Industrial fans run at fixed speed by default, which means they consume the same power whether the facility is operating at full capacity, partial load, or requiring minimal ventilation. A variable speed drive changes this by allowing the fan to run at the exact speed the application requires at any given time. Because fan power consumption follows the cube law, dropping proportionally to the cube of the speed reduction, even modest speed reductions produce significant energy savings.
This article explains how VSDs work with industrial fans, which fan types are compatible, and how to estimate the savings potential for a specific installation.
What a variable speed drive does
A VSD, also called a variable frequency drive or VFD, controls the speed of an AC motor by varying the frequency of the electrical supply to the motor. Standard grid power is delivered at a fixed frequency, 50 Hz in Australia, which drives the motor at its nameplate speed. A VSD adjusts this frequency, reducing it to slow the fan and increasing it up to the motor’s rated speed to run the fan faster, allowing precise speed control without mechanical intervention.
On an industrial fan, the VSD sits between the electrical supply and the fan motor. The operator or building management system sets the required speed, the VSD adjusts the motor frequency accordingly, and the fan runs at the target speed continuously until conditions change.
Browse Fanquip’s variable speed drives to see the available specifications.
Why small speed reductions produce large energy savings: the cube law
Fan power consumption does not decrease in proportion to speed. It decreases in proportion to the cube of the speed reduction. This relationship is described by the fan affinity laws. Reduce fan speed by 20%, to 80% of full speed, and power draw drops to approximately 51% of its original level, a saving of nearly 49%. Reduce speed by 50% and power draw drops to approximately 12.5% of its original level, a saving of 87.5%.
In practical terms, a 5.5kW axial fan running at 80% of full speed for eight hours per day consumes less than 2.9kW during that period. Running the same fan at 50% speed for the same period consumes approximately 0.69kW. Over a full year of operation, the compounding effect of this reduction is substantial, particularly for facilities running multiple large fans continuously.
For a related look at how fan energy consumption scales with speed across Fanquip’s range, see how industrial fan energy consumption relates to speed.
The cube law applies most directly to pure fan power consumption. Real-world savings depend on the system resistance characteristic, meaning how the ductwork or building resistance affects the fan’s operating point, the duty cycle, meaning how often the fan runs at reduced speed, and motor efficiency at partial load. These factors mean actual savings may differ from the theoretical maximum, but the directional effect is consistent: running fans slower saves significantly more energy than the speed reduction alone suggests.
Which Fanquip fans are compatible with variable speed drives
Axial flow fans. Bothbelt drive and direct drive axial flow fans are compatible with VSDs. On belt drive fans, a VSD complements the mechanical speed adjustment already available via pulley ratio change, allowing finer control than pulley ratios alone permit. On direct drive fans, the VSD is the primary speed control mechanism.
Browse the axial flow fan range, including belt drive and direct drive models.
Centrifugal fans. Centrifugal fans, including forward curved, backward inclined, and materials handling variants, are compatible with VSDs. Centrifugal fans in variable volume ventilation systems, such as process ventilation where extraction demand changes with production levels, are strong VSD candidates because they often run at partial capacity for significant periods.
Roof fans and wall fans. Roof fans and wall fans used in facilities with variable ventilation demand, such as food processing plants where cooking processes are intermittent or warehouses where occupancy varies, can be fitted with VSDs to match extraction rate to actual demand rather than running at full speed continuously.
For applications across industries where axial fans are already in use, see axial flow fan applications.
Applications where VSDs deliver the highest return
Facilities with variable ventilation demand. Warehouses, food processing plants, and manufacturing facilities where ventilation demand varies across the day or across shifts are the highest-value VSD candidates. A fan running at 70% speed during low-demand periods and 100% during peak periods uses significantly less total energy than a fan running at full speed continuously, and the savings compound over a full year of operation.
Large fans with high motor ratings. The absolute energy savings from a VSD scale with motor size. A 5.5kW fan running at 80% speed saves more kilowatt-hours per year than a 0.75kW fan at the same speed reduction. Facilities with multiple large axial or centrifugal fans running continuously, common in mining and metals operations with large ventilation systems, are the most commercially compelling VSD candidates.
Make-up air and supply systems. In facilities where supply fans introduce fresh air to match extraction rates, VSDs allow supply and exhaust fans to be balanced dynamically rather than at a fixed ratio. This prevents over-pressurisation or under-pressurisation of the facility as production conditions change.
Compliance with building energy efficiency requirements. The National Construction Code 2022 Section J includes energy efficiency requirements for mechanical ventilation systems in new commercial and industrial buildings. VSDs are one of the engineering controls that can be used to meet demand-controlled ventilation requirements, though NCC compliance should always be verified against the specific project requirements rather than assumed.
For a related energy optimisation approach, see energy cost savings from industrial fan system optimisation.
Agricultural facilities also see strong returns, with grain stores and livestock buildings experiencing significant seasonal and diurnal temperature variation that makes fixed-speed ventilation inefficient for large parts of the year.
