Anatomy of the Silent Fan

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A distilled summary of fundamental design elements and key factors that affect the acoustic performance of DC axial fans in the role of PC cooling. This information is presented apart from our fan test methodology or actual fan review articles to keep information overload at bay. NOTE: A Postscript Q&A features Dorothy Bradbury, UK fan maven extraordinaire.

Nov 12, 2006 by Mike Chin

The electric, multi-bladed, axial fan has been around since the late 1800s, starting originally as a ceiling fan for well-to-do households in the U.S. Today, they are visibly ubiquitous as air-blowing devices for human comfort all over the world, especially in warmer climates and seasons. Electric axial fans are also employed in almost every sector of industry, mostly for gaseous transportation and cooling ventilation of machinery. Their first application in the personal computer was to provide cooling airflow in the power supply. Now, axial fans can be found on every type of computer component.



Not a reproduction of an early ceiling fan. This one is inspired by Wright… Wilbur and Orville. (Photo courtesy of www.ceilingfan.com) 

The body of information about electric fans is voluminous. With a history that extends back to the first practical use of electricity, the scientific and engineering knowledge about electric fans seems endless. There is no desire or advantage in trying to reproduce or reference all of this extensive information. The focus in this article is on those aspects of DC axial fans that are particularly relevant for quiet operation in cooling computer components.

A simple way to think about an axial fan is that it is the marriage of a impeller (or propeller) and an electric motor. The impeller has blades extending from a hub that is spun by the motor. The electric motor has a shaft with a wire armature electromagnet that connects to the impeller’s hub. A rotary switch or commutator reverses the direction of the electric current twice every cycle, to flow through the armature. The electromagnets push and pull against permanent magnets on the motor housing. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the motor going in the proper direction.

Two key mechanical parts in a DC axial fan are critical to its acoustics: The bearings that hold the shaft in place, and the design of the blades that interact with the air. Other factors include structural aspects (such as rigidity, dynamic balance, and the internal self-damping characteristics of the materials the fan is made of) and the commutator frequency. All of these factors will be touched upon in this article.

BEARINGS

I. Ball vs. Sleeve

The main types of bearings used in axial fans are ball bearings and sleeve bearings. When choosing between them, the engineer usually considers the following factors shown in the table below (from Ball vs. Sleeve: A Comparison In Bearing Performance (pdf) by Melody Williams of NMB Technologies):

Criteria Ball Bearing Sleeve Bearing
Fan Longevity Longer life Shorter life
Heat Endurance Higher Lower
Fan Mounting Options Vertical, Shaft Center Line Parallel, Perpendicular Vertical
Noise Emission Quieter at High Speeds Quieter at Low Speeds in Early Life
Parts Precision Non-precision
Lubricant Less Evaporation More Evaporation
Contact Point Line
Cost More Expensive Per Unit Less Expensive Per Unit

Ms. Williams favors the ball bearing for many reasons, the primary being long term reliability. For silent computing enthusiasts, the issue is not so cut and dry. Key highlights are cited from the comparison article.

Temperature: The chart below uses the L10 method to illustrate fan performance at various temperatures. L10 refers to the point of time at which 90 percent of a large population of these fan types will continue to work. On average, a fan that operates for 50,000 hours will run continuously for over five years.

Temperature Ball-Bearing Fan Sleeve-Bearing Fan % Difference

25°C
95K Hours 80K Hours 18%

40°C
75K Hours 52K Hours 44%

50°C
63K Hours 40K Hours 58%

60°C
54K Hours 30K Hours 80%

70°C
45K Hours N/A

Fan Mounting: Fan orientation does not affect the longevity of ball-bearing fans because of preloading, the procedure by which manufacturers build an initial side load, such as a spring or a wave washer against the balls of the bearing. When mounted in vertical positions, sleeve-bearing fans can maintain life spans comparable to their ball-bearing counterparts. However, when sleeve-bearing fans are mounted in any position other than vertical, the fan’s life span decreases.

