[Elecraft] KPA-1500 Efficiency As A Function of Input Power

[Bob McGraw K4TAX]

Yes yes and yes.    Amps are designed with the fixed output matching 
network chosen for a value of impedance and energy transfer to be at 
"rated power".  Running any amp at reduced power then does not confirm 
to the design of the output matching network and the efficiency 
decreases.   You aren't doing any favor to yourself, the amp, or others 
to run the amp otherwise.

73

Bob, K4TAX


On 8/17/2018 8:24 AM, Charlie T wrote:
> I really do NOT understand why this data is so surprising.
> It all seems to me to be perfectly normal with the amplifier's highest
> efficiency occurring at near max output.
> Which curiously, I would assume,  is the way the amp was designed.
>
> Think zero output with zero drive = zero efficiency.
> Apply some drive, read some output and the efficiency goes up from there.
>
> 73, Charlie k3ICH
>
>
>
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On
> Behalf Of john at kk9a.com
> Sent: Friday, August 17, 2018 8:57 AM
> To: elecraft at mailman.qth.net
> Cc: charles at k5ua.com
> Subject: [Elecraft] KPA-1500 Efficiency As A Function of Input Power
>
> Interesting data, Charles.  I assumed that running the KPA1500 (or KPA500)
> at lower power would keep the fan from running as much. I knew that
> efficiency changed with output power but I did not expect that it was this
> significant. If your data is correct there is 1000++ watts of heat to
> dissipate no matter what power level you use.
>
> John KK9A
>
>
> From: charles k5ua
> Date: Fri Aug 17 01:17:49 EDT 2018
>
> Upon further testing, I have found that the efficiency of the KPA-1500 is
> varies greatly with driving power.  The following table illustrates the
> relationships between exciter power, voltage, current, power-in(voltage x
> amps), power-out, efficiency, and dissipated power.
> The following test was made with the KPA-1500 into a dummy load and readings
> from the KPA-1500 utility software.
>
> (Exciter)(Amps) (Voltage) (Power-In) (Power-Out) (Efficiency Pout/Pin)
> (Dissipated Pwr)
>     10w     29a     52.7v     1528w       375W        375/1528 = 24.5%
> 1205w
>     15w     37a     52.7v     1950w       620w        620/1950 = 31.7%
> 1335w
>     20w     43a     52.6v     2262w       861w        861/2262 = 38.0%
> 1413w
>     25w     47a     52.6v     2472w      1060w       1060/2472 = 42.8%
> 1451w
>     30w     51a     52.6v     2682w      1227w       1227/2682 = 45.7%
> 1420w
>     35w     53a     52.5v     2782w      1380w       1380/2782 = 49.5%
> 1412w
>     40w     55a     52.5v     2887w      1497w       1497/2887 = 51.9%
> 1382w
>     45w     56a     52.5v     2940w      1600w       1600/2940 = 54.4%
> 1313w
>     50w     57a     52.5v     2992w      1703w       1703/2992 = 56.9%
> 1315w
>     53w     59a     52.5v     3097w      1825w       1825/3097 = 58.9%
> 1306w
>
> The remarkable thing is that DISSIPATED POWER appears to be relatively
> constant from 375w output through 1825w output.  This implies the
> KPA-1500 will need to dissipate nearly the same amount of heat at low power
> output as at high power output, if I am interpreting the data correctly. I
> would like to know from Elecraft if this is normal behavior.
>
> Charles  K5UA
>
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