Air Handling Unit (AHU) Fan Motor: Replacement, Inverter and Maintenance Criteria

An air handling unit (AHU) fan is the centrifugal fan in the supply or exhaust section of the unit that pulls air through the filters, coils and heat recovery elements and pushes it into the ductwork; in today’s units it is mostly a plug fan, in older ones a belt-driven scroll fan. Its motor is a standard three-phase induction motor, and what decides the selection is not a single operating point but the range the unit moves through in service: the pressure difference between clean and dirty filters, day and night airflow, and the inverter frequency range. The motor must cover the heaviest end of this range and still run properly at the lightest end.

This article looks at AHU operation from the motor’s point of view: what changes as filters load, how to select the motor when converting from a belt-driven fan to a plug fan, what to watch at low night speed, and how temperature and humidity inside the unit affect the motor. Motor replacement, terminal box connection and inverter parameters are live electrical work; they must be done by a qualified electrician. Once you have the right model, you can compare it among the 120 motors on our Buy Electric Motors Online page.

General supply topics for fan and pump motors in HVAC systems are covered in our HVAC fan and pump motors article, and the full fan motor range on our Fan Motors page.

Supply and exhaust fans in the AHU: where is the motor, and in which air?

"AHU fan" is not an operating principle but the name of an application; in principle the fan inside is centrifugal. A typical unit has two fans: the supply fan takes outdoor (or mixed) air through filters and coils and sends it to the rooms; the exhaust or return fan draws room air back and discharges part of it outdoors. For the motor, the two fans live in different conditions.

In a plug fan unit the motor sits right in the airstream and gives its own losses to the supply air as heat; this small load is already part of the design calculation. A motor working downstream of a cooling coil can see condensation inside the frame after long standstills. When replacing it, compare the IP rating and permitted ambient temperature on the old nameplate with the new motor; positioning the terminal box with its cable entry facing down prevents water from collecting.

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How does the operating point shift as filters load?

A large share of the unit’s pressure loss is in the filters, and this loss grows as the filters collect dust. Filter manufacturers’ data sheets give the recommended final pressure drop according to EN 13053 as initial pressure drop + 100 Pa or three times the initial pressure drop, whichever is lower [1]. A fan at fixed speed delivers less air as this rises; an inverter that holds airflow constant (pressure or flow control) raises the speed instead, and the motor draws more power. The motor must be sized for the dirty filter at its change point, not for the clean filter.

Example: 20,000 m³/h (5.56 m³/s) supply airflow. Ductwork and diffusers 400 Pa, coils 150 Pa, heat recovery and other components 160 Pa. Initial pressure drop of the pre-filter 50 Pa, of the bag filter 90 Pa. Assuming a fan efficiency of 0.65, P_shaft = Q·Δp / η_fan:

At the same airflow, filter loading raised shaft power by about 24%. A motor sized for 7.3 kW would be overloaded as the filters approach their change point. You can enter your own unit’s values in the Pump & Fan Motor Power calculator and calculate both conditions separately.

There is no single figure for the power range: motor power changes with unit size, airflow, total pressure, filter class, heat recovery type, impeller and speed. As a guide: in one manufacturer’s catalogue, a plug fan range offered for AHU applications goes up to 37 kW with IEC induction motors [2].

Converting from a belt-driven scroll fan to a plug fan: how to select the motor

A common job when refurbishing older units is to remove the belt-driven scroll fan and put a plug fan into the same section. Simply copying the old motor’s kW can give the wrong result, because two things change: the belt loss disappears, and fan speed now comes directly from the motor shaft instead of the pulleys. Let us use the change point from the previous example (9.0 kW shaft power) and an impeller speed of 1250 rpm given by the fan manufacturer for the new plug fan. The torque needed at this speed is about 68.6 Nm.

• Belt-driven: 9.0 / 0.95 ≈ 9.4 kW required; in the old unit this is usually done with an 11 kW 4-pole motor and pulleys.
• Plug fan with 4-pole 11 kW (DRG-160M-4): rated torque 71.7 Nm; about 43 Hz for 1250 rpm. About 96% of the torque is used, and the motor’s own cooling fan has slowed down. Very little headroom.
• Plug fan with 4-pole 15 kW (DRG-160L-4): rated torque 97.8 Nm; load about 70%. Frame 160L, shaft 42 mm.
• Plug fan with 6-pole 11 kW (DRG-160L-6): rated torque 108.3 Nm at 970 rpm; about 64 Hz for 1250 rpm. In this range usable torque drops to about 84 Nm and the load is about 82%. If the fan manufacturer allows this frequency, it is the way to stay at the same kW.

