Vane-Axial Fan with Guide Vanes and Its Electric Motor

A vane-axial fan is a cased axial fan with fixed vanes placed just behind the impeller that straighten the swirling air and turn it into pressure; at the same diameter it delivers more pressure than a plain duct-type axial fan and therefore needs a stronger motor. For example, a 1,000 mm fan delivering 54,000 m³/h at 1,200 Pa works out to a 30 kW, 4-pole motor; in this class the DRG-200L-4 draws 53.8 A at 400 V and uses the 200L cast iron frame.

This article explains what guide vanes mean for the motor: why a bigger motor is needed, what happens to motor current when the blade angle changes, starting and mounting. If you have the figures, you can find the right frame among the 120 models on the Buy Electric Motors Online page. At these powers, wiring, starting and the direction-of-rotation check must be done by a qualified electrician.

What do guide vanes do, and how does it differ from a plain duct fan?

An axial impeller pushes the air forward but also sets it spinning. In a plain tube-axial fan this swirl carries on down the duct and much of its energy is lost without doing any work. In a vane-axial fan, fixed profiled vanes after the impeller turn this rotating flow parallel to the axis; one manufacturer's catalogue describes this as converting the rotational energy at the impeller outlet into useful work [1]. The result: higher static pressure at the same airflow.

In terms of terminology, a vane-axial fan works on the axial principle; as a housing and installation type it is a duct fan, flanged into the duct. What sets it apart is the guide vane section inside the casing. Main uses are long systems that need high pressure: industrial exhaust and process air, mine and tunnel ventilation, ship engine rooms, and ducts with resistive elements such as filters or silencers. One manufacturer describes these fans as suited to mine and marine applications and gives static pressure up to 10 in. w.g. (about 2.5 kPa) [2].

The motor's role: direct drive inside the guide vane hub

In most vane-axial fans the impeller is mounted directly on the motor shaft and the motor sits in the inner body at the centre of the casing. The guide vanes do two jobs here: they straighten the air and act as the struts tying the inner body to the casing. The motor is in the air stream; in clean air that means extra cooling. In hot or dirty air the motor is moved outside: in belt-driven versions the motor is outside the casing and the impeller runs on a separate shaft in its own bearings [2]. Which layout is chosen depends on the temperature and content of the air.

Who is responsible for what: Impeller diameter, number and angle of blades, guide vane design and operating point are the job of the fan manufacturer or design engineer. The motor supplier's job is to provide a motor matching the given shaft power, speed, mounting type and ambient conditions. Cases that need certification: Gassy mines and explosive atmospheres require an ATEX/IECEx certified motor, and smoke extraction requires a fan-motor set certified to EN 12101-3; the smoke-exhaust versions of the fans in the catalogue above are supplied with this certification [1]. DRG's standard motors are not sold with these certificates; they are recommended only for normal ventilation where no certification is required. Details on mines and tunnels are in the article on mine and tunnel ventilation motors.

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Information needed to select the motor

• Fan nameplate: airflow (m³/h), total or static pressure (Pa), impeller diameter
• Blade angle setting and the power the fan draws at its maximum angle (from the fan curve)
• Drive: direct or belt; is the motor in the air stream
• Air temperature, humidity and dust; whether the atmosphere requires certification
• The old motor's nameplate: kW, rpm, frame, voltage (is it 400/690 V)
• Motor mounting: by its feet (B3) or flange on the plate in the inner body
• Shaft diameter × length and how the impeller hub is fixed
• Starting: star-delta, soft starter or VFD
• Cable route out of the casing and location of the junction box

Power: why do guide vanes call for a bigger motor?

Fan shaft power is found with P_shaft = Q·Δp / η_fan (Q in m³/s, Δp in Pa, P in W). Guide vanes improve efficiency somewhat, but their main effect is that the fan reaches a higher pressure; as pressure rises, shaft power at the same airflow rises in proportion. Example: 1,000 mm fan, 54,000 m³/h = 15 m³/s, 1,200 Pa, assuming a fan efficiency of 0.75: P_shaft = 15 × 1,200 / 0.75 = 24 kW. With direct drive there is no drive loss; adding a 10% margin gives ≈ 26.4 kW and a standard 30 kW motor is chosen. Pushing the same airflow through a system needing only 600 Pa would take 12 kW (same efficiency assumed): twice the pressure means twice the motor power. Use the Pump & Fan Motor Power calculator with your own figures.

Power depends on impeller diameter, airflow, pressure, blade angle and number, speed, system resistance, air density and temperature, and drive loss if belt-driven. So no fixed kW can be assigned to a vane-axial fan. A sourced example: in one manufacturer's catalogue, 630–1600 mm vane-axial fans are supplied with 2-, 4-, 6- and 8-pole B3 IEC motors; the motor table runs from 1.5 kW (800 mm, 6-pole) up to 200 kW (1600 mm, 4-pole) [1]. The same catalogue lists 30 kW and a 200L frame for 1,000 mm 4-pole fans [1], consistent with the calculation above.

