For any specific motor, the integrated value of BEMF during half of an electrical cycle is a constant as shown by the shaded gray area in #T4509467-37. This value is constant regardless of whether the motor runs fast or slow. The DRV10974 device monitors this value and uses it as a criterion to determine if the motor is in a lock condition.

The DRV10974 device uses the integrated BEMF to determine the Kt value of the motor during the initial motor start. Based on this measurement, a range of acceptable Kt values is established. Then, during closed-loop motor operation the Ktc (Kt calculated) value is continuously updated. Finally, the Ktc value is checked to see if it is within the range between ½ Kt and 2Kt. If the Ktc value goes beyond the acceptable range, a lock condition is triggered as shown in #T4509467-38. Note, there is a blanking period of 0.3 s after the transition from open loop to closed loop where the abnormal BEMF lock is momentarily disabled. The device uses this time to finalize the Kt value that Ktc is compared against.

Figure 7-9 BEMF Integration

Figure 7-10 Abnormal Kt Lock Detect