[Elecraft] KPA500 cooling and efficiency

[ANDY DURBIN]

Bob,


I have made no changes to the cooling of my KPA500.  I also have no idea to what extent the designers of the KPA500 studied the thermodynamics of the design.   I have, however, been surprised at how quickly it heats up.    I abandoned a heating test with the amplifier only producing 100 W output when the PA temperature reached 70 deg C after only 1 minute 45 seconds.  At that point the temperature was close to asymptotic and fan speed was 5 so it may not have gone much higher.


My KPA500 seems horribly inefficient when compared to my TS-590S running a similar power output.  The data for the heating test showed a PA efficiency of 23%, far short of the "approximately 50%" stated in the specification.


I'm willing to share my test data but I know of no means to post it on this stone age "reflector".


73,

Andy k3wyc


________________________________
From: Bob McGraw K4TAX <rmcgraw at blomand.net>
Sent: Monday, May 21, 2018 6:57 PM
To: ANDY DURBIN
Subject: Re: [Elecraft] KPA1500 fan speeds and fan noise

Andy:

There are basically two types of radial blade fans and specific
application for each one.   Those that "blow" and those that "suck".
There is a difference in the design of the fan blades and the
application for a specific fan.   Also some are designed to have "air
over the motor".  This is a point affecting motor life and bearing
lubrication.

Those that "blow" are designed to pressurize a box or enclosure to a
specific  pressure measured in inches or millibars of H²O. Reducing the
air exhaust area or restricting air exhaust flow, thus increasing
internal pressure, can cause a blade stall.  This actually reduces the
amount of air measured in CFM that a given fan can deliver.

The other type  of fan, those that "suck" are designed to exhaust air
from a box or enclosure to a specific value measured in inches or
millibars of H²0.     There is always some amount of restriction to
airflow entering, usually being contributed by the obstruction i.e. air
vents,  to air entering the box.  The perforated air vent on top of the
KPA500 as an example.

While I agree that increased airflow is beneficial to keeping
electronics cool, one must assure that added cooling efforts do not
create a vortex in the given area to be cooled.   This could cause a
void of air movement and thus reduced cooling.    The KPA500 {I can't
speak for the KPA1500} are surely engineered and evaluated {brick on the
key}  to work within design limits of the solid state devices along with
cooling other components.    In some cases, ones added cooling efforts
which may actually cause a partial blade stall, same as the stall effect
of an airplane wing, and thus cooling is actually reduced and, in many
cases.  In some instances, overall fan noise being increased.

Take care in the application of added cooling efforts.

I run my KPA500 with the FAN CTL at Normal.   It will step through all 6
levels as needed, otherwise it is off.    I find no reason for it to run
all of the time at the lowest speed. Depending on mode and length of
transmission, I find the fan comes on and will ramp up as needed.
I've not seen any reported temperature above 65ºC with a heat sink upper
limit of 90ºC.

73

Bob, K4TAX