Variable frequency drive Vector or V/F control mode in fan application

Q:
In fan application, should we run the variable frequency in vector control mode or v/f mode? what are the pros and cons of both the modes. what are the methods to find out the correct running of the motor?

A:
It depends on the application. If it is a small fan or easy application go with V/F. As a rule V/F is going to be less accurate, lower current and less money. You can buy a vector VFD and run it in V/F mode.

If you get into kilns with fans that are reversing and being back driven you may want vector with external braking resistors. On large fans you may not have a choice, only vector variable frequency drives, but you may need to increase the slip to reduce current.

Most VFD's have a tuning procedure you can do or you can get motor constants from motor manufacturer. I think the best is to get a VFD that constantly tunes to the conditions while running, like Gozuk.

Vectors really do well below 15 hz with speed and torque control. Vectors are used in hoists where you need torque at 0 hz. The Vector senses the motor speed and adjusts the output to match the command speed. It can sense with an external device/encoder or it can sense the slip by the current waveform compared to output voltage wave form, sensorless vector.

A V/F basically just puts out the commanded frequency and does not care about changing load or slip.
If it is a centrifugal fan then it would be cube law loaded. The HP required will increase or decrease to the cube of the speed. The motor would start with no load and the load increases to 100% at 100% speed. High starting/break away torques are usually not required. As a result the motor and variable frequency drives usually do not require an added service factor.
Most of these applications do not require high accuracy speed/flow regulation so v/f is all that is needed.
If the fan is in a test facility (like a wind tunnel) and it is necessary to control the flow or pressure accurately for extended periods while measurements are taken then the vector control mode may be required.