Suggestion: Reference:
1. Bimal K. Bose "Modern Power Electronics and AC Drives," Prentice Hall PTR, 2002,
Page 388 Section 8.5 Sensorless Vector Control:
... A speed signal is required in indirect vector control in the whole speed range and in direct vector control for the low-speed range, including the zero speed start-up operation.
Reference 1 covers the AC drives such a way that an engineer, designer, educator, student and enthusiast will find it very informative. It has many 5 star reviews.