Single-phase inverters used for prison van air conditioning

Tranz Corporation has supplied Lenze AC-Tech SMV inverters for air conditioning systems in prison vans commissioned by the New Zealand authorities; the drives offer advantages in terms of simplified controls and easy program transfer.

A most unusual application for frequency inverters has emerged in New Zealand: providing air conditioning for prison vans. Lenze AC-Tech SMV inverters were supplied by the local agent Tranz Corporation, which 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 prisoners. In addition to their main purpose of transporting prisoners all over the country, the new vans would be called upon to act as temporary holding cells outside 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 an electric motor that takes over when the van is parked (at this point the van is connected to the 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' that 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.

SMV inverters

Lenze AC-Tech SMV sensorless vector drives are excellent for such small to medium air conditioning drives. An IP65 enclosed version is available up to 22kW, but Tranz offered the IP31 variant that is available for single-phase supply to 2.2kW or three-phase to 37kW. The inverter enables the motor speed to be adjusted to match demand - in this case a set temperature - thereby saving considerable amounts of energy compared to simple on/off control. Other features of these variable-speed drives include a compact footprint, and 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. Paul Londrigan of Tranz Corporation explains: "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. 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".

Follow the link for more information about Lenze AC-Tech SMV inverters.

Lenze Ltd

6 Abbey Court Road
Priory Business Park
MK44 3WH

+44 (0)1234 753200

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