• Calender Machine Specialist

Conventional Calender Machines & Drives

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In India, Bangladesh, Sri Lanka, Pakistan and many other countries still many conventional calenders are in use. Those calenders were originally from England, Italy, Germany and other countries. They are very old, many decades old, calenders with bush bearings and leaver loading system having AC motor drives. Maintenance costs of these machines are very high. Power bills are frightening. User feels and believes that the expenditure involved in improving their efficiency by changing drive system, etc will be beyond the imagination. Well, may be they are true, depending on the calendering charges they get towards job work. But, they ignore the vital point that the investment will come back in no time to their unit in the form of power saving, better quality and speed of production. The operator is often curious to know the comparison of DC drive over AC slip ring induction motor because the drive on which he is depending is not at all helpful in smooth running of the machine. So, for the benefit of conventional old calender machine user, here is a brief. Given below are a few points and general calculations regarding the drive, etc. The effect of speed controls by resistance in rotor winding of slip ring induction motor will be:

• Increase in rotor losses due to increase in slip.
• Reduction in cooling due to reduced speed.
• Reduction in efficiency of motor.
• For constant torque operations higher size of AC motor is required than DC motor, viz, for speed variations 70% to 100% the required size of AC motor will be 1 - 2 times the size of DC motor.
• Efficiency of AC motor at arranged 90% load is 90% and power factor will be 97%.

In view of the above, the efficiency of 60 HP slip ring induction motor at synchronised speed at 90% load is 91 x 90 that equals to 82% approximately. Corresponding efficiency of 50 HP DC drive motor with auto transformer rectifier drive is 82% approximately. In general, energy losses are mainly proportional to power losses in rotor starter resistors of slip ring induction motor. For 60 HP motor with speed range of 70 - 100% at an average speed of 80%, energy losses amount to 4.0 kW approximately. So, energy loss for two shifts running of 14 hours a day at the rate of says Rs 6 per unit [approximately cost of unit] will be 4kw x Rs 6 x 14 hours = Rs 336 per day. For 300 days in a year working, loss will be rest, 336 x 300 = Rs 100800. The investment cost of 60 HP slip ring motor with rotor starter for 70 - 100% efficiency, speed regulation starter, main switch fuse unit, etc works out to say Rs 250000.00. Similarly cost of 50 HP DC drive motor with AC/DC converter may cost approximately Rs 370000.00. So to say, the payback time on investment will be about [370000.00 - 250000.00] = Rs 120000.00. Hence, payback period will be -- 120000 divided by 100800 = 1.1904 years. Say in 12 - 14 months one can get back his investment. Finally to say, if one wants the efficiency of old and conventional calenders to be enhanced then one should opt for DC motor in place of AC motor. Remember, simply by changing the motor one can get efficiency in drive only but to get overall improvements in performance of calender you will have to change the bearings from existing bush type to ball/roller bearing and convert the load system to hydraulic or pneumatic system. Please note the costs taken into consideration, in the above presentation, are purely for calculation purpose and the costs are not actual costs as available in the market today.