Noise: Sleeve-bearing fans run more quietly than ball-bearing fans. This is true for applications that have very low fan speeds. The noise advantage of sleeve bearings is much reduced in applications that require faster fan velocities.

Parts: Typically, sleeve bearings deteriorate under high temperatures because they are made from porous, powered metals from a sintering process. Also, sleeve-bearing fans can develop a high micro-hardness that makes secondary machining difficult. Steel parts make ball-bearing fans more exact. They are precision-ground and super-finished.

Lubrication: Sealed-for-life ball-bearing systems use thicker lubricants that have more additives, and are less subject to evaporation. While the lubricants within sleeve-bearing fans have a greater concentration of oil, the sleeve-bearings’ bushings can only hold a fixed amount of lubricant. Since there is no periodic recharging of the oil, the lubrication within a sleeve-bearing system is more likely to evaporate.

Point of Contact: Sleeve bearings are line-contact bearings; there is broad contact between the shaft and bearing that generates a good deal of friction. Ball bearings are point-contact bearings, which generate minimal friction. Previously, opponents of ball-bearing fans argued that the ball-bearing system could lead to brinelling or indentations in the raceway. Yet, defenders of the ball-bearing system believe that if a ball-bearing system is assembled correctly, each component will fit perfectly, eliminating the potential for parts damage.”

Comair-Rotron, another fan manufacturer, has a somewhat different perspective on the issue of ball versus sleeve:

Most ball bearing fans are noisier by 1 to 3 dBA over their counterpart sleeve-bearing fan. Also the additional noise is somewhat pure tone in nature. Therefore, the annoyance level is considerably higher than with the sleeve-bearing fan. This higher noise level is also in the higher frequency ranges, which makes it even more annoying.

Sleeve bearing fans, generally speaking, can easily sustain multiple shocks of 80 g’s with duration of 11 msec without impacting noise at all. This is not true for ball bearing fans. Figure 3 shows what can happen to ball bearing fan noise if the fan is subjected to 40 g’s (11 msec duration). This is a very important factor since the equipment manufacturer has no control over how this equipment is treated after the fan is installed, particularly in shipment. It is quite common for a ball bearing fan to be noisy before it is even used just from the handling of the equipment it is installed in.

Typically, sleeve bearing fan noise does not increase due to life. This remains true up until the system begins to fail due to loss of oil. However, ball bearing fans can begin to get noisy in a very short time. This increase in noise is due to many facts, such as grease channeling, loss of grease, damaged bearing camouflaged by the grease, etc. Also, as time goes on, the grease may begin to dry out which allows for a very noisy fan, but it will continue to run for a long time. This brings up an interesting point: the reason for the use of ball bearing fans is to extend the fan life past sleeve bearings. However, if usable life were defined to end when the fan became noisy, it is quite possible the sleeve bearing fan would out live the ball bearing one.”

SPCR’s Take on Sleeve vs. Ball

The advantages of ball bearings for general application in
computer equipment are clearly spelled out in the NMB comparison. For silent computing, the advantages of ball bearing fans are only relevant if the acoustics at low and very low speed are very good. The advantages of sleeve bearings are certainly relevant for silent computing. The Comair-Rotron article shows why PC silencers might prefer sleeve bearing fans. They are…

  • Quieter than ball bearings, especially at low speed, and stay quieter throughout their life — the most compelling aspect for SPCR
  • Good for >3 years continuous use in up to 60°C; longer in cooler conditions — good enough for most well-designed PCs used in quiet home environments. It should be noted that concerns about >5yr lifespans for fans become moot; most PCs do not have much longer usable lifespan.
  • Less prone to damage during shipping and handling. This is a conjecture I made in the Fan Testing Methodology article (Editor’s Note at the bottom of page 2); Comair-Rotron gives support to my suspicion.

