[ARC5] Need Help with Variac Diagnosis

[Bruce Long via ARC5]

The breaker tripping at no load might be in-rush current but if that is the case it should only trip the breaker - say- 1 out of three times.  If you close the power switch at the peak of the 60 Hz sine wave peak, the in-rush current will be very small.  Large in-rush currents happen when you close the power switch near the sine wave zero crossing point when voltage is at a minimum --- no I did not get that wrong, power on at V=zero is the worst case.  

The drop in output voltage could be consistent with a change in the line voltage or consistent with a small steady state voltage drop in your in-rush limiter.  It is not clear from your text what conditions existed when you made the two secondary side voltage measurements.

      From: Mike Feher <n4fs at eozinc.com>
 To: 'Phillip Carpenter' <carpenterpa at tds.net>; arc5 at mailman.qth.net 
 Sent: Thursday, February 19, 2015 2:05 PM
 Subject: Re: [ARC5] Need Help with Variac Diagnosis
   
Is it possible that you do not have it connected correctly? There is minimal
inrush current with no load, makes me think you used the wrong taps for the
input. Regards - Mike 

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 


-----Original Message-----
From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Phillip
Carpenter
Sent: Thursday, February 19, 2015 2:01 PM
To: <arc5 at mailman.qth.net>
Subject: [ARC5] Need Help with Variac Diagnosis

I have a General Radio W20MT3 autotransformer, this one handles 20 amps, or
about 3,000 watts. It came without meters so I've added a Simpson AC
voltmeter in parallel on the load side and a matching Simpson Ammeter in
series with the reset beaker on the load side.

I'm having two issues: 1) the unit trips my 20A house breaker at no load. I
believe this is caused by inrush current so I have ordered a 20A inrush
current limiter to help solve that problem. 2) originally the voltmeter read
120 volts when the Autotransformer was rotated full, but now it only reads
117 volts. If I switch the output to 140 volts the voltmeter only reads 134
volts. Something has started acting like a resistor and dropping the
voltage.

Does anyone have an idea what could be causing this? I plan to use this
Autotransformer to drive my tube radios, such as the ARC-5s, as well as my
Hallicrafters SR-2000 ( which is why I have the 20A version). The AC voltage
here runs 120-121 volts, so I want to roll it back to 115-117 volts.

I can't figure why the output voltage dropped below 120 or 140, depending on
the winding setting. Could there be some metal shaving has fallen into the
winding that is shorting it?

Any thoughts from those with experience using a Variac is very much
appreciated!

Phil

Sent from my iPhone
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