Water and Wastewater
Boost sets, transfer pumps and blowers sized for peak demand and running against control valves for most of the year.
Application example
Fitting Variable Speed Drivesto Fixed-Speed Pumps and Fans
Duty Surveys • Drive Sizing • Harmonics & Cabling • Proven Savings • UAE
- Direct-on-line starter: replaced by, Variable speed drive
- Motor cable: replaced by, Screened motor cable
- The process: measured by, Pressure or flow signal
- Throttling valve or damper: opened fully, Nothing fitted
- Pump, fan and motor: left as they are, Nothing fitted
A great deal of industrial plant runs at one speed: full. The pump was sized for the worst case that might occur on the hottest day with the dirtiest filter, that case happens rarely or never, and the surplus is thrown away across a throttling valve or a damper. The motor draws what it draws, the valve gets hot, and the energy difference is simply lost.
A variable speed drive lets the machine run at the speed the process actually needs. On a centrifugal pump or fan the relationship between speed and power is steep, so a modest reduction in speed is a disproportionate reduction in energy — which is why this is one of the few efficiency measures where the physics does most of the arguing for you.
At the pump or fan
An electrical job, with the pump left alone
The pump, impeller and pipework stay as they are. The work is in the motor circuit, plus one signal from the process for the drive to follow, and the same measurements are taken before and after so the saving is shown rather than claimed.
- Water
- Signal
- FMeasuring point
- What we fit
- FFlow, logged firstLogged over a normal operating period before anything is sold, to show how much of its life the pump spends throttled.
- ZValve opened fullyIt stays fitted for isolation but stops setting the flow, and its position becomes a diagnostic.
- PThe signal the drive followsDischarge pressure wired back to the drive, so the speed matches what the process needs rather than the worst case.
- IMotor current and hoursCurrent, torque and running hours from the drive, which is condition monitoring you have already paid for.
- JPower, before and afterLogged in the survey and again after commissioning, so the saving is a measured difference rather than an estimate.
What changes
A drive in the circuit, and the pump left alone
A drive retrofit is an electrical job. The pump, the impeller and the pipework are not touched — what changes is what the motor is fed and what decides its speed.
Added or replaced
Everything here lives in the motor control centre or beside it.
- The drive itselfSized on measured load, not on the motor's nameplate rating
- Screened motor cable and glandingNot optional on a drive; it is what keeps the emissions where they belong
- A process signal to control onPressure, flow, level or temperature — the drive needs something to follow
- Harmonic mitigation where requiredChokes or filters, decided by a study rather than by habit
Left alone
Which is what keeps the work inside an electrical isolation rather than a plant shutdown.
- The pump or fanSame machine, same impeller, same duty point at full speed
- The motor, usuallyChecked for insulation and bearing suitability; replaced only if it fails that check
- The pipework and dampersThe throttling valve stays fitted, but stops being what sets the flow
- The process itselfIt gets the same flow or pressure, produced with less energy
The throttling valve is left in place and opened up rather than removed. It remains available for isolation, and its position becomes a diagnostic rather than a control.
What the job involves
The drive is the cheapest part. Most of the engineering is in deciding whether it belongs there at all and in the details that decide whether it survives.
A duty survey first
Logging actual flow, pressure and power over a normal operating period, so the drive is sized and justified on how the machine really runs rather than on its nameplate. This is also the step that finds the machines where a drive will save nothing — a pump that genuinely runs at full duty most of the time is better left alone, and saying so is part of the job.
Drive selection and control mode
Sized on measured load with the right overload rating for the duty, and configured to follow the process rather than a fixed reference — pressure on a boost set, level on a transfer pump, differential pressure on a fan. A drive running at a manually set fixed speed is a more expensive way of doing what the machine did before.
Harmonics and power quality
Drives draw non-linear current, and enough of them on one supply becomes a power quality problem. Whether line chokes are sufficient or a filter is needed is settled by a study against the site's supply and existing drive population, not by assuming the last installation's answer applies here.
4
Cabling, earthing and the motor itself
Screened cable properly glanded at both ends, correct earthing, and cable length within the drive's limit or with an output filter to suit. The motor's insulation and bearings are checked for inverter duty — an older motor on a long cable run is where shaft currents and insulation stress show up, months after everyone declared the project finished.
How it runs
From a survey to a proven saving
The order matters because the first step is the one that can, and sometimes should, stop the project.
01 / 04 STAGES
- Measure
Log the machine as it runs
Power, flow and pressure over a representative period, including the quiet times. The result either shows a machine spending most of its life throttled — in which case the case makes itself — or it does not, and the money is better spent elsewhere.
- Design
Size the drive and the installation
Drive rating, enclosure and cooling for an ambient that is a real constraint in this region, cable specification and route, harmonic mitigation, and where the control signal comes from. Panel modifications are drawn and approved before anything is ordered.
- Install
Fit within an electrical isolation
Drive installed in or beside the motor control centre, motor cable replaced with screened, control signal wired in, and the starter left in place or removed as the design dictates. The process itself does not need to be broken into.
- Prove
Commission and measure again
Control loop tuned so the machine follows the process without hunting, minimum speed set so the pump is never run below where it can deliver, and then the same measurement taken again. The saving is a measured difference, not a calculated projection.
Minimum speed is the setting most often left at default and most often the cause of a complaint. A centrifugal pump run too slowly delivers nothing while consuming power.
What a drive actually gives you
Energy proportional to the duty the process needs rather than to the worst case the pump was sized for.
Soft starting, which removes the mechanical and electrical shock that shortens the life of couplings, belts and the supply.
Control of the process variable itself — pressure, level, temperature — rather than of a valve position that approximates it.
Less wear on the throttling valve, which was absorbing the surplus and being eroded by it.
Motor current, torque and running hours available on the network, which is condition monitoring you have already paid for.
Where it fits
Machines that spend their lives throttled
Manufacturing
Process pumps, cooling fans and extraction systems that were specified once for a worst case and have run flat out ever since.
Power and Utilities
Cooling water, condenser and draught fan duties where the load genuinely varies and the machines are large enough that a few per cent matters.
Questions
Common questions about drive retrofits
No, and this is the most important thing to get right. The saving comes from reducing speed on a centrifugal machine with a variable demand. A positive displacement pump, or a centrifugal one that genuinely runs at full duty around the clock, will save little or nothing and may cost you in drive losses. The duty survey exists to tell the two apart before you spend anything.
Usually, after a check. Insulation condition, bearing type and cable length together decide it — inverter supplies stress insulation and can produce shaft currents that damage bearings, particularly on older motors and long runs. Where the motor is marginal the honest answer is to replace it or fit an output filter, and to say so before the drive goes in rather than after a bearing fails.
They can, and it is worth taking seriously. Harmonic current distorts the supply and enough drives on one transformer will affect other equipment; the switching also demands proper screened cable and earthing to stay out of nearby instrument signals. All of this is manageable and well understood — it just has to be designed rather than discovered.
Get in touch
Point us at a pump that runs against a throttled valve, and we will measure it before we sell you anything.
A duty survey with logged power, flow and pressure, and an honest answer about whether a drive is worth fitting.