Backward-Curved Centrifugal Fan and Motor Selection: The Non-Overloading Power Curve

On a backward-curved centrifugal fan, shaft power rises to a peak as airflow increases and then falls again. That is why a motor selected for the peak power on the fan curve will not overload at constant speed, even if the duct is removed, the damper is opened or the filter is changed. This is called a non-overloading power curve, and it makes the backward-curved fan the centrifugal fan with the most predictable motor selection.

Below we calculate the curve of a direct-drive backward-curved fan point by point, show where the peak power occurs, in which cases this guarantee breaks down (higher speed, cold and dense air) and what an inverter does. You can choose suitable motors from the 120 models in our online store; for general information see our Fan Motors page.

What is a backward-curved radial fan and where is it used?

A backward-curved fan is a centrifugal (radial) wheel whose blades lean against the direction of rotation. Compared with a forward-curved wheel it has fewer blades (usually 8–12), and they are longer and heavier [1]. The blade can be a flat plate, a curved plate or an airfoil profile; all three belong to the same family. "Backward-curved" is a blade type: the operating principle is centrifugal, and the housing may be a scroll or there may be none at all.

Applications: air handling units, cabinet extractors, industrial exhaust after the filter, process air and combustion air fans that need medium to high pressure in clean or filtered air. The housing-less backward-curved wheel is the basis of plug (plenum) fans; a forward-curved wheel is unstable without a housing and does not suit this job [1]. Air carrying sticky dust and material is a job for straight radial-blade fans, not for the backward-curved wheel.

Non-overloading power curve: where is the peak?

On a backward-curved blade the air leaves the wheel at an absolute velocity lower than the tip speed; most of the energy is converted to pressure between the blades. As airflow increases, pressure falls, and beyond a certain point the product of airflow × pressure starts to shrink; shaft power follows. One manufacturer document states that a motor selected for the peak power will not overload at constant fan speed despite changes in system resistance or airflow [1]; another describes the power curve of this wheel as non-overloading [2].

For the example we used a simplified curve: a single-inlet backward-curved fan with the wheel mounted directly on the motor shaft, 560 mm wheel diameter, 1450 rpm (tip speed about 42.5 m/s). The design point is 10,800 m³/h at 898 Pa; a fan efficiency of 0.74 was assumed at this point: P_shaft = Q·Δp / η_fan = 3.0 × 898 / 0.74 ≈ 3,640 W.

Although airflow rises from 7,200 to 18,610 m³/h, more than 2.5 times, shaft power never exceeds 3.72 kW. With the duct removed, power is even lower than at the design point. This fan's free-delivery airflow is about 23,070 m³/h; the design point is 47% of it. One manufacturer document recommends keeping the selection between 40% and 85% of free delivery for quiet and efficient operation [1].

What does the motor do in this fan, and who is responsible?

Two arrangements are common on backward-curved fans. With direct drive the wheel sits on the motor shaft; the motor carries the wheel's weight and unbalance force on its bearings. In a plug fan the motor usually stands behind the inlet side, close to the airflow inside the cabinet. In belt or coupling arrangements the motor is outside the scroll housing and the wheel turns on its own bearing-mounted shaft. Either way, the motor is a standard three-phase induction motor providing the power and speed the fan needs; there is no separate motor type called a "backward-curved fan motor".

Which fan is used, and how its curve, wheel and housing are designed, is decided by the fan manufacturer or the engineer who designs the system. Our part is to supply the right motor for the shaft power, speed, mounting and ambient conditions that this decision produces. Some plug fans come with permanent-magnet or EC integrated motors; DRG does not sell these motors, and in that case a motor replacement should be discussed with the fan manufacturer.

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Motor selection at peak power and three cases where the guarantee breaks

Direct drive has no drive loss; motor power = peak P_shaft + margin. In the example the peak is 3.72 kW, so a 4 kW motor is enough: DRG-112M-4, 1450 rpm, 7.95 A at 400 V. The margin is about 7.5%; because the motor is selected at peak power, there is no need to add a separate allowance for an "open door" case as on forward-curved fans. The guarantee breaks down only in these three cases:

• Higher speed: the guarantee holds only at constant speed [1]. Going above 50 Hz with an inverter or fitting a larger pulley increases the peak power with the cube of speed.
• Cold, dense air: power is proportional to air density. If the curve is given for 20 °C, a fan drawing −10 °C outdoor air sees density about 1.11 times higher, and peak power rises from 3.72 kW to about 4.14 kW. A 4 kW motor then exceeds its limit; choose 5.5 kW (DRG-132S-4) or limit speed during cold starts.
• Different wheel or blade angle: a changed wheel means a new curve; the old peak power is no longer valid.

For the market range: one manufacturer catalogue lists its single-inlet backward-curved series up to 2,800 Pa and 40 kW shaft power, its double-inlet series up to 3,700 Pa and 160 kW, and its backward-curved plug fan series up to 2,800 Pa and 80 kW [3]. Another document states that this wheel is used up to 20 inches water gauge (about 5,000 Pa) [1]. The power of your own fan is calculated from diameter, airflow, pressure, speed, system resistance, air temperature and drive losses; for a quick calculation use the Pump & Fan Motor Power Calculator.

