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Oriental Motor
Cooling Fans Overview

Cooling Fans Overview

Structure of Cooling Fans

The following explains the structure of axial flow fans, centrifugal blowers and cross flow fans as well as how these fans blow air.

● Axial Flow Fans

The propellers (fan blades) located in the circular flow path between the cylindrical hub and casing are used to force-feed air in order to generate air flow in the direction of the axis of rotation.

Since air flows along the axis of rotation, the structure is kept compact. Capable of generating a large air flow, axial flow fans are suited for applications requiring ventilation cooling where the entire space inside the equipment must be cooled.


● Centrifugal Blowers

The centrifugal force of the cylindrically positioned runner (forwardfacing blades) generates rotational flows roughly perpendicular to the axis of rotation. The generated rotational flows are aligned in a uni-direction through scroll action, and the pressure rises accordingly.

Since the exhaust outlet is reduced to focus air in a specified direction, these blowers are used for spot cooling. The static pressure is also high, which makes them a suitable choice when cooling equipment through which air cannot flow easily or when blowing air using a duct.


● Cross Flow Fans

A cross flow fan has an impeller similar to that of a centrifugal blower, but both sides of the fan are covered with side panels and thus no air enters from the axial direction. As a result, air flows that pass through the impeller are generated. Cross flow fans utilize these air flows. Since a long cylindrical impeller is used to blow air, air travels over a wide width. Also, uniform air can be achieved because air is exhausted sideways along the circumference of the impeller.


Air Flow – Static Pressure Characteristics

● Pressure Loss

When air flows along a certain path, air flow resistance is produced by anything in the path that inhibits the flow.

Comparing the cases illustrated in Fig. 4 and Fig. 5, we see that the device shown in Fig. 4 is almost empty, so there is almost no air flow resistance in the device and little decline in the air flow. By contrast, there are many obstructions of the air flow in the device shown in Fig. 5, which increases air flow resistance and decreases air flow.

This situation is very similar to the role of impedance in the flow of electrical current: when impedance is low, the current flow is large, and when impedance is high, the current flow is low. The air flow resistance becomes the pressure energy that increases the static pressure within the device. This is called pressure loss. Pressure loss is determined using the following formula:



In terms of the fan, this formula says that to achieve a certain air flow (Q), the fan must be able to supply static pressure sufficient to increase the pressure inside the device by


● Air Flow – Static Pressure Characteristics

Fan characteristics are generally expressed in terms of the relationship between air flow and the static pressure required to generate such air flow, given as air flow – static pressure characteristics. As an example, assume the air flow required is Q1 and the accompanying pressure loss of the device is P1.

When the fan characteristics are as shown in Fig. 6, the fan is capable of a static pressure of P2 at an air flow of Q1. This is more than sufficient for the required air flow, since it exceeds the required static pressure
value of P1.

Since pressure loss is proportional to the square of the air flow, if the air flow needs to be doubled, then the fan chosen must be capable of providing not only twice the air flow but four times the static pressure, as well.


● How to Measure the Air Flow – Static Pressure Characteristics

Two methods are available for measuring the air flow – static pressure characteristics: the air-duct measurement method via the pitot tube, and the double chamber measurement method.

Oriental Motor employs the double chamber method, which offers higher accuracy than the air-duct method and is used worldwide. Moreover, Oriental Motor uses measuring equipment conforming to AMCA (Air Moving and Conditioning Association) standard 210, a fan measurement standard that is recognized worldwide. In the double chamber method, air flow – static pressure characteristics of fan to be measured are obtained by measuring the pre- and post-nozzle differential pressure (ΔP) and the pressure within the chamber (Ps), as shown in Fig. 7.


Oriental Motor's double chamber equipment is a measuring device with the highest level of general utility that may be used regardless of whether the fan is equipped with an intake or outlet tube.

Since this method allows the speed of the fluid flowing through the nozzle to be determined from the pressure differential between chambers A and B, the air flow (Q) can be expressed as a product of the flow speed (V) through the nozzle, the nozzle area (A), and the flow coefficient (C), as shown below.


