Humidity and Mounting Conditions for Cooling Tower Fan Electric Motors

A cooling tower is a heat exchanger that cools process water or air-conditioning condenser water by bringing it into contact with air. The fan on top of or beside the tower continuously discharges warm, humid air. This is why the electric motor that turns the tower fan works in one of the most humid operating environments in industry: splashing water droplets, condensation, open air, sun and frost can all occur over the life of the same motor. In this article we explain clearly what to watch out for regarding humidity and mounting when choosing and replacing a cooling tower fan motor, and what a standard motor does and does not cover.

Tower fan drive arrangements: where does the motor stand?

On cooling towers the connection between fan and motor is made in several different ways. The amount of moisture the motor will face depends largely on this arrangement.

• Gearbox drive (with drive shaft): On large towers the fan is connected to a slow-running right-angle gearbox. The motor usually stands outside the fan stack and transmits power to the gearbox through a long drive shaft. The purpose of this arrangement is to keep the motor away from the humid air flow.
• Belt and pulley drive: On medium-sized towers the motor is connected to the fan shaft by belt. The motor is placed on a platform beside the tower casing or close to the air inlet area; depending on its position, it may be directly exposed to splashing water.
• Direct-coupled drive: On small and packaged towers the fan propeller is fitted directly onto the motor shaft. The motor usually runs in a vertical position, just below or above the fan and inside the humid air flow.

If the motor is inside the air flow, it is continuously cooled by humid air and the risk of condensation is at its highest. If it is outside the flow, it faces only outdoor conditions. Before replacement, note which of these three arrangements the motor is in.

What does IP55 cover, and what does it not?

All of our models on sale are IP55-rated. Knowing exactly what this rating means prevents false expectations on a cooling tower.

• First digit (5): Protected against dust. Ingress of dust is not completely prevented, but it does not enter in amounts that would impair the motor's operation.
• Second digit (5): Protected against low-pressure water jets from any direction. Rain, splashing droplets and briefly hosing with water fall within this scope.

The situations IP55 does not cover are also clear:

• The motor running continuously inside the water curtain, the water escaping from the drift eliminators or dense water vapour.
• Condensation water accumulating inside the frame. The IP rating defines protection against water from outside; it does not prevent the air inside the motor from cooling and condensing.
• Immersion of the motor in water or high-pressure washing.
• The effect of chemically treated water vapour, salty environments or aggressive atmospheres on paint and metal surfaces.

For this reason, IP55 is an adequate starting point for many positions on a tower; but if the motor is directly inside the humid air flow or exposed to constant splashing, it may not be enough on its own. Additional needs such as a condensation drain hole, anti-condensation heater, extra surface protection or a higher protection rating are stated in the order and their suitability is assessed together. These should not be assumed to be part of the standard model.

Condensation: the silent risk for tower motors

Tower fans often run intermittently: when the water temperature drops the fan stops, and when it rises it starts again. The motor heats up while running and cools down when it stops. The humid air inside the cooling frame condenses, and water droplets settle on the winding, bearings and terminal box. This cycle can reduce insulation resistance within a few weeks.

Practices that reduce condensation

• Plan the motor position so that condensation water can collect at the lowest point. In vertical mounting, which end shield the drain will be in should be discussed in the order.
• After long standstills, especially after the winter period, measure the winding insulation resistance before the first start.
• Check the terminal box cover gasket and the cable gland at every service; position the cable entry facing downwards so that water does not run along the cable into the box.
• If the fan will be stopped for long periods in winter, assess whether a measure to keep the motor dry is needed and state this need in the order.

Mounting arrangement: horizontal and vertical layouts

On gearbox and belt-driven towers the motor is usually horizontal and foot-mounted (B3); in the belt arrangement it sits on a slide rail, and in the gearbox arrangement it is connected to the drive shaft by a coupling. On direct-coupled packaged towers, however, the motor runs in a vertical position: if the propeller is above the motor the shaft faces up, if below, the shaft faces down.

