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A paper machine example illustrates that large electric drive systems have special requirements, for example, special inverter packaging with a separated dc converter to save space and cost, and provide the best reliability.
High machine speeds require the controls and drives to respond quickly to speed and torque changes, and web constraints require the control system to closely coordinate the speed and torque of the drives to avoid web breaks that can cause costly downtime.
In web-handling applications, large numbers of variable-speed drives are used, for example, in the paper industry on paper machines and coaters, and in the metals industry on rolling mills, galvanizing lines, and pickle lines. For these applications, ac motors are preferred to dc motors. The electric drive system consists of variable frequency ac drives and variable-speed ac induction motors.
Although ac synchronous motors are used for the higher power levels, most of the web handling is done with lower horsepower motors in the five to 500 hp range. These systems employ up to 200 motors to ensure reasonable tension, and the motor speeds and torques are controlled and coordinated by a master controller over a high-speed network, using feedback from the machine and inputs from the process control system or mill control. This article looks at the controls on a paper machine to illustrate the drive system used.
Variable speed drives
The main parts of a low voltage ac drive are illustrated by the two circuits in Figure 1, above.
1. ac to dc converter supplied by 400 to 690 V ac, three-phase power
2. Voltage filtering section consisting of capacitors
3. dc to ac inverter using pulse width modulation (PWM) to generate the variable frequency power output to the motor
The last part is a variable-speed, three-phase, ac induction motor.
Converter
Several...