The measurement of air flow – static pressure characteristics uses an auxiliary blower to control the pressure in chamber B, altering the pressure in chamber A. Thus, each point on the characteristics curve can be measured. Oriental Motor's measuring equipment is connected to a computer, providing extremely precise measurements in a short period of time.

Changes in Air Flow – Static Pressure Characteristics Using Two Fans

By using two fans featuring the same characteristics together, you can change the characteristics of the fans.


As shown in Fig. 9, the maximum air flow is approximately twice as large using two fans.

● Changes in the Air Flow – Static Pressure Characteristics with Installation of Accessories

When installing a fan in equipment, the safety and reliability of the overall apparatus can be significantly improved by attaching accessories such as finger guards and filters. However, these parts produce air flow resistance, affecting fan characteristics and fan noise. This should be taken into account when selecting fans and accessories.

The graph in Fig. 10 shows data regarding pressure loss caused by its accessories for a frame size 119 mm (4.69 in.) sq. fan. The filter causes the most significant pressure loss, while the finger guard causes little loss.


The graphs in Fig. 11 show how characteristics may change with installation of accessories, using the MU1225S-21 as an example.


As the Fig. 11 shows, the greater the pressure loss caused by accessories, the greater the reduction in air flow – static pressure characteristics.


● What is Noise?

We generally refer to sounds that are unpleasant to us as "noise." In the case of fans, the rotation of the fan blades causes air pressure fluctuation and generates noise. The greater the change in air pressure, the louder the resulting noise will be.

● Measurement of Noise

The noise level of Oriental Motor fans is measured in the A-weighted sound pressure level at a distance of 1 m (3.3 ft.) from the intake side (at a point above the center line of the intake side).




● Composition of Noise

This section explains the noise level when using two fans, each of which produces 40 dB of noise. Noise is expressed in decibel units, and noise cannot be determined simply by adding individual noise levels. The value that expresses this combined noise is found by determining the energy of the noise and then using it to calculate the increase in sound pressure. The relationship between sound energy (J) and sound pressure (P) is
expressed in the following formula:


ρ = Air density,
c = Speed of sound propagation

Using the above formula, the noise level can be expressed in decibel unit as follows:


In this formula the noise level is expressed in decibels based on the reference energy of Jo. As the noise energy for n fans is n times that of a single fan, the sound pressure obtained by this formula will be:


In other words, when n fans are operated simultaneously, the increase in noise is equal to 10 log n [dB].

In this example, if two 40 dB fans (n = 2) are operated simultaneously, the increase in noise level is equal to 10 log 2 or 3 dB, and the combined noise level is 43 dB.

The following explains the combined noise level when a 40 dB fan and a 50 dB fan are operated simultaneously. Again, the combined noise level is not given by a simple arithmetic sum but is obtained as follows:


If 40 dB of noise is combined with 50 dB, the resulting increase in noise level is only 0.4 dB. In other words, the noise level is always controlled by the larger of noise values, so it is important to suppress the noise of the fan producing greater noise.


● Distance and Noise

The noise level decreases as the distance from sound source increases.

The decrease in noise level due to distance is given by the following formula:


In the following example the noise level at a distance of 2 m (6.6 ft.) from the intake side of fan, whose noise level is 40 dB at a distance of 1 m (3.3 ft.) from the intake side of fan, is calculated. Since r2 = 2 m (6.6 ft.), r1 = 1 m (3.3 ft.), and SPL1 = 40 dB, substituting in the formula gives:


Thus, at a distance of 2 m (6.6 ft.), the noise level decreases by 6 dB. The value 20 log r2/r1 in the above formula represents the ratio between two distances. Thus, if the values used above were 3 m (9.8 ft.) and 6 m (19.7 ft.), the result would have been the same.

Therefore, if the noise level at a certain distance is known, the noise level at another distance can be estimated.



Permanent split capacitor motors contain an auxiliary winding offset by 90 electrical degrees from the main winding. The capacitor is connected in series with the auxiliary winding, causing the advance of current phase in the auxiliary winding.

Motors employ vapor-deposition electrode capacitors recognized by UL. This type of capacitor, which uses a metallized paper or plastic film as an element, is also known as a "self-healing (SH) capacitor" because of the self-healing property of the capacitor element.

