What do modulation transformer ratings really mean?

[KE6DF]

Modulation transformers are usually rated in watts.

So when a transformer has a catalog rating of 125 Watts, what does that mean?

Does it mean the transformer can output 125 Watts continuously with a sine wave input?

But, of course, voice isn't even close to a sign wave.

Or is 125 Watts the peak, instantanious power the transformer can pass without saturating?

Usually, a 125 Watt transformer is rated to be used with a 250 watt final amplifier.

But, with voice output, the modulation transformer will be outputing way less than 125 watts when used in a 250 watt AM transmitter.

So what does the wattage rating mean?

[W7TFO]

This has been addressed many times over many years, both in theory and in practicality.  Articles abound in both the ARRL and Jones handbooks, also in QST, Radio, and surplus modification magazines.

What it really boils down to is reliability under continuous service.  

To handle hi-fi, aka 'broadcast' audio, the response and distortion specs come into play and are the most important aspect.

To actually get a transformer to pass everything from 50Hz to 15KHz at power is no mean trick.  The only way to do it is to use high permeability iron, generous copper, and intelligent winding techniques.  Circuit designs also are set up for maximum performance and impedances are specified to the Ohm.  

Here you have more size, weight, and expense.


For communications grade performance, the target is limited response (say 300Hz to 3500Hz) and package size.  This assumes intermittent voice duty, DC in the secondary, and generally matched impedances.

To wit, look at the modulation iron from an ART-13.  Very small, easily held in one hand, but legendary in the ability to handle a lot more power than designed for due to limited frequency response and winding technique.

Another example is a lot of the "poly-pedance" types with multiple tapped windings.  They can be made to work in many circuits, and the power handling ability is directly proportional to the audio bandwidth running thru it.

I have seen transformers factory rated at 175 Watts doing a fine job in ham transmitters of 1kW input with a narrowband audio channel and properly set spark gaps added.  

Unfortunately, the days of ordering a transformer for a few dollars and 'guinea-pigging' it in a rig just to see where it will yield is over.

Research as to what worked OK in the past would be a good idea, and patient looking for a good vintage part is still viable.

Bottom line:  You can mod between 50 and 80% overload if just using restricted voice bandwidth.

73DG

[KE6DF]

This has been addressed many times over many years, both in theory and in practicality.  Articles abound in both the ARRL and Jones handbooks, also in QST, Radio, and surplus modification magazines.

Thanks for the response.

But I think you may have taken my question as more general than was intended.

I was asking specifically what the wattage ratings mean and they were generated by the manufacturer.

[W7TFO]

It means no fudging or guesswork if you stay within what they say;  a 125-Watt transformer modulates a 250-Watt transmitter 100% 24/7 regardless of duty.

73DG

[KM1H]

Only if its commercial or military grade. The economy grade is ICAS rated at its maximum listed ratings which I believe was 50% duty cycle back then. I would think some of the windbags on the bands better have commercial grades

So now you have to ask yourself....do you feel lucky to run economy grade with heavily processed audio and 125% positives anywhere near it max?

[N4LTA]

How much more can you get out of one with all the plate current diverted through a modified Heising circuit?


Pat
N4LTA

[k4kyv]

Only if its commercial or military grade. The economy grade is ICAS rated at its maximum listed ratings which I believe was 50% duty cycle back then...  do you feel lucky to run economy grade with heavily processed audio and 125% positives anywhere near it max?

I don't think duty cycle is an issue with most modulation transformers, even "economy" ones.  I have never seen a modulation transformer overheat even when operated well beyond its specified ratings.  Not to say it wouldn't with a pure sinewave tone modulating 100% running 24/7, but we don't operate those transformers that way.  Even heavily processed voice modulation is inherently  low duty cycle.

Modulation transformer failure is usually due to insulation break-down.  High positive peaks (150% or more) with an ancient transformer running at its maximum rating might be a different matter.  Operating the transformer without a load is a sure way to crap it out, but I suspect the majority of failures are due to transient peaks.

