Car Park Jet Fan Motor: Thrust, Reversible Operation and Motor Selection

A car park jet fan motor is chosen according to the thrust (in newtons) the fan must produce and the mode it will run in: a 400 mm jet fan blowing air at 22 m/s delivers about 73 N of thrust, and a 2.2 kW, 3000 rpm three-phase motor (for example DRG-90L-2, 4.35 A at 400 V) is enough for that job. The real distinction is not power but duty: if the fan only clears exhaust gases (CO) as part of normal ventilation, a standard motor can be fitted; if it also has to sweep smoke during a fire, the motor must be part of a fan set certified to EN 12101-3.

In the second case, fitting "the same kW" is not a solution; the motor of a certified set is supplied by the fan manufacturer. DRG Motor is a manufacturer, supplier and seller of electric motors; our standard motors are not sold with smoke extraction certification, and the models suggested in this article are for normal ventilation mode only. You can find suitable models for that scope among the 120 models on the Buy Electric Motors Online page.

What is a jet fan and how does it replace ductwork in a car park?

A jet fan is an axial fan whose casing is like a tube open at both ends, blowing a short, fast jet of air. "Jet fan" is not the name of an operating principle or a casing type; it describes the purpose: the impeller inside is axial and the fans are installed as free units hung from the ceiling. For low-ceiling car parks there are also flat, box-shaped jet fans with a centrifugal impeller; the purpose is the same, the principle differs.

In conventional ventilation, air is collected through ceiling ducts and grilles. A jet fan system has no ducts: fans hung from the ceiling at set intervals give the air speed and direction; each jet drags the surrounding air along, and a river of air forms inside the car park, flowing towards the next fan and finally to the exhaust shaft. Fresh air enters from the ramp or a supply shaft, polluted air leaves through the main exhaust fan in the shaft. The job of jet fans is not to throw air outside but to push and steer the air inside the car park.

Why is thrust (N) more important than airflow?

A ducted fan is defined by pressure and airflow. A jet fan faces no duct resistance, so it produces no static pressure; its value is the momentum it gives to the air. That is why the first column in catalogues is thrust: T = ρ · Q · v (ρ air density kg/m³, Q airflow m³/s, v outlet velocity m/s). A fan blowing the same airflow through a narrower outlet at higher speed gives more thrust, but its power rises with the cube of velocity. In one manufacturer's catalogue, a 315–450 mm axial jet fan range delivers 25–98 N at high speed with installed motor power of 0.55–3 kW [1]. That range belongs to that product family; for another jet fan with a different diameter and velocity, power is calculated separately.

Worked example: 400 mm, 22 m/s

Assumptions: outlet diameter 0.40 m, outlet velocity 22 m/s, air density 1.2 kg/m³, fan efficiency including silencers 0.45, motor margin 15%. As a jet fan produces no static pressure, the jet's dynamic pressure (½ρv²) is used for Δp in the formula.

• Outlet area: π × 0.40² / 4 = 0.126 m²
• Airflow: 0.126 × 22 = 2.76 m³/s (about 9,950 m³/h)
• Thrust: 1.2 × 2.76 × 22 ≈ 73 N
• Dynamic pressure: 0.5 × 1.2 × 22² ≈ 290 Pa
• Shaft power: P_shaft = Q·Δp / η_fan = 2.76 × 290 / 0.45 ≈ 1.78 kW
• Requirement with margin: 1.78 × 1.15 ≈ 2.05 kW → standard 2.2 kW motor

As the impeller is mounted directly on the motor shaft, there is no belt loss. The result matches the 2.2 kW installed power of the catalogue model of the same diameter [1]. To check with your own values you can use the Pump & Fan Motor Power calculator; it does not replace the fan curve and is only a cross-check.

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Normal ventilation mode vs smoke mode: what changes for the motor?

The same fan may be called on for two different duties, and for the motor they are completely different jobs.

In normal mode, CO and vehicle exhaust are monitored by sensors and the motor works under familiar conditions. In smoke mode the fans sweep smoke in a set direction to the exhaust shaft; the motor stays inside the hot gas.

For fans working in smoke mode, EN 12101-3 defines classes that state temperature and duration together; for example 200 °C for 120 minutes for F200, 300 °C for 60 minutes for F300 and 400 °C for 120 minutes for F400 [3]. The certificate is issued for the fan and motor tested together as a set. Therefore, in a jet fan with a smoke duty, the motor is replaced with the fan manufacturer's certified spare motor or with its approval; fitting a standard motor invalidates the set's certification and puts fire safety at risk. DRG's standard motors are not sold with this certification. A DRG motor can only be recommended for jet fans that have no smoke duty and perform normal ventilation only. Where the line lies is set by the project's fire scenario; if you are unsure, ask the project engineer.

Single speed, two speeds or reversible?

In a car park jet fan, pole selection is driven by the diameter and the required jet velocity. A 400 mm impeller turning at 2895 rpm has a blade tip speed of about 61 m/s; this speed is needed to create a high jet velocity in a short casing. That is why the high speed of axial jet fans in the 315–450 mm class is usually 2-pole; 4-pole motors appear on larger diameters or in projects with tight noise limits.

Two speeds are a car-park-specific need: at night and in low-traffic hours half speed is enough. When speed is halved, thrust drops to about a quarter and power to about an eighth. In our example about 18 N of thrust and around 0.2 kW of shaft power remain instead of 73 N; the catalogue model of the same diameter also lists 18 N at low speed [1]. Two speeds are obtained in two ways: a two-speed (pole-changing) motor, or a single-speed motor + inverter. The models in our store are single-speed; if two speeds are needed, this is solved with an inverter, and if the inverter has no role in the fan's fire scenario this arrangement causes no problem.