The caveats against sleeve bearing fans are…

  • Don’t use them in any orientation other than vertical. Not only do they not last as long, they quite often make more noise in positions other than vertical.
  • Don’t expect them to last as long in very hot conditions. Ball bearing fans in the same hot conditions will last longer though they get noisier, which is why they are almost universally used in rarely-attended server machines that are locked away in air-conditioned rooms. Ball bearings screech as they wear out so there is some warning of pending failure. In contrast, sleeve bearings usually stop working without warning. This does not necessarily spell disaster, as most PCs have some features that warn of high temperatures. Also, overheating of a component causes instability in the PC which will usually be investigated by the user. (If the PC is left on while the user is absent, then the fan failure could turn into a more expensive problem; all the more reason to turn it off when you’re not there.) For the average personal PC user, it may be wise to routinely replace a sleeve bearing fan used on a hot heatsink after a few years of service.

II. Modified Sleeve Bearings

Many manufacturers have developed variants of sleeve bearings that try to combine the positive aspects of sleeve bearing fans along with merits of ball bearing fans. The better known of these different bearing types include Hydro Wave by Panaflo (now part of NMB), Hypro by Adda, Sintec by EBM/Papst, and most recently, Fluid Dynamic by Sony. The Hydro Wave and Sintec bearing types go back a number of years; the FD bearing is relatively new for fans, although it has been used in hard drives since around 2001.

While precise details differ, and engineers from different camps would probably debate relative merits with some conviction, the broad goals of these modified sleeve bearing technologies is to…

  • reduce friction across the shaft / bearing surface,
  • eliminate the relatively quicker loss of lubricant in the bearing,
  • allow great freedom in working position (i.e., not just vertical), and
  • improve high temperature performance and longevity

Details of how these bearing types work are contained in company-sourced documents, which, as you would expect, contain strong partisan opinions. Nevertheless, they are well worth studying if you want to learn more about fan bearing design and the impact they have on fan acoustics.

  • Panaflo Hydro Wave Bearing by Panaflo (Panasonic) (quoted from the Anglia (UK) web site) The Hydro Wave bearing is found on fans from a number of different brands, but mostly on Panasonic fans. The brand is now part of NMB, but HW fans are still listed in the NMB-MAT DC axial fan section of Panasonic’s industrial products site. The HW fans have model numbers that start with the letters FBA.

    “HWB addresses the traditional weakness of sleeve bearings… axial friction. By utilizing a unique system, the thrust plate “floats” on a circulating film of oil, which greatly reduces the axial friction and the bearing’s deterioration. The HWB pattern on the bearing virtually eliminates thrust plate contact which is depicted above. As the shaft rotates it acts as a pump constantly circulating the oil in the radial, as well as the axial direction. The sealed system and reservoirs of oil maintain the proper lubrication to minimize mechanical contact and prolong the fan’s life expectancy. The radial direction analysis shown above is essentially the same as a traditional sleeve bearing. Panasonic’s unique HWB reduces the axial contact.”

  • Adda, another major DC fan maker for the PC industry, confusingly refers to a proprietary bearing called Hypro: HY – “Hydro-dynamic wave” and PRO – “Oil protection system” about which they claim similar strengths.
    1. By means of reducing the surface friction area between the shaft and the bearing, the bearing temperature will be reduced naturally when HYPRO fan operates.
    2. The unique bearing construction can store and maintain more oil volume with an oil-cycling protection.
    3. The bearing is made of a new alloy material harder than bronze.
  • Sintec Bearing by EBM/Papst (quoted directly from the EBM/Papst web site) This bearing type is found only on certain EBM/Papst models.


Fig. 1: Lubricant circuit in Sintec bearing

“In fact, the single most important factor affecting the reliability of a fan is the composition and reliability of the lubrication system and not the fatigue life of the bearing itself… Papst has conducted comparative life tests which show that fans with particular sleeve bearing systems can now achieve similar reliability to those with ball bearings. i.e. an L10 lifetime of 80,000 hours of operation at an ambient temperature of 40°C.