Mounting arrangements are offered according to the frame: B3, B5, B14, B34, B35 and vertical V1/V3/V5/V6/V15 options are available; the flange size is confirmed with the order. Two points are particularly important in vertical operation:

• Axial load: The weight of the propeller and the thrust it produces place an axial load on the motor bearings. The propeller weight and the direction of fan thrust should be stated in the order.
• Water in shaft-up arrangements: On a vertical motor with the shaft facing up, rain and drip water can collect around the shaft towards the front end shield. In this arrangement, top protection and shaft sealing are discussed separately.

For dimensions and flange options, you can refer to the technical drawings and the question on mounting arrangements.

Speed, starting and the frequency inverter

Tower fans are large-diameter, slow-turning propellers. In the gearbox arrangement the motor speed is selected according to the gearbox ratio, so the number of poles must be kept the same as the old motor; otherwise the tip speed of the propeller changes and the fan balance may be upset. The same rule applies to direct-coupled packaged towers.

Large propellers have high inertia and starting takes a long time. In addition, in windy weather a stationary propeller may begin to turn in the reverse direction; if the motor is energised in this situation, it must first stop the propeller and then accelerate it, making the start even heavier. This is taken into account when choosing the starting method.

Many towers use a drive to adjust fan speed according to water temperature. 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. On towers the drive is usually in the panel room and the cable to the motor may be long; this information must be shared in the order. For details, see the question on inverter operation.

Example models: for gearbox and belt tower drives

The models below are examples of powers and speeds that may be found on gearbox or belt-driven tower fans. The fan power of your tower is determined by the tower manufacturer's data and the existing motor nameplate.

At the same 15 kW power, the 6-pole 180L-6 and the 4-pole 160L-4 are in different frames; both the number of poles and the frame code of the replacement motor should be compared with the old motor. The cast iron frame is the preferred construction on outdoor and vibrating tower platforms. All three models are IP55 with class F insulation. The current price is on the product pages.

Hypothetical example: renewing the motor on a belt-driven tower

The following sequence is hypothetical; it is not a real customer application. Suppose that on a belt-driven cooling tower on a factory roof, the motor's insulation resistance is measured low every spring, and it is finally decided to renew the motor. The nameplate reads 15 kW and 1,465 rpm.

• Keep poles and frame: 15 kW and 4 poles point to the model with code 160L-4; if the old frame code is also 160L, the slide rail and pulley alignment are retained.
• Question the position: If the motor is right in front of the air inlet louvre, it is directly exposed to splashing water. If possible, moving the motor a little further away or protecting it from above and from the side is planned.
• Report condensation: Because the fan stands idle for long periods in winter, the need for a condensation drain and for keeping the motor dry at standstill is stated in the order.
• Check the cable: The orientation of the terminal box is discussed so that the cable entry faces downwards, and the gland and gasket are renewed.
• Note the drive: If the fan runs on a drive, the cable length between drive and motor is shared in the order.

Conclusion and what to send before ordering

With a cooling tower fan motor, the main issue is the operating environment rather than the power. IP55 provides a good basis against splashing and rain; but if the motor is continuously inside the humid air flow, goes through condensation cycles, or runs vertically with the shaft up, additional measures are needed and it is essential that they are stated in the order. To determine the right model together, send us the following information:

• Power, speed and a nameplate photo of the existing motor
• Drive arrangement (gearbox, belt, direct-coupled) and the motor's position relative to the air flow
• Mounting arrangement (horizontal foot-mounted, flange, vertical shaft up or shaft down)
• Network voltage, starting method and, if there is a drive, the cable length
• Environment: splashing, condensation, winter standstills, chemically treated water or salty air

Review suitable models and current prices on the buy electric motors page. For sector-specific information, see the HVAC sector motors, fan and extractor motors, ventilation motors and, for pumps in the tower circuit, the pump motors pages. To ask for the right model with your motor's nameplate data, send us a nameplate photo or contact us.