Commercial kitchen exhaust fans are the workhorses nobody thinks about. They run for 12 to 18 hours a day at a single flat speed, regardless of whether the kitchen is flat-out during dinner service or ticking over with two breakfast orders. Every hour they run at full speed when they do not need to is money going straight to the switchboard meter.
A variable speed drive (VSD, sometimes called a VFD) fixes that. It is one of the cheapest, highest-return upgrades you can make to a commercial kitchen, and it is almost always a straight retrofit, no new ductwork, no new canopy.
Here is how it works, what it costs, and why the next step after installing one is usually smart monitoring.
What a VSD Actually Does
A variable speed drive sits between your mains supply and the exhaust fan motor. It converts the incoming 50 Hz mains into a variable-frequency output, which lets the motor run at any speed from about 20% to 100% of its nameplate rating.
Instead of being stuck with a single on/off fan, you get a dial. When the kitchen is quiet, the fan runs at 50% and draws a fraction of the power. When the char-grill gets fired up, it ramps to 100% in a couple of seconds.
The reason this matters so much for fans is a bit of physics called the affinity laws. Fan power does not scale linearly with speed, it scales with the cube of it. In practice:
| Fan speed | Airflow | Power draw |
|---|---|---|
| 100% | 100% | 100% |
| 80% | 80% | ~51% |
| 60% | 60% | ~22% |
| 50% | 50% | ~13% |
Drop the fan to 80% during off-peak and you are already using half the electricity. Drop it to 50% during prep and clean-down and you are using an eighth.
What a Realistic Saving Looks Like
Take a mid-sized restaurant with a 3 kW exhaust fan running 14 hours a day, 6 days a week, at a commercial tariff of around $0.35/kWh.
Before VSD: fan runs at 100% the whole time:
- 3 kW × 14 h × 6 d × 52 w = ~13,100 kWh per year
- That is roughly $4,600 per year in exhaust fan electricity alone.
After VSD: fan averages around 65% speed across the day (full speed during service, ramped down outside peak):
- Average power draw drops to ~28% of rated = ~0.84 kW
- 0.84 kW × 14 h × 6 d × 52 w = ~3,670 kWh per year
- Annual cost: ~$1,280
Net saving: ~$3,300 per year, on a unit that typically installs for $2,500-$4,500 supplied and fitted. Simple payback sits around 12-18 months.
The numbers vary with fan size, tariff, and operating hours, but the shape is the same across most commercial kitchens we inspect: VSDs are one of the few energy upgrades that pay for themselves inside two years without any change to kitchen operation.
Secondary Benefits Beyond the Power Bill
The electricity saving is the headline, but it is not the only thing a VSD brings:
- Less wear on the motor and belts. Soft-starting at reduced frequency eliminates the inrush-current slam that kills motors and shreds drive belts over time.
- Quieter kitchen. Anyone who has worked under a flat-out exhaust fan knows the difference when it drops to 60%. Staff conversation becomes possible; noise complaints from neighbours drop.
- Smoother makeup air. Pressure balance in the kitchen becomes more stable, reducing the door-slamming and odour back-draw problems that plague oversized systems.
- It helps you meet AS 1668 airflow targets. A VSD lets you tune the actual delivered airflow to the design spec, something a fixed-speed fan cannot do once it has been installed.
What a Retrofit Actually Looks Like
A typical retrofit on an existing exhaust fan takes one day on-site. The job is:
- Isolate the supply, remove the existing motor starter.
- Install the VSD in the switchboard or in a dedicated enclosure close to the motor.
- Wire the motor through the drive, including any required EMC filter and output reactor for long cable runs.
- Configure motor parameters, set speed ranges and ramp times.
- Add a simple control interface, either a wall-mounted keypad, a switch-selectable high/low, or (better) an automated schedule.
- Commission with actual airflow measurement to confirm the kitchen still meets its design extract rate at its chosen speeds.
The last step is the one most installers skip and is exactly where compliance risk creeps in. A VSD is only a win if the lower speeds still achieve capture at the canopy during cooking.
The Missing Piece: Smart Monitoring
Once a VSD is installed, the obvious question is "how do we know it is actually saving what it is supposed to?" Most kitchens run on one of three control strategies:
- Manual high/low switch: reliable, but depends on staff remembering to use it.
- Time-of-day schedule: good for predictable hours, clumsy when trading hours change.
- Demand-based control: the drive ramps automatically based on duct temperature, pressure, or cooking activity. Best results, most configuration.
Whichever strategy you pick, you cannot manage what you cannot measure. This is why we are rolling out smart monitoring as an add-on to VSD retrofits in 2026. A small module wired into the drive reports runtime, actual speed, motor current, and duct conditions to a cloud dashboard, so operators see exactly how much they are saving each week, and we see the fault signatures before a motor fails.
If you are running a kitchen on a fixed-speed exhaust fan and it is more than a few years old, a VSD is almost certainly the best return on capital you can make on the mechanical side of the business.
Want a free VSD assessment? We will visit, measure your current draw, and tell you what a retrofit would save on your actual tariff and operating hours. Call 0423 063 061 or request a quote through the contact page. $0 call-out across Sydney Metro and Wollongong.
Energy savings in this article are indicative and based on typical Sydney commercial tariffs and operating patterns. Actual results depend on your fan, hours, tariff, and control strategy, which is why we measure before we quote.