Cereal solution

As part of The Weetabix Food Company’s ongoing improvements programme, energy and productivity reviews are undertaken throughout the year.

It was during one such review that system integrator MCS Control Systems, suggested its variable-speed drive partner, Sentridge Controls, carry out an energy appraisal for the company.

After looking at all applications from vacuum packing to air compressors, an area of high energy use identified was the extruders, which use up to 30% of the site’s energy consumption.

After product is fed into a hopper, it is then measured into the extruder screw.

The extruder mixer transfers the product forward through the barrel under a controlled temperature and pressure through a faceplate.

From here it is extruded into a long hollow tube before being cut into short lengths prior to cooking.

A speed signal from the system’s PLC to the motors, synchronises the extrusions with the rotating cutter.

Portable energy loggers were installed on the incoming supply to monitor and measure kW, kVA, kVAr, power factor, current and voltage.

As a result, eight ABB industrial drives, five ABB general purpose drives and low voltage ABB motors, rated from 0.75 kW to 250 kW, were installed during a two week phase, part of which was the plant’s shutdown period.

The equipment was selected because the incumbent drives supplier was not able to offer a drives and motors matched pair.

Following the installation, further measurements were taken and a 20% drop in energy consumption was observed.

The results showed that by changing to AC drives, Weetabix would save some £28,000 per year, with a payback within three years.

Aside from the energy savings, there were other benefits, says Dave Chapman, MCS Control Systems’ site engineer: “The new AC drive technology brings greater torque control and speed accuracy, thanks primarily to the motor control platform, direct torque control (DTC),” he says.

“Unlike DC drive technology which needed a tachometer feedback arrangement to maintain accuracy, using DTC, no encoder feedback is needed, which reduces complexity and cost; yet we are able to get a far better control of the process resulting in a constant extrusion rate.”

Allan Norman, manufacturing manager at Weetabix’s extruded line, says: “The benefit of the repeatability function is that the endproduct, the Weetos, are produced with consistent length, thickness of the tube and density. DC drive technology is subject to surging which affects the product output.”

Because the speed and torque control are now so accurate, the rotating knife that cuts the extruded wheat, can do so in a more uniform manner, he adds.

This means that the cooking time of the end product is optimised; there is minimal temperature fluctuation which further saves energy.

Weetabix says it has also eliminated maintenance on the drive train, as there are no carbon brushes to replace, no forced cooling, and feedback devices are eliminated as the gearboxes are protected by implementing a torque limit control on the AC drive.

It estimates that engineering maintenance costs and plant downtime have been reduced by almost £20,000 per year, providing an overall estimated cost saving of £48,000 per year.

Meanwhile a by-product of the AC motors is that the noise and heat levels have fallen.

Working in the area of the DC drive train would require ear defenders, with noise levels as high as 130 decibels (dBA). Now the levels are within the regulation 80 dBA.

“A key factor in our choice of drive and motor controls remains the engineering skills of the people involved,” says Norman.

“They provide the same team and are prepared to tackle and explore any new engineering avenues that might benefit our production.”

He says the overall installation was supplied with guaranteed cost savings or money back to the difference.

“This probably accounts for the fact that, following installation, we started the extruders on Sunday evening and they ran without a hitch until turned off on Friday night.”