Variable speed drives (VSDs) are used regularly in industrial control systems due to their power saving potential and flexibility. There’s a wide range available to suit many applications. This includes IP20 models, which are designed to mount VSD’s inside a switchboard, and IP54/66 models for external mounting.
There’s no right or wrong answer when it comes to deciding which option is most suitable for your application. Here are some important factors you need to consider.
Requirements for Ventilation and heat
Variable speed drives tend to generate a lot of heat for their size. It’s essential that this heat is pulled away from the drive to prevent overheating and avoid component damage and shut-downs.
An external VSD avoids this problem by providing a way for heat to disperse into the room’s ambient air mass. An internally mounted switchboard is located in a smaller environment and therefore needs forced ventilation to pull cool air inside the cabin and force warm air out.
Ingress protection and weather-proofing
Weather-proofing can be a misleading term. While an external IP66 VSD can handle heavy rain, it cannot withstand direct sunlight in a hot climate for extended periods. If a control system will be situated in a place where it’s exposed to weather directly, it might be better to mount VSD’s internally, using air-conditioners or shrouded ventilation to reduce liquid ingress while cooling the cabinet effectively.
An experienced switchboard builder can estimate the heat load and design a suitable cooling system.
One of the major advantages internally mounted drives have is that they can be wired and set-up for testing in a factory before they are delivered.
External VSD’s can’t be tested in the same way because they need additional cabling, and require more time for installation and commissioning. On saying that, VSD’s frequently have restrictions for maximum cable length, and this may require IP54/66’s to be mounted close to the motor. This can mean savings on the cost of shielded motor cable.
What’s the Cost?
The cost of the IP54/66 VSD is usually 20-50% higher than the corresponding IP20 internal drive. In terms of larger projects this may not be a significant saving, but it can be a serious factor for smaller installations.