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Mitsubishi Electric has revealed how industrial inverter drives are being used to help bring cold-stored fresh produce back to ambient temperature to ensure it remains in perfect condition for sale.

Crop Systems designs and installs potato and crop storage controllers and equipment for farmers, growers and produce suppliers.

Ray Andrews from Crop Systems said: ‘If you take, say, potatoes out of cold storage and let their temperature rise in an uncontrolled manner, you risk impairing flavour, texture and appearance.

‘You can also make them more prone to shattering and other handling-related damage.

‘In the past this was acceptable, but the constant march of quality standards means we have to reduce this problem and thus give our customers an edge over their competitors,’ Andrews added.

The basic concept Crop Systems has developed is a 14-bay batch-warming facility.

Potatoes are transferred from storage to a bay, where a controller is set to provide gradual warming to a set-point temperature over a specified period of time.

The target temperature varies between 4C and 10.5C, and the time is typically three to five days.

This is not a simple, open-loop, linear warming – parameters such as humidity and carbon dioxide (CO2) concentration are monitored and controlled throughout the process.

Andrews said: ‘The bays use the same control technology as the main bulk stores, but on a smaller scale and with different software.

‘Basically we are using fans with inverter feeds so that their speeds can be varied, sensors around the bay or stores to monitor environmental conditions, and a computer to read the sensors and adjust the inverters accordingly.

‘In the not-too-distant past, the philosophy in the stores was to have large, fixed-speed fans running flat out regardless of conditions – a setup that made it impossible to truly optimise conditions.

‘We developed our Evolution controller to give many varying levels of control through different types of sensors located around the store; from this, we can adjust the fan speed according to the exact requirements.

‘From here, it was a natural step to develop the warming bays along the same lines,’ he added.

Looking at storage, newly harvested potatoes have a high moisture content, which needs to be reduced over the first few weeks.

The Evolution is designed with an automatic load cure program, which keeps the fans running at an economical speed until the stock has settled down.

The potatoes also emit heat, which, if left to build up, would compromise storage.

The fans adapt their speed to compensate for different conditions.

Andrews said: ‘We have also developed an automatic fogging program so that we can better address the distribution of ‘cleaning-in-place’ fog within the storage area, and we have proven that “stirring” and re-circulating the air at reduced air speeds helps give far better efficacy and extends the fogging intervals dramatically.

‘This speeds the payback period to less than 18 months in some cases,’ he added.

Central to the storage processes above is the use of an F740 variable-speed drive from Mitsubishi.

The F740 is the fan-rated version of the F700 range of drives and offers motor efficiency during constant-speed operation and acceleration periods, and optimum torque patterns, which accurately match the power delivered to that required at any instant.

The speed of optimisation is effectively instant, so the drive is always giving maximum performance and efficiency to the load.

The F740 was selected for this fan-drive application for several reasons, including its ease of commissioning, removable terminal block, on-board EMC filter, predictive maintenance monitor, trip-less reliability, and ability to optimise energy usage.

Mitsubishi Electric Automation Systems

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