[Elecraft] 120V vs 240V

[Edward R. Cole]

Important reasons for running HP amps with 240vac.

I used to run a smaller PS that provided 2500v at 400mA = 1000w dc 
input with 600w RF out on my 2m-8877.  That PS ran on 120vac and drew 
8.3 amps on ordinary house wiring and I saw some flicker in the 
lights on CW.  I don't recall what the voltage sag was.

Now I run 3700v at 720 mA = 2664w dc (1400w RF) which represents a 
10.9 amp load on 244vac (my line voltage).  I see about 2-3 volts 
line -voltage sag so that is 33w dissipation in the 8-4 wiring. If I 
had tried running with 120vac the load would be 21.8 amps which would 
exceed the rating of the wiring and likely trip breakers.  The power 
supply is capable or 1.5amps dc load so represents a max load of 5kW 
and that hits over 20A which is my current breaker rating.  Of course 
this would produce 2500w RF which exceeds legal level by a lot.

Important stuff to consider when running significant RF power.  The 
KPA-500 will likely run on 120vac with no problem but better if run on 240v.

------------------------------
Message: 16
Date: Wed, 16 Feb 2011 10:35:27 -0600
From: Scott Ellington <sdelling at facstaff.wisc.edu>
Subject: Re: [Elecraft] 120V vs 240V
To: Elecraft Reflector <elecraft at mailman.qth.net>
Message-ID: <74E5832A-97F4-494F-8B36-377DDC47183C at facstaff.wisc.edu>
Content-Type: text/plain; CHARSET=US-ASCII

Actually, the percentage voltage drop at 120 V is FOUR times that at 
240 V. (For the same power, wire gauge and length.)  Since the 
KPA500, like most tube-type amplifiers, uses an unregulated power 
supply, the extra drop can significantly reduce output.  For example, 
suppose the amplifier can put out 500 W with a perfectly regulated 
input of 120 or 240 V.  Say the line voltage, at 240 V, drops 6 volts 
at full output.   That's a 2.5 percent drop, which reduces RF output 
by about 5 percent, to about 475 W.  At 120 V, the drop is 12 volts, 
10 percent, which reduces the output by about 20 percent, to about 
400 W.  You just lost 75 more Watts, each of which cost you $4.

Keep in mind that 500 W output requires about 1 kW input, that the 
power factor of the power supply is considerably less than 1, and 
that the high peak currents make the effects of line voltage drop 
even worse.  When calculating effective line drop for the KPA500, I 
would use something like 2 kVA, about 17 A at 120 V. The above 
example corresponds to a run of 220 feet of AWG 12 (440 feet total), 
admittedly a very long run.

Bottom line:  If you are adding wiring, use 240 V.  Run the amplifier 
on 120 V only if you have no choice, or if the run from the 
distribution panel is under about 50 feet.


Scott  K9MA





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