Web handling operators used to be told that current and torque for a dc drive were related. For the past 10 years we have used ac motors and this is NOT true for ac drives. AC drives draw current any time they are energized, even if the motor is stopped. Some significant current is used to establish a magnetic field inside the motor.
Above 80% current, torque and current rise together. That means at 100% motor current, we get 100% of the motor rated torque. At 150% motor current we get 150% torque. Most NEMA motors suffer breakdown torque at 220% current.
In web handling (extruders excluded), we do not run above 80% current. So current is not related to torque where we earn our living. Current measurement is important for protecting the motor. Torque measurement is useful for web handling.
What can be done? It is bad to give current meters (Amps) to the operators. It is good to give torque meters (ft-lb or N-m) to operators. All vector drives have a reliable torque signal. It is best to scale torque by the roller radius and the web width to present PLI or N/m to the operator.
Above 80% current, torque and current rise together. That means at 100% motor current, we get 100% of the motor rated torque. At 150% motor current we get 150% torque. Most NEMA motors suffer breakdown torque at 220% current.
In web handling (extruders excluded), we do not run above 80% current. So current is not related to torque where we earn our living. Current measurement is important for protecting the motor. Torque measurement is useful for web handling.
What can be done? It is bad to give current meters (Amps) to the operators. It is good to give torque meters (ft-lb or N-m) to operators. All vector drives have a reliable torque signal. It is best to scale torque by the roller radius and the web width to present PLI or N/m to the operator.
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