Reversible jet fans blow in both directions; what makes this possible is a symmetrical blade profile that behaves the same in both directions, not any special motor design. The motor's direction is changed by swapping two phases or by the inverter's direction command; the impeller must come to a full stop before reversing. Panel work must be done by a qualified electrician.

Motor selection checklist

• Fan nameplate: model, diameter, thrust (N), speed, installed power.
• Old motor nameplate: kW, rpm, voltage and connection, IEC frame, mounting type, shaft diameter.
• The fan's duty: normal ventilation only, or also part of the smoke scenario? (If so, a certified set is required.)
• Single speed, two-speed motor or inverter; is reversible operation needed?
• Starting method: direct, star-delta or inverter?
• Mounting: motor inside the casing, in the middle of the airstream; flange or foot mounted (from the fan manufacturer's drawing).
• Environment: humidity, dust and exhaust soot build-up in an enclosed car park; position of the terminal box and cable entry.

Clarifying responsibilities from the start makes the job easier. Fan selection, the fan curve, impeller and casing design, and the number and layout of fans in the car park are the work of the fan manufacturer and the project engineer. The motor supplier's job is to provide a motor that suits the given shaft power, speed, mounting and ambient conditions.

Mounting, starting and motor connection

In a jet fan the motor sits in the middle of the casing, right behind the impeller, and the air passing through the jet cools it. The motor is usually fixed to the casing via a support; depending on the fan model this connection may be foot-mounted (B3) or flange-mounted (B5/B14). Hanging the fan from the ceiling is a separate matter from the motor's IEC mounting type; the exact mounting type of the motor is taken from the fan manufacturer's drawing and noted in the order. DRG motors are supplied in B3/B5/B14/B35 mounting as specified in the order note.

For starting, one manufacturer's own practice is direct-on-line starting for motors up to 2.2 kW and star-delta starting for 3 kW and above [1][2]. One detail matters here: star-delta starting requires the motor to be delta-connected on a 400 V supply, i.e. a 400/690 V winding. In the DRG range, motors up to 3 kW are 230/400 V, and 4 kW and above are 400/690 V. If your existing panel is star-delta and you will fit a 3 kW motor, discuss this at the outset. For starting options see the star-delta starting tool.

Matching with DRG motors

The DRG range covers 0.25–11 kW in aluminium frames and 0.55–355 kW in cast iron, 1000/1500/3000 rpm, 3-phase, 230/400 V or 400/690 V; B3/B5/B14/B35 mounting is set by the order note. In car park jet fans, power is small and the fan hangs from the ceiling, so the light aluminium frame stands out. The model and price table in the middle of this article is for jet fans in normal ventilation mode; it does not mean every jet fan power on the market is a DRG product.

For the 400 mm fan in our worked example, DRG-90L-2 is suitable. For larger 4-pole fans, models such as DRG-100L1-4 are considered. The shaft diameter and flange size of the existing fan must match the new motor; compare them with the old motor's nameplate and dimensions. For the general power and speed logic in axial and radial fans, see our ventilation fan motor article, and for the effect of speed reduction on power, our affinity laws article. All fan applications are on our Fan Motors page.

DRG motors for car park jet fans

• DRG-90S-2 – 1.5 kW – 2,895 rpm – 90S aluminium
• DRG-90L-2 – 2.2 kW – 2,895 rpm – 90L aluminium
• DRG-100L-2 – 3 kW – 2,895 rpm – 100L aluminium
• DRG-100L1-4 – 2.2 kW – 1,450 rpm – 100L aluminium

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Car park jet fan motor: FAQ

How many kW is a jet fan motor?

There is no fixed value; thrust, diameter and outlet velocity decide it. In one manufacturer's catalogue the installed power of 315–450 mm car park jet fans is 0.55–3 kW [1]. For your own fan, check the power on the nameplate or the calculation from thrust and diameter.

Can a standard motor be fitted to a smoke extraction jet fan?

No. In fans with a smoke duty, the motor is part of the set certified together with the fan to EN 12101-3; replacement is made with the fan manufacturer's certified motor. DRG's standard motors are not sold with this certification.

Why does a jet fan run at two speeds?

Half speed is enough in low-traffic hours; when speed is halved, power falls to about an eighth and noise drops. Full speed is used when pollution rises or in emergencies defined in the project.

How is the direction of a jet fan reversed?

On a three-phase motor two phases are swapped, or a direction command is given on the inverter; the impeller must first come to a full stop. Only fans with symmetrical (reversible) blades give the same thrust in both directions. This work must be done by a qualified electrician.

Send us your fan nameplate and the old motor's nameplate, and we will determine the right model together: phone +90 (542) 666 11 11, e-mail [email protected], contact page. DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the date of delivery (warranty certificate). For current prices, see Buy Electric Motors Online.

Sources

1. Kruger – Induced Jet Fan IJB & IJC II Series (catalogue, PDF) – thrust and installed power table, high/low speed, starting note.
2. Kruger – Jet fan product page – direct-on-line up to 2.2 kW, star-delta from 3 kW.
3. BS EN 12101-3 – Smoke and heat control systems, Part 3: Powered smoke and heat control ventilators – F200/F300/F400 temperature-time classes.