In this conversion, the fan manufacturer decides which frame and impeller the motor connects to and the base and inlet cone dimensions; the exact mounting type comes from their drawing. The old motor’s slide rail and pulleys are no longer used. Fitting the 11 kW motor left over from the belt fan to the plug fan "because the power is the same" can be a choice where torque is at its limit, as shown above.

Day and night speed: the motor at low frequency

Office, school and shopping centre units are set to low airflow at night or outside occupied hours. Since fan power falls with the cube of speed, shaft power drops to about 22% when airflow is cut to 60%: about 1.6 kW instead of 7.3 kW in our clean-filter example. The details of this relationship are in our affinity laws article. For the motor, low speed is usually not a problem, because the load falls quickly with speed and the slower cooling fan does not cause overheating. Points to watch:

• The inverter minimum frequency must not go below the value set by the fan manufacturer and the building controls.
• Any impeller resonance speeds should be entered as skip frequencies in the inverter.
• At low speed the supply/exhaust airflow balance must be kept, otherwise building pressure is upset.
• If the motor stands still for long periods (unit off at weekends), short periodic runs help against condensation.

Checklist for replacing an AHU fan motor

Fan selection, the fan curve, the impeller and the unit design are the job of the fan or AHU manufacturer or the design engineer. The motor supplier’s job is to provide a motor suited to the given shaft power, speed, mounting and ambient conditions. For a replacement, collect the following:

• Supply and exhaust airflow and external static pressure from the unit nameplate.
• Filter classes, clean and change pressure drops (from the differential pressure gauge).
• Fan type: belt-driven scroll, IEC-motor plug or EC integrated (if EC, the motor is not bought separately).
• Old motor nameplate: kW, rpm, frame, voltage, current, IP rating.
• Shaft diameter, mounting type (B3, B5, B14, B35) and base dimensions.
• Inverter frequency range and control mode (constant flow, constant pressure, constant speed).
• Motor position: after the cooling coil, on the exhaust side, or out of the airstream.
• Night or weekend operating schedule.

Matching with DRG motors

DRG Motor is a manufacturer, supplier and seller of electric motors. Our range covers 0.25–11 kW in aluminium frames and 0.55–355 kW in cast iron frames; 1000, 1500 and 3000 rpm; 3-phase, 230/400 V or 400/690 V. B3, B5, B14 and B35 mounting is specified with an order note. This range is for AHU fans with IEC motors; fans with EC integrated motors are outside the DRG range. The models in the price table above were chosen around our worked example and up to the 37 kW limit mentioned in the source [2]; they include the 4- and 6-pole options from the example. For dimension checks, use our technical drawings page. DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate).

DRG motors for AHU fans

• DRG-160M-4 – 11 kW – 1465 rpm – 160M cast iron
• DRG-160L-6 – 11 kW – 970 rpm – 160L cast iron
• DRG-160L-4 – 15 kW – 1465 rpm – 160L cast iron
• DRG-180L-6 – 15 kW – 978 rpm – 180L cast iron

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AHU fan motor: frequently asked questions

Why does an AHU fan motor trip on overload?

A common cause is filters past their change point, with constant-flow control raising the speed. Check the differential pressure gauge, motor current and inverter frequency together; a motor sized for clean filters is overloaded with dirty ones.

How many kW is an AHU fan?

There is no fixed value; it is calculated from airflow, total pressure and fan efficiency. In our example, 20,000 m³/h gave about 7.3 kW shaft power with clean filters and about 9.0 kW at the change point.

Can the same motor be kept when converting from a belt fan to a plug fan?

Not always. The belt loss disappears, but speed now comes directly from the motor, so torque at impeller speed must be recalculated. In our example the same 11 kW was at its limit as a 4-pole motor; a 6-pole 11 kW or a 4-pole 15 kW was more suitable.

Does an AHU fan motor overheat at low night speed?

With a fan load, usually not: power falls with the cube of speed, so motor heating falls too. Do not set the minimum frequency below the fan manufacturer’s value, and watch for condensation during long standstills.

Send the AHU nameplate and a photo of the old motor’s nameplate via WhatsApp to +90 (542) 666 11 11 or to [email protected]; other channels are on our contact page. All models and current prices are on the Buy Electric Motors Online page.

Sources

1. Camfil – EcoPleat G data sheet (PDF) – recommended final pressure drop acc. to EN 13053
2. Nicotra Gebhardt – RLM Evo plug fan – AHU plug fan, IEC induction motors up to 37 kW