Whenever I run a modulation transformer or reactor anywhere near its maximum rated power or more than about 50% of its maximum  rated voltage, I mount it on insulators and float the entire case and core away from ground.  Grounding the frame puts additional unnecessary stress on the insulation between core and winding and serves no useful purpose.  I take similar precautions with power supply filter chokes.  Also, spark gaps across the PRIMARY winding are a good idea.  Never place gaps across the secondary without also having a set (a gap between midtap and each side) across the primary.  A spark-over across the secondary may induce dangerous transients in the primary and destroy the transformer, so a gap across the secondary only is worse than no gap.  Of course, spark-overs and arc-overs in the final amp stage may produce the same effect, one more reason to maintain a set of gaps across the primary. Some transmitters use a relay to short out the secondary of the modulation transformer for CW or linear operation; those relay contacts may inadvertently function as a spark gap and generate transients when they spark over.

ICAS vs CCS ratings is a concept concocted by RCA right after WWII, primarily applicable to tubes.  Dangerous transient voltages can just as easily destroy a transformer whether they occur 0.1% of the time or at every voltage cycle.  Same thing with solid state devices such as  transistors and diodes. It takes just one zap to puncture the (usually paper) insulation of a transformer or the P-N barrier of a solid state device.

[KM1H]

Intermittent amateur service goes back to at least 1938 in this Thordason catalog.
Plus they had a full series specifically for amateurs....translation: economy

http://www.bunkerofdoom.com/xfm/THORDARSON_400C/Thordarson400C.pdf

RCA may have copied the idea but cheap hams and articles in QST and Radio preceeded them by a long time.

I don't think duty cycle is an issue with most modulation transformers, even "economy" ones.  I have never seen a modulation transformer overheat even when operated well beyond its specified ratings.  Not to say it wouldn't with a pure sinewave tone modulating 100% running 24/7, but we don't operate those transformers that way.  Even heavily processed voice modulation is inherently  low duty cycle.

Thats the whole point....ham grade could be made cheaper and lighter because it was built strictly for low duty cycle. But if some gasbag locked the mike for an hour the iron, copper and insulation could get to dangerous temperature levels. In addition ham grade iron wasnt designed for the current hi-fi fad and trying to ram all that low frequency power thru them is not healthy.

[Opcom]

One problem with power ratings is that the MFGR will give a power level but not a minimum frequency at which the power can be transformed. The two values are directly related.

The ART-13 transformer may be a 100W unit, but they don't say it only does 300Hz.
The 90W unit on my shelf weighs as much as four of the ART-13 transformers. It is rated down to 100Hz.

There is more to the spec than power. The lowest frequency has to be considered and is determined by the inductance and the amount of magnetism the core can take (size is generally an indication of that).

There is a power bandwidth sometimes given, where the trans will for example handle 100 watts from 50Hz to 5000Hz -3dB. That means the power at 50Hz and 5000Hz is only guaranteed to be 50W, but somewhere inside that range it rises to 100W (possibly from 70.7Hz on up, see article below).

Then there are units that say 125W from 200-7000Hz +1dB. That unit will do 100W at 200Hz and up, but at some point it will do 1db more.

All those things have to be taken into account and where any part of the information is missing it is a crap shoot until the transformer is tested in a circuit.

If you will derate the transformer then it will handle lower frequencies. A dirty trick is to know the core area and figure the wattage at 60Hz as laid out in the handbooks, as if it was a power transformer. Then extrapolate from there for 120Hz, voice service, etc. That is not completely accurate because of core materials and other factors but a shotgun is often good enough to hit a burglar in a dark room.

Same for the ol' 400Hz transformer thing. BTW compare a 1KVA 400Hz transformer to a 1KVA 60Hz one.


I give an example: suppose you select an
OPT which can handle 50 Watts at 30 Hz.
Then this transformer can handle 50/2 = 25
Watt at 30/1.414 = 21.2 Hz. Or the
transformer can handle 50*2 = 100 Watt
at 30*1.414 = 42.4 Hz.
The rule behind all this is: “the power
capability doubles when the frequency is a
factor 1.414 larger. The power capability
halves when the lowest frequency is divided
by 1.414 (square root 2)”.
-M.V.

Menno Vanderveen wrote a paper or two on power handling vs frequency and gave this formula. It applies to correctly designed systems but seems like a good rule.
http://plitron.com/wp-content/uploads/Atcl_1.pdf