Mixed-Flow Fan and Electric Motor Selection

The motor of a mixed-flow fan is a standard three-phase induction motor that carries the impeller directly on its shaft, and its power is calculated from the fan's flow and static pressure; the motor usually sits inside the fan casing, in the middle of the airstream. For example, a direct-drive mixed-flow fan moving 10,800 m³/h at 450 Pa static pressure needs about 2.1 kW of shaft power if the fan efficiency is assumed to be 0.65; with a 15% margin this becomes 2.4 kW, and a 3 kW 4-pole motor is chosen. Only in small-diameter duct models (up to roughly 400 mm) is the motor built into the impeller; in that case the fan is replaced, not the motor.

This article explains why the mixed-flow fan is treated separately from axial and centrifugal fans, how the motor works in this fan, which data you need for selection and which motors in the DRG range fit these fans. You can see suitable motors and current prices among the 120 models on the Buy Electric Motors Online page. Overview: Fan Motors.

What is a mixed-flow fan and where is it used?

Mixed flow is the name of an operating principle; it is not a housing name such as wall, duct or roof type, nor a purpose such as smoke extraction.

The impeller hub is cone-shaped and the blades are set at an angle on that cone. Air enters the impeller along the axis; as it passes between the blades it is pushed forward and also thrown somewhat outward, so it leaves the impeller at an oblique (diagonal) angle to the axis. That is why it is also called a diagonal fan or hybrid fan. Fixed straightening vanes behind the impeller reduce the swirl and turn the air back onto the duct axis.

Its place between axial and centrifugal

An axial fan moves a lot of air for its diameter but at low pressure. A centrifugal fan produces high pressure, but the air turns 90 degrees, the housing grows and the duct connection becomes awkward. The mixed-flow fan sits between the two: air enters and leaves in a straight line, so it is installed in series in the duct like an axial fan, yet it delivers higher pressure than an axial fan of the same diameter. For the same pressure it fits into a narrower space than a centrifugal fan. Because it turns the flow smoothly, it is generally quieter than an axial fan doing the same job.

These features make the mixed-flow fan stand out in the following applications:

• Ducted extract and fresh air lines in enclosed car parks (normal ventilation mode).
• Central bathroom and toilet extract and shaft extraction lines in hotels, hospitals and housing.
• Kitchen and hood extract, as an in-duct fan after the filter.

Where does the motor sit in this fan and how does it work?

Three drive arrangements are found in mixed-flow fans, and the arrangement determines motor selection.

• Integrated external rotor motor (small duct fans): In one manufacturer's catalogue, an entire series of mixed-flow circular duct fans is supplied with external rotor motors matched to the impeller [3]. The rotating part of the motor is inside the impeller hub; there is no separate motor with a shaft end and flange. On failure, the same fan or a new fan with the same flow and pressure is bought. DRG does not sell external rotor or integrated EC fan motors.
• Direct drive (medium and large fans): The impeller is mounted on the shaft of a standard motor fixed to a support in the middle of the casing. The motor is in the airstream. Fan speed is motor speed; there are no pulleys and no belt losses. One manufacturer's catalogue states that direct-drive mixed-flow fans can be supplied with any commercially available cast iron motor appropriate for the fan size [2].
• Belt drive: The motor sits outside the casing or on a base attached to it and turns the fan shaft by belt. Fan speed is set by the pulleys and the motor is protected from the airstream; this arrangement is preferred for hot or greasy exhaust.

Running inside the casing, in the air, has two consequences for the motor. The good side is that air flows over the motor frame all the time. The risky side is that the air being handled also touches the motor: oil mist in kitchen exhaust sticks to the frame and impairs cooling, and in humid exhaust condensation can carry water into the terminal box. The hotter the air, the harder it is for the motor to cool; so air temperature is a key input for motor selection.

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Data needed to select the motor, with a worked example

Selecting the fan, the fan curve, and the impeller and housing design is the job of the fan manufacturer or the design engineer. The motor supplier's job is to provide a motor that matches the shaft power, speed, mounting and ambient conditions resulting from that selection. Before asking for a motor for a mixed-flow fan, fill in this checklist:

• Operating point from the fan label or catalogue: flow (m³/h) and static pressure (Pa).
• Impeller diameter and fan speed; direct drive or belt drive.
• The old motor's nameplate: kW, rpm, IEC frame, voltage, mounting type (foot or flange).
• Shaft diameter and length, key size of the impeller hub.
• Maximum temperature and content of the air handled: oil mist, moisture, dust.
• Fan position: horizontal duct or vertical shaft (the motor shaft may point up or down).
• Starting method and panel: direct-on-line, star-delta, inverter; any day/night speed requirement.
• Whether the fan is part of a smoke extraction system (see the warning below).

Shaft power is found with P_shaft = Q·Δp / η_fan (Q in m³/s, Δp in Pa, P in W). Example: a hotel bathroom extract shaft must discharge 10,800 m³/h (3 m³/s) at 450 Pa static pressure, including duct, grille and silencer resistance. If the fan efficiency is assumed to be 0.65, shaft power is 3 × 450 / 0.65 ≈ 2,080 W. With direct drive there is no belt loss; with a 15% margin the requirement is about 2.4 kW and the standard rating is 3 kW. If the speed is reduced to 75% by an inverter at night, power falls with the cube of speed: 0.75³ ≈ 0.42, so shaft power drops to about 0.88 kW. You can run the same calculation with your own values in the Pump & Fan Motor Power tool.

