Stupid physics question: Will impedance effect RPM?

kevinshi · Post by **kevinshi** » Mon Feb 07, 2005 8:18 am

I have two identical fans being driven by the same fan controller (Hardcano 13) at nominally the lowest speed setting (as in both are cranked all the way down). The Hardcano reports ~950 RPM for one and ~750 RPM for the other.

The only difference I can think of that could reasonably explain this is that the 'slower' one has far more intake impedance (and more exhaust impedance for that matter). I assume from a physics perspective this is expected, eh?

flyingsherpa · Post by **flyingsherpa** » Mon Feb 07, 2005 8:44 am

well an easy way to test that would be to remove them from your system and test them both in free air conditions. or swap their locations and see if the trend is consistent.

my first impression is that yes, it's possible... but there could be other factors causing it. manufacturing diffferences, slight voltage differences from different lines on the fan controller (you have a voltmeter?), etc. with some fairly easy tests like above you could at least rule some things out.

burcakb · Post by **burcakb** » Mon Feb 07, 2005 11:03 am

That's not necessarily a given. My Globe fans speed UP with greater impedence.

ferdb · Post by **ferdb** » Mon Feb 07, 2005 2:30 pm

Typically fans draw less current and spin faster with lower flow (increased airflow impedance). I would suspect an electrical or fan difference in your case, not an airflow difference.

kevinshi · Post by **kevinshi** » Mon Feb 07, 2005 10:31 pm

Sigh. OK, I guess I'll take the case apart *again* and test them in uniform conditions.

Now you all have me worried that my hack soldering job (replaced 4 pin connectors with 3 pin connectors) is the culprit...

I'll repost in a few days (yes it will take me that long to get around to this!