Inverter’s stall-prevention feature keeps ice cream moving

During the automated production of ice cream it is vital that there are no unscheduled process stops as this would render the batch unfit for use. 

To eliminate any risk of process stoppages in its ice cream machines Kälte-Rudi has installed a compact Mitsubishi Electric FR-E700 series frequency inverter into its Diagonal-Freezer which features a diagonally installed production cylinder and agitator with kneading function for batches of between four and 16l. Markus Moosmann, technical director electrical engineering at Kälte-Rudi, explains: "The first step involves pasteurising the additives. It would, of course, also be possible to use an already pasteurised finished mixture as a base for adding flavouring. However, for most of our customers it is important to produce their own ice cream mix. The liquid is then poured into the ice cream machine where air is bound into the liquid to give it a creamy consistency as well as more volume. Larger machines, with a four-cylinder compressor can process up to 16l of mix, producing 20 to 25 litres of ready-to-eat ice cream."

During this process, the drive can be pushed to the limit as ice cream is difficult to handle. When the machine is mixing several litres of ice cream, it is only the movement of the ice cream that keeps it ‘liquid’. If the paddle should stop working, the viscous mass would turn into a solid block of ice that would have to be defrosted in order to remove it from the machine. Like any dairy product, you cannot refreeze ice cream that has melted. If the motor stopped unexpectedly during the mixing process, the whole batch would be rendered useless. 

Mitsubishi Electric recommended inverters from its FR-E700 series. The integrated stall prevention ensures that the paddle can continue to turn even when the motor is operating within its overload range, as the inverter reduces the output speed for a short time.

Kälte-Rudi first installed an FR-E740 in a prototype and carried out numerous tests. Moosmann said: "Tests have shown that the stall prevention can be used to control overloading. If the motor ever goes into its overload range, the inverter will continue to operate at a reduced speed, so the mixing of the ice cream continues and the product remains intact."

With the FR-E700 series inverter, the machine now even continues operation when the motor reaches its performance limits. As a result, the availability of the machine and production efficiency have been improved. Finally, the resource-saving inverter has indirectly contributed to the achievement of savings on additives, power and water consumption. 

Besides stall prevention, the inverter enables a measurable reduction in power, between 15 and 40% depending on the machine and its configuration. 

Having network capability with Profibus connection was particularly important to Kälte-Rudi, and the inverter offers this, allowing the company to monitor operation of the inverter in real-time at any time. If a problem should occur, the service engineer can tackle it on site by simply connecting a single programming cable to the HMI. No additional special cable is required. Diagnostics blocks can be programmed and any data can be read out via the software immediately.

The integrated safety-off function, in accordance with EN 12855, also enables a high level of operational safety as the control and regulation circuits of the inverter operate independently from each other. For emergency cases, it provides a safe and direct stop of the paddle.