“Behind this improvement in sleeve bearings lies significant developments in sintering technology, in particular the unique Sintec process used by Papst. This process uses a special metal powder that is pressed into the required shape at very high pressure and then sintered at high temperature. The material created in this way is porous, with the pore volume taking up about 15 to 30% of the bearing. The pores are then filled with lubricant using a vacuum soaking process and a re-circulating reservoir is created to ensure continuing operation over long periods.



Fig 2: The compact Sintec bearing guarantees excellent alignment and a large oil reservoir.

“This unique design of Sintec bearing ensures that lubrication is maintained even when the fan is at rest, so that the bearing is never dry, even during the crucial startup phase. At rest the capillary effect forms a film of lubricant between the shaft and bearing so that lubricant never drains away from the bearing. Then when the fan starts to rotate, a slight pressure difference causes a hydrodynamic pressure wave to be set up in the bearing gap as a result of the rotary movement of the shaft causing the lubricant to circulate around the bearing. This hydrodynamic state produces a lubricating bulge at the narrowest part of the bearing gap, exactly where friction should be greatest, ensuring that the shaft and bearing no longer touch and lubricant is always supplied to exactly where it is needed.

“To ensure re-circulation, the oil pressure that is greatest at the narrowest place of the bearing gap forces the lubricant into the pores of the sintered bearing. To compensate for this, oil flows out from the sintered metal in areas of lower pressure and circulates to the areas of high pressure where it is needed. This re-circulating circuit creates a stable state as in Fig 1.

“To allow the Sintec bearing to take up the radial load of the fans, two bearing positions with sufficient axial spacing are required. The ideal solution is to use double sintered bearings with two separate bearing positions in a single sintered part. The main advantages are improved alignment and a considerably larger oil reservoir due to the larger bearing volume.

“The design also features an additional axial bearing disc which ensures that a stable position is reached in both axial directions. By taking up the axial lateral forces in either direction, fans can be mounted with the shaft in a non-horizontal plane. Under this condition a film of lubricant is formed between the rounded end of the shaft and the disc and a state is achieved similar to that normally seen in a radial bearing.”

Fluid Dynamic Bearing (PDF document, linked directly from Jaro Thermal) There appears to be a handful of brands offering fans that employ the FDB bearing, and they generally identify Sony as the maker or inventor of the bearing itself. No information about FDB could be found on any of Sony’s English-language sites. The fan brands we found that use FDB bearings are Jaro Thermal, Arctic Cooling, and Scythe. It’s clear that Scythe does not manufacture its own fans. Whether Arctic Cooling also employs OEMs is not clear. However, it is clear that Jaro does manufacture its own fans; it’s possible that Jaro may be the actual manufacturer of all the FDB fans mentioned above.

The text below is quoted from the web site Green Supply Line:

“The FDB fan’s unique design eliminates surface contact between the shaft and bearing, which translates into lower noise, higher reliability, and a longer operating life. The fan also uses a patented “vacuum impregnating oil” that eliminates air bubbles and oil leakage, which means no oil leakage due to changes in air pressure.

“The FDB bearings are manufactured with an extremely thin, herringbone-shaped groove on the interior wall to ensure a constant oil layer. This provides an exceptionally low noise level as low as 19.7 dB for a 92 x 92 x 25 mm fan, said the company. Life expectancy is guaranteed for 80,000 hours at 25 degrees C. Click the link for a look at the FDB bearing’s structure.

“The life of this model is achieved by a V-shaped groove that generates the dynamic fluid pressure on the inner surface of a sintered metal sleeve allowing dynamic pressure to build up in the center of the shaft, said Dennis Eisen, vice president of sales for Jaro Thermal. In addition, the less vibration caused by the bearing results in the lower noise, he said.

“The FDB fans may replace conventional dual ball-bearing systems, offering a higher maximum operating temperature up to 90°C compared with 70°C for fans with dual-ball bearing systems. The operating temperature range is -10°C to 90°C. The FDB fans are RoHS compliant.”

The following table was extracted from Jaro’s brochure, which suggests that the FDB is the most advanced and best performing of all fan bearings.


