VFD vs Impeller Trimming Guide
There are various ways in which a pump can achieve a duty point but one question that repeatedly comes up is should the duty point be achieved by impeller trimming or via VFD?
To explore the differences between a VFD and impeller trimming a summary is provided below:
| VFD Advantages | VFD Disadvantages | Impeller Trimming Advantages | Impeller Trimming Disadvantages |
| High efficiency | More expensive | Cheaper | System calculations must be correct |
| Lower NPSH | Electronics can fail | Less reliance on electronics | Higher rotational speed means lower lifetime and limited by motor RPM |
| Wider range of duties | Larger sized motor required and forced air cooling can be required at low RPMs | Less efficient than VFD | |
As impeller wears speed can be increased which means built-in redundancy | Impeller trimming | System modifications likely to mean new pump and motor | |
Energy savings 2 pumps running at 50% save 1/2 of 1 at 100% | |||
Longevity by reducing pump speed on components such as bearing and seals | |||
Exact RPMs can be selected as normally motors are only available in set RPMs | |||
Can eliminate the need for surge suppression equipment as pump can be slowed down gradually | |||
Variable frequency drive pumps do have more advantages due to their design. VFD Pumps are manufactured with impellers at the full diameter and hence are more efficient as the internal clearances between it and the casing are always the same distance.
This also means 2 pumps running at 50% save ½ the energy of one pump operating at 100% speed.
As such pumps are more efficient the NPSH is lower, and as the pump can be run across a wide range of RPMs the available duty range is also larger.
As the impeller wears the speed can be increased which means it has built-in redundancy as parts wear. Such pumps are typically plug and play or require a lot less set up than pumps where an impeller has been trimmed and the exact system loses are unknown leading to the system being balanced by the manual operation of valves.
VFD pumps are more expensive and electronics can fail over time and if the VFD is mounted on the pump it can mean the entire unit requires replacing which is why we always recommend a separate VSD.
The payback of such equipment is typically a lot shorter though.
Impeller trimming is a lot cheaper, and has less reliance on electronics however caution must be taken to ensure the selection is correct, otherwise the impeller and motor may require changing.
Furthermore, as impellers are trimmed it means that the clearances between it and the casing are larger which means the unit is less efficient the more that the impeller is trimmed.
Are you looking for a pump to match your exact duty point, multiple duty points or looking to save energy by using a VFD?
Contact North Ridge and we will work through your selection with you.
