Anatomy of the Silent Fan

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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.

* * *

Discuss this article in the SPCR Forums.



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