A transformer operating above its rated winding temperature degrades its insulation at an accelerating rate. As a widely cited industry rule of thumb, each 6°C to 8°C rise in winding temperature above the rated limit roughly halves the insulation’s remaining service life. In Australian conditions, summer heat and peak electrical load arrive simultaneously, creating the greatest thermal stress on distribution and power transformers exactly when they are needed most. Forced-air cooling using transformer cooling fans extends a transformer’s safe operating capacity and protects insulation life.
This guide covers when forced-air cooling is required, which industries rely on it, and how to select the right Fanquip transformer cooling fan unit.
How transformer cooling works: natural versus forced air
Most oil-filled distribution transformers cool through natural convection, known in the industry as ONAN (Oil Natural, Air Natural). Heat from the core and windings transfers to the transformer oil, which circulates by convection to radiator fins on the outside of the tank. The fins dissipate heat to the surrounding air by natural convection and radiation. This requires no moving parts and no external power, but it has a fixed heat dissipation capacity that diminishes as ambient temperature rises.
When natural convection is insufficient, whether under high load, in high-ambient-temperature environments, or following a load increase, fans are added to the radiator fins. This configuration is known as ONAF (Oil Natural, Air Forced). The fans force air across the fins, increasing the heat transfer coefficient and allowing significantly more heat to be dissipated at the same ambient temperature. ONAF cooling can typically increase a transformer’s rated capacity by 15% to 30% compared with ONAN operation, though the actual uplift depends on the specific transformer design and should be confirmed against the manufacturer’s documentation rather than assumed.
A related configuration, OFAF (Oil Forced, Air Forced), adds pumps to force oil circulation in addition to fan-forced air. OFAF is typically reserved for large power transformers with very high thermal loads and sits outside the scope of the fan-based cooling covered in this guide. Dry-type transformers, which use air rather than oil as the cooling and insulating medium, rely on natural air cooling in smaller units and are a different category from the oil-filled, radiator-cooled transformers this article addresses.
Cooling fans may be factory-fitted on larger transformers or added retrospectively when load growth or ambient conditions exceed the transformer’s ONAN cooling capacity. The decision to add fans is typically triggered by load monitoring showing the transformer running close to its thermal limit, by temperature alarm activations, or by a planned load increase that would otherwise exceed the ONAN rating.
When transformer cooling fans are required
Load growth beyond ONAN capacity. Distribution transformers in industrial facilities, substations serving growing manufacturing operations, and infrastructure substations supplying new commercial developments may experience load growth that exceeds the transformer’s original ONAN cooling capacity. Adding forced-air cooling fans to the radiator fins is often more cost-effective than transformer replacement and can restore or exceed the original rated capacity.
High ambient temperature environments. Transformers installed in Australia’s northern states, in unshaded outdoor substations, or in enclosed plant rooms with poor natural ventilation may experience elevated ambient temperatures that reduce the temperature differential between the transformer and the surrounding air. This reduces natural convection efficiency. Forced-air cooling maintains cooling performance largely independent of ambient temperature by actively driving airflow across the radiator fins.
Ageing transformers with reduced thermal margins. Transformer insulation degrades over time, and an older transformer with degraded insulation may have reduced thermal tolerance compared with its nameplate rating. Adding cooling fans can reduce winding temperatures and slow further insulation degradation, extending service life beyond what natural cooling alone would allow.
See transformer cooling methods for a broader overview of cooling approaches across transformer types, and what does a transformer cooling fan do for the fundamentals.
Increased operational reliability requirements. In critical facilities where transformer failure causes significant operational or safety consequences, adding cooling fans provides a margin of thermal safety that ONAN operation alone does not. Running transformer cooling fans in parallel with natural convection reduces peak winding temperatures during high-load periods, even when the transformer is technically within its ONAN rating.
For guidance on identifying overheating risk before it becomes a failure, see transformer overheating prevention in industrial facilities.
The Fanquip transformer cooling fan
The Fanquip transformer cooling fan is designed and manufactured in Australia specifically for Australian conditions, including high ambient temperatures, UV exposure, salt air in coastal locations, and the electrical standards applicable to the Australian grid. The IP56 motor rating, higher than the IP55 standard fitted to general-purpose fans, suits outdoor substation environments where the fan is exposed to weather, dust, and washdown during maintenance. The continuously rated design means the fan is built for sustained 24/7 operation without thermal derating, which matters for a cooling application where the fan needs to remain fully operational precisely when thermal stress is highest.
Three models are available, covering free air volumes of 2,300 L/s, 3,000 L/s, and 3,700 L/s, with motor power of 0.37kW or 0.75kW across a fixed 600mm diameter. The number of fans required and the model selected depend on the transformer’s radiator fin area, the required heat dissipation, and the available mounting positions on the transformer tank. Fanquip’s team can assist with fan count and model selection based on the transformer’s nameplate data and cooling configuration.
Browse the transformer cooling fan product page, or the full transformer cooling fan category.
