Variable-frequency drive for a pump – when does it actually reduce energy consumption?

A variable-frequency drive (VFD) allows the pump motor’s speed to follow the demand of the system. In a variable-flow system it can reduce energy use, particularly where flow is currently restricted by a valve or diverted through a bypass. However, savings are not automatic: they depend on the hydraulics, process requirements, equipment efficiency and the hours spent at each operating point.

Before upgrading, establish how much electricity the complete pump–motor–control system draws under actual operating loads. Only a comparison of options delivering the same duty can show whether a VFD is the best solution.

When can a variable-frequency drive reduce a pump’s energy consumption?

The greatest potential is in systems where the required flow changes over time but the pump runs at a fixed speed for many hours. If excess flow is throttled at a valve or returned through a bypass, speed control can reduce the energy losses associated with this method of control.

Start by looking for four operating conditions:

• Partly closed control valve: the system does not need the pump’s full capacity for much of the year.
• Continuous bypass flow: some of the pumped liquid is returned even though the process does not use the full flow.
• Variable process demand: for example, machine cooling requires different flow rates depending on current production.
• Several pumps in parallel: there may be scope to optimise the number of running pumps and control the speed of one or more units.

Where the required flow and pressure are virtually constant and the pump operates near its optimum point, fitting a VFD alone may not deliver meaningful savings. The drive’s own losses must also be considered. The relationship between control method and power draw is described, among other sources, in the U.S. Department of Energy’s guide to centrifugal pump control.

The system curve matters more than a claimed savings percentage

A centrifugal pump cannot operate at an arbitrarily selected point. Its flow and head are determined by the intersection of the pump curve and the system curve. Changing the rotational speed shifts the pump curve, but the system still requires a certain pressure and flow.

The system curve has two distinct components:

• Static head: arising, among other things, from elevation differences, vessel pressures or the minimum pressure required by the process. It does not disappear when the flow is reduced.
• Friction losses: resistance in pipes, valves and other components, which usually decreases as flow falls.

Why does a high static head limit the benefits of speed control?

Suppose a system requires a total head of 50 m at a flow of 100 m³/h: 20 m is static head and 30 m is friction loss. If friction loss varies approximately with the square of flow over the range considered, it falls to 30 × (60/100)² = 10.8 m at 60 m³/h. The system therefore still needs about 30.8 m, not the 18 m obtained by simply multiplying the entire 50 m by 0.6².

This calculation illustrates the effect of the static component; it is not a substitute for establishing the actual operating point from the pump curves. At too low a speed, the pump may be unable to overcome the required static head at all. A similar limitation is noted by the Hydraulic Institute in its answers on variable-speed pumps.

Does the cube law always apply to a pump?

For the same pump, the affinity laws indicate, in simplified terms, that at comparable hydraulically similar operating points, flow is proportional to speed, head to its square, and shaft power to its cube. They are useful for an initial assessment, but they do not mean that reducing speed by 20% will cut the system’s electricity consumption by 49%.

That simplification ignores static head, shifts in the operating point, changes in pump and motor efficiency, VFD losses and process constraints. The actual effect must be calculated for the complete system and its operating hours, not from motor supply frequency alone.

How can energy savings from a pump VFD be estimated?

The analysis must cover the same required duty: flow, pressure, temperature, fluid quality and production conditions. Comparing two different volumes of liquid pumped may show lower consumption without necessarily indicating better efficiency.

1. Build a demand profile: determine how many hours per year the system operates at each flow rate and what pressure must be maintained.
2. Measure the existing operation: log flow, suction and discharge pressure, valve positions, operating time and active electrical power drawn from the grid at the same time.
3. Model the modified system: use the manufacturer’s curves to identify feasible operating points and pump limits; account for motor and VFD efficiency at part load.
4. Calculate energy for each mode: multiply grid power by operating hours and sum the results for the full year. Include material auxiliary loads as well.
5. Compare alternatives and costs: consider not only drive purchase and installation but also controls, measurements, downtime, servicing and running costs.

Example calculation for a pump operating 6000 hours per year

Consider a hypothetical valve-controlled system where flow demand varies during the year. The table below uses illustrative electrical power drawn from the grid before the upgrade and after installation of a VFD. The flow required by the process is the same in both options.

In this scenario, the difference is 78 000 kWh/year, or around 31%. At full flow, power draw after the upgrade is even slightly higher because the VFD also incurs losses. Most of the savings arise during part-load operation.

The table values are assumptions used to illustrate the method, not a forecast for any particular site. A real project requires measurements and performance curves to establish these values. The approach based on measuring power, flow and operating time is consistent with the principles of the FEMP Measurement and Verification Guidelines, version 5.0.