Jaro’s FDB-promotional comparison.
Note: 1B1S = “1 ball, 1 sleeve” bearing; 2BB = two ball bearing

This is the end of our look at fan bearings for now, but the list of fan bearing types does not stop here. There are different types of ball bearing fans, such as dual and single ball bearing fans, mixed sleeve / ball bearing fans, ceramic ball bearing fans, magnetic levitation bearing fans, and so on. Our experience has taught us to be wary of so-called revolutionary designs, and most ball bearing fan variants don’t sound as good as the best sleeve and modified sleeve bearing fans. When we run across quiet fans using other types of bearings, we’ll tell you about those bearings and update this article.

FAN BLADE DESIGN

Looking into propeller, impeller, or fan blade design can sink you quickly into aerodynamics, physics and other disciplines that are arcane and mind-boggling for the average person, certainly for this writer. The topic of spinning axial blades has received intense, prolonged study for over a century, in aviation (propellers for airplanes and helicopters, turbines in jet engines, etc.) and in marine-related applications — just think of the money and intellectual resources lavished on the development of silent propellers for nuclear submarines alone. Not having as much military value, it’s not likely that blade design for axial fans has received quite as much attention, but the technology does go back over 120 years.

Elsewhere, we’ve mentioned the difficulty of trying to correlate aspects of blade design to practical results. That comment bears repetition. The best we can really do is to try and identify the various elements of blade design. Changes in any one of these elements can have an impact on airflow / noise performance, and there are endless variations.

Much thanks go to long time SPCR supporter Neil Blanchard, for creating the following illustrations. Here, Neil compares the variations in the design of two relatively quiet fans.

Neil Blanchard’s Blade Design Observations


Intake side

On the intake side, the blades on both fans are swept forward, but to different degrees; the NMB’s sweep further forward. The blade area on the NMB is greater, and the pitch (angle of attack) of the leading edge is lower. The Nexus has a larger radius on the leading tip, and the edges of the blades are more blunt. The edge of the hub on the Nexus has a larger radius.

On the exhaust side, the frames are quite different; the struts on the NMB are rounder, but they are nearly parallel to the trailing edges. The struts on the Nexus, on the other hand, intersect with the tailing edges only at one point, which would seem to reduce air turbulence. Also, the strut with the wires is quite wide on the NMB, which probably creates a larger “shadow” in the air flow.


Exhaust side

 

 

An article by Comair-Rotron on Fan Acoustic Noise identifies several primary causes of noise in fans, including some related directly to the blades:

Vortex Shedding Broad band noise generated by air separation from the blade surface and trailing edge. It can be controlled somewhat by good blade profile design, proper pitch angle and notched or serrated trailing blade edges.

Turbulence Turbulence is created in the airflow stream itself. It contributes to broad band noise. Inlet and Outlet disturbances, sharp edges and bends will cause increased turbulence and noise.

Speed The effect of rotational speed on noise can best be seen through one of the fan laws:

  • dB1 = dB2 + 50 log10 (RPM1 / RPM2)

Speed is an obvious major contributor to fan noise. For instance, if the speed of a fan is reduced by 20%, the dB level will be reduced by 5 dB.”

OTHER FACTORS

  • Acoustics in general are affected by all mechanical / structural aspects, and this holds true for fans. The strength of the frame which houses the fan, the strength and precision of all the parts, the internal damping characteristics of the materials from which the parts are formed — all of these factors have an impact on the acoustics of the fan. Suffice it to say that very high precision in parts, good dynamic balance of the rotor and fins, high strength and rigidity in the mechanical structures, and very good damping of all internal resonances are important aspects of good fan acoustics.
  • Commutator Switching Frequency Noise is described by JMC

    “The stator motion is a square wave that is switched on and off before and after the peak torque position. This motion causes a small amount of undulation in motor torque, producing an audible noise caused by the lower frequency commutation operation. Each small torque causes a minute contracting of the entire fan structure and results in an audible clicking noise while the fan is operating.”

