Power transformer stress

Power transformer stress

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Articles and helpful comments appear everywhere about reducing heat in power
transformers, or increasing transformer life, by changing from vacuum tubes to
solid state rectifiers. Most of the articles and helpful suggestions make some
very silly statements about heat, such as “the power transformer is weak.
Changing to solid state removes 10 watts of heat from the weak Heathkit power
transformer, and extends transformer life”.

You will find this advice everywhere for many pieces of equipment, from
guitar amplifiers to amateur radio transmitters. Unfortunately, the advice is
often completely opposite of the truth. We can show how this works by looking at
what causes heat inside transformers.

Replacement transformers are difficult to locate, and replacing damaged power
transformers is often time consuming or expensive. Many of us,
restorers or users
of old radios and audio amplifiers in
particular, would
like to extend the
life of power
transformers in old
tube style equipment. One of the
ways to increase transformer life is to correct the thing causing failures.

The problem is, most people guess at the cause of failures. They really don’t
know why something 20 to 100 years old failed, and they want to do something to
not have another failure in a 20-100 year old part. Since they can only fix
something they can control, they blame something they can easily do something
about. This is the pattern, even though they often have no idea what caused the
failure, or if the thing they are changing is actually helping.  The
purpose of this article is to quantify changes, and logically improve component

Transformer Failures

Transformer failures, outside of manufacturing defects like nicked or scraped
enamel insulation and poor solder joints, generally come from three sources:

1.) A transformer winding is subjected to significant heat from operation
beyond transformer manufacturer’s design or operational ratings, and the heat
destroys otherwise adequate insulation

2.) A transformer is designed or constructed with inadequate or inferior
insulation through design oversights

3.) A transformer has been subject to moisture or other external physical
contaminants or stresses, and the contamination or physical stress has
deteriorated insulation or internal connections

Preventing or reducing failures require