Wall-Mounted Axial Fan Motor: Replacing the Motor of an Extractor or Ventilator

When the motor of a wall-mounted axial fan burns out, it can be replaced by a standard three-phase motor of the same power, speed, shaft size and mounting, provided the old one is a separate IEC motor fixed to the fan’s support arms by feet or flange, with the impeller on its shaft end. If the motor is buried inside the impeller hub and its outer shell turns with the impeller (an external-rotor motor), it cannot be replaced on its own; the fan itself has to be changed. For a wall fan with a separate motor, the kW, rpm and shaft diameter on the nameplate are enough; you will find suitable models among the 120 listed on our Buy Electric Motors Online page.

In a wall fan the motor faces the open air or a dusty indoor space and works in the middle of the airflow; failures are usually linked to moisture, dust build-up or overcurrent caused by a wrong blade setting. That is why you need to understand why the old motor failed before fitting the new one. Removal, wiring and the direction-of-rotation check must be carried out by a qualified electrician; if the fan is high up on a wall, safe working at height must also be considered.

What is a wall-mounted axial fan?

A wall-mounted axial fan sits on a wall plate or on a ring surrounding the impeller and blows straight out or in without being connected to a duct. “Axial” describes the operating principle: air enters and leaves parallel to the axis. “Wall-mounted” belongs to the casing and installation group; it tells you where the fan is fitted. On site, the same fan is called an extractor when it blows air out and a ventilator when it pushes air in; mechanically, the only difference is the direction of flow.

With no ductwork, the fan works against very little resistance: the plate, the guard, a shutter if fitted, and the building’s own air inlets. High airflow at low pressure is this fan’s defining feature. Warehouses, livestock buildings, greenhouses, workshops, production halls and general exhaust points are typical places of use.

How is the motor attached to the fan, and where does it sit in the airflow?

In industrial wall fans, the motor is fixed to a platform at the centre of three or four support arms welded or bolted to the plate. Depending on the manufacturer, it is attached by its feet (B3) or flange (B5 or B14); which one is used can be seen on the fan drawing or on the removed motor. The impeller is fitted directly to the motor shaft with a key and retaining bolt; there is no belt or coupling. One manufacturer’s catalogue clearly states that its 300–1250 mm plate and ring axial fans use IEC induction motors with the impeller coupled directly to the motor shaft [2].

The motor is in the airflow; for this reason many manufacturers limit the temperature of the air. One manufacturer’s data sheet for a wall and duct axial series states that the motor sits directly in the airflow and that the air temperature is limited to 40 °C [1]. Air can flow from motor to impeller or from impeller to motor; this direction must stay the same when ordering and replacing, because the motor’s cooling and the direction of the axial force on the bearings depend on it.

The split of responsibility applies here too: the fan’s airflow, impeller, blade pitch and plate design are the job of the fan manufacturer or the project engineer. The motor supplier provides the motor that matches the shaft power, speed, shaft size and mounting the fan requires.

Can a standard IEC motor replace the burnt-out motor?

The answer depends on how the motor is built into the fan. The table below summarises the most common situations on site.

In an external-rotor motor the stator is fixed in the centre and the rotor turns outside it as one piece with the impeller hub. These motors have no separate shaft end; the impeller is the motor housing. The EC axial fans in one manufacturer’s catalogue are built this way and are sold as complete fans with a wall plate [3]. DRG does not sell external-rotor or EC motors; for these fans the honest answer is to replace the whole fan. If you are selecting a new fan, choosing a wall fan with a separate IEC motor means that next time only the motor will need replacing.

Small single-phase wall fans are also mostly integrated-motor units and fall outside DRG’s three-phase range, which starts at 0.25 kW. With separate IEC motors another detail matters: some motors built for wall fans have no cooling fan of their own and are cooled by the fan’s air, whereas a standard motor has a fan and cowl at the rear and may be slightly longer. Measure that the new motor’s overall length (L) clears the guard and the wall before fitting it.

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Motor replacement checklist

• Nameplate: kW, rpm, voltage (230/400 V or 400/690 V), current and IEC frame code.
• Shaft diameter and length (D × E), key width: the impeller hub is machined to this size.
• Mounting: fixed to the platform by feet (B3) or flange (B5 / B14); hole spacing.
• Airflow direction: from motor to impeller, or from impeller to motor?
• Overall length (L): is there clearance to the guard and the wall?
• Cable entry and terminal box orientation: rainwater must not run into the box.
• Blade pitch: mark or measure it before removing the impeller.
• Cause of failure: water ingress, bearing noise, dust build-up, frequent overload trips.

Blade pitch, current and motor power

On most wall fans the blade pitch is adjustable; one manufacturer’s catalogue states that it is set with the fan at standstill [2]. As the angle increases, each blade takes more air per revolution: airflow and pressure rise, and so does shaft power. The same motor may exceed the nameplate current at a steep angle. If the blades were removed during maintenance and refitted without marks, this may be why the motor overheated and burnt out.

Pitch is not the only factor: impeller diameter, speed, airflow, system resistance (guard, shutter, building air inlets) and air density also change the power demand. The calculation uses P_shaft = Q × Δp / η_fan (Q in m³/s, Δp in Pa, P in W).