Selecting a VFD for an industrial pump: technical limitations

Minimum flow and the pump’s safe operating range

Not every speed is permissible. The pump manufacturer specifies limits for flow, speed and stable operation, as well as efficiency curves and the required net positive suction head (NPSHr). Prolonged operation at very low flow may cause overheating of the fluid, internal recirculation, vibration or accelerated wear.

The design should account for minimum continuous flow, the preferred operating region around the best efficiency point and an adequate margin between the net positive suction head available in the system (NPSHa) and that required by the pump (NPSHr) throughout the control range. These terms are defined in the Hydraulic Institute’s terminology database. In a multi-pump arrangement, how the units interact as the number in operation changes must also be checked.

Motor, VFD and electrical installation

A “30 kW” rating on both the motor and drive is not sufficient proof of correct selection. Check the motor’s actual current, the drive’s permissible current after derating for ambient temperature and installation conditions, overload requirements and operation at low speeds. At low speed, the motor’s self-cooling may be less effective.

Winding insulation, bearing currents, motor cable length and screening, electromagnetic compatibility and any required output filters also matter. Component selection and protective measures depend on the documentation for the specific equipment. We cover this in more detail in our article on selecting a VFD for an industrial motor.

Pressure and flow control and operation without demand

The VFD should respond to the system’s actual needs. Depending on the process, control may maintain flow, differential pressure, tank level or another variable. Sensor location and setpoint are important: maintaining excessive pressure around the clock can undermine expected savings.

During commissioning, define minimum and maximum speed, acceleration and deceleration ramps, responses to dry running, signal loss and overpressure, and operation when there is no demand. Selected pumping functions, such as no-flow detection and sleep mode, are described in the Danfoss VLT AQUA Drive design guide. Their availability and settings must be confirmed for the chosen model.

What should the upgrade design, settings and acceptance testing cover?

An efficiency assessment does not end when the drive starts the motor. The work can be divided into stages:

1. Assessment: inventory the pump, motor, valves, controls and protective devices; confirm process requirements and the operating profile.
2. Hydraulic calculations: establish the system curve and operating points at different flows, assessing minimum flow and suction conditions.
3. Electrical verification: select the drive, cabling, protection, cooling, filters and interfaces with the control system.
4. Commissioning: configure the controller, speed limits, ramps, alarms and emergency modes; for multiple pumps, also configure the staging sequence.
5. Acceptance testing: measure input active power, flow and pressure in representative operating modes and check stability, vibration and temperature.

After an upgrade, compare energy consumption before and after for a comparable amount of useful work. A lower frequency reading on the VFD alone is not evidence of savings. Where the result is to support an investment decision or verification of benefits, an energy efficiency audit of the specific measure in Poland may be helpful.

When might replacing the pump or changing the hydraulics be better?

A VFD cannot correct every system design problem. Before buying one, consider other measures, particularly if:

• the pump is oversized and operates far outside the manufacturer’s preferred range most of the time — impeller trimming or a different pump may be an option;
• poorly sized pipes, filters or fittings cause substantial losses — assess the hydraulics first;
• the process requires constant flow — another solution may be more appropriate than continuous speed control;
• several pumps operate together — better sequencing and selection of the number running may improve performance without speed control on each unit;
• the pump is unnecessary for much of the time — reducing operating hours may matter more than reducing speed.

Compare these options on a consistent technical and economic basis, taking reliability and production requirements into account. Guidance on equipment selection and life-cycle costs is provided in the Europump and Hydraulic Institute guide to pump system life-cycle costs.

How should a specific pumping system be assessed?

A complete set of measurements is not needed at the outset. Useful starting information includes pump and motor identification, performance curves, a system diagram, available flow and pressure data, power measurements, details of valves and bypasses, and an approximate operating schedule.

Practical takeaway: a VFD is worth considering where the system genuinely needs variable flow and pump speed can safely be reduced. The decision should be based on grid energy use and process parameters across the entire operating profile, not a blanket promise of percentage savings.

If you are considering such an upgrade at your facility, describe the system and email the available data to audytor@energytrend.pl. This will help identify which calculations, measurements and alternatives should be investigated before a decision is made.

Technical references

• U.S. Department of Energy – Control Strategies for Centrifugal Pumps with Variable Flow Requirements.
• Hydraulic Institute – Pump FAQs, including system curves and variable-speed operation.
• Hydraulic Institute – definitions of pump performance parameters and operating ranges.
• U.S. Department of Energy, FEMP – Measurement and Verification Guidelines, v5.0.
• Danfoss – VLT AQUA Drive FC 202, Design Guide.
• Europump / Hydraulic Institute – Pump Life Cycle Costs, Executive Summary.