Date: Mon, 3 Feb 2014 08:24:17 -0700
From: "Jim Garland" <4cx250b@miamioh.edu>
To: "'Roger \(K8RI\)'" <k8ri@rogerhalstead.com>, <amps@contesting.com>
Subject: Re: [Amps] Air temps leaving the amp.
This raises an interesting issue. How does one reduce the impeller speed on
a blower with a 120 VAC motor (capacitor start)? I've done some tests on
several blowers (3370 rpm) and find the rpm doesn't vary appreciably as the
voltage is varied over a significant range. Once the voltage gets low
enough, of course, the blower stalls, but the rpm doesn't vary with the
voltage. I suspect this is an inherent property of induction motors. I first
discovered this effect years ago when I tried to reduce the blower speed of
a Rotron blower in an 8877 amp. I switched in a 24V transformer to lower the
voltage from 120V to 96V, but discovered it didn't make much difference.
73,
Jim W8ZR
## Use PWM to lower the speed.
On another note... air exhaust temps are sorta meaningless...and are
tube specific. Some tubes like a 3CX-3000A7 are more eff at extracting
heat vs other tubes.. like a 3CX-6000A7. 67 cfm for 4 kw diss for the 3x3 vs
205 cfm for 6 kw diss for the 3x6. The hot air exhaust is higher on the
3x3...cuz
it extracts more heat per cfm.
## use a manomometer to measure pressure. If its good, then forget the
exhaust temps.
Or just point a fluke IR at the anode under normal op condx.
## On gigavac ham series vac relays... their power ratings are meaningless.
2.5 kw
means 2000 vdc at 1.25A..which of course has nothing to do with the normal 7A
into 50 ohms.
Jim VE7RF
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