40 n PROCESS AUTOMATION AND HAZARDOUS AREAS February 2025 www.drivesncontrols.com Applying pressure to achieve explosionproof motor operation Plant operators often cannot nd suitable replacements for obsolete motors. In particular, the choice of explosion-proof drive systems is limited. Until recently, the only alternative was to buy a standard motor with the required explosion-protection rating and adapt the installation site and/or the driven machine, sometimes at considerable expense and taking a long time. An alternative approach, which oers more choice and exibility, is to adapt a standard motor for use in potentially explosive atmospheres. One way of doing this is to purge the air from inside the motor so that no pockets of potentially ammable gases remain in its casing. By ushing the motor with an inert gas mixture, the gases are removed from the frame before the motor is started. During operation, a constant positive pressure is maintained to prevent the ingress of explosive gases. The terminal boxes of these motors oer type Ex eb increased safety ignition protection. For example, the German motor-maker Menzel Elektromotoren applies this procedure to its stock motors to oer competitively priced Ex p motors in the 2G and 3G categories. The pressurised motor technique is typically suitable for squirrel-cage motors with shaft heights from 400mm. It is available for all voltage ranges and with air-to-air or air-towater heat exchangers (cooling types IC 611, IC 616 or IC 81W). The German technical inspection service TÜV Nord has certi ed Menzel to produce large pressurised squirrel-cage motors with type Ex p ignition protection for use in Zone 1 hazardous areas where an explosive gas/air mixture is likely to occur in normal operation. To achieve this certi cation, TÜV subjected a petroleum industry compressor motor to a gas displacement, leak and pressure tests. Menzel recently built an Ex p compressor motor for an Austrian chemical company in just three weeks. The MEBKSW squirrel-cage motor was modi ed in various ways and tted with an air purge system that allows it to be ushed with an inert gas mixture before startup to remove potentially explosive atmospheres and to maintain a constant positive pressure during operation, thus preventing the ingress of hazardous gases. The motor was labelled II 3G Ex pzc IIB T3 Gc. Additional work included: converting the motor to water cooling; adjusting its frame size from 560 to 630; repositioning its terminal boxes; and preparing the neutral box for the installation of current transformers. In another recent project, Menzel manufactured a 1.635MW, 6kV Ex p replacement compressor motor for an oil re nery, which matches the original motor in all of its mechanical and electrical interfaces. The motor, based on Menzel’s MEBKSL series, is labelled II 3G Ex pzc ec ic IIC T3 Gc for zone 2. Before shipping, the motor was subjected to tests including load and type testing. • In another project – this time not involving pressurisation – Menzel has built a 1.12MW dust-explosion-protected motor for an Italian our mill. The motor-maker upgraded a standard 690V squirrel-cage machine for converter operation, and adapted its dimensions to match those of the original motor, allowing a smooth replacement. It also manufactured an oversized terminal box to customer speci cations. The upgraded motor provides a nominal torque of 11.575kNm. It has type Ex tc ignition protection (protection by housing) and an Atex II 3D Ex tc IIIB T125°C DC label for use in Zone 22 areas. It can rotate in both directions and oers IP55 protection and IC 611 cooling (via an air-to-air heat exchanger). n Adapting standard motors for use in explosion-proof applications can be costly and time-consuming. A more cost-e ective and quicker approach can be to purge the air from inside a motor so that no pockets of potentially ammable gases remain. This Ex p compressor motor, based on a standard squirrel-cage machine, was tted with an air purge system that allowed it to be ushed with an inert gas mixture before startup
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