Variable speed drives (VSDs) are becoming more common in industrial control systems due to their flexibility and potential for significant power savings. There’s a huge range of drives available for a myriad of applications including IP20 models for mounting VSD’s inside a switchboard and IP54/66 models for mounting externally. Which option is best for your application? There’s no right or wrong answer, but here’s a few points to consider:
Heat and ventilation requirements
Variable speed drives generate a large amount of heat in operation for their size. It’s critical to ensure this heat is drawn away adequately from the drive to avoid overheating, unexpected shut-downs and component damage. In a cool and dry environment, an externally mounted VSD bypasses this problem by allowing the heat to disperse in the large ambient air mass in the room. An VSD mounted inside a switchboard however, is in a much smaller environment and generally requires forced ventilation to draw cool air into the cabinet and force the warm air outside.
Weather-proofing and ingress protection
The word ‘weather-proof’ can be misleading: while an externally-mounted IP66 VSD can technically handle heavy rain, in most cases it will not withstand hours of direct sunlight in a hot climate. Where a control system is situated directly in the weather, it may be preferable to locate VSD’s inside the switchboard, with shrouded ventilation or air-conditioners to minimise liquid ingress while effectively cooling the cabinet. A good switchboard builder will be able to calculate the expected heat load and design a cooling system to suit.
Installation considerations
Internally mounted variable speed drives have one clear advantage over external drives: they can be fully wired, set-up and tested in the factory before they get to site. External VSD’s on the other hand require additional cabling and more installation and commissioning time. That said, many VSD’s have a maximum motor cable length restriction – this may necessitate IP54/66 VSD’s mounted closer to the motor. This can also translate into cost savings on shielded motor cable.
Cost
An IP54/66 VSD will generally cost 20-50% more than the equivalent IP20 internally-mounted drive. This may not be a huge cost difference in the context of a larger project, but can be a genuine consideration for smaller installations.
