[R-390] Variacs and solas: additional thoughts.

Sat Apr 30 15:20:34 EDT 2005

When I use a variac I always have a voltmeter hooked up to the output at all 
times, because mine will also go up to 140 volts with 117 line voltage,

Bob Young

----- Original Message ----- 
From: "Roy Morgan" <roy.morgan at nist.gov>
To: "W. Li" <wli98122 at yahoo.com>; <r-390 at mailman.qth.net>
Sent: Saturday, April 30, 2005 2:08 PM
Subject: [R-390] Variacs and solas: additional thoughts.

> At 10:02 PM 4/29/2005 -0700, W, Li wrote:
> > >(DO NOT bring it up slowly on a variac!)....
> >
> >I was re-reading an old e-mail of yours, is your advice
> >based on the fact that an unmetered Variac could put
> >over 125VAC onto the primary of the power transformer?
>
> W. Li,
>
> Actually, it can be much worse.  Many variacs are wired for overvoltage 
> and
> would deliver up to 150 volts if they are ment to be run from a 115 volt
> line.  Even if a variac has a meter, it's easy to operate it incorrectly
> and not notice the meter until after some damage has been done.
>
> I just did two experiments:
>
> 1) With a second variac (a GR W5MT3), I powered a STACO Adjust-A-Volt 
> rated
> for 120 volts input, 7.5 amps output.  This thing is both useful and
> dangerous in that it has a two-position power switch: "120 Volts"and "140
> Volts".  I fed it 130 volts to simulate a 117 volt variac on a 124 volt
> line.  With it switched to the 140 volt output, the output was 150 volts
> running a 4+ amp halogen work lamp.
>
> 2) I re-arranged things and fed the General Radio W5MT3 with the
> STACO.  The GR unit is rated at 115 volts input, 5 amps output, 0-135
> volts. It is set up for overvoltage and has the normal ON-OFF power
> switch.  I ran it with 121 volts input (the house line voltage at the 
> time)
> and the output at max setting was 142 volts running a 100 watt lamp.  I
> then increased the input voltage to 124, which I get as line voltage from
> time to time, and the output was 146 volts.  If I run it on normal line
> voltage of 122, it produces something like 147 volts. If I set the dial to
> "115", I get whatever is the line voltage at the time, normally 121 to 
> 124.
>
> So, take your pick, 146 to 150 volts input to your 50 year old radio rated
> at 115 or 117 volts. I'd expect trouble.
>
> The danger with the STACO transformer is that you can easily push the
> switch to "140 volts' instead of "120 volts", and if the knob is set full
> up, you get 150 volts output.
>
> A further note on variacs in general:
> The fusing or circuit breaker arrangements can be important.  The GR W5MT3
> has a circuit breaker on it and the STACO has a fuse.  My guess (without
> investigating it just now), is that both of them are in the INPUT of the
> unit. This means that if there is a short cicuit or heavy overload on the
> ouput, and you start at 0 volt setting and increase the setting slowly,
> you'll be overloading the windings as to current, and the fuse or circuit
> breaker will not trip or blow.  In my opinion, variacs should be fused or
> have circuit breakers in BOTH input and output. This will protect the 
> thing
> wherever an overload would occur.  I have had such failures burn the low
> windings of a small variac in a DC power supply, and recently got a 
> one-amp
> GR W-200B one-amp variac with evidence of the same thing.
>
> Incidentally, if you run into a 400 cycle variac, it can be run on 24 
> volts
> AC. (The allowable voltage is proportional to the frequency.) This can 
> lead
> to a convenient low voltage power supply.  If you hook a 400 cycle variac
> to 60 cycle supply at it's rated voltage, expect smoke in a hurry.
>
> Some notes on constant voltage "sola" transformers:
> Some are the "harmonic neutralized" type and some are not.  All these
> things operate on a magnetic circuit basis with non-linearities and very
> high circulating currents.  Part of the current is harmonics of the supply
> voltage due to the non-linearities.  Thus, the output waveform may well be
> regulated to 115 or 120 volts RMS, but have very high peaks in a distorted
> waveform.  It seems to me that this could create high B+ voltages in
> rectifier power supplies.  Filaments may be very happy at the right RMS
> voltage but the B+ may rise very high due to peak rectification in the 
> supply.
>
