the motor.