Several things come to mind, none of them good.Originally posted by paultwang@Apr 25 2006, 05:07 PM
What are the effects of jamming a fan from rotating, besides that it doesn't move air?
i.e. what happens to current, power dissipation, and what side effects. I am concerned with brushless DC fans, computer fans.
Hello SparkMan,Originally posted by SparkMan@Apr 27 2006, 04:00 PM
It's been a while since my motor theory class, so I am not fully sure of this answer:
Besides the effect on whatever the fan is supposed to cool, what happens to the fan itself -
DC Motors are a different animal than AC, So this might be more correct for AC motors. But basically in a motor voltage is applied to a coil of wire, and it comes in many configurations, but the two main ones are the coil is either the warped around the base (non-moving part), or the moving spindle (I beleive its called a stator).
The voltage creates a magnet field, and there are once again 2 more possibilites on the next part: Some motors use magnets, and the magnet fields produced by the electrified windings (Electromagnetic Fields) react with the permanant magnet fields, thus forcing the stator to spin. Others use 2 electric fields, one created with the base windings, and one created with the stator, in opposing forces.
My teacher had a theory he called the "happy coil". Basically, when a voltage is first applied to a coil winding, the coil tries to oppose it. When a motor starts spinning, this initial opposition is commonly reffered as "Counter Electromagnetic Force", or CEMF. As the motor picks up speed, CEMF decreases. When CEMF is the lowest, the coil is "happy"
When there is a lot of CEMF, current draw is higher, so when the motor is first powered up, Current draw is highest, and when its running full speed, current is lowest.
Now, when you jam a motor, you are increasing CEMF to the max, so current increases the same.
This, if jammed long enough, can and/or will burn up the motor.
Another lesson in electronics - if you don't use your learned knowledge, you'll forget it. This motor theory is what I can remember, I may be worng on some things, but that's basiclly the deal with motors.
CEMF is a pain with motors. Big huge motors have starters and caps to try to resolve this issue. If you ever noticed your house lights dim when the Furnace kicks on, you start a power tool, washer, etc, its CEMF that does that.
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by Jake Hertz
by Jake Hertz