[R-390] Variacs and solas: additional thoughts.
Bob Camp
ham at cq.nu
Sat Apr 30 15:42:38 EDT 2005
Hi
A few observations:
Variac's are always fused on the input side - thank the national
electric code for this one. Rarely you will see one with dual fuses,
but if it only has one fuse you'll see it on the input.
Constant voltage transformers have one other oddity. When you check
the output voltage with a fairly normal AC voltmeter you never quite
know what you will get. A true RMS meter will give you one thing, and
a peak reading / calibrated to read RMS meter will give you something
very different.
Take Care
Bob Camp
KB8TQ
On Apr 30, 2005, at 3:20 PM, ROBERT YOUNG wrote:
> 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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