This noise is most often heard as a rapid clicking or buzzing. JMC’s solution to commutator switching noise is to put the frequency up to 25KHz, where it is inaudible to human beings. This is the main feature of their PWM fans.

  • PWM speed control and commutator switching and work in very similar ways, and they cause similar noise. Pulse-width modulation (PWM) circuits in fan speed controllers were first marketed to PC enthusiasts a few years ago. PWM switches the power to the fan on and off rapidly, which results in a series of pulses. When the frequency of these pulses is fast enough, the fan spins steadily because of its momentum. There are a number of advantages of PWM over linear voltage control, especially for higher power fans.
    However, the downside is a potential increase in clicking noise, very similar to commutator switching noise.Application Note 58 from TelCom Semiconductor explains the issue of Suppressing Acoustic Noise in PWM Fan Speed Control Systems (pdf). (Much thanks to cpemma for finding this document!)

    “Stator excitation is a square wave that is switched ON 45° before peak torque position, and switched OFF 45° after peak torque position. This excitation causes a small amount of ripple in motor torque at the frequency of commutation. Each small torque “burst” causes a minute flexing of the entire fan structure, and results in a faint (but audible) “ticking” noise while the BDC fan is operating (Figure 2A). Acoustic PWM noise is generated in exactly the same way. When the PWM pulse turns on, a step change in torque occurs within the fan, the profile of which matches the rise time of the PWM pulse (Figure 2B). This impulse torque is articulated by the fan structure as audible noise. This is true mostly in larger fans (i.e., fans with operating currents in excess of 300 mA), since they generate a greater amount of torque and have larger size and mass. This effect is more pronounced at low operating speeds (i.e., low PWM duty cycle): the lower the PWM duty cycle, the greater the percentage of time the fan is OFF (quiet), and the more noticeable the acoustic noise caused by the PWM becomes.”

    TelCom Semiconductor’s solution to PWM induced fan noise is to “slow the slew rate of the PWM drive signal to the fan”. In essence, the edges of the square wave pulses are rounded, and the voltage spike reduced. As a result, the acoustic click or spike is dramatically reduced.

  • A larger fan can move more air at the same noise level than a smaller fan. Alternatively, it can be quieter moving the same amount air as a smaller fan. This is assuming that all aspects of the fans except for diameter or depth are identical. It is an outcome that’s predicted by the fan laws, and also confirmed experimentally with SPCR’s own hands-on, ears-open testing. Larger blades don’t have to spin as quickly to move the same amount of air.For example, a typical 120x25mm fan spinning at 1,000 rpm can move ~40 cfm. The best quiet 120x25mm fans can do this at <22 dBA@1m. In comparison, an 80x25mm fan has to spin at some 3,000 rpm to achieve the same airflow, and its noise will be a minimum of 30 dBA@1m. Subjectively, the higher pitched tonal aspects of the 80mm fan sound at this speed will make it seem more than twice as loud. In order reach ~22 dBA@1m level, the 80x25mm fan speed has to be reduced to ~1,500 rpm, at which point, the airflow drops to about half of the 120mm fan at 1,000 rpm.

    The broad transition in CPU heatsinks from fans under 80mm size around the year 2000 to fans as large as 120mm by 2005 was driven mostly by the need for greater cooling capability without further escalating noise, which had reached absurdly high levels in “performance” PCs. 120mm (and now larger) fans also appeared as PC case ventilation fans during that period. These developments have helped to lay the foundations for a much broader realization of quiet computers.

EXTERNAL FACTORS

All the factors discussed thus far have been internal to the fan itself. There are external factors that also impinge on fan acoustics. The major ones are noted here:

Fan Load (Electrical) Noise varies as the system load varies. This variation is unpredictable and fan dependent.

Impedance refers to vent openings that may not be large enough to allow 100% of the fan’s airflow, restrictive dust or protection grills, or very densely packed heatsink fins that represent high resistance to airflow. Fans make more noise with greater impedance.