Fanquip VSD specifications
Fanquip’s variable speed drives are available in single phase or three phase input configurations, covering the range of fan motor supply voltages used in Australian industrial applications. The IP20 enclosure suits indoor panel-board installations, while the IP66 enclosure is rated for outdoor or washdown environments where the VSD must be mounted adjacent to the fan in an exposed location.
Both enclosures are EMC compliant, which is relevant where the VSD is installed near sensitive electronic equipment or in facilities subject to electromagnetic compatibility requirements. The user-friendly interface allows speed setpoints to be adjusted without specialist programming tools.
Pricing is by enquiry. VSD sizing depends on the motor kW rating of the fan being controlled, the supply voltage, and any specific environmental requirements. Fanquip’s team can confirm the correct VSD specification for an existing or new fan installation.
Calculating your potential energy savings
The following is a worked example to illustrate the calculation method. Actual results depend on the fan’s real operating profile, the system resistance curve, the applicable electricity tariff, and motor efficiency at partial load, so treat the figures below as illustrative rather than a guaranteed outcome.
Take a 3kW direct drive axial flow fan running 16 hours per day, 5 days per week, approximately 4,160 hours per year, at full speed.
At $0.30 per kWh, a representative Australian industrial electricity rate that varies by state and retailer, annual energy cost at full speed is 3 kW x 4,160 hours x $0.30, which equals $3,744 per year.
If the fan instead runs at 80% speed for half of its operating hours, typically off-peak periods, the calculation changes. Power at 80% speed is 3 kW x 0.8³, which is 3 x 0.512, equalling 1.54 kW. Energy cost for those 2,080 hours is 1.54 kW x 2,080 hours x $0.30, equalling $960. Energy cost for the remaining full-speed hours is 3 kW x 2,080 hours x $0.30, equalling $1,872. Total annual cost with the VSD is $960 plus $1,872, equalling $2,832. The saving is $912 per year, approximately 24%.
This example illustrates the calculation method rather than a promised result. Fanquip’s team can assist with a site-specific savings assessment based on your fan’s actual operating profile and local electricity tariff.
VSD installation and compatibility considerations
Motor compatibility. VSDs require IE2 or IE3 rated motors for reliable variable speed operation. Confirm motor compatibility before retrofitting a VSD to an existing fan. New Fanquip fans are supplied with IP55 motors as standard, so confirm the motor’s VSD compatibility when ordering.
Harmonics and EMC. VSDs can introduce electrical harmonics into the supply. In facilities with sensitive electronic equipment or where power quality requirements apply, select an EMC-compliant VSD, confirmed across Fanquip’s range, and consider whether additional line filters are required.
Control integration. VSDs can be integrated with building management systems or control panels via standard analogue or digital inputs. A facility running automated demand-controlled ventilation can have the VSD respond to temperature, COâ‚‚, or occupancy sensors rather than requiring manual setpoint adjustment. This also reduces mechanical stress during motor start-up and shutdown compared with fixed-speed direct-on-line starting, extending equipment life and reducing wear on bearings and blades.
For general fan servicing intervals that apply alongside VSD-equipped installations, see maintenance tips for industrial fans.
Frequently Asked Questions
How much energy can a variable speed drive save on an industrial fan?
Because fan power consumption follows the cube law, a 20% speed reduction saves approximately 49% of the fan’s power draw, and a 50% speed reduction saves approximately 87.5%. Actual savings depend on how often and how much speed is reduced in operation, the fan’s system resistance curve, and motor efficiency at partial load.
Can a variable speed drive be retrofitted to an existing industrial fan?
Yes, in most cases. The motor must be compatible with variable frequency operation, meaning IE2 or IE3 rated. Fanquip can confirm compatibility for fans supplied by Fanquip and advise on the correct VSD specification for the motor rating and supply voltage.
What is the difference between IP20 and IP66 variable speed drives?
IP20 enclosures are rated for indoor panel-board installation in clean, dry environments. IP66 enclosures are rated for outdoor or washdown applications where the VSD is mounted in an exposed location. Choose IP66 for VSDs installed adjacent to fans in food processing, agricultural, or outdoor environments.
Can a variable speed drive be used with both belt drive and direct drive fans?
Yes. VSDs are compatible with both belt drive and direct drive axial flow fans. On belt drive fans, the VSD complements the mechanical speed adjustment available via pulley ratio change. On direct drive fans, the VSD is the primary speed control mechanism.
Do I need a VSD for every fan in my facility?
Not necessarily. VSDs deliver the greatest return on fans that run for long hours at variable load. Fixed-speed operation is appropriate where the fan always runs at full capacity and no variable demand exists. Fanquip’s team can assess which fans in a facility are strong VSD candidates based on their operating profiles.
Next steps
If your facility runs axial or centrifugal fans continuously, or if ventilation demand varies across the day or across seasons, a VSD is likely to deliver a measurable reduction in running costs. Browse Fanquip’s variable speed drives and accessories range, or contact Fanquip’s team with your fan’s motor rating and operating hours to get a site-specific savings estimate.