Although most of the previous capacitors used paper elements, the plastic film capacitor has become a mainstream model in recent years due to the growing demand for compact design.

● Capacitance

The use of a capacitor with a different capacitance may cause excessive motor vibration and heat generation or may result in torque drops and unstable operation. Be sure to use the capacitor included with the fan. The capacitor's capacitance is expressed in microfarads (μF).

● Rated Voltage

Using a capacitor exceeding the rated voltage may cause damage and then smoke or ignite. Be sure to use the capacitor included with the fan. The rated voltage of the capacitor is expressed in volts (V). The capacitor's rated voltage is indicated on the surface of the capacitor case. Take proper precautions, since the capacitor's rated voltage is different from that of the fan.

● Rated Conduction Time

The rated conduction time is the minimum design life of the capacitor when operated at the rated load, rated voltage, rated temperature and rated frequency. The standard life expectancy is 25000 hours. A capacitor that breaks at the end of its life expectancy may smoke or ignite. We recommend that the capacitor be replaced after the rated conduction time. Consider providing a separate protection measure to prevent the equipment from being negatively influenced in the event of capacitor failure.

Overheat Protection Device

If a fan in run mode locks due to overload, ambient temperature rises rapidly, or the input current increases for some reason, the fan's temperature rises abruptly. If the fan is left in this state, the performance of the insulation within the fan may deteriorate, reducing its life and, in extreme cases, scorching the winding and causing a fire. In order to protect the fan from such thermal abnormalities, our fans recognized by UL and CSA Standards and conform to EN and IEC Standards are equipped with the following overheat protection device.

● Thermal Protector

The MRS Series, MB Series (impeller diameter ϕ80 mm (3.15 in.) or more) and MF Series fans contain a built-in automatic return type thermal protector. The structure of a thermal protector is shown in the figure below.

The thermal protectors employ bimetal contacts, with solid silver used in the contacts. Solid silver has the lowest electrical resistance of all materials, along with a thermal conductivity second only to copper.

Operating Temperature of Thermal Protector


(The fan winding temperature, where the thermal protector is activated, is slightly higher than the operating temperature listed above.)


● Impedance Protected

The MU and MB Series (MB520 and MB630 type) fans are equipped with impedance protection. Impedance protected fans are designed with higher impedance in the fan windings so that even if the fan locks, the increase in current (input) will be minimized and the temperature will not rise above a certain level.


● Decibels (dB)

Noise level is expressed in decibel units (dB). When the noise level is expressed based on the linear scale, with the minimum level of noise audible to the human ear being 1, the maximum level of noise the human ear can withstand is expressed in such a substantial figure as 5 million. In contrast, if noise (sound pressure level) is expressed in decibels, then


Therefore, the range of sound pressure audible to the human ear can be conveniently expressed as 0 to 130 dB.

● A-Weighted Sound Pressure Level

It is generally said that the audible range of the human ear is between 20 Hz and 20 kHz. Moreover, low frequency and extremely high frequency sounds are not disturbingly loud to the human ear.

For this reason, an accurate indication of loudness as perceived by the human ear cannot be achieved simply by measuring sound pressure without taking frequency into account. Therefore, measurements of the sound pressure level must be corrected according to frequency in order to accurately reflect the human perception of loudness. This corrected level is called the A-weighted sound pressure level.

The graph below compares the corrected measured values (A-weighted sound pressure level) with the uncorrected measured values (C-weighted sound pressure level).


● Flammability Grade

The flammability grade represents the degree of fire retardancy for plastic materials used in equipment parts. The generally accepted standards for flammability grade are the UL Standards (UL94, STANDARD FOR TESTS FOR FLAMMABILITY OF PLASTIC MATERIALS FOR PARTS IN DEVICES AND APPLIANCES). The UL Standards provide the flammability of plastic materials based on the burning rate, duration of burning from the onset of fire, fire ignited by a dripping substance and other items. Flammability grade is rated in four different grades, as shown in the table below.


Cooling fans use blades and frames with materials that receive the highest grade in this classification, V-0.

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