Altitude could also be said to be a part of the load. When the air is thinner, airflow is reduced for a given RPM.

Vibration-induced noise may originate from the fan, but can be exacerbated and amplified by resonant panels to which the fan is mounted. Mechanical isolation or decoupled mounting techniques are often required for the quietest fan operation.

FINAL NOTES

It is difficult to sum up all of the various complexities around DC axial fan design and implementation into an intelligent paragraph or two; nonetheless, it’s a task expected of the author of an article such as this. Keeping it simple to avoid revealing too many of the holes in my grasp of the topic, I would suggest that the Ideal Silent Fan has the following characteristics:

  • Advanced bearing that allows long life and mounting in any position while remaining extremely quiet, even while spinning as fast as 1,500~2,000 rpm, with a benign acoustic signature that scales smoothly as speed is reduced.
  • Aerodynamic structural design that maximizes laminar airflow and keeps vortex shedding and turbulence to a minimum.
  • High precision parts assembled with perfect dynamic balance from non-resonant materials to ensure minimal vibration.
  • Ultrasonic commutator switching speed, very low start voltage (say 3.5V), built-in silicone vibration decoupling, built-in switchable thermal / manual speed control, availability in sizes from 60x15mm to 140x38mm.
  • And as long as we’re asking for the sky, priced at <$10 for the smaller sizes and no higher than $30 even for the biggest.

Any fan makers who have products that come even close, please contact me asap. 😉

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Reference Sources

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POSTSCRIPT overleaf, Nov. 13, 2006:
Dorothy Bradbury on “What determines the rated speed of a fan?”

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Discuss this article in the SPCR Forums.

POSTSCRIPT, Nov. 13, 2006

** Dorothy Bradbury ** on “What determines the rated speed of a fan?”

During the usual round of proof-reading and editing that takes place for each article at SPCR, Russ Kinder, a senior member of the editorial team, asked…

“One topic of discussion not covered [in the Anatomy article] that has always interested me is what the underlying engineering differences are between a high-speed fan and a low-speed fan — ie, what makes one fan spin at 2,000 rpm and another at 4,000 when the same voltage is applied? Is the difference in the motor (more windings?), or in the commutator? There’s some confusion about this in our reader base as well, as I’ve seen more than one person in the SPCR forums suggest erroneously that the difference between say an H1A and an L1A is only a resistor hidden somewhere inside the hub.”

I was only sure that it’s not a resistor in series or parallel at the input voltage; if it was, the power ratings of all the models in a fan series would be the same, and they are not. Beyond that, I could only think it would have to be the commutation frequency because making and stocking different types of armature windings for all the fans that manufacturers make would be a complete headache. But that was only a conjecture.

I had the opportunity in the last couple of hours (ah, the wonders of the web!) to exchange a couple of emails with Dorothy Bradbury, the British fan expert mentioned in our first 80x25mm fan roundup. She runs the web site http://www.dorothybradbury.co.uk/, subtitled “SILENT & INDUSTRIAL FANS”. I decided to forward the comments from Russ to her. Her thorough response came back promptly, in somewhat shorthand form.

I’ve posted Ms. Bradbury’s response in the original form except for minor [identified] clarifications to ensure I don’t introduce any misinterpretations. Thank you, Ms. Bradbury! And, Russ, you will be happy to finally have your answer.

Dorothy Bradbury’s answers to Russ’s questions:

What makes one fan spin at 2000 rpm’s and another at 4000 when the same voltage is applied?

1. DC fans…

  • Motor is fixed (stationary)
    • wires go from the PCB directly to stationary coils
    • thus the motor is commutatorless & brushless
  • Rotor-Blades, Rotor-cap & Magnet rotate around the fixed motor
    • the magnet is inside the rotor cap inside the rotor

2. DC [input voltage] does not directly drive a fan…

  • Apply DC to the coil of a fan directly: the rotor moves 90-degrees and stops
  • Apply DC to each coil of a fan in sequence: the rotor will move at the speed of that sequence

3. DC fan speed is based upon switching frequency…

  • Motor-IC: switching frequency set by resistor or capacitor
  • Transistor: switching frequency set by resistor or capacitor

4. DC fan speed is based upon voltage applied…

  • DC voltage drives the frequency of switching
  • DC voltage drives the electromagnetic field

However a DC fan can’t spin faster than the speed at which
DC current is switched on/off/on/off around the coils inside.

