- Posted on
Tue May 22, 2012 10:00 pm
-
roussell
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- Posts: 1108
- Joined: Aug 18, 2008
- Location: Alabama
Well, the short answer is motors are wire differently depending on their application and sometime that mean that a motor's speed can't be adjusted.
Here's the long answer:
There are a few of ways to change the speed of an AC induction motor. First off for US 60Hz AC, you can count the number of poles in the stator of the motor and divide that number by 7200. So, if you looked into the attic fan motor and counted four poles, doing the math you'd come up with 1800 RPM which is the no-load speed of the motor - the loaded rpm of the motor in that case would most likely be 1725 rpm which is common for most general purpose AC motors - especially in belt drivin applications. As you can see by the equation for 60Hz: 7200(number of poles)=RPM, you can vary the speed of the motor by changing the number of poles that are active in the stator. Multi-speed box fans, old ceiling fans and other general purpose AC induction motors do this by having more poles and electrically switching them in and out of circuit to change the speed. As an example, if you have a 12 pole motor, you can get three speed levels 1800, 900, and 600 rpm by powering up 4, 8, or 12 poles of the motor.
The second way is to add capacitance to the motors field windings. A capacitor placed inline with the field windings effectively changes the phase angle fo the current in relation to the voltage and in essence acts as if the motor was getting a lower frequency supply. Remember the constant 7200 above being based on US 60Hz current ? Well 50Hz current uses the constant 6000 so in our four pole motor example, the no-load speed at 50Hz would be 1500rpm instead of 1800. The capacitors are the small black cubes that are usually stuffed inside the light housing on the fan.
The third common way it the way devices like the FanLinc and other aftermarket speed controllers work. The work by altering the AC signal before it gets to the fan, again tricking the fan motor into think that the frequency has been altered. Early electronic speed controllers worked similar to electronic dimmers and simply chopped the AC waveform into something resembling a series of shark fins - that works well for lights, but would cause motors to hum (the hum was the actual windings vibrating with sudden collapse of the magnetic field associated with the modified sine wave).
Now, onto the reason it most likely doesn't work with your attic fan: I presume your attic fan motor is a shaded pole, internal rotor induction motor with 4 poles, unless it has a bulge on it's side, in that case the bulge is from a start capacitor and then is a split-phase capacitor start induction motor. If it's a belt driven fan then the motor probably has 4 poles on it's stator, possibly more if it's direct drive. in contrast your ceiling fan will have anywhere from 8 to 14 poles and is probably an shaded pole external rotor. By design, the ceiling fan motor will require less in-rush current during startup than the attic fan, also more importantly, it will require substantially less current to continue running than the attic fan. The ceiling fan's multiple poles also lend themselves more easily to being 'fooled' by the altered sine wave produced by the Fanlinc than does the attic fan. The attic fan works with the fanlinc in High speed because the Fanlinc is delivering an unaltered sine wave to the attic fan motor when it's on high. THe only thing that comes into play then is the current capacity of the fanlinc, which again most assuredly has to be less than what the attic fan requires.
Hope this helps explain things a little bit. If you take some pictures of the attic fan, I might be able to help more, or better yet, if you can get a picture of the motor plate (the little plate/sticker on the moror with all the info on it) I might can offer a better description of what you have electrically.
Terry
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