Motors have two useful outputs – RPM and torque. Motors have two inputs – volts and amps. Actually, ac motors have three inputs volts, frequency, and amps. Ac motors must maintain a close relationship between volts and frequency. You will often see references to Volts/Hertz or V/Hz for a motor or drive.
We note that volts and frequency (rising together for an ac motor) control the RPM. We would expect that amps would then control the torque of the motor. It turns out that is almost true for a dc motor. It is not true for an ac motor. The best we can say for an ac motor is that increased torque requires increased current. Above 80% current, the current and torque become linear. Unfortunately, web handling drives rarely run at 80% of the motor’s rating.
The reason the ac induction motor current is not linear with torque is that a portion (10-30%) of current is used to provide magnetizing flux for the induction motor. The magnetizing current does not become negligible below an 80% load.
The tension in the web is related to torque. We see that we cannot use ac motor current to estimate tension.
Estimated torque can be read from the drive. We can trust this torque value for a properly tuned drive. Tension can then be calculated from the drive’s estimated torque value.
There is no point in providing an ammeter for the web handling operator. A torque meter has some usefulness. A tension meter calculated from torque and scaled to PLI or N/m is the ideal meter or display for the operator.