Use of VFD in Medical Vacuum Control Panel
One of the new technology in recent years is the utilization of variable frequency drive (VFD) in medical vacuum control panel. The purpose of using a VFD is to save energy by lowering the peak demand charge, eliminate hard start stop and match the pump performance to the demand of the hospital. Axecon manufactures VFD panel for many industries and have a dedicate product page just for it that provides all the benefit of using a VFD panel.
While VFD are a great technology, the execution of this technology is also important to utilize the full potential of the saving VFD can bring.
Sharing a Single VFD Between Pumps
The cost of adding VFD in the panel can be quite significant. As manufacturers push to reduce the cost of the product, some performance trade offs are necessary to accomplish this goal. The use of single VFD in the medical industry is quite common. The saving can easily be seen. In a non VFD duplex panel, the power circuit would have 2 motor starter (contactor and overload combine). Upgrading to a panel with single VFD, would would need to add another set of contactor to the mix but you only need to add 1 VFD to this application. Since NFPA 99 size the system with 1 on reserve, the pump will always run under VFD unless lag pump is called to run. This situation should ideally never happen. This would be an ideal application for this type of design. But once we move on to triplex and up, the saving starts to fade away.
In triplex and up, the system demand would be all pumps minus 1. When the vacuum level drops, the first pump will come in as a VFD operated pump, which provides you have the energy saving mention above. Once the demand continue to increase, the 2nd pump will need to be called for. But the 2nd pump will need to run full speed as it's no longer operated by the VFD. At this point, all the saving mention before are no longer applicable to this unit since it's now operating as a direct driven pump. This situation gets worse as more pumps are added to the system.
Extra Point of Failure
As the design of single VFD requires to share the VFD between pumps, each pumps gets 2 sets of contactor: one for direct drive when it's a reserve or lag pump, and one for VFD drive. This is to isolate the pump to allow the VFD to operate only 1 pump. In a triplex situation, you end up with 6 contactors, all of which is a point of failure, plus the VFD itself. This makes these type of system more difficult to maintain and diagnosis.
Solutions? Multi VFD
The best execution to utilize the VFD technology is to apply a VFD to each pump. By doing so, you effectively increase the energy saving in the panel and reduce point of failure by reducing the number of parts. As each pump is now operating on VFD regardless of when it comes in, the peak demand charge is now lower, providing the operator the energy saving that was intended which using VFD. As well, by using VFD for each pump, we eliminate the use of contactors and overload. This works because each VFD is dedicated to each pump which provides the overload protection by each VFD. The increase in cost compare from single VFD and multi VFD is small as manufacturers are saving some cost by not installing contactor and overload but instead, offsetting the cost of the VFD with it.
Axecon's 5000 Series Control Panel provides the industry leading multi VFD control panel for contactless claw vacuum operation. As each VFD is dedicated to each pump, the reliability of these panel far exceed the single VFD control panel. If you need a new system or replacement control panel, feel free to reach out to us and our team is here to help you with the correct application of VFD in medical vacuum systems.
