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Inverters prevent prisoners feeling hot under the collar

14 October 2010

A most unusual application for frequency inverters has emerged in New Zealand - providing air conditioning for prison vans. SMV inverters were supplied by the local Lenze agent Tranz Corporation who won the order because the SMV has features that simplify the control system. In addition the ability to transfer programs in seconds saves time on commissioning.


Tranz Corp became involved in the prison van project following a new initiative in New Zealand to keep prisoners separate from each other in single cells during transits. Previously, prisoners were kept together in a large communal holding area within the vans. However, this arrangement resulted in many altercations between prisoners – and some fatalities! The authorities reacted by ordering vans with segregation for prisoner. In addition to their main purpose of transporting prisoners all over the country, the new van would be called upon to act as temporary holding cells outside of court houses or police stations.

This requirement to use the vans as a temporary holding facility meant that a supplementary ventilation system was required. As a result, the vans are fitted with air-conditioning packages, which comprise the air-conditioner itself, a diesel motor that drives the system when mobile, and a electric motor that takes over when the van is parked. At this point the van is connected to the mains supply.

The diesel engine is shut down and the electric motor started with connection to a single phase mains supply. A delayed start of the electric motor is essential to allow the diesel engine to come to a complete stop, and disengage when the mains cable is connected. Failure to do this would cause serious harm to the equipment. Here Tranz Corp was able to offer an advantage on the SMV drive that competitive drive suppliers for the ventilation control system could not. The SMV has an integral "start on brake feature", which provided the required delay period without any requirement for additional external equipment. This factor was very important in what was an extremely competitive tender for the prison van project.

The Lenze AC-Tech SMV inverter range is ideal for such small to medium air conditioning drives. An IP65 enclosed version is available up to 22kW, but Tranz offered the IP31 variant which is available for single phase supply to 2.2kW or 3phase to 37kW. The inverter allows the motor speed to be adjusted to match demand, in this case a set temperature, saving considerable energy compared to simple on/off control. A compact footprint includes an easy-to-use control panel with buttons and a programmable status display.

A further feature of the SMV inverters saves time on commissioning, particularly for future installations. "Thanks to the EPM memory chip, we will now be able to save build time and reduce costs by supplying the next batch of drives pre-programmed" said Paul Londrigan of Tranz Corporation. "The EPM is a memory chip with its own rugged plastic enclosure that plugs directly into the front of the drive. It allows an entire set of operating parameters to be copied in a matter of seconds, and commissioning of the drive without having to turn it on".

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