What to Look For When Buying an Electric Motor for a Scroll Fan

A scroll fan is a radial (centrifugal) fan named after the spiral shape of its housing. It draws air in along the axis, throws it outwards and converts it into pressure thanks to the spiral casing. It is widely used in ventilation, extraction, drying, combustion air and process lines. When buying an electric motor for these fans, power and speed are of course the first questions; but the type of connection, direction of rotation, mounting position and the environment in which the fan operates are just as decisive. This article lists what to check before ordering a scroll fan motor, in the form of a buyer's checklist.

Get to know the fan first: where is the motor on the fan?

Scroll fans are combined with the motor in three basic arrangements. Without knowing which arrangement you have, it is not possible to choose the right motor.

• Direct-coupled (impeller on the motor shaft): The impeller hub is fitted directly onto the motor shaft. This is common on small and medium-sized fans. The motor carries the weight and unbalance of the impeller directly in its own bearings. The motor is usually held by a flange fixed to a bracket or to the casing.
• Coupled: The fan has its own shaft in bearings; the motor sits on a base by its feet and is connected by a flexible coupling. The impeller load stays in the fan bearings.
• Belt and pulley: The motor is usually placed on a slide rail beside or below the fan casing. Fan speed comes from the motor speed and the pulley ratio; this provides adjustment flexibility but places a side load on the shaft.

For a motor bought for a direct-coupled fan, the shaft end, flange and bearing load become critical. On coupled and belt-driven fans, the foot connection and the shaft end dimension come to the fore.

Speed: why is the number of poles not changed on a scroll fan?

The pressure and flow a scroll fan produces are determined by the impeller diameter and the speed. The fan manufacturer designs the impeller for a certain speed; this is usually the speed of a 2-pole (3,000 rpm synchronous) or 4-pole (1,500 rpm synchronous) motor. The value read on the nameplate is the rated speed: for example 2,895 or 2,905 rpm on a 2-pole motor, and 1,450 or 1,460 rpm on a 4-pole motor.

On a direct-coupled fan, changing the number of poles changes the entire characteristic of the fan. If the speed halves, the pressure falls to a quarter; if the speed doubles, the fan power rises to around eight times and the impeller may exceed its strength limit. For this reason, keeping the number of poles shown on the nameplate is the basic rule when ordering a replacement motor. If the speed needs to be changed, this is done with the fan manufacturer's approval and usually through the pulley ratio or a frequency inverter.

Speed adjustment with a frequency inverter

Our three-phase series are suitable for speed control with a frequency inverter; for long periods of full-torque operation at low speed an external fan may be required, and if the cable between drive and motor is long a dv/dt or sine filter is considered; these conditions are stated with the order. Since torque on a fan load decreases as speed falls, the risk of overheating at low speed is generally limited; however, if operation above 50 Hz is planned, the suitability of the motor and the impeller must be assessed separately. Details are on the inverter operation page.

Direction of rotation and casing position

On scroll fans, the direction of rotation of the impeller must match the opening direction of the spiral casing. A radial fan rotating in reverse still moves some air; this is why the error may not be noticed at first glance. However, flow and pressure drop noticeably, the current differs from what is expected and the system runs inefficiently.

• Look at the direction-of-rotation arrow on the fan casing; if there is no arrow, determine the direction from the opening direction of the casing.
• On three-phase motors, the direction of rotation is reversed by swapping two phases. Before the motor is connected to the impeller, the direction is checked with a short run.
• The cooling fan of standard motors is usually designed to work in both directions. Even so, stating the required direction of rotation in the order allows a possible unidirectional cooling arrangement, especially at higher powers, to be discussed in advance.

The casing position (outlet facing up, sideways or down) does not directly change the motor selection, but it affects the position of the motor and access to the terminal box. Which side the terminal box should be on should be considered in advance so that cable entry can be made easily.

