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Starting a sweet revolution in productivity

06 October 2019

Harri Vaara explains how sugar beet processors are increasing productivity and energy efficiency through the use of variable speed drives (VSDs). 

Intense operating cycles are a significant factor in the rapid ageing of sugar processing machinery. During the harvest period a plant can run 24/7 for several months, followed by long idle periods. Operating conditions in the plants, such as heat, humidity and dust, are also tough on equipment. 

Furthermore, some sugar processing plants are situated in remote areas – where the local electricity network can be weak – and this has led to some producers looking to make their plants energy self-sufficient. 
VSDs can adjust the frequency and voltage of the motor supply, controlling the speed or torque of alternating current (AC) electric motors. Compared to the traditional method of running a motor (where the motor is run at full speed and its process output controlled by valves, gearboxes or dampers) this method can result in major energy savings of up to 60%.  

Improving reliability
Installing VSDs can also help improve process reliability as well as reducing maintenance needs and downtime because the need for mechanical drive components is less. Processes are smoother, preventing sudden shocks to the gearbox, motor and other machinery.

A wide range of operational data from processes can be provided by a VSD integrated into broader control systems. Connecting the electrical controls to higher level systems can help to boost a plant’s reliability and efficiency through predictive maintenance and process optimisation.

Sugar beet preparation consumes a lot of energy during the preparation stage – conveying, washing and slicing – so this is a key stage which can be optimised to improve energy efficiency. 

VSDs can be used to control the speed of the slicing machine, the better the control, the more efficient the process flows and the more likely it is that slices are produced in the right quantities. Therefore, installing VSDs will have direct correlation to optimising process flow. 

In the later stages of the process which require presses and mixers, each motor can be operated by its own VSD with built-in master-follower functionality. Even though a number of motors drive each press or mixer, for each machine driven, one VSD is configured as the master and the others as followers. The followers then mimic the master in terms of speed and torque via a communication link. 

This makes it easier to control the load when its driven by a number of motors and downtime can be reduced as multiple VSDs and motors provide redundancy. So, in the event that one motor fails, the process will continue. 

Process pumps are used in sugar beet plants to transform water and juice and this can make up around 35% of the total energy consumption within a plant. Traditionally, valves are used to control the flow and the pump motor is just left to run at full speed. The flow is controlled by adjusting the valves, but this is really inefficient. Introducing VSDs into this part of the process allows the pump speed to be easily controlled and ensures the motor only runs as much as it is actually needed. The number of potentially unreliable mechanical components is reduced, and the amount of energy saved increased. 

Recovering energy
Sugar plants normally have around ten centrifuges that work by fast acceleration, spinning at normal speed and then fast deceleration. The cycle time depends on the size and mass of the centrifuge and is typically around three minutes – 20 cycles per hour.  

VSDs, with regenerative functionality, are ideal in this application as they can recover energy during the deceleration phase. For example, one sugar cane plant in the USA has used a regenerative VSD to recycle and transform its energy efficiency. Originally, an inefficient legacy drive controlled one of plant’s largest centrifuges. Fitting a regenerative drive to provide both speed and torque control has cut the cycle time by 20%. It also allows the motor to act as a generator during the deceleration phase. This enables energy to be recovered and then transferred via the drive to an adjacent centrifuge. As a result, the plant can maintain full power even during times when power is limited.

Another sugar beet plan, in Finland, is now using VSDs widely in almost every part of its production process. This plant installed a total of 1 MW by fitting 144 VSDs, including 36 in the power plant. A further 108 VSDs are now being used in the production process, with a total power of 6 MW. 60% of the 108 VSDs are used to operate pumps, 25% control presses, mixers and centrifuges and 15% run other applications. In around one year or less the energy savings from installing these drives will result in payback. 

Similarly, Mirpurkhas Sugar Mills in Pakistan, which produces 7,500 tons of sugar cane every day upgraded its cane milling process. The steam turbine used to operate the crusher mill has been replaced with a VSD and high voltage induction motor. Originally, 650 to 700 kW of steam energy was required to drive the crusher. Now, just 350 to 400 kW is required – an improvement of 40% in energy efficiency.

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