Paper Mill Process:
The raw materials for Paper Mill Process undergo two processes before the paper is available: The pulp is made from the raw materials. This pulp is converted to paper in paper making machines. The drives required for making pulp are different from those required for Paper Mill Process.
The conversion of raw materials to pulp is accomplished either completely by mechanical process or by a combination of chemical and mechanical processes.
When the pulp is made by purely mechanical means, the logs of wood cut to 1 m length are ground in big grinding machines. The grinders operate at a constant speed, the speed of operation being in the range of 200-300 rpm. No speed control is required. No load starting of the grinders is possible. The power ratings of the motor required are relatively large. The grinders require a very large power and low speed. For such operation, synchronous motors are useful. Geared drives may be used for reduction in speed. A cycloconverter fed synchronous motor can be operated at low speeds and the gears can be completely dispensed with. The problems of starting can also be avoided. A converter fed synchronous motor is also suitable. The motors can drive the load from a separate chamber and are protected from the humid atmosphere.
In the second process, which combines mechanical and chemical processes, the logs of wood are first chopped into smaller pieces. These are treated with suitable chemicals, simultaneously beating the pieces to pulp by means of heaters. The beaters require starting on load and against a large inertia, due to a large disc on which the knives are mounted. The load characteristics of the beaters are also random. The speed of operation is less than 200 rpm.
The rating of the motor ranges to thousands of watts. For driving beaters, slip ring induction motors are suitable. The desired starting torque of the motor can be achieved by a proper rotor resistance. For processes, such as chipping and refining, synchronous motors are employed as they are available in large power ratings. An s.f.c. fed synchronous motor may be employed for beaters.
The conversion of pulp is effected in several stages or sections. In these sections the water is removed froMthe pulp and it is pressed to sheets of Paper Mill Process which are finally wound up on a mandrel. These sections are wire (couch) section, pressing section, dryer, calender and reel section.
The Paper Mill Process making machine should satisfy the following requirements.
- The speed of the paper machine must be constant in view of economy while forming the sheets of paper.
- A speed control range of 10:1 is required so that it is suitable for performing several jobs.
- The speeds of individual sections should be varied independently to allow an elongation of 5% of the web on the wet end of the paper. The quality of the paper is decided by this elongation. To allow free hanging of the web between sections, at the dry end of the paper a definite amount of tension is required and it must be regulated. The successive sections must be run at speeds with a definite difference. This relative speed between the sections also affects the pull on the paper. It must be regulated so that there is no tearing of the paper.
- The arrangement should be capable of taking up sag.
- Even with correct speeds in the last two sections, uneven drying of the paper may cause variations in tension, which must be taken care of by suitable tension control.
- The motor must be capable of inching in order that the wire be cleaned up.
- Every section must be able to run at crawling speeds.
- The starting and acceleration of the sections must be smooth as well as quick. The starting system should be such that peaks of starting current may be avoided, besides obtaining sufficiently high accelerating torques for fast acceleration.
The Paper Mill Process making may employ either group drive or individual drive for several sections. In the group drive a line shaft is driven by the motor with different gear arrangements or belts as power transmitting equipment to drive different stages of paper making.
The drive motor may be either a dc motor or ac motor. DC motors having Ward Leonard speed control provide a lossless smoothly variable speed for the sections. Ac motors controlled from variable frequency sources are available now-a-days and they can provide the required lossless smooth speeds. In conventional ac systems an ac commutator motor may be used for stepless speed control. This is more compact when compared to a Ward Leonard controlled dc motor. The constancy of speed required for a Paper Mill Process cannot be maintained with an ac commutator motor, as the speed of the motor falls with load on the motor. The speed range is limited. It has sluggish transient behaviour compared to a dc drive. With dc motors, static Ward Leonard control can also be employed.
In the case of individual drives each section has its own drive motor. The speeds of these motors are varied by means of voltage variation. The changes in the speeds of a motor with respect to the others can be achieved by field control.