Straight Radial Blade Fan: Motor Selection for Material Handling and Dusty Air

A straight radial blade fan is a centrifugal fan whose blades run straight out from the hub, so it can move air carrying sawdust, dust, fibre or granules without clogging; the motor that drives it is not a special “radial fan motor” but a standard three-phase induction motor chosen for the fan’s shaft power, speed and the start of a heavy wheel. In the example in this article, a belt-driven fan conveying 5,040 m³/h of chip-laden air at 4,200 Pa calls for a 15 kW, 4-pole DRG-160L-4: 27.3 A at 400 V, 1465 rpm, 97.8 Nm rated torque. You can browse suitable models among the 120 models in our online store.

What sets this fan apart is not moving air with little power but surviving dirty air. The price is paid on the motor side: power demand rises with airflow, the wheel is heavy because it is made of thick plate, and the start takes a long time. Choosing the motor correctly means accounting for both. The panel, starter and overload setting must be handled by a qualified electrician, and nobody should work near the wheel or belts before the power is locked out.

What is a straight radial blade fan and where is it used?

By operating principle this is a centrifugal fan: air enters the wheel axially at its centre, is thrown outward by the blades and leaves through the scroll housing. In Turkish, “radial” and “centrifugal” usually mean the same thing; “straight radial blade” is not a separate fan principle but a wheel type of the centrifugal fan. As a blade family it is the third group, alongside forward-curved and backward-inclined wheels.

An engineering note states that this wheel usually has 6 to 12 flat, equally spaced blades and few ledges for material to cling to [1]. The air passing over the blade surface also limits build-up [3], so the fan largely keeps itself clean. Typical uses:

• sawdust and chip extraction lines in woodworking shops,
• pneumatic conveying lines that carry granules, paper trim or textile fibre in air,
• extraction systems where material passes through the fan, and process air requiring high pressure,
• exhaust lines with sticky or abrasive particles.

What does the motor do in this fan, and who selects what?

The motor turns the wheel through belts and pulleys, through a coupling, or with the wheel mounted directly on its own shaft. The same engineering note says these fans are mostly belt-driven because of the heavy shock loads and harsh conditions [1]. Belts bring two benefits: the fan speed can be set to any value between two pole speeds, and impacts on the wheel do not hit the motor shaft and bearing directly. Direct drive is mostly seen on small, lightly loaded fans.

The material does not pass over the motor; the motor sits outside the airstream, beside the housing or on a base. Even so, dust around the fan settles on the motor fins and fan cover, so an IP55 frame and regular cleaning matter. In belt-driven layouts the motor is usually B3 foot-mounted on a belt-tensioning slide rail; direct drive may use a flange or foot mounting. The exact mounting type comes from the fan manufacturer’s drawing.

The split of responsibilities is clear: fan selection, the fan curve, the wheel, the housing and the pulley ratio are the job of the fan manufacturer or the design engineer. The motor supplier’s job is to provide a motor that suits the given shaft power, speed, mounting type and ambient conditions. For the scroll housing itself and the fan laws, see our article on the centrifugal (scroll) fan motor.

How do you find the motor power of a material handling fan?

The basic relation is P_shaft = Q·Δp / η_fan (Q in m³/s, Δp in Pa, P in W). In a material handling line Δp must include the duct, the cyclone and the material load together; the fan manufacturer gives this value along with the quantity of material conveyed. Example assumptions:

• airflow 1.4 m³/s (5,040 m³/h), total pressure 4,200 Pa,
• assuming a fan efficiency of 0.62, shaft power: 1.4 × 4,200 / 0.62 ≈ 9.5 kW,
• divided by 0.95 for belt losses ≈ 10.0 kW,
• with a 20% margin for the rising power curve ≈ 12.0 kW → next standard size 15 kW.

The margin comes from this wheel’s power curve: as airflow increases, so does power [1]. When the line runs empty, an intake is left open or the cyclone hatch is opened, system resistance falls, the fan moves more air and the motor can exceed its nameplate current. There is no self-limiting power peak as on a backward-inclined wheel. To try the calculation with other values, use the Pump & Fan Motor Power calculator.

Be careful with power ranges: one manufacturer’s catalogue lists radial bladed series with 225–2,650 mm wheel diameters and static pressure up to 11,440 Pa [2]; that is a fan range, not a motor power range. We do not quote a single sourced motor power range for this fan type; power is set by application data: diameter, airflow, static pressure, blade width, speed, material load, air density and temperature, and belt losses.

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Heavy wheel: starting time, starting current and overload

A straight radial blade wheel is welded from thick plate and often carries extra wear liners; its inertia is larger than that of a backward-inclined wheel of the same diameter. In the example, take a wheel inertia of 6 kg·m² on the fan side and a fan speed of 1800 rpm. Referred to the motor shaft, the inertia becomes 6 × (1800 / 1465)² ≈ 9.1 kg·m². Assuming an average accelerating torque equal to the motor’s rated torque, the starting time is t ≈ J·ω / T ≈ 9.1 × 153.4 / 97.8 ≈ 14 s. That is longer than a standard overload relay comfortably tolerates.

