Beat the Rush
Thank you to everyone who sent in questions to firstname.lastname@example.org . We have taken one of the top questions and answered it below!
Frequently asked Question:
Should we install a vfd to control our pump?
Variable frequency drives, or VFDs, do pretty much what their name suggests. They drive electric motors at variable frequencies providing significantly more control than “On” or “Off”. Briefly put, VFDs convert incoming AC power at a fixed frequency to an internal DC voltage and then back to AC power at a variable frequency for a motor. This variable frequency control has some disadvantages, but in the right application these may not matter when weighed against the benefits. For the purpose of this post we will be discussing VFDs only when used with induction motor centrifugal pumps, mostly because that’s what we’re good at.
Some things to think about
Maybe the most obvious point to consider about driving your centrifugal pump from a VFD is the initial cost. VFDs are the most expensive starting method for your pump when comparing stick prices, there’s no way to hide that. But if the total cost of ownership is considered, this might not be the case, especially with the prices of VFDs continuing to drop. If your application is one that typically requires frequent start-stop cycles, peak power utility costs and pre-mature motor failure due to inrush currents may be something you want to consider. Eliminate one or two motor re-winds over the life of your pump and your VFD may have paid for itself.
One of the less obvious things to consider when considering a VFD is the effect it may have on your local power system and other electrical device operating from the same source. When connected directly to a power source many VFDs, especially cheaper ones, can negatively affect the supplies power quality. If you’re equipment will be operating in an area with tight power quality restrictions from your utility provider or VFD load comprise most of your power consumption, this may be something you need to consider. In many cases this is not an issue, especially if the system you are running is secondary to your main power requirement, like a discharge sump pump.
An issue that should absolutely be addressed during the design phase of a VFD pump system is the choice of motor and power cable. Standard induction motors and power cables are not designed to work directly from the power output of a VFD. Motors especially have a limited speed range that they can safely operate, usually referred to as “maximum turn-down”. This should be strictly enforced. Motors designed for operation from VFDs, usually called “inverter duty”, are always the preferable option to a standard induction motor for variable speed control. Inverter duty motors are physically larger for the same power output to handle higher heat generation at lower speeds and have higher voltage insulation for the voltage spikes generated by VFDs. Both features mean that a motor can be expected to operate longer when driven from a VFD and may take advantage of the full output speed range.
Once the VFD design challenges have been met, there are some distinct advantages to running a centrifugal pump from a variable speed drive. Dryly put, these can be summarized quite quickly. A VFD Driven centrifugal pump:
- Can operate across a variety of duty points
- Can operate with closed loop control, to maintain system set points
- Often does not require impeller trimming
- May compensate for wet-end wear over it’s the service life
But maybe a couple hypothetical application would be more interesting to think about.
If your application requires consistent flow across varying medium densities, then a VFD is your solution. Add flow meter to the programmable logic control embedded in most new VFDs, and you have a pumping system with closed-loop flow control. Capably of maintaining constant output without operator intervention.
Limited sump size combined with large flow rate requirements can create a tough situation, especially for pumps started across the line. The large inrush current means that motors will generate a lot of heat, limiting the number of allowable starts before causing permanent damage to a motor. By sizing a pump to handle anticipated upset conditions and then turning down for normal operation with a VFD, it is possible to match the pump flow with the sump’s normal fill rate while allowing head room for upset conditions. With control over the pumps flow rate, it is then possible to keep the pump running consistently, removing the need for frequent start/stop cycles and the associated inrush current.
Even working within the limitations of a VFD, there are a lot of situations where the correct VFD and motor selection can provide solutions to tricky problems. A VFD may not be the solution to every problem, but they can absolutely make the difference between a big problem and a challenging application. If you have such an application, ask your pump pros if a VFD is a solution you should consider!
Until Next Time,
RJ & The Hevvy Electrical Team!