We use electric motors and drives in web handling because the torque can be controlled so well. Induction motors are typically used. By itself, an induction motor has a predictable torque curve, but this curve is not suitable when a specific speed and torque are required. Torque is created in an induction motor based on slip. Slip in this motor is the amount the motor runs slower than the actual frequency would normally drive the motor. A 4 pole motor would run at 1800 RPM when driven at 60 Hz. You will notice the motor nameplate shows the motor RPM as 1750 or 1760 RPM. The 50 or 40 RPM difference is the slip.
For the motor without drive, the RPM must slow to increase torque. Torque can be increased up to 3 or 4 times rated by slowing (slipping) the motor up to 8 or 10%. Call for more torque and eventually, the motor will slip to its breakover value, and torque will decrease instead of increasing. The motor will stall and come to a stop.
A scalar drive (Volts/Hertz - V/Hz) will act as described above. The operator will set the desired speed and if the motor even starts it will slip to a slower speed determined by the load. A pump or fan will be very happy with a V//Hz drive. This won't do for web handling.
Vector or Direct Torque Control (DTC) drives do a better job holding RPM while delivering the required torque. This control is achieved by increasing the electric frequency to the motor to compensate for the slip required to give the torus required. In addition, the drive controls three phases of current to the motor. The result is a drive running at the correct speed to keep the web moving and with the correct torque to hold tension.
In review, an ac motor without a proper vector or DTC drive will not provide adequate speed and torque control for Web Handling. With the proper drive, well-tuned and calibrated, the web will run at the correct tension.