Inverters have the right mix to make dough

Roberts Bakery has improved the whiteness and texture of its sandwich loaves by fitting Mitsubishi Electric's latest generation of inverter drives to its dough mixers.

Roberts Bakery has improved the whiteness and texture of its sandwich loaves by fitting Mitsubishi Electric's latest generation of inverter drives to its dough mixers so that it can profile the operating speed as mixing proceeds.

Previously the mixers' motors were star-delta started and ran at fixed speeds, but in-house tests had shown that by reducing the speed at the start of the mix, the flour better absorbs the water.

This creates a uniform size of air bubble in the finished loaf which leads to more even baking.

The mixers are then ramped up to full speed for the rest of the mix.

"Like most Industrial bakeries we don't mix for a specific time, but to the target of 11Wh/kg of flour, which takes a little over 3min", explains Senior Software/Projects Engineer Steven Hepworth.

"The inverters start off being time controlled for the initial slow mix, but then switch to monitoring their own power consumption to complete the cycle".

This control regime was straightforward because the F700 drives are fitted with both timer and energy meter as standard, the latter usually being used for efficiency monitoring.

Once the dough is mixed the mixer impeller slows down while the drum tips through 120 degrees to empty it into a hoist bowl.

"This speed is critical too".

"If we were to empty without the mixer rotating, the dough would stick to the impeller; if we went at full speed the dough would break up and go everywhere but where we wanted it".

There have been four mixers fitted with the new drives; each holds 200 or 225kg of flour, plus all the other ingredients.

All the drives are F700 110kW units from Mitsubishi Electric.

The order for the new drives was placed with Warrington based Mitsubishi distributor, Newton Tesla (Electric Drives).

Frank Roberts Engineering designed the control circuits, the software and modified the existing wiring.

Newton Tesla engineers installed the new control cabinets and, working to an agreed schedule, it was possible to install and commission the inverters without any down time or loss of production.

The control circuit has been designed so the bakery also has the option of switching between star-delta or inverter control.

This is partly to monitor the improvement in dough texture between the two methods of control, but it also helps reduce downtime in the unlikely event of an inverter failure.

The F700 was introduced last year and features both optimum excitation control and optimum torque patterns, ensuring that the drive is always giving maximum performance and efficiency.

Thus the F700 is able to automatically optimise energy usage for every possible load profile.

This is significant for bakers because the dynamic characteristics of the load change radically as the dough is formed; the F700 can detect this and constantly re-optimise its settings to follow the changing conditions.

The drives are controlled by Mitsubishi Q-series PLCs (programmable logic controllers).

The mixers are configured with two hoists and are all networked over Ethernet to form part of the plant-wide Scada control system.

"We work the PLCs and drives quite hard", says Hepworth.

"The mixers are paired, two to each hoist and alternate so that the hoist is almost constantly lifting another load of dough to the divider, where it is cut and shaped into 900g dough pieces ready for proving and baking".

"Not only does the Q series control this and feeding live data onto the Scada, it is also controlling the mixers' tilting mechanisms - which are again controlled by Mitsubishi inverters".

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