Thank you for all your posts.
Let me clarify my question: I do need to find a better solution (motor + drive) for the new cooling fan meeting the following three requirements: start up the fan smoothly, realize the speed of the fan variable in 50-100% range with accuracy around 2% of the full speed; minimize equipment cost and operation cost. No other factor is important.
Dougmose, WRIM and resistor starter is my second option too; can the resistor handle the high temperature when the motor is running continuously in lower speed?
Jbartos, VFD can achieve the best speed control for the whole range, if the cost is acceptable. Can you give me some budget price for the VFD system including motor, transformer and VFD? (Note: the motor stator voltage is 10KV)
Magna drive seems to be a good solution, compared to VFD, how much cheaper is it?
By the way, in your opinion, what’s major issue if the fluid coupling with variable speed control is used for this application?