I recently worked on a drive for a paper machine. The wet end drives for paper machines often require more than one driven roller per section. This applies to the forming section and the press section. In this case, the press section was driven with 5 motors.
Balancing the load between 5 motors all running above their base RPM required some care. DC drives were used throughout, but this story would read almost the same if AC drives were used.
The first consideration is that the DC motor voltage be kept below motor nameplate voltage. Motor was adjusted with the shunt field for each of the 5 motors. For an AC drive, simply setting the rated voltage would do the job.
The next consideration was balancing the load between the 5 motors. As a starting point, the motors were balanced to run at the same percentage. That is to say if the large motors ran at 70% torque, the smaller motors should run at 70% torque.
Load sharing was accomplished in two ways. One drive was the speed master for the press section. It's speed regulator set the speed for itself and the other 4 motors. Two additional drives (follower drives) powered 2 of the motors. That left 2 motors (helper motors) to be shared on 2 of the 3 press drives.
Control for the follower drives is fairly standard and is included as an optional control scheme for sharing torque between two drives. It involves communicating speeds and torques between two or more drives and coordinating the torque. For modern AC drives, this may involve a dedicated copper or fiber cable between the affected drives and setting several parameters to activate load sharing.
Helper motors are not generally used with AC drives - a separate drive for each motor is recommended. Control for the DC helper drives was accomplished by carefully setting the shunt field current for the motors. Note that setting the field for one motor will affect the load on the other motors.
I suggest balancing the load for a machine section such as this be done at half speed and half voltage and without web. Close all nips and establish normal operating condition for the section. This will avoid the possibility of overloading or over-voltaging any motor. Set the section master speed first using its gear ratio and roller diameter. Then verify the load sharing for follower drives. Ensure that load adjustment works. Then adjust the helper motor fields to balance the load percentage with the main motor in the section.
To complete the section commissioning, test at full speed. The final test is an Emergency Stop from full speed. We expect the section is brought to a standstill at maximum deceleration with no faults on any motor or drive.