Forward-Curved Centrifugal Fan: Why Does Motor Power Rise with Airflow?

On a forward-curved centrifugal fan, shaft power rises with airflow: the more freely the fan discharges, the harder the motor works. That is why a forward-curved fan with a short duct, a removed filter or an open service door can overload a motor that runs comfortably at the design point and trip its overload relay. The motor is sized for the system's lowest-resistance condition, not for the dirtiest filter.

Below we calculate this behaviour with three operating points of a single example fan, and explain how to set the motor margin, the overload relay and the damper. You can choose motors of the right power and speed from the 120 models in our online store; general fan motor information is on our Fan Motors page.

What is a forward-curved fan and where is it used?

A forward-curved fan is a centrifugal wheel with many short, scoop-shaped blades whose concave face points in the direction of rotation. It is also called a sirocco or squirrel-cage wheel. "Forward-curved" is a blade type, not an operating principle: the principle is centrifugal (air enters along the axis, turns 90° and leaves at the rim), and the housing is usually a scroll. Because the wheel is light, slow and compact, it moves high airflow at low pressure quietly.

Typical uses are low-pressure, compact applications: air handling units and packaged air conditioners, fan-coil units, kitchen hood extractors, warm-air heaters and the combustion air fan of small burners. Heavy dusty industrial exhaust is not a job for this wheel; its light construction and the way dust collects between the blades limit it to clean or filtered air.

Why does the power curve rise with airflow?

A forward-curved blade throws the air faster than the wheel's own tip speed. As airflow grows, so does the energy carried by the air leaving the blades; even though pressure falls, the total power the wheel gives to the air keeps increasing. One manufacturer document states that the power curve rises constantly and fairly steeply from shut-off to free delivery [2]. There is no peak point as on a backward-curved wheel.

For the example we used a simplified curve: a double-inlet, belt-driven forward-curved fan, 400 mm wheel diameter, fan speed 950 rpm (tip speed about 19.9 m/s). The design point is 7,200 m³/h at 350 Pa. Assuming a fan efficiency of 0.55, P_shaft = Q·Δp / η_fan = 2.0 × 350 / 0.55 ≈ 1,270 W. As system resistance changes, the same fan moves to these points:

Although pressure drops from 350 Pa to 200 Pa, shaft power rises by about 30%. As the filter clogs, the fan gets an easier job; when the filter is replaced or the door is opened, the load increases. The site complaint "we fitted a new filter and the motor started tripping" comes from this curve.

The other end of the curve matters too: if this wheel is throttled too much, it enters the dip in the pressure curve and the airflow pulsates. One manufacturer document recommends keeping the selection at or above 45% of free-delivery airflow [1]. The example fan's free-delivery airflow is about 13,150 m³/h; the design point is 55% of it and the dirty-filter point 47%.

What does the electric motor do in this fan?

On larger forward-curved fans the motor sits outside the scroll housing on an adjustable base and turns the wheel through belts and pulleys; the airstream does not pass over the motor, which cools itself with its own fan. The belt lets the fan speed be chosen independently of the motor speed. On small fans the wheel is mounted directly on the motor shaft, or the motor is built into the wheel hub.

Most small forward-curved fans in fan-coils, hoods and packaged units come with an integrated external-rotor, single-phase or EC motor. These motors are designed together with the wheel; DRG does not sell such integrated motors, and a standard IEC motor cannot be fitted directly in their place. In that case the part that is replaced is usually the fan set itself.

The division of responsibility is clear: fan selection, the fan curve, and the wheel and housing design 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 and ambient conditions. The calculations in this article are for sizing the motor correctly; they do not replace the fan curve.

Click to see current electric motor prices and order nowSEE PRICES →

How to size the motor: margin, overload relay and damper

On a forward-curved fan the motor power comes from the shaft power at the highest airflow the fan can actually reach: motor power = P_shaft / η_drive + margin. Taking a belt drive efficiency of 0.95, the motor load in the example is about 1.34 kW at the design point and about 1.74 kW with the service door open.

• 1.5 kW motor: enough at the design point, but about 16% overloaded with the door open. The shaft power a 1.5 kW motor can carry is about 1.43 kW, which means the fan must not exceed 8,400 m³/h.
• 2.2 kW motor: covers every condition, including the open door. If there is no permanent element limiting the airflow, this is the safe choice.
• 10% speed increase via pulleys: power rises with the cube of speed (1.1³ ≈ 1.33). The 1.27 kW shaft power at the design point rises to about 1.69 kW and the motor load to about 1.78 kW; a 1.5 kW motor cannot carry this setting.

The overload relay is set to the motor's nameplate current; for example 3.3 A at 400 V for DRG-90L-4. Raising the setting because the relay trips does not solve the problem; it heats the winding. The real cure is to limit the airflow: an adjustable damper, a fixed throttling plate or a speed limit on the inverter. At commissioning, run the fan with the door closed, filters fitted and ducts connected; do not run an "open door test" without measuring the current. For overload and fuse selection you can use the Overload Relay, Fuse and Contactor Selection tool.

