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Overriding fan speed with tach/voltage leads

Discussion in 'Electronic Repair' started by Fpbear II, Feb 19, 2007.

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  1. Fpbear II

    Fpbear II Guest

    I have a 12V 0.8A Nidec TA450DC fan in some equipment and I need to slow the
    fan down to about 75% of its speed. It has four leads: red, black, yellow,
    and green. I guess that yellow is the RPM sensor and green is the high/low
    voltage sensor.

    I tried using a 5-ohm resistor as well as a zener diode on the red lead.
    This did not work because the equipment senses something is wrong, and the
    fan won't spin. To try another strategy, is there something I can place on
    the green or yellow lead to trick the system into reducing the fan speed?
  2. Fpbear II

    Fpbear II Guest

    I discovered that when I cut the green wire, the fan is blowing even faster.
    This may provide some clue but I'm not sure how to -lower- the speed.
  3. Jim Land

    Jim Land Guest

    The usual way to slow down a DC fan is to drop the voltage below 12v
    until you get the speed you want. But it sounds like you've tried that
    and it didn't work. What kind of equipment is it hooked up to?
  4. Franc Zabkar

    Franc Zabkar Guest

    The datasheet ( refers to
    Alarm/Tachometer/Thermal Speed Control/PWM Speed Control options.

    You may be lucky and have the PWM speed control option, in which case
    you could control the speed with a 555 timer running at a 75% or
    greater duty cycle. I don't know what the "alarm" is, but you could
    find out by monitoring the voltage on the wire(s) while heating the
    fan, stalling it, or varying its supply voltage. This assumes that the
    relevant signal is not OC, in which case you would need a pullup

    - Franc Zabkar
  5. Fpbear II

    Fpbear II Guest

    Thanks Franc. I just discovered, I was able to use the resistor method on
    the positive wire -after- I cut the green wire. I had to disable the
    thermosister before it would accept the resistor.
  6. James Sweet

    James Sweet Guest

    Pick up one of the little speed control boxes made for computers. They
    plug in series with the fan and use PWM to control the speed. Any place
    that sells parts for high performance PCs will have them.
  7. Franc Zabkar

    Franc Zabkar Guest

    Brushless DC fans use Hall effect sensors for electronic commutation.
    If you must chop the supply to these types of motors, then you should
    ensure that the PWM frequency is much lower than the fan's rotational
    frequency, otherwise the speed control logic will be disturbed.

    This old post illustrates the strange things that happen when external
    PWM control is applied to a PC CPU fan that has its own internal speed

    - Franc Zabkar
  8. AZ Nomad

    AZ Nomad Guest

    Not any of the fans I've ever come across. They usually use a chip to
    generate AC out of the DC supply and drive an AC motor.
  9. Franc Zabkar

    Franc Zabkar Guest

    According to the following article, a BLDC motor is essentially an AC

    "BLDC motors have come to dominate many applications: Consumer devices
    such as computer hard drives, CD/DVD players, and PC cooling fans use
    BLDC motors almost exclusively."

    "A brushless DC motor (BLDC) is an AC synchronous electric motor that
    from a modeling perspective looks very similar to a DC motor.
    Sometimes the difference is explained as an electronically-controlled
    commutation system, instead of a mechanical commutation system,
    although this is misleading, as physically the two motors are
    completely different."

    "Because the controller must direct the rotor rotation, the controller
    needs some means of determining the rotor's orientation/position
    (relative to the stator coils.) Some designs use Hall effect sensors
    or a rotary encoder to directly measure the rotor's position. Others
    measure the back EMF in the undriven coils to infer the rotor
    position, eliminating the need for separate Hall effect sensors..."

    "Although BLDC motors are practically identical to permanent magnet AC
    motors, the controller implementation is what makes them DC. While AC
    motors feed sinusoidal current simultaneously to each of the legs,
    (with an equal phase distribution), DC controllers only approximate
    this by feeding full positive and negative power to two of the legs at
    a time."

    - Franc Zabkar
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