DRG range: aluminium frame 0.25–11 kW, cast iron 0.55–355 kW; 1000, 1500 and 3000 rpm; 3-phase, 230/400 V or 400/690 V; mounting B3, B5, B14 or B35 by order note. 8-pole motors are not in the range. The table below shows, with current prices, 4- and 6-pole cast iron models in the 7.5–37 kW range common on vane-axial fans; current values are for setting the overload relay.

Adjustable blade angle and motor current

On most vane-axial fans the impeller blade angle can be adjusted. One manufacturer sets the pitch angle in the factory to suit the operating point [3]; another catalogue states that the blades can be adjusted at the hub but the impeller must be rebalanced after each adjustment [1]. For the motor the rule is simple: a larger angle makes the blade take more air, airflow and pressure rise together, and shaft power climbs quickly. So increasing the angle on site "to pull a bit more" can push the motor beyond its nameplate.

After changing the angle, run the fan at full load and measure the current on all three phases. For the DRG-200L-4, for example, the measured current must not exceed 53.8 A at 400 V; if it does, reset the angle or choose a bigger motor from the fan manufacturer's curve. The overload relay is set to this nameplate current; it is not raised to stop nuisance tripping.

Speed, starting and mounting

Vane-axial fans are usually large in diameter and tip speed (v = π·D·n/60) limits the pole choice. On a 1,000 mm impeller the tip speed is about 77 m/s with 4 poles (1475 rpm) and about 51 m/s with 6 poles (980 rpm); with 2 poles (2960 rpm) it would reach 155 m/s, which is not used in practice at these diameters. So 2-pole motors suit small diameters, while 4 and 6 poles dominate above 800 mm; where low noise is wanted, 6 poles are chosen and the same airflow is achieved with a larger angle or diameter. Poles are not changed on the same fan unless the blade angle and impeller are reselected accordingly.

Starting: The inertia of a large impeller lengthens the start. With direct-on-line starting, the 30 kW motor's starting current is about 393 A, using the data sheet ratio Is/In = 7.3. Because the motor is 400/690 V, it can be star-delta started on a 400 V supply; in star the current drops to about a third, around 131 A. For details see the star-delta starting tool. A VFD both softens the start and adjusts airflow without touching the blade angle.

Mounting: Bolting the fan into the duct and the motor's IEC mounting type are separate things. In the cited catalogue the motors are B3 foot-mounted and fixed by their feet to the plate in the inner body [1]; designs with flange-mounted motors also exist [2]. The exact type comes from the fan manufacturer's drawing and the old motor; for a fan in a vertical duct, vertical mounting is written into the order. For all fan applications see the Fan Motors page.

DRG motors for vane-axial fans

• DRG-160L-4 – 15 kW – 1465 rpm – 160L cast iron
• DRG-180L-4 – 22 kW – 1470 rpm – 180L cast iron
• DRG-200L-4 – 30 kW – 1475 rpm – 200L cast iron
• DRG-225S-4 – 37 kW – 1485 rpm – 225S cast iron

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Vane-axial fan motor: FAQ

What does vane-axial fan mean?

An axial fan with guide vanes: fixed vanes after the impeller straighten the swirling air and turn it into pressure. A plain tube-axial fan has no such vanes.

How many kW is a vane-axial fan motor?

There is no fixed value; it is calculated from airflow, pressure and fan efficiency. Example: for 54,000 m³/h at 1,200 Pa on a 1,000 mm fan the shaft power is 24 kW and the chosen motor is 30 kW, 4-pole.

If I increase the blade angle, will the motor cope?

Not always. Shaft power rises quickly with the angle; after adjustment measure the current on all three phases and confirm it stays within the nameplate value. The impeller must also be rebalanced.

How many amps does a 30 kW 4-pole fan motor draw?

The DRG-200L-4 draws 53.8 A at 400 V and 56.6 A at 380 V; being 400/690 V, it can be star-delta started on a 400 V supply.

Send us the fan nameplate, the blade angle setting and the old motor's nameplate, and we will work out together the DRG model that meets the operating point. Phone +90 (542) 666 11 11, e-mail [email protected], contact page. For all models and current prices see Buy Electric Motors Online.

Sources

1. CVS Air – CVS-A/GV Axial Vane Fan, technical catalogue (PDF) – 630–1600 mm, up to 285,000 m³/h; adjustable blades, rebalancing after adjustment; 2/4/6/8-pole B3 IEC motors; motor table 1.5–200 kW; smoke-exhaust versions certified to EN 12101-3
2. Hartzell Air Movement – Adjustable and Fixed Pitch Vaneaxial – static pressure up to 10 in. w.g. (about 2.5 kPa); mine and marine use; belt-driven versions with the motor out of the airstream
3. Systemair – AXC-E axial fans (product page) – pitch angle set in the factory to the operating point; guide vane for higher pressure