> It  has been noted on the reflector that "sola" type voltage stabilizers
> create both noise and a lot of heat, especially when they are lightly
> loaded.  The good advice given was to run them well loaded at all times to
> avoid trouble.  It may be that 20 to 40 percent loading will bring down 
> the
> waveform peaks to reasonable levels, but some experimentation is in
> order.  Installation instructions advise making sure there is plenty of 
> air
> circulatoin to avoid overheating.  The Sperior Electric Company makes Sola
> brand regulators still, and their website has interesting reading.
>
> I have a 230 volt input unit here that runs the photo darkroom to avoid
> troublesome changes in enlarger lamp brightness due to changes in line
> voltage.  When it is switched on, there are significant overvoltage
> transients, and I make sure that nothing but light bulbs are on the line
> when I start it up.  A surge supressor outlet strip might help protect the
> few solid state things I use (timer and enlarging densitometer), but it
> also might get a whacking big surge each time I start up.  The 
> Oscilloscope
> will tell the truth in the matter.
>
> By the way, two of  the three capacitors in this thing failed open at one
> point.  Each was rated at 8 uF and 660 volts ac. The output voltage
> collapsed to near zero.  I was able to locate a fairly high voltage
> (440vac) motor starting capacitor of the right capacitance that was about
> one fifth the size of the original three caps. It seems to work fine.  The
> voltage on the capacitor during operation is normally way above 440 volts
> ac, but it was all I could find, and they normally have breakdown voltages
> very much in excess of the rating.
>
> I once had a Sola made for 50 cycle operation (for use in Europe). The
> thing did not work well at all on 60 cycles. It regulated at some 150 
> volts
> output, and over a  limited range of input voltage.  As I understand it, 
> it
> is not feasible to modify the 50 cycle units to work on 60 cycles.  I sent
> it to someone in a 50-cycle country and he was happy to get it.
>
> If you have a sola transformer that's rated at 120 volts and you want to
> get 115 volts out of it, add a voltage bucking transformer, or see if 
> there
> is room to add a few turns of wire in series with the normal output over
> the existing winding and arrange the connection to reduce the
> output.   I've not added bucking turns to a sola but it's an attractive
> possibility.
>
> Normally, voltage regulator transformers will operate well just a bit 
> above
> their rated output power (they are rated in volt-amps, not current, 
> because
> of the the way they work and the harmonic content of the output.)  Above a
> certain point, however, they collapse and go into a low-output-voltage
> condidition. This is good, because they are inherently current
> limiting.  Apparently, there is no harm to the thing and it automatically
> recovers.
>
>
> Here are some questions to investigate:
>
> Variacs:
> 1) What particular makes and models of "variacs" are set up for 
> overvoltage
> as made?
> 2) Do small variacs behave differently than bigger ones?  (I doubt it.)
> 3) Are fuses or circuit breakers normally in the input as made?
> 4) What errors are found in the dial readings due to line voltages being
> higher than the unit was made for?
> 5) Are the voltmeters found on variacs at all accurate?
>
> Constant Voltage Transformers:
> 1) Do small ones behave in a way similar to large ones?  (I have examples
> from 60 volt-amps up to one Kilowatt.)
> 2) What peak voltages come out of the harmonic neutralized type, and what
> from the non-netutralized ones, as a function of loading.
> 3) Does harmonic content in the output lead to high voltages in rectifier
> power suppllies?  Are choke input plate supplies affected in the same way?
> 4) What are the overload characteristics of these things?  Are they the
> same for small and bigger units?
> 5) Where above rated output do these things collapse, and do all such
> transformers behave this way?
>
>
> Sooo many projects, sooo little time!
>
> Roy
>
>
>
>
>
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Cell 301-928-7794
> Work: Voice: 301-975-3254, Fax: 301-948-6213
> roy.morgan at nist.gov --
>
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