Speed, tip speed, balancing and inverter

To produce the same pressure, a backward-curved wheel needs a higher tip speed than a forward-curved wheel [1]. The 560 mm wheel in the example turns at about 42.5 m/s at 1450 rpm. That is why 4 poles (1500 rpm class) and, for small diameters, 2 poles (3000 rpm class) are common on backward-curved fans; DRG-112M-2 runs at 2905 rpm, DRG-132S1-2 at 2930 rpm.

The pole number comes from the wheel diameter and the wheel's maximum permitted speed; it is not changed freely on the motor side. Driving the same 560 mm wheel with a 2-pole motor raises the speed about 2.02 times; power rises about 8.25 times (from 3.72 kW to about 30.7 kW) and tip speed to about 86 m/s. This can exceed the wheel's strength limit; it is not done without the fan manufacturer's approval.

High tip speed makes balancing critical. With direct drive, the wheel-to-shaft fit (28 mm shaft and 8 mm key on the 112M frame), key clearance and balancing the wheel together with the motor determine vibration. The radial and axial loads that the wheel's weight puts on the motor bearings are checked against the fan manufacturer's data; the bearing on the 112M frame is 6206-ZZ-C3. The motor mounting type depends on the arrangement: on plug fans usually B3 on a base or a B5 flange on the frame, V1 on vertical fans; the exact type is taken from the fan manufacturer's drawing.

Running on an inverter: airflow control and energy

The backward-curved fan suits every control method [2], but the real energy saving comes from speed control. According to the affinity laws, at 80% speed the example fan delivers 8,640 m³/h at 575 Pa, and shaft power drops from 3.64 kW to about 1.86 kW. Throttling to the same airflow with a damper does not reduce power nearly as much. Detailed calculation in our affinity laws article.

• The inverter output current must cover the motor's 400 V nameplate current (7.95 A in the example).
• Keep the upper frequency limit at 50 Hz; above it, peak power and tip speed grow.
• Because torque falls at low speed on a fan load, a self-ventilated motor is usually enough; watch the temperature during long running at very low speed.
• If the wheel–motor system has a resonance speed within the range, skip it on the inverter.

Checklist before ordering and DRG matching

• Fan curve: design point and peak shaft power of the curve.
• Fan speed, wheel diameter and the wheel's maximum permitted speed.
• Lowest temperature of the air drawn in (cold start).
• Drive: direct, coupling or belt; wheel hub bore and key.
• Motor mounting type and bearing load data.
• Is there an inverter, and what is the frequency range?
• Integrated EC/permanent-magnet motor? If so, talk to the fan manufacturer.

DRG range: aluminium frames 0.25–11 kW, cast iron 0.55–355 kW; 1000/1500/3000 rpm; three-phase 230/400 V or 400/690 V; mounting B3/B5/B14/B35 with an order note. Not every backward-curved fan power on the market is a DRG product; find the value from your own fan's peak power. For starting methods on large wheels with long run-up times see the Motor Starting Methods and Star-Delta tool, and for general selection on industrial fans our industrial fan and extractor motor article. DRG Motor is a manufacturer, supplier and seller of electric motors. DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate).

DRG motors for backward-curved fans

• DRG-112M-2 – 4 kW – 2905 rpm – 112M cast iron
• DRG-112M-4 – 4 kW – 1450 rpm – 112M cast iron
• DRG-132S1-2 – 5.5 kW – 2930 rpm – 132S cast iron
• DRG-132S-4 – 5.5 kW – 1460 rpm – 132S cast iron
• DRG-132M-4 – 7.5 kW – 1460 rpm – 132M cast iron
• DRG-160M-4 – 11 kW – 1465 rpm – 160M cast iron

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Backward-curved fan motor: FAQ

Why doesn't the motor overload on a backward-curved fan?

Because shaft power peaks with airflow and then falls. If the motor is selected for this peak power, the load does not exceed the motor power at constant speed, whatever airflow the fan runs at. In the example fan, airflow changes by more than 2.5 times yet power never exceeds 3.72 kW.

What speed should a backward-curved fan motor have?

Whatever speed the fan manufacturer specifies. On medium and large diameters 4 poles (1450–1470 rpm) are common, on small diameters 2 poles (2905–2955 rpm). Changing the pole number changes power with the cube of speed.

Can a backward-curved fan run above 50 Hz on an inverter?

Only if the fan manufacturer allows it for the wheel and the motor can handle the new peak power. In the example, a 10% speed increase raises the peak power to about 4.95 kW; a 4 kW motor cannot carry it.

Is a plug fan backward-curved or forward-curved?

Plug (plenum) fans are built with backward-curved or airfoil wheels. A forward-curved wheel depends on its scroll housing and is unstable without it.

Send us your fan curve or the old motor's nameplate, and we will confirm the DRG model that suits the peak power. 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. Aerovent – Fan Performance Characteristics of Centrifugal Fans (FE-2400) – non-overloading power curve of the backward-inclined wheel, motor selection at peak power, selection range and pressure limit
2. Greenheck – Product Application Guide: Fan Applications (FA/126-08) – non-overloading horsepower curve of the backward-inclined fan and clean-air use
3. Comefri – Global Product List – pressure and shaft power limits of single/double-inlet backward-curved and plug fan series