This may be an often overlooked cause but it was the problem in my case. I had a fan which has a 400 mm sweep. A few months ago I sent it for rewinding and after it came back started running slower than before. The rewinding agency and the local electrician concluded and tried to convince me that factory original winding has an edge over a refurbished job and hence the lack of speed. After a lot of tinkering I compared it to the same fan installed in my bedroom and found out that the blade angles may be the culprit. It so happened that while removing the fan and refitting it, the blades got twisted and two out of four actually drooped a bit lower than the recommended angle. I spent 30 mins adjusting the angle an the position of the four blades using a reference point from the (dangled a thread and marked where each blade should be touching the leading the trailing edge) and to my surprise a small adjustment to the blades made a significant increase to the RPM. I should have noticed the fan actually getting...