Fans and pumps account for a large share of industrial electricity use, and most of them run flat out against a damper or valve that throws part of their output away. Fitting an inverter, also called a variable speed drive or VSD, lets the motor run only as fast as the process needs. Because fan power falls with the cube of speed, the savings are far larger than intuition suggests. This guide explains the physics, the practical steps, and how to work out whether a retrofit will pay.

Why speed control saves so much

Centrifugal fans and pumps obey a set of relationships called the affinity laws. For a given fan and system:

• Air flow is proportional to speed.
• Pressure is proportional to speed squared.
• Power is proportional to speed cubed.

The third law is the one that matters. Slow a fan to 80 percent speed and it delivers 80 percent of the flow, but draws only about 51 percent of the power. At 50 percent speed it draws roughly 12.5 percent. A fan that spends most of its life throttled to 70 percent flow is wasting around two thirds of its energy if it is achieving that flow with a damper rather than a speed reduction.

How flow is usually controlled, and why it wastes energy

Traditional methods of reducing fan output all keep the motor at full speed:

• Outlet dampers restrict the discharge. The fan works against a higher resistance, and power falls only slightly. This is the least efficient method and still the most common.
• Inlet guide vanes pre-swirl the air entering the impeller and are somewhat better, but still leave the motor at full speed.
• Bypass or recirculation moves the full flow and simply dumps the surplus. There is no saving at all.
• On-off cycling saves energy but stresses the motor, belts and bearings, and gives poor process control.

An inverter does what none of these do: it reduces the energy going into the motor rather than wasting it after the fan has done the work.

What an inverter does

An inverter takes the fixed 50 Hz mains supply, rectifies it to DC, then synthesises a new AC output whose frequency and voltage can be varied. A standard induction motor’s speed follows the supply frequency, so reducing the output from 50 Hz to 40 Hz slows the motor by 20 percent. Modern drives also add soft starting, which removes the inrush current and mechanical shock of direct-on-line starting, built-in motor protection, and communications so the drive can be controlled from a building or plant management system.

Where the savings are largest

The best candidates share three features: a centrifugal fan or pump, a duty that varies or is permanently below full output, and long running hours. Typical examples:

• Air handling and extract fans in factories, warehouses and cleanrooms, where demand varies with occupancy, temperature or process load.
• Dust and fume extraction sized for the worst case and throttled the rest of the time.
• Cooling tower and condenser fans that only need full speed on the hottest days.
• Boiler forced and induced draught fans, which follow boiler load.
• Drying and process air fans set up once by damper and never adjusted.
• Circulating and transfer pumps on systems with variable demand.

Positive displacement equipment, such as screw compressors and gear pumps, does not follow the cube law, and savings there are proportional rather than cubic. They can still be worthwhile but the case is different.

A worked example

Consider a 30 kW extract fan running 6,000 hours a year, throttled by damper to 75 percent of its rated flow. Assume the throttled fan still draws around 85 percent of full power, or 25.5 kW.

• Current consumption: 25.5 kW x 6,000 h = 153,000 kWh per year.
• With an inverter at 75 percent speed, power falls to roughly 0.75 cubed, or 42 percent of rated, plus about 3 percent for drive losses: roughly 13.5 kW.
• New consumption: 13.5 kW x 6,000 h = 81,000 kWh per year.
• Saving: 72,000 kWh per year. At 25p per kWh that is about 18,000 pounds a year.

An installed 30 kW drive typically costs a fraction of that, so payback is measured in months. The figures are illustrative; your own fan curve, running hours and tariff will change them, but the shape of the result is the same for most throttled fans.

Practical steps for a retrofit

1. Survey the candidates. List every fan and pump over about 5 kW, its running hours, and how its output is currently controlled. Clamp a power meter on the largest for a week to see the real load profile.
2. Check the motor. Most induction motors built in the last twenty years are suitable for inverter duty. Older motors may need checking for insulation condition, and motors on long cable runs may need an output filter to protect the windings from voltage spikes.
3. Decide how the drive will be controlled. The saving only appears if speed actually falls. Link the drive to a pressure, temperature, air quality or flow sensor, or to the plant control system, so speed follows demand automatically. A drive left at a fixed manual setting still saves energy compared to a damper, but far less than one that follows the process.
4. Open the dampers. Once the drive is controlling flow, any damper should be fully open or removed. A drive fighting a half-closed damper wastes the investment.
5. Set minimum speeds. Motors need airflow over their cooling fins. Below about 20 to 30 percent speed a standard motor can overheat under continuous load, and some fans have resonant speeds that must be skipped. The drive can be programmed to avoid both.
6. Consider harmonics and EMC. Drives draw non-sinusoidal current. Larger installations may need line reactors or harmonic filters, and all need screened motor cables and proper earthing to meet EMC requirements.
7. Commission and measure. Record consumption before and after. This proves the saving and provides the evidence for the next project.

Beyond energy

Energy is the headline, but drives bring other benefits. Soft starting extends the life of belts, bearings and couplings. Precise speed control improves process stability. Reduced fan speed cuts noise sharply, since fan noise falls with speed even faster than power does. And the drive’s diagnostics give maintenance teams early warning of a failing motor or bearing. For related coverage of motor and drive technology, see MEPCA’s report on Festo’s CMMT-ST servo drive and the maintenance section.