Retrofitting Liquid Resistance Slip Ring

You do not automatically attain savings just by using a VFD. The savings that can be realized using a VFD in a centrifugal pump application are due for the most part in comparing it to OTHER means of flow or pressure control, typically mechanical systems that perform this function. The savings come from the fact that using speed reduction of the motor will take advantage of the Affinity Laws to waste LESS energy in accomplishing the variable conditions desired than these mechanical means.

But in comparing it against a slip ring motor using a liquid rheostat, you must understand that you are ALREADY getting most of those benefits with your existing control system. Swapping it out for a VFD is not going to attain any appreciable gains in efficiency. Maintenance reduction, yes. Simplicity (by comparison) for younger people who are used to VFDs but may have no previous experience with slip ring motors, let alone liquid rheostat speed control, yes. But energy, not much if any. VFD salespeople may tell you otherwise, but remember, they have an agenda.

If your only goal however is to reduce losses in the system to improve energy efficiency, look into a rotor slip energy recovery system for your existing motor, they can be retrofitted to existing liquid rheostat controllers. There is a company called Flomatcher that makes LR controls, who also bought another company called Marathon that made rotor energy recovery systems. They only list it now as a built-in component of their LR controller, but if you ask them nicely, they will probably discuss a rotor slip recovery system for your existing LR controller.