[Collins] Transformers, was 32S-3 wire

[Dr. Gerald N. Johnson]

On 8/26/2013 3:36 PM, antqradio at sbcglobal.net wrote:
> Doctor J's comment about higher then design line voltage made me
> think a bit out of the box.
>
> I was always under the impression, mistaken or not, that 117 vac was
> just the geometric mean for the line voltage limits of 110 to 125
> vac.  Designers specifying 117 vac instead of the high limit of 125
> vac may have inadvertently created a problem for the rest of us.

117 IS the geometric mean between 110 and 125 volts. But many devices 
were specified at 115 with little range in the specifications. I used to 
have a 1/3 hp electric motor that said 115 on the nameplate and it drew 
rated current with no shaft load at 122 volts. It was on my metal lathe, 
then I moved it to an air compressor and burned it up when the piston 
rings froze in the winter when I neglected to unplug its cord.

Power companies tend to prefer voltage running high at light load, then 
they get fewer complaints about low voltage. Each standard transformer 
tends to have about 4% impedance which means if they regulate the 34.5 
KV area distribution voltage, there's up to 4% drop when the substation 
step down transformers to 13.8 KV distribution and then another 4% drop 
in the distribution transformers down to 120/240. If they regulate 8 or 
10% high (to allow for some conductor voltage drop) then the inherent 
transformer drop won't need a lower voltage regulator. Some tend to run 
even higher when the utilities board requires a minimum of 108 or 110 
and they have designed the power system to occasionally (as on hot days) 
load distribution and substation transformers to twice nameplate 
currents. A classic reference manual says doing that to the transformers 
may cut their life in half, from 40 years to 20 years. So the power 
companies run high voltage which makes resistive loads, like electric 
water heaters, electric heaters, electric ranges, and incandescent lamps 
draw more power, hence more revenue. Also induction motors draw less 
current unless the core goes into saturation at higher voltages, 
improving the customer voltage regulation by keeping currents a little 
lower for motor loads.
>
> I would hope that any well designed power transformer would not
> saturate at 125 vac on the primary and thus operate at higher then
> ambient temperatures.  Obviously any transformer with primary taps
> for 115, 120, 125 is in the well designed category.

In power transformer design, the most economical design operates as 
close to saturation on peaks as possible to save on both iron and 
copper. It may not be the most economical for operation or the coolest 
operating transformer but its the cheapest to build. In documents for 10 
million a year fluorescent ballasts I've seen changes of one layer of 
lamination more or less which I believe were in response to changing 
costs of transformer iron and copper. In that production quantity if 
they didn't cause overheating and saved a penny a unit, the engineer 
made his annual salary for the company in one change. Besides the 
lamination count they changed the turns count and I don't know what that 
did for the performance or efficiency where they expect the core to 
saturate to regulate the secondary current.
>
> That said, if anyone notices that the power transformer is running
> too hot to touch in any piece of electronic equipment and that
> transformer does not have primary taps; I would recommend that you
> install a power resistor in the primary lead to drop a few volts and
> run the transformer with 115 vac on the primary.  Much easier to do
> this then to add a bucking transformer, especially when space is at a
> premium.

Definitely easier but not a great idea for a receiver or transmitter. If 
the resistor is chosen to lower the voltage for the operating condition, 
it won't do nearly as much for the standby condition and varying supply 
voltage heats up the tube heaters more on standby and raises the 
unregulated power supply voltages so you get more tube cathode 
temperature changes that can induce delamination of the cathode coating 
and will cause drift of unregulated oscillators when turned on from standby.

I much prefer the bucking transformer, it can easily be a filament 
transformer properly wired and the regulation is very close to that of 
the line even with varying loads.

A variac works well, but unless its hidden some visitor will turn the 
knob to see what it does and a variac these days can be costly compared 
to a filament or low voltage DC supply transformer.
>
> The power transformer is, for the most part, the single most
> expensive component in just about any piece of equipment.  I am sure
> that cost pressures have resulted in the design of power transformers
> with too little iron and copper in them so that they are now close to
> magnetic saturation at present day line voltages.

Some that Radio Shack used to sell had a very high operating temperature 
as if designed for 100 instead of 120 but was labeled 120 volts. I don't 
think they sell transformers anymore.
>
> Much better to prolong transformer life by reducing the internal heat
> then trying to find a replacement transformer for your favorite
> receiver. Jim
>
Very true.
>
73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.