AC Drive V/F Control

V/F control was the original methodology developed by AC drives manufacturers. One of the weaknesses in V/F drive is starting torque and lack of control. Vector control methodology was developed in the 80s to give better control at lower frequencies but could not go to zero speed. Therefore a third methodology, Field Control, was developed that can give full torque at zero speed.

A fan application does not need torque at zero speed and really normally (unless very large) need much torque at very low speeds. A V/F drive will do fine as will a vector drive. Efficiency should be similar in both technologies. Due to volume, cost is probably similar. All ac drive manufacturers still have V/F technology for multi-motor applications. Both vector and field methodologies must have a single load, if multiple motors are used V/F must be used.

It is the best way to try it. Set a fan drive in V/F mode and measure motor current, and set in vector control mode and measure the motor current in same mechanical environment. Lower current draw in open loop vector on the same application (high inrush current application), the drives controlling the current more precisely in open loop vector mode.

Also, as a side note, most ac drives now have energy savings mode which will adjust the acceleration to keep current low while getting up speed (different AC drives manufacturers do this in different ways), as opposed to following the defined ramp, as most fans are not critical acceleration so you may consider turning on this function for additional cost savings, the larger the motor the more the savings.