Industries that use transformer cooling fans
Smelters and refineries. Large industrial facilities with on-site power distribution substations running high continuous loads are primary transformer cooling fan users. Smelting operations draw significant sustained current that keeps transformers at or near thermal limits during production runs.
See smelter and refinery electrical infrastructure requirements for related infrastructure considerations.
Mining and metals. Mine site substations powering processing plants, crushers, and conveyors often operate in remote locations with high ambient temperatures and limited maintenance access. Transformer cooling fans rated for dust and heat, with IP56 protection and continuous duty, suit these environments.
See thermal management in mining operations for broader context.
Oil and gas. Upstream and downstream oil and gas facilities include on-site transformers powering compressors, pumps, and processing equipment. Continuous operation, high ambient temperatures, and critical uptime requirements make forced-air cooling standard practice in this sector.
Manufacturing. Large manufacturing facilities with high-demand electrical loads, including motor drives, process heating, and compressed air systems, may push distribution transformers to their ONAN limits during peak production. Adding cooling fans is often lower cost than transformer uprating.
Power utilities and substations. Grid-connected substations managing distribution at 11kV, 22kV, or 33kV include transformers that may require forced-air cooling to handle peak summer demand. Utility asset managers specifying replacement cooling fans for existing transformer installations represent a recurring procurement audience for this product.
How to select the right transformer cooling fan
Matching fan output to radiator requirements. The correct fan selection depends on the transformer’s radiator fin area and the required airflow to maintain safe winding temperatures under the maximum expected load and ambient conditions. This data is available from the transformer manufacturer’s nameplate or design documentation. Fanquip offers three models covering 2,300 L/s, 3,000 L/s, and 3,700 L/s, with the correct model selected based on the airflow specification from the transformer documentation.
Number of fans. Most oil-filled distribution and power transformers have multiple radiator panels, and each panel typically requires one fan. The transformer’s cooling schedule, usually shown on the nameplate, specifies the number of ONAF fans required to achieve the rated forced-air cooling capacity.
Environmental specification. The IP56 motor and hot dip galvanised casing on Fanquip’s transformer cooling fan suit standard outdoor substation environments. For coastal installations exposed to salt air, marine-grade finishes or stainless steel casing options should be confirmed with Fanquip at the time of enquiry. The optional finger guards should be specified for any installation where the fan is accessible during transformer operation, stopping foreign objects from entering the fan.
Contact Fanquip’s team with the transformer’s nameplate data to confirm the correct fan model, quantity, and any site-specific finish requirements.
Maintenance and replacement
Transformer cooling fans run continuously for extended periods, often years, between maintenance inspections. The continuously rated design of Fanquip’s unit means the motor is built for this duty, but routine inspection of blade condition, motor bearing integrity, and mounting hardware should still be included in the transformer’s scheduled maintenance programme. Automatic fan activation on temperature threshold is standard in most ONAF configurations, so confirming the control circuit and temperature sensor calibration should also form part of routine checks.
When a cooling fan requires replacement, whether due to motor failure, blade damage, or end of service life, the impeller is available as a replacement part, allowing blade replacement without a full unit change. Contact Fanquip for replacement impeller availability for your specific transformer cooling fan model.
Frequently Asked Questions
What is a transformer cooling fan used for?
Transformer cooling fans force air across a transformer’s radiator fins to increase heat dissipation beyond what natural convection alone can achieve. They are used when a transformer’s load exceeds its natural cooling capacity, when ambient temperatures reduce the efficiency of natural convection, or when maximum operational reliability is required at high load.
What is the difference between ONAN and ONAF transformer cooling?
ONAN (Oil Natural, Air Natural) cooling relies on natural convection to dissipate heat from the transformer’s radiator fins. ONAF (Oil Natural, Air Forced) adds fans to the radiator fins to increase airflow and heat dissipation. ONAF cooling can typically increase a transformer’s rated capacity by 15% to 30% compared with ONAN operation, depending on the specific transformer design.
How many transformer cooling fans does a transformer need?
The number of fans required depends on the transformer’s radiator panel configuration and the cooling schedule shown on the transformer’s nameplate. Most transformers with multiple radiator panels require one fan per panel to achieve the rated ONAF cooling capacity.
Are Fanquip’s transformer cooling fans suitable for outdoor substation installation?
Yes. Fanquip’s transformer cooling fan features an IP56 motor suitable for outdoor and exposed environments, along with a hot dip galvanised casing. The fan is manufactured in Australia to suit Australian conditions, including high ambient temperatures, UV exposure, and coastal environments.
Can a transformer cooling fan be retrofitted to an existing transformer?
Yes, in most cases. Retrofitting requires mounting brackets suited to the transformer’s radiator panel configuration and electrical connection to the transformer’s control circuit. Fanquip can confirm the correct fan specification for a specific transformer based on its nameplate data.
Next steps
If your transformer is showing temperature alarms, running close to its rated capacity, or is due for a load increase, forced-air cooling is worth assessing before considering a full transformer replacement. Browse the transformer cooling fan range, or contact Fanquip’s team with your transformer’s nameplate data to confirm the correct fan model, quantity, and finish for your site conditions.