Ask a plant engineer what their largest single electricity consumer is and you’ll usually hear "the compressors" or “the furnace”. Both are good answers. But in a lot of Victorian manufacturing plants, if you actually metered it, the hydraulic power units would be near the top of the list.

Here is why. A fixed-displacement pump has one setting: Full flow. It doesn’t know or care whether the machine it feeds is cutting, indexing, or sitting there while an operator changes a tool. When the machine is working, that flow goes where it should. When it is not, the same flow goes over the relief valve, back to tank, converted entirely into heat. The motor draws close to the same current either way.

On a typical press or shear line, the percentage of actual working time is somewhere between a quarter and a third. The rest is positioning, loading, waiting, changeover. For all of that time, you are paying full freight for flow you are throwing away - and then paying again for the cooler that removes the heat it made.

Putting a number on it

Take a 55 kW power unit on a two-shift operation. Round numbers, but the shape of them is right for a lot of plants:

The saving is about 81,000 kWh a year, or roughly 55%. At a delivered rate of 20 c/kWh that is $16,200 per year from one power unit.

Two things to notice about that table. First, there is no saving at all while the machine is working - a VSD does not make hydraulics more efficient under load, and anyone who tells you otherwise is selling something. The entire saving comes from the hours the machine is not doing anything. Second, the existing idle figure of 33 kW assumes you already have some form of unloading in place. If your unit dumps over relief at full pressure with no unloader, the idle draw is much closer to the loaded figure and the saving is larger again.

Then there are two more savings that most calculations miss.

Maximum demand.

Large sites pay for peak demand as well as consumption. A power unit that runs continuously contributes its full draw to the site peak whenever that peak occurs. One that only spins when a cylinder calls for flow often does not. If the retrofit takes 25 kW off your maximum demand and you are on a demand tariff in the order of $13/kVA/month, that is another $3,900 a year - but only if the unit is genuinely running at the moment your site peaks, which is worth checking rather than assuming.

Heat.

Every kilowatt not wasted is a kilowatt the cooler does not have to reject. Fans and circulation pumps run less, oil runs cooler, and seal and hose life goes up with it. Call it $1,000 and some deferred maintenance you will never see on a spreadsheet.

That takes the total to around $21,000 a year from one 55 kW unit.

What it costs

A properly executed VSD retrofit on a unit that size - drive, motor suitability check, pressure transducer, control integration, installation and commissioning - typically lands between $28,000 and $45,000. Take the middle of that at $35,000.

Payback: about twenty months. On a unit that will run for another eighteen years.

Manufacturers put the headline higher. HYDAC quotes energy savings of up to 70% for its KineSys variable-speed drive units, depending on machine cycle. That figure is achievable on the right duty - a machine with long idle periods and a modest average load - but it is a ceiling, not a planning assumption. Use your own duty cycle.

Where it does not pay

Be honest about this, because the projects that fail give the whole idea a bad name.

If your machine runs at near-constant flow demand - some continuous casting, coil processing and extrusion duties do - the idle fraction is small and so is the saving. If the unit is only running one shift, halve everything above and the payback stretches past four years. And if the motor is already at end of life, you are not comparing a VSD against zero, you are comparing it against a like-for-like replacement, which improves the numbers considerably.

There is also money on the table through the Victorian Energy Upgrades program. Motor and industrial efficiency projects can be claimed through project-based activities using measured and verified savings, which suits a hydraulic retrofit better than the standard appliance activities do. One caveat that catches large plants: Victoria's largest energy users - around a hundred businesses - are exempt from the VEU program by default and must opt in to access incentives. Worth confirming which side of that line your site sits on before you build a business case around it.

Start with a meter, not a quote

The single most useful thing you can do is put a logger on the HPU circuit for a normal fortnight and compare the draw against the hours the machine was actually cutting. Most plants have never measured this. The gap between the two lines is your project, and it is usually bigger than anyone expected.

Final Thought

Flotek is Victoria's leading hydraulic services company. If you want to know what your power units are actually costing to run, we can log them and give you the numbers before anyone quotes you anything. In some cases a new VSD unit doesn’t stack up from a financial perspective. Why not find out – because the savings can be enormous.