24VDC Fan motor won't run

[chipwitch]

Hi everyone.  Been a while since I've been here.  Work has been keeping me from electronics for a year or so.  As I was only just beginning to learn a thing or two, before I was pulled away, I fear I'll have to start all over. 

Anyway, I'm working on a small project for which I needed a blower fan.  I salvaged a squirrel cage blower from a copy machine a couple years ago and thought it'd be ideal for my little project.  It draws a little over 3/4 amp, according to the tag.  There are 3 conductors, black, red and yellow.  I expected black and red to be negative and positive, with white as a speed sensor.  Unfortunately, the motor rotates a few degrees and stops.  Spinning the squirrel cage with voltage applied doesn't help.  Bearings seem okay and cannot detect any physical blockage preventing rotation.  Rotates easily by hand, but "steps" every 90 degrees. 

I can see the edge of a small circuit board inside, but the case is riveted.  Before I drill out the rivets and inspect it, I thought I'd ask if there's the possibility I'm doing something wrong.  I'm assuming the pcb is just part of the speed sensor.  Is it possible the pcb does more?  Like, senses voltage to protect the motor?  I'm using an old laptop ps, 19vdc 3000mA.  Wasn't planning on it as a permanent solution, just testing the fan.  Or, is the yellow wire something other than speed sensor?  Could it be a stepper with the case as a conductor?  I can feel 4 "steps" per revolution if I rotate by hand.

Thanks for the input

[djQUAN]

There are 3 conductors, black, red and yellow.  I expected black and red to be negative and positive, with white as a speed sensor.

which is it? I presume it is a tachometer output which pulses according to rotor RPM. Used for detecting stuck fans.

Fans with PWM input are usually 4 wire. two for power, one for PWM input and another for tach output.

Powering it through the red and black should make it spin without anything needed to be connected to the third wire.

[chipwitch]

Oops... yellow

[mikeselectricstuff]

It's possible there's an enable input.
It's not reasonable to expect a 24V 750mA blower to have much in common with PC fans.

[Ian.M]

Its also possible its is a BLDC motor and needs an external controller to feed it a two or three phase waveform.  The PCB may just be to anchor the winding ends and provide convenient terminations for the flex wires.   I suspect that you may find the black is the common and there is equal resistance to both the red and yellow wires, with double that between red and yellow.  A two phase motor will have 4N positions it 'cogs' at and resistanse as just described. A thre phase motor will have 6N positions ad equal resistance between any pair of wires.

[chipwitch]

I appreciate the input.  Checking the resistance, I get 115k between red and black.  Does that seem high?  Yellow/red is zero and about 20k for yellow/black.

Also, I made this observation which I think is pertinent...  There is a brief time delay between "attempts" to start the motor.  In other words, when I energize the black and red leads, the motor lurches.  I immediately open the circuit.  If I then immediately close the circuit again... nothing happens.  However, if I wait approximately 1 second (maybe less) before closing the circuit, the motor will once again lurch.  This behavior is repeatable.  It's as if a capacitance is being discharged during which time the motor is being locked out.  Does that make sense?

Yes, I believe the motor is brushless.  And the label says it is 24vdc.  The pcb to which I referred earlier is integral to the fan assembly.  Would such a label be appropriate if a multi-wave form controller was needed?  Would that not then be considered ac?

[Ian.M]

That's got an internal controller.  %DEITY% only knows whether it takes a logic signal  for on/off, or a voltage or a PWM for speed control.  Continued experimentation with applying power without knowledge has a high probability of FUBARing it.  As you didn't take measurements before salvaging it, if you cant google up anything from its part number, you have no choice but to drill out the rivets and open it, and hope you can identify the controller and find a datasheet.

[lm3baker]

I appreciate the input.  Checking the resistance, I get 115k between red and black.  Does that seem high?  Yellow/red is zero and about 20k for yellow/black.

Definitely has an internal controller. The first thing i'd do is place a current meter in series (so I can tell if something is going wrong), the second is apply voltage through a high impedance (like a 100k ohm (one hundred thousand ohms) resistor) to the yellow (while powering the red and black) , and measure the voltage between the yellow and black. I'd expect the fan to start, with a clamp at 3.3 or 5 volts on the yellow.

[Connoiseur]

Oddly enough, I have a similarly behaving fan from an old Mercury PC. The fan runs when its yellow wire is connected to Vcc and black to ground