Power range, speed and pole selection

No fixed kW can be assigned to a mixed-flow fan. Power is set by impeller diameter, flow, static pressure, blade angle and number, speed, system resistance, air temperature and density, and drive losses if belt-driven. To show the scale with figures from a manufacturer's catalogue: in a mixed-flow fan series built in 450–2,000 mm diameters, for 4,000–380,000 m³/h and 200–2,300 Pa, the largest permitted motor frame for direct drive is given as 80 at 450 mm, 180 at 1,000 mm and 355 at 2,000 mm [1]. These figures are not a power range; the fan's motor power is still calculated from the application data.

In a mixed-flow fan, pole selection is tied to the impeller tip speed. Tip speed is v = π·D·n/60: a 630 mm impeller reaches about 48 m/s at 1450 rpm and about 95 m/s at 2895 rpm. The impeller and housing are designed for one of these speeds. On a direct-drive fan, fitting a 2-pole motor instead of a 4-pole one doubles the speed; by the affinity laws, shaft power rises about eightfold (about 16.5 kW for the 2.1 kW fan in the example). This burns the motor and strains the impeller. In hotel and residential extract, noise limits favour 4- and 6-pole motors; 2-pole is used only on small, high-pressure fans and only if the fan manufacturer permits it. You will find the speed–power relationship in detail in our affinity laws article.

Mounting, starting and connection

Mounting the fan in a duct, shaft or ceiling and the motor's IEC mounting type are separate matters. In direct-drive mixed-flow fans the motor is fixed to the support plate inside the casing either by its feet (B3) or by its flange (B5 or B14); which one depends on the fan design. If the fan sits in a vertical shaft, the motor shaft points up or down and the motor runs vertically on its flange (similar to V1). The exact type is taken from the fan manufacturer's drawing; at DRG the mounting type is specified with an order note.

There is one point to watch when starting: one manufacturer's catalogue states that in its mixed-flow fans, motors up to 2.2 kW are started direct-on-line and motors of 3 kW and above with star-delta [1]. To use star-delta on a 400 V network, the motor must be wound for 400/690 V. In the DRG range, motors up to 3 kW are 230/400 V and 4 kW and above are 400/690 V. So if the panel of a 3 kW mixed-flow fan is built for star-delta, a 230/400 V 3 kW motor will not suit it; either the panel is changed to direct-on-line or inverter, or a 400/690 V 4 kW motor is chosen. See the star-delta starting page for details. The connection must be made by a qualified electrician.

• Check the direction of rotation against the arrow on the casing before commissioning; a mixed-flow impeller running backwards loses a large share of its flow.
• In greasy or humid exhaust, clean the motor frame periodically; a layer building up on the frame makes the motor run hot.

Mixed-flow fans used in car parks and shafts often also extract smoke in a fire. A fan and motor set that will run in smoke mode must be certified to EN 12101-3; DRG's standard motors are not sold with this certification. A DRG motor can be recommended only for fans running in normal ventilation mode. General framework: smoke extraction motors.

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; 1000, 1500 and 3000 rpm; 3-phase, 230/400 V or 400/690 V; mounting B3, B5, B14 and B35 (with an order note). Among mixed-flow fans, the match is direct-drive and belt-driven models with IEC motors; small duct fans with integrated external rotor motors are outside this range. For the 3 kW requirement in the example, the DRG-100L2-4 (1450 rpm, 6.02 A at 400 V) suits direct-on-line starting, and the DRG-112M-4 (4 kW, 400/690 V, 7.95 A at 400 V) suits a panel that keeps star-delta.

DRG MOTOR motors are guaranteed against material and manufacturing defects for 2 years from the delivery date (warranty certificate). Axial vs radial comparison: ventilation fan motor.

DRG motors for mixed-flow fans

• DRG-100L2-4 – 3 kW – 1450 rpm – 100L cast iron
• DRG-112M-4 – 4 kW – 1450 rpm – 112M 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
• DRG-160L-4 – 15 kW – 1465 rpm – 160L cast iron

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Mixed-flow fan motor: FAQ

What is the difference between a mixed-flow fan and an axial fan?

Both are connected in a straight line in the duct. The cone-hub impeller of a mixed-flow fan discharges air diagonally and produces higher pressure than an axial fan of the same diameter, which is why it is preferred for long, high-resistance ducts.

If a mixed-flow fan motor burns out, is only the motor replaced?

Not if it is an external rotor motor built into the impeller; then the fan is replaced. If the impeller sits on the shaft of a standard motor, a three-phase motor with the same kW, speed, frame and mounting type as its nameplate can be fitted.

Which motor should I buy for a car park extract fan?

If the fan only provides normal ventilation, a standard three-phase motor at the power and speed on the fan label is suitable. If it also extracts smoke, the fan and motor set must be certified to EN 12101-3; that motor is obtained from the fan manufacturer.

How many amps does a 3 kW mixed-flow fan motor draw?

The 4-pole DRG-100L2-4 draws 6.02 A at 400 V and 6.34 A at 380 V. Set the overload relay to the nameplate current.

Send us the fan label, the old motor's nameplate and the panel's starting method, 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, visit Buy Electric Motors Online.

Sources

1. Kruger – MXA Series Axial Mixed Flow Fan (catalogue) – 450–2,000 mm, 4,000–380,000 m³/h, 200–2,300 Pa; largest motor frame by diameter; direct-on-line up to 2.2 kW, star-delta from 3 kW.
2. Greenheck – Model QEID Direct Drive Mixed Flow Fans, Sound & Air Performance Supplement – use of commercially available cast iron motors in direct drive.
3. Systemair – KD Circular Duct Fans – small duct fans with mixed-flow impeller and external rotor motor.