FIG. 1 shows a known motor drive (the terms variable speed drive or variable frequency drive are also used to refer to a motor drive—the terms are used interchangeably in this document), indicated generally by the reference numeral 1. Motor drives generate significant heat during use and therefore require cooling. The variable speed drive 1 shown in FIG. 1 includes a back channel 2 that provides the bulk of the cooling for the electrical circuitry within the drive 1. Nevertheless, this is not sufficient and an air inlet 3 and air outlet 4 are provided on the front of the drive to provide an additional air flow for cooling purposes.
In order for sufficient cooling to be provided, it is important that the air inlet 3 and air outlet 4 are not blocked. This is not generally a problem when the drive 1 is mounted indoors in a relatively clean environment. However, the air inlets and outlets provide a weakness in the structure of the cabinet that is particularly problematic if the drive 1 is mounted outdoors (where it may, for example, be required to withstand rain) or in a particularly harsh indoor environment.
It is possible to mount the drive 1 within an enclosure box in order to protect the drive from, for example, rain, sand or excessive dust. However, such an enclosure must allow sufficient air to flow into the air inlet 3 and away from the air outlet 4, otherwise the thermal performance of the drive 1 will be adversely affected.
A known solution to this problem is to provide a cabinet to protect the drive 1. A commonly used cabinet is the so-called NEMA 3R cabinet (as rated by the National Electrical Manufacturers Association). NEMA 3R cabinets typically involve mounting the drive within the cabinet. Fans are typically provided at the bottom of the cabinet to draw air in for cooling the drive. The air is typically expelled out of the side of the cabinet. In addition to cooling fans, heaters are often provided in order to be able to heat the cabinet to prevent condensation when the drive is not operational. Other possible features include filters and mounting feet to raise the cabinet off the ground.
Cabinets such as NEMA 3R cabinets are expensive. Moreover, such cabinets are necessarily larger than the drive that is being enclosed (substantially larger in the case of a typical NEMA 3R implementation). The additional expense and/or the additional space requirements are unacceptable in many circumstances.
The present invention seeks to address at least some of the problems outlined above.