The DRG-160L-4’s starting current is 7.8 times rated: about 213 A direct-on-line. Because the motor is wound for 400/690 V, it runs in delta on a 400 V supply and is suitable for star-delta starting; in star the line current drops to about 71 A and the starting torque falls to one third, about 71.7 Nm. Acceleration then slows down. In practice three measures are applied together:

• Start with the damper or gate closed: since power rises with airflow on this wheel, a closed damper reduces the starting load.
• An overload trip class suited to heavy starting, or a soft starter or inverter.
• Switch from star to delta only once the motor approaches rated speed; an early switch causes a second current surge.

Use the star-delta starting tool for the calculation and the overload, fuse and contactor selection page for the protection devices.

Speed, tip speed and wear: how many poles?

In a radial fan the pressure is set by the wheel tip speed: v = π·D·n / 60. For the example’s 0.8 m diameter and 1800 rpm, the tip speed is about 75.4 m/s. With abrasive material, high tip speed wears the blades quickly, so the fan manufacturer often enlarges the diameter and keeps the speed low. As a result, 4- and 6-pole motors with belts are common on these fans. The pulley ratio is the fan speed divided by the motor speed; in the example 1800 / 1465 ≈ 1.23. Direct drive with a 2-pole motor stays with small fans handling light dust.

Wear, loss of balance and bearings

As blade tips wear or material sticks to the blades, the wheel’s centre of gravity shifts. The resulting vibration travels through the belts to the motor shaft and bearing; on the DRG-160L-4 this bearing is 6309-C3 and the shaft is 42 × 110 mm. If vibration or bearing noise rises on the motor, check the wheel before replacing the motor: renewing the worn blades and rebalancing the wheel often cures the problem at its root. Belt tension should not be excessive either; an overtightened belt puts unnecessary radial load on the motor bearing.

Motor selection checklist and DRG match

• Fan nameplate or data sheet: airflow, total pressure, fan speed, shaft power
• Material conveyed: type, quantity, abrasive or sticky
• Drive: belt (pulley diameters), coupling or direct
• Old motor nameplate: kW, rpm, frame, mounting type, shaft diameter
• Wheel inertia, or wheel weight and diameter (for the starting time)
• Starting method and current overload setting
• Environment: dust, humidity, temperature

The DRG range, stated plainly: aluminium frames 0.25–11 kW, cast iron frames 0.55–355 kW; 1000, 1500 and 3000 rpm; 3-phase, 230/400 V or 400/690 V; mounting B3, B5, B14 and B35 by order note. This does not mean the range covers every fan on the market; a match is made when the power and speed the fan needs fall within it. Current prices and short specifications of the cast iron models often chosen for belt-driven radial fans are in the price table above.

Details of the 15 kW motor from the example:

• Power: 15 kW
• Rated speed: 1465 rpm
• Current 400 V: 27.3 A
• Rated torque: 97.8 Nm
• Starting current Is/In: 7.8
• Shaft D × E: 42 × 110 mm
• Bearing: 6309-C3
• Voltage: 400/690 V (Δ/Y)

For dimension checks see the technical drawings page, and for an overview our Fan Motors page. The general steps for selecting a motor for dusty extraction systems are in our industrial fan and extractor motor article.

DRG motors for radial blade fans

• DRG-132M-4 – 7.5 kW – 1460 rpm – 132M cast iron
• DRG-160M-4 – 11 kW – 1465 rpm – 160M cast iron
• DRG-160L-4 – 15 kW – 1465 rpm – 160L cast iron
• DRG-180M-4 – 18.5 kW – 1470 rpm – 180M cast iron
• DRG-180L-6 – 15 kW – 978 rpm – 180L cast iron

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Radial blade fan motor: FAQ

How many kW should a radial fan motor be?

There is no fixed value; airflow, pressure, material load and belt losses decide it. In the example, 1.4 m³/s and 4,200 Pa gave about 12 kW and a 15 kW motor was chosen. For a replacement, the old motor’s nameplate power is the reference.

Why does a sawdust extraction fan trip the overload on start?

The heavy wheel takes a long time to accelerate and the motor draws high current meanwhile. Start with the damper closed, use an overload class suited to heavy starting or a soft starter, and do not switch from star to delta too early.

Should a radial fan run at 1500 or 3000 rpm?

When abrasive material is conveyed, a low tip speed is wanted, so 1500 or 1000 rpm motors with belts are common. 3000 rpm direct drive is preferred on small fans with light dust. The final decision follows the fan manufacturer’s speed.

How many amps does a 15 kW radial fan motor draw?

The DRG-160L-4 draws 27.3 A rated current at 400 V. Starting current is about 213 A direct-on-line and about 71 A in star-delta.

Send us your fan nameplate, the old motor’s nameplate and the pulley diameters, and we will confirm the right DRG model together: phone +90 (542) 666 11 11, e-mail [email protected], contact page. For all models and current prices: Buy Electric Motors Online.

Sources

1. Twin City Fan – Fan Performance Characteristics of Centrifugal Fans (FE-2400) – straight radial wheel with 6–12 flat blades, power rising with airflow, mostly belt-driven because of shock loads
2. Twin City Fan & Blower – Full Line Catalog – radial bladed series: 225–2,650 mm wheels, static pressure up to 11,440 Pa, airflow up to 240,900 m³/h
3. U.S. DOE – Improving Fan System Performance: A Sourcebook for Industry – airflow over radial blades limits build-up; dirty airstreams and material handling