5. Actual switching depends on [the particular] technology…

  • Transistor simply switches from one to the next; smoothing capacitor is used to offset the jolt (adds cost)
  • Motor-IC is a complex multi-phase motor drive
    • overlaps switching from one coil to the next (smoothness)
    • use back-EMF from the trailing coil to the next (efficiency)
    • vary motor drive phase & frequency (torque)

6. Some Motor-IC tricks actually block some signals.

  • Switch current from one coil to the next coil
    • the powered off coil becomes a generator (magnet moving)
    • the powered on coil becomes a motor (moves the magnet)
  • The coil becoming a generator creates back-EMF
    • that current can be sensed (counted) to detect RPM
    • that current can be re-used to create more efficiency
  • Some Motor-ICs re-use that current and so do not offer Tacho.

Tacho is not necessarily sensed by the magnet moving past a
hall-effect sensor on a PCB and counting the resulting pulses.
That is done on transistor fans; motor-ICs do it in software.

I’ve seen more than one person in the SPCR forums suggest erroneously that the difference between say a [Panaflo] H1A and an L1A is only a resistor hidden somewhere inside the hub.

Not as they mean — ie, not directly. 🙂

A resistor reducing voltage would dissipate heat
exactly where you don’t want it, near the bearings. Instead a resistor is used to set the switching frequency;
basically consider the resistor value acting as a multiplier.
You can also use a capacitor (transistor drive fans often do).

Re: Panaflo fans. There are several motor-ICs available.
The best, unsurprisingly, is Panasonch, but it is not always a
Panasonic motor-IC fitted – generally 1BX are not Panasonch.
There are ~5 on 12V, 2-3 on 24V. Minebea side use
a different chipset which is far more consistent, but there
are also 2 main versions — as well as many other variables.

This applies to all fan makes — transistor drive can vary
the design of circuitry, and acoustic characteristics change.

Windings are generally optimised to minimise copper cost,
mainly because it is always in short supply and expensive.

Overall, a DC fan spins due to the frequency being turned
around the coils which pulls the rotor around with it.
Frequency of that switching affects actual fan speed.

* * *

Dorothy went a couple steps further, and posed and answered two of her own questions to further clarify the above.

So how does an AC fan work?

An AC fan works based on voltage & frequency

  • Voltage sets the first RPM component
  • Frequency sets the second RPM component

Hence the same fan will spin faster at 60Hz than 50Hz

  • hence AC fans have 2 figures based on frequency

In a DC fan the frequency is set (and varied) within the fan. They just work on DC for greater efficiency reasons – an AC fan is limited to mains frequency and can’t use complex overlapping, timing, phasing and so on.

What do you mean by DC fans with motor-ICs use phasing?

A motor has a physical set of coils

  • for most fans it is turned 4-pole or 6-pole or 8-pole
  • a 4-pole has 2 coils, an 8 pole has 4 coils
  • more poles require space & are needed for huge fans

A motor can be made to have multiple sets of coils

  • a motor-ic switches current around the coils
  • that switching can be done at the RPM frequency
  • or it can also be done at a far higher frequency
  • in effect creating 3-phase or multi-phase motors

An AC fan is limited to just 50Hz or 60Hz frequency, it can’t use fancy motor-ic drive – unless external.

Playing around with phase allows you to improve motor efficiency beyond a simple transistor drive – so you have less thermal wastage, less bearing heating, more torque. The big downside is motor-ic cost, R&D cost and so on.

So a fan is not a simple DC device, it’s actually complex.

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