Last issue, we discussed several factors which can influence the decision regarding the type of VSD – stand-alone module or packaged drive – to utilize for your application. These factors included the configuration of the drive you are seeking to replace (if this is not a new application); available space; ambient conditions in/around the installation site; and peripheral equipment to be connected to the drive.
Other factors can play important roles as well. For instance, safety and code compliance can heavily impact the VSD configuration to choose. For instance, packaged drives which are UL labeled provide assurances that the assembly is properly built, tested, and suitable for its rated output. While not all installations require UL labeling, the assurances it provides can more than offset the nominal cost for obtaining it. And while many stand-alone drives are UL labeled, a packaged unit ensures that the upstream disconnecting means, overcurrent protection, and any associated peripherals are specified and installed in compliance with applicable codes – which can translate to less work by field engineering and installation personnel.
The installation of any VSD requires properly rated disconnecting means and over-current protection installed in conjunction with the drive. (A basic VFD power wiring installation is shown in Fig. 1 below.) These components must be specified to provide adequate protection for sensitive VSD electronics while confining shutdown to only the affected equipment under fault conditions. A properly designed packaged drive includes fast-acting semiconductor fuses to protect the drive transistors and rapidly remove power to the drive and motor, thereby minimizing the likelihood of damage to the drive.
The above is not to imply that the proper specification and installation of the VSD and its associated components can’t be done when installing a stand-alone drive – that’s necessary in any case. However, the disconnect and fuses in Fig. 1 would either require their own enclosure, or would otherwise be field-installed in a customer-supplied cabinet along with the drive. Because a packaged drive comes with the drive module, disconnect and fuses already installed and interconnected, the installation is less labor-intensive and field-side costs will be lower. Whether these lower costs will offset the additional cost of the packaged drive will depend on many things; the drive rating, cost of materials, labor rate and experience, and cost of compliance, just to name a few.
To break this down in more detail, let’s compare the engineering and installation cost impacts of a VSD module versus a packaged unit. (Note that the following tasks assume a basic drive application using either V/Hz or Open Loop control for the VSD.) From the summary below, you can see that the amount of effort required of site personnel is significantly reduced when purchasing a packaged drive.
| VSD module | Packaged VSD |
| Approx. capital cost of VSD module (e.g. 37.5kW/50HP VT rated, NEMA 1): US$4,750 | Approx. capital cost of packaged VSD (e.g. 37.5kW/50HP VT rated, NEMA 1): US$6,490 |
| Engineering: ** Specify drive rating ** Determine cable req’ts. ** Protection coordination ** Specify disconnecting means ** ID installation locations for all peripheral components | Engineering: ** Specify drive rating ** Determine cable req’ts. |
| Procurement: ** Obtain quote for VSD ** Obtain quotes for peripheral components ** Obtain installation quotes ** Issue PO for VSD ** Issue POs for all peripheral components ** Award installation bid(s) ** Receive and inspect all components | Procurement: ** Obtain quote for VSD ** Obtain installation quotes ** Issue PO for VSD ** Award installation bid(s) ** Receive and inspect all components |
| Installation: ** Install VSD ** Install peripheral components ** Install VSD line, load wiring ** Install wiring between VSD and peripheral components ** Field-test VSD and peripheral components as an integrated assembly | Installation: ** Install VSD ** Install VSD line, load wiring ** Field test VSD assembly |
One other, less tangible factor may affect your decision as well – the capability and experience of your on-site and contracted resources. If your Engineering and/or installation personnel are not experienced with VSD specification and installation, all other things being equal it may make economic sense to choose a packaged unit, where the design and specification of inter-related components is already done for you by qualified outside resources.
Have you been faced with determining the most cost-effective drive configuration to use? How did you resolve it? Please share your story in our Comments section. And if you have any questions related to drive application or specification, please contact us at info@joliettech.com or call us toll free at 866-492-9888.
Thanks for reading!
Regards,
Jay Baima
Joliet Technologies
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