seattlejoe1970 wrote:
... as for the fans speeding up and drawing less current if restricted.... i would have thought that Impossible. Could you help me understand why that is.... |
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Uncomfortable Politician getting a pain-in-the-butt question wrote:
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First - the proof:
Block the suction of a vacuum cleaner with your hand.
What happens - the motor slows & grinds to a halt (as "common sense" dictates)??
NO!!! It speeds up! (Can you hear the
whir getting faster or higher pitched? I know it from memory!)
Why?
Cavitation. Air stops, fan "chops" through air and doesn't move it. Hence less friction (speeds up) and less power required (current down).
Cavitation - same happens in powerful propellor boats - connect power too quick and the prop just spins a while before it starts to "grip" and push water thru - the boat equivalent of a wheel-spin (burn out, snakey, etc).
And that too is ONE reason cars can overheat when revving too high OR thermostats are removed. (The other/s being jet-streaming, and changed hydrodynamic flow profiles.) That's another story - but some still think overheating is because the water travels "so fast" it hasn't got time to "lose the heat". (LOL!)
I tried arguing the car overheating, but it wasn't till ~3 years later I learned why [Thanks Dean!] . (I just knew intuitively etc that it wasn't the "not enough time to dump the heat..." Like what - yet is had time to pick it up? Does that mean to heat a house faster, keep the heater fan on LOW? Etc.)
There are conformal sprays - maybe $20 a can? But I would not "conform" a commercial product (warranty etc) - they should be doing that themselves whether for dust or moisture.
But I had the same dilemna years ago for some critical equipment (UPS) - specify filters and hence FORCE maintenance else failure?
And it was Dene or Dean that convinced me (just as above) that monitoring for HIGHER fan current (to indicate blockage) would NOT work! (So very D'Oh! In retrospect that is!)
The final solution - conformal coating but with filter supplied (at base of unit). The site could decide whether to keep or remove filters.
The 2 or 4 fans on top of the units each had a spring-loaded air-flap that would trigger an alarm after a few seconds. IE - allo for flapping, but if no air, then flap closed and alarm was raised.
FYI - air-flaps seem primitive or inelegant, but it's another brilliant design.
What are you trying to measure? Airflow. Not current or fan speed etc.... (Sure - maybe hot-wire sensors etc, but as long as it is air-flow that is measured.)
As to what temp to measure... oh boy!
Components cannot exceed their maximum operating temperature hence sensors are place on the components or heatsinks etc. But that is an "averaging" method - even if one sensor per device, it still won't sense instantaneous overheating (eg current).
And usually it's the big components you monitor - eg, output transistors or FETs etc, input transformers or chokes.
Individual components cannot be monitored economically. Besides, it they overheat, it is usually some failure which you will find out about anyhow...
The other temp consideration is not immediate failure, but lifetime.
A common rule for electronics & electrical is that every 10C or 15C increase is a HALVING of its life. EG - maybe a battery at 45C lasts half as long as at 35C (lifespan - not capacity!), or an amp at 50C lasts half as long as at 35C.
And also, the higher the temp, the greater the chance of failure (which may be another way of stating the above "lifetime" reduction).
So - what to measure?
Intake air? (Will you shut it down if it's too hot? Or to meet warranty conditions - if it's warranted up to 45C, then have a 45C intake-air sensor?)
Exhaust air?
Case temp? (If output heatsinks are ok and inputs are okay, but case to "too hot", is there a problem or just a hot car?)
Amps should have thermal protection. Usually a bi-metal temp switch (Klixon etc) on their heatsinks etc. These are usually cheap on-off switches ($4-$6 retail).
I'm experimenting with a 90C klixon as my car's fan switch (the car uses an 82C thermostat).