For a sense of the market range: one manufacturer catalogue lists its double-inlet forward-curved series up to 2,200 Pa total pressure and 40 kW shaft power, and its single-inlet series up to 2,400 Pa and 30 kW [3]. Another manufacturer document writes that this wheel can be used up to 6 inches water gauge (about 1,500 Pa), while most applications stay at 3 inches (about 750 Pa) or below [1]. The power of your own fan is calculated not from these limits but from airflow, pressure, wheel diameter, speed, air temperature and drive losses. For a quick calculation there is the Pump & Fan Motor Power Calculator.

Speed and poles: pulley ratio or 6-pole direct drive?

Forward-curved fans run at low speed; the example fan's 950 rpm is typical. There are two ways to reach this speed:

• 4 poles + belt: DRG-100L1-4 runs at 1450 rpm; for a fan speed of 950 rpm the pulley ratio is about 1.53. Fine-tuning the speed by changing the ratio is easy, but remember that every increase has a cubic effect on power.
• 6-pole direct drive: DRG-100L-6 runs at 949 rpm, DRG-112M-6 at 955 rpm. If the wheel sits on the motor shaft, belt losses and belt maintenance disappear; but speed can then only be adjusted with an inverter.

With direct drive the wheel hub must match the shaft diameter: the shaft is 28 mm on the 100L frame and 24 mm on the 90L frame. A 2-pole motor is not fitted directly to a forward-curved wheel unless the fan manufacturer allows it; the light wheel is not designed for the higher tip speed. For scroll-housed fans we explained pole and pulley selection with examples in our centrifugal fan motor article.

Motor mounting depends on the fan arrangement. On belt-driven fans the motor is usually B3 foot-mounted on a belt-tensioning slide rail; direct drive may need a flange (B5 or B14) or foot + flange (B35). The exact type is taken from the fan manufacturer's drawing.

Checklist before ordering and DRG matching

• Fan label or fan curve: airflow, pressure, fan speed, shaft power.
• Nameplate of the old motor: kW, rpm, frame, mounting type, shaft diameter.
• Drive: belt (both pulley diameters) or direct coupling.
• The system's lowest-resistance condition: is the filter removed, is there a service door, where is the damper?
• Overload setting and measured running current (in at least two conditions).
• Environment: humidity inside the AHU, kitchen grease, outdoor installation.
• Is the fan motor integrated (external-rotor/EC)? If so, the fan set is replaced.

The DRG range covers 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 is specified with an order note. The small and medium powers where forward-curved fans usually fall are inside this range; do not treat every forward-curved fan power on the market as a DRG product, find the value from your own fan's data. For dimensions see the technical drawings page. For air handling and HVAC applications our HVAC fan and pump motors article also helps. 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 forward-curved fans

• DRG-90L-6 – 1.1 kW – 945 rpm – 90L aluminium
• DRG-90L-4 – 1.5 kW – 1445 rpm – 90L aluminium
• DRG-100L-6 – 1.5 kW – 949 rpm – 100L aluminium
• DRG-100L1-4 – 2.2 kW – 1450 rpm – 100L aluminium
• DRG-112M-6 – 2.2 kW – 955 rpm – 112M cast iron
• DRG-100L2-4 – 3 kW – 1450 rpm – 100L cast iron

Click to see current prices and order these motors nowSEE PRICES →

Forward-curved fan motor: FAQ

Why does the fan motor trip after the filter is changed?

On a forward-curved fan a clean filter lowers system resistance, airflow rises and shaft power increases. In the example fan the shaft power is 1.14 kW with a dirty filter, rises to 1.27 kW at the design point and reaches 1.65 kW with the door open. If the motor was not sized for this rise, the overload trips.

Does testing a forward-curved fan without ducts harm the motor?

Yes, it can. Running without ducts or with the door open moves the fan towards free delivery, where this wheel reaches its highest power. Test with the system connected, or start with the damper throttled and measure the current.

What kW motor does a forward-curved fan need?

There is no fixed value. Find the shaft power at the highest airflow the fan can reach, add drive losses and go up to the next standard power. In the example fan this means a 2.2 kW motor instead of 1.5 kW.

What is the difference between forward-curved and backward-curved fans?

On a forward-curved fan power rises steadily with airflow; on a backward-curved fan the power curve peaks and then falls. The forward-curved wheel is smaller and quieter but its motor is more sensitive to system resistance; the backward-curved wheel reaches higher pressure and speed.

Send us your fan label and the old motor's nameplate, 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, see Buy Electric Motors Online.

Sources

1. Aerovent – Fan Performance Characteristics of Centrifugal Fans (FE-2400) – rising power curve of the forward-curved wheel, pressure limits and selection range
2. Greenheck – Product Application Guide: Fan Applications (FA/126-08) – forward-curved fan power rising from shut-off to free delivery
3. Comefri – Global Product List – pressure and shaft power limits of double- and single-inlet forward-curved series