Mounting arrangement: foot-mounted, flange, vertical

The most common arrangements on scroll fans are B3 (foot-mounted), B5 or B14 (flange) and B35 (foot and flange). On roof-mounted or vertical-axis fans the motor may run in a vertical position (V1 shaft down, V3 shaft up). In vertical operation the bearings also carry the weight of the impeller in the axial direction; for this reason the mounting position must be clearly stated in the order.

B3, B5, B14, B34, B35 and vertical V1/V3/V5/V6/V15 arrangements are offered according to the frame; the flange size is confirmed with the order. A flange being called "B5" does not mean it matches the bolt circle on your fan bracket. For dimensions, see the technical drawings; the arrangements available are summarised in the question on mounting arrangements.

Environment: hot air, humidity and dust

The air carried by a scroll fan may not be the same as the environment in which the motor runs. Two situations need to be distinguished here.

• Motor outside the air stream: On coupled and belt-driven fans the motor usually runs outside the conveyed air flow, with its own cooling. Even if the conveyed air is hot, the motor should remain outside the hot zone and, if necessary, be protected by a heat shield.
• Motor close to the air stream: On direct-coupled fans the motor is adjacent to the fan casing; if the casing is hot, heat passes to the motor through the flange and shaft. On fans carrying hot gas this arrangement must be assessed separately, and a layout in which the standard motor would run inside the hot gas must be avoided.

Our models on sale are IP55-rated with class F insulation. For a fan outdoors and directly exposed to rain, overhead protection is planned. If the dust load is high, regular cleaning of the cooling fins is necessary. In closed, clean environments the aluminium frame makes installation easier thanks to its light weight; in vibrating and harsh environments a cast iron frame can be considered.

Example models: in typical scroll fan power ratings

The models below are examples of powers and speeds commonly found on ventilation and extraction type scroll fans. The power requirement of your fan is determined by the fan manufacturer's data and the existing motor nameplate; this list is an example, not a recommendation.

Note the voltage column: the 100L-2 model is rated 230/400 V (Δ/Y) and is connected in star (Y) on a 400 V network; the 112M-2 and 132S-4 models are rated 400/690 V (Δ/Y) and are connected in delta (Δ) on a 400 V network. This difference matters in panels where star-delta starting is planned: star-delta starting is done with a 400/690 V rated motor suitable for delta operation on a 400 V network. The current price is on each model's product page; other powers are in the aluminium frame models list.

Hypothetical example: renewing the motor of a roof extractor

The following sequence is hypothetical; it is not a real customer application. Suppose that the motor of a scroll extractor on a plant's roof has started to make bearing noise over the years and is to be renewed. The nameplate shows 4 kW and a speed of around 2,900; the motor is flange-mounted on the fan bracket and the impeller sits directly on the shaft.

• Keep the number of poles: 4 kW and 2 poles point to the model with code 112M-2.
• Verify the connection: The flange bolt circle on the bracket and the shaft diameter are compared with the technical drawing; it is confirmed that the impeller hub will fit the new shaft.
• Check the direction: At first energisation the motor is run briefly without the impeller, and the direction is compared with the casing arrow.
• State the environment: Because the position is on the roof and exposed to rain, overhead protection is planned and this is noted in the order.
• Confirm the supply: If the panel uses star-delta starting, it is checked that the 400/690 V rated model is suitable for delta operation at 400 V.

Conclusion and information before ordering

The right motor for a scroll fan is chosen when the fan's connection arrangement, number of poles, direction of rotation, mounting position and environment are assessed together. Keeping the power and speed from the nameplate is the right starting point in most cases; flange and shaft dimensions are confirmed separately with the order. Send us the following information:

• Power, speed and a nameplate photo of the existing motor
• Connection arrangement of the fan (direct-coupled, coupling, belt)
• Mounting arrangement (B3, B5, B14, B35 or vertical) and, if flanged, the bracket dimension
• Network voltage and starting method (direct-on-line, star-delta, drive)
• Application and environment (outdoor, hot air, dust)

Review suitable models and current prices on the buy electric motors page. Information specific to fan applications is on the fan and extractor motors page. To ask for the right model with your motor's nameplate data, send us a nameplate photo or contact us.