Example: take a wall fan exhausting 12,600 m³/h from a barn against 90 Pa of resistance including the shutter and guard. 12,600 m³/h = 3.5 m³/s. Since an unducted wall fan has low efficiency, assuming a fan efficiency of 0.45 gives P_shaft = 3.5 × 90 / 0.45 = 700 W, i.e. 0.7 kW. Adding a 25% margin for a dusty shutter and small deviations in blade setting gives ≈ 0.88 kW; the next standard rating is 1.1 kW. If the fan is designed for 6 poles, the matching model is DRG-90L-6: 1.1 kW, 945 rpm, 2.69 A at 400 V, shaft 24 × 50 mm. For your own data, use the Pump & Fan Motor Power calculator.

The simplest check after fitting the motor is a current measurement: with the fan running normally, the current measured with a clamp meter must not exceed the nameplate value (2.69 A in the example). If it does, the blade pitch is brought back to the fan manufacturer’s value; the overload relay is set to the nameplate current (overload, fuse and contactor selection).

For scale on the market: in one manufacturer’s data sheet, the three-phase IEC motor table of a wall and duct axial series runs from 0.09 kW to 4 kW [1]. DRG’s range should be stated separately: 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; B3, B5, B14 and B35 mounting with an order note. Very small wall fans below 0.25 kW are outside this range. The table below shows models selected from the 4- and 6-pole power range common in wall fans.

Speed selection and outdoor conditions

On a wall fan, speed is set by the fan manufacturer according to impeller diameter and noise limits. Large wall fans commonly use 6 poles (about 950 rpm) or 4 poles (about 1450 rpm); near residential areas, lower speed is preferred for noise. When replacing, use the same number of poles as the old motor: fitting a 4-pole motor to a 6-pole fan raises speed by about 1.5 times and shaft power, which follows the cube of speed, by about 3.5 times, overloading both motor and impeller.

On a wall fan facing outdoors, the motor’s enemies are rain, dust and birds. Practical measures:

• Use a gravity or motorised shutter on the outside face; when the fan stops, rain and wind do not reach the motor.
• Position the terminal box and cable gland facing down or sideways.
• If the motor has condensation drain holes, make sure they end up at the lowest point.
• Fit guards on both faces; nests and leaves unbalance the impeller.
• In barns and dusty workshops, clean deposits off the motor housing regularly; the housing cannot shed heat through a layer of dust.

Check the motor’s protection rating and ambient limits on the nameplate and technical data; compare shaft and flange dimensions on our technical drawings page. For power calculation examples that include wall fans, see our ventilation fan motor article; for starting and pulley topics on industrial extractors, our fan and extractor motor selection article; and for all applications, the Fan Motors page.

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 wall-mounted axial fans

• DRG-801-6 – 0.37 kW – 890 rpm – 80 aluminium
• DRG-90S-6 – 0.75 kW – 935 rpm – 90S aluminium
• DRG-90L-6 – 1.1 kW – 945 rpm – 90L aluminium
• DRG-90L-4 – 1.5 kW – 1445 rpm – 90L aluminium
• DRG-100L1-4 – 2.2 kW – 1450 rpm – 100L aluminium
• DRG-112M-6 – 2.2 kW – 955 rpm – 112M aluminium

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Wall-mounted axial fan motor: FAQ

My wall extractor motor burnt out — can I replace just the motor?

Yes, if the motor is fixed to the support arms by feet or flange and the impeller sits on the shaft end; a three-phase motor of the same kW, speed and shaft size is fitted. If the motor is inside the impeller hub (external rotor), the motor alone cannot be replaced; the whole fan is changed.

Why does my wall fan motor keep tripping the overload?

The most common causes are an increased blade pitch, water in the motor, a bearing failure and overheating from dust on the housing. First measure the running current and compare it with the nameplate, then return the blade pitch to the fan manufacturer’s value.

Is an extractor motor different from a ventilator motor?

No. The same axial fan is an extractor when it blows air out and a ventilator when it pushes air in. The motor is the same; the only difference is the flow direction and, with it, the way the impeller is fitted.

Can a 6-pole motor be used instead of a 4-pole one on a wall fan?

If the fan was designed for 4 poles, a 6-pole motor lowers the speed and the airflow drops markedly. The number of poles should follow the fan’s design speed and stay the same as the old motor.

Send us the old motor’s nameplate, the shaft size and a photo of the fan; together we will establish whether the motor can be replaced and which DRG model fits. Phone +90 (542) 666 11 11, e-mail [email protected], contact page. For all models and current prices, visit Buy Electric Motors Online.

Sources

1. Åkerstedts – Axial fan LPMA (data sheet) – wall and duct type; 3-phase IEC motor table 0.09–4 kW, motor in the airflow, air up to 40 °C.
2. Dynair – Plate mounted and ring axial fans AC-A / AC-B – 300–1250 mm, IEC induction motor, impeller directly on the motor shaft, pitch set at standstill.
3. Hidria – EC Axial fans technical catalogue (2020) – axial fans with external-rotor EC motors and wall plates.