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Variable Frequency Drive on Major Electrical Motors

Description

The AC induction motor typically operates at 80% efficiency when under full load, but the efficiency drops dramatically at lower loads. Applications like Pulper, Turbo, Paper machine main drive etc. are having constant variation of load. The power consumption by these equipment motors must be optimised to have cost effective and environment friendly operations. The task of a variable speed electrical drive is to convert the electrical power supplied by the mains into mechanical power with a minimum loss. This case study describes how variable frequency drive helps conserving energy. 

Implementing Industry

Daman Ganga Paper Mills is a medium sized company engaged in recycled packaging products. Company produce a range of recycled products such as Paper, Core-Board, Cores & Tubes, Fibre-Drums, Partition Boards, Insulation Boards, Acoustic Boards, etc. The company started its operation in the year 1982, with present production capacity of Kraft Board 60 Tons per day, with annual turnover of 50 Crores.

Implemented technology / technique

BEFORE:

  • The waste paper on the conveyor is added to the pulper along with water. The role of the pulper is to break apart the paper sheets into individual pulp fibres while leaving the contaminants as large as possible. The process conditions in the pulper are characterised by strong pressure shocks from falling bales, vortex formation from fast rotating agitators as well as heavy abrasion from foreign substances such as wires, glass, stones, etc. thus the load on the motor of 200 HP varies continuously. 

  • Turbo Separator is functional in separating unwanted material from pulp like metal pieces and heavy water resistant particle pins, this machine is efficient and reliable. Besides, it also acts as a plastic catcher in some plants and thus it is exposed to varying loading at motor of 120 HP.

  • In paper machine, the paper produced varies in terms of Length, Grade, Fibre Concentration, Quality variation which in turn affects the loading on the motor which is of fixed 200 HP capacity.  

  • When the motor is operating at light load for extended periods, the motor's efficiency falls due to the over-fluxing of the windings for the particular torque required to drive the load. At a constant terminal voltage this flux, often referred to as magnetising current, is fixed and accounts for around 30-50% of the motor’s total losses.

AFTER:

By installing VFD, when the frequency applied to an induction motor is reduced, the applied voltage must also be reduced to limit the current drawn by the motor at reduced frequencies. Apart from energy saving, VFD provides following benefits:

  • Motor is driven only as fast as needed in order to get the right speed and process control required

  • Soft start/stop means less wear on couplings, belts and motors. Controls prompts the operator as and when system needs attention

  • Industry continues to reuse about 150 m3 of treated water per day as process water with better quality than in past, resulting in improved paper quality. 

The VFDs on the major motor were installed by the industry.

Economic benefits

Capital investment to install the VFDs was Rs. 17,00,000 and with minimum maintenance costa total savings of Rs. 20,79,000 per annum was accrued giving return on investment in just 10 months.

Environmental benefits

The total electrical power consumption has reduced from 265 kWh per Ton of paper to 250 kWh per ton of paper.

Contact details 

Mr. Banjul Shah, Director; banjul.shah@damanganga.com