about AC Motor Drives

I have heard of vector control never vectorel.

Best check with the motor manufacture on that one. If you reduce speed to

100 rpm for very long the motor will get hot and eventually fail. My experience says that not all motors work well with VFD's.

Answer-1) You cannot use Vector control for multiple motors off of one drive anyway. Only V/Hz.

To answer your question however, Vector control allows the VFD to react more accurately to changes in the motor and load so as to maintain accuracy at any speed. To do this, the Vector Control drive must create and maintain a mathematical model of the motor power, torque and speed curves, then somehow determine the rotation speed, compare it to the commanded speed, and tweak it's output to correct for any error in the two. True Vector Drives determine rotation speed by using an encoder to feed back the speed into the microprocessor. Open Loop vector drives (a.k.a. Sensorless Vector) use subtle changes in the current waveform going to the motor to determine speed without an external device, although technically it is still a closed loop. Neither can accurately determine anything about rotation speed if there is more than one motor connected however.

here is some additional detail.

Answer-2) In V/Hz mode, you can realistically expect accurate speed control down to a 4:1 ratio, so in your case, 375RPM. Below that and your accuray starts to suffer. Also,remember that as the motor is turning slower, so are the motor's cooling fans, so it often becomes necessary to add external fans at lower speeds as well.

You're welcome in advance...