[Collins] Transformers

Thu Aug 29 14:33:38 EDT 2013

Carl, welcome to the fray.
Like you, I have more then my fair share of old radios.  I have seen the same issues with power transformers and share your concerns when transformers are too hot to touch.

At my workbench, I have rewired a 1 KVA, 220 to 115 volt auto-transformer to be a one-to-one isolation transformer so that I have less of a chance for shock and other calamities while working on AC/DC All American Five radios.  Interesting to note that the output voltage is around 117 volts when the input line voltage is at 125 volts.  In the early morning, this voltage can go as low as 113 volts as the power company backs off on the line voltage during low demand.

Yes, the bucking transformer method is superior in efficiency to using a power resistor to lower the operating voltage for power transformers that run hot.  A bucking transformer doesn't get as hot as the power resistor when it is doing it's job.  But in reducing the heat stress on a power transformer, the series resistor still gets the job done.

For small power transformers, let's say under 120VA the series resistor is dissipating under 10 watts to drop about 8 volts from the ac main and it will allow the power transformer to run 10 to 15 (or more depending on core saturation) degrees F cooler.  There is an added benefit in that the peak current in a tube rectifier is also reduced because the series resistor is acting as a current limiter to large current spikes.  Although a better job is done in reducing peak rectifier current by adding 100 or so Ohm power resistors in series with the rectifier plates.  But this discussion point is for another time.

In looking at the Mouser online catalog, a Hammond 266VB6 dual primary and dual secondary transformer goes for $11.90 each.  It has a two 3.15 volt 1.2 amp secondaries so it could be configured as Jerry mentioned below and wired to buck from 6.3 volts down to 1.6 volts.  It will require a 3 X 2 X 2 inch mounting area and weighs just under 3/4 pound.  Other voltages are available but at higher cost.  Also in Mouser is a Xicon 280-CR10-13-RC 10 watt power resistor (I looked at a 13 ohm unit) that goes for $0.64 in unit quantity.  This would work well for a 70VA transformer and drop around 8 volts from the AC line.

Jerry mentioned that the voltage regulation is not as stiff as a bucking transformer and I can see his point.  But with the series resistor, as voltage drops so will the current demand and the associated voltage drop across the series resistor.  So one would think that voltage regulation would be a bit better with the series resistor then with a fixed ratio bucking transformer.
Jim

________________________________
 From: Carl <km1h at jeremy.mv.com>
To: geraldj at netins.net; collins at mailman.qth.net 
Sent: Thursday, August 29, 2013 10:08 AM
Subject: Re: [Collins] Transformers

----- Original Message ----- From: "Dr. Gerald N. Johnson" <geraldj at netins.net>
To: <collins at mailman.qth.net>
Sent: Wednesday, August 28, 2013 9:57 PM
Subject: Re: [Collins] Transformers

> 
> 
> On 8/27/2013 10:26 AM, antqradio at sbcglobal.net wrote:
>> "The harm is in the poorer regulation avoided with the bucking
>> transformer and many an early Collins rig is rated at 115 volts, not
>> 117."
>> 
>> As I understand it, the only way to get any degree of voltage
>> regulation in a power transformer is to operate it near saturation.
>> Otherwise, the transformer's secondary voltage is determined solely
>> by turns ratio.  Near saturation, with the flux density in the core
>> at maximum, any increase in input power just heats copper and core
>> material because there can be no increase in flux density.  If the
>> transformer was designed to operate at these higher temperatures then
>> no harm is expected.  But if this is not so, then the transformer
>> will just cook itself to destruction, most commonly resulting in a
>> shorted turn.
> 
> Not quite. When you saturate a core solidly, the instantaneous impedance goes very low, the slope of the B/H curve and so the current spikes. You have to include some series impedance to keep from drawing hellacious peak currents. And that's not how a Sola works. The sola includes a resonant winding and a loosely coupled winding. When the applied voltage rises that resonant winding's core moves towards saturation reducing its inductance and moving its resonant frequency away from 60Hz (or whatever design frequency) and that compensates by lowering the coupled voltage to the load winding so keeping it constant. Its not possible to change the working frequency of a Sola by changing the capacitor, I've seen engineers try it and fail. You have to change the core stack, the winding and the capacitor.
> 
> Otherwise you improve transformer regulation by reducing its internal impedance. Less leakage inductance and lower winding resistance, while avoiding saturation in the normal operating range. One way to accomplish both is with higher permeability core material, one I used more than 50 years ago was Selectron E IIRC, it was a tape wound grain oriented silicon steel. Then the tape winding was shaped into a pair of U's and cut apart. Because it had higher permeability and a higher allowable peak flux it could operate at more volts per turn than standard E-I laminations and so fewer turns for lower R and leakage inductance. A typical pole transformer with that for core has 1 to 2% impedance instead of 4 to 5% for conventional transformers. But when you drive it into saturation the input currents rise more than hellaciously, more like astronomically because the B/H curve turns FLAT.
> 
> Speaking of transformer impedance. That is conventionally measured by shorting the secondary (ies) and then applying low voltage to the primary raising that low voltage until the rated primary (and secondary) current is reached. The percentage of rated voltage applied is the % impedance. What it means is that with a super powerful primary supply that doesn't vary with load that putting full load on the transfomer will drop its secondary voltage(s) by that percentage. E-I transformers typically have 4 to 5%, tape wound grain oriented silicon steel are typically under 2%. Losses are smaller too with the better core so utilities like that design but don't like the initial cost. I have wound transformers with it.
> 
> So that means the inherent line impedance in your house is a few percent of the total power capability of the distribution transformer supplying the place. For a 25KVA transformer that can mean the normal load resistance for full load at 240 volts is 104 amps, and that is an impedance at 240 of 2.3 ohms so a 4% impedance transformer has a series impedance of .09 ohm. If you try to run a transformer load into saturation that's the series impedance of the power system. The primary wiring contribute very little to the impedance seen at the house voltage. So little its not worth computing, I have on occasion but it was a waste of time. The last transformer impedance is very predominant. Its impedance is higher than the secondary wiring in most cases.
> 
> In my PHd research I was working on communicating from load to substation using LF energy. I worked a couple years on trying to drive a LF current against that impedance and to find it at the substation. After burning up many a transistor and SCR, I gave up on the idea and drew a sequence of half cycle currents instead that I was able to detect at the substation. I had both substation, 7200 volt distribution, and distribution transformers in my basement lab. I supplied the substation transformer from 120 volts through a variac and current transformer hooked to my scope so I was able to observe core saturation very clearly while looking for my signal currents accompanied by ordinary load currents. I used a current transformer on the substation 7200 volt secondary with filtering to notch out 60 Hz and harmonics and to detect my 15Hz signaling by my sequencing of half cycle currents. Four half cycles per bit, one phase or the other with respect to the
 line voltage. Coding would have to have allowed for detecting the initial phase which has been solved for phase shift keying communications circuits so i didn't have to invent that too.
>> 
>> Any of the two methods already mentioned to back off from saturation
>> by reducing the input voltage, will obviously reduce the inherent
>> voltage regulation that comes from operation near saturation.  I have
>> no problem with the bucking transformer method.  It is just that
>> finding room for it in the equipment is problematic.
> 
> I think it should not be in the equipment but should be one bucking transformer for all the vintage equipment in the shack. The bucking transformer can be fairly small because its only handling the change in voltage, not the entire load VA.
> 
> I know one area ham and electrician (now retired) who installed a bucking transformer for his whole house and I think his shop and office years ago. He's moved and I don't know if he moved the bucking transformer or not.
>> 
>> Cost is also an issue, new filament transformers are not cheap.  And
>> finding one with a non standard secondary voltage is next to
>> impossible, so one is stuck with a compromise unless you can rewind
>> the bucking transformer to suit your needs.  That said, power
>> resistors are obviously less expensive then a bucking transformer and
>> can be easily sized according to need and they can be made to fit in
>> smaller spaces by series connecting several to arrive at the optimum
>> value.
> 
> Standard bucking transformers stocked by electrical distributors all over come in two types. One has two 12 volt secondaries and two 120 volt primaries. The other has two 16 volt secondaries and two 120 volt primaries. So with 120 volt primary you can get buck/boost voltages of 6, 12, or 24 with the first type (half power when the primary is connected for 240 but running on 120) or 8, 16, and 32 with the second type. 32 just happens to be very handy in industry when needing to run equipment made for 208 volts on 240 or visa versa.
> 
> Then there are universal rectifier transformers with multiple primary and secondary taps, getting less common but they still exist.
> 
> And the bucking transformer wired with the low voltage in series with the "primary" allowing applying 125 volts to the sum rated at 132 or more moves it further from saturation where its energy efficiency is enhanced.
>> 
>> 
>> Saving power transformers from self destruction is the issue, not the
>> best method for doing so.  Power transformers will benefit from
>> operating at a lower temperature, regardless of the method used to
>> get there. Jim
> 
> Yes, but in the long run the bucking transformer will run up fewer KWH on the electric bill and produce less heat in the hamshack while giving the radio circuits better voltage regulation.
> 
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

I'll add my .02 to this as I have been a bucking transformer proponent and user for at least 30 years now in a home enviroment.

I have a large collection of vintage ham/commercial/military/consumer electronics all protected from the 123VAC or higher line voltage by bucking transformers. For the various vintage radios up in the living areas, mostly 30's consoles from Philco, RCA, Zenith, and others, the transformer is mounted under the chassis in a no added holes arrangement. These are common 120 to 12V 2-3A open frame items from various sources such as All Electronics, Parts Express, Electronic Goldmine, and others selling new ones at a deep discount. Many are also installed in customer restorations including various "boatanchors" when there is space available.

Tests had revealed that many radios were using marginal transformers when built when the average line was 110-117VAC and were already well into partial saturation at 122-123V. Hallicrafters and National had a large percentage of sets built that way and wind up with shorted transformers with even low levels of electrolytic capacitor leakage.

Further tests and analysis show that the most efficient configuration for wiring a bucking transformer is as an autotransformer. Not by much but if the power is on it at all times it helps a bit.

My various benches for modern SS gear, vintage stations, and repair areas are all fed by a large dedicated transformer for each bench. These are all in the 9-13V range and 10-20+A purchased at hamfests and a local electronics surplus shop where prices are by weight and not usefulness to hams as big amp filament iron or for a DC supply.
Considering that a 20A bucker is capable of around 2400W in a rather small package they are easy to locate under or on a bench.

One downside to leaving them always energized is that they do draw magnetizing current 24/7 and I eventually rewired so that all benches have their own master switch.

When operating at the lower voltages the heat reduction, even in modern SS transceivers, is dramatic.  On vintage tube sets its possible to hold onto transformers without getting burns. With "antique" wire and paper insulation any help is a good thing.

I would suggest derating buckers to no more than 75% of maximum; and maybe more if its from China based upon reports Ive read. All of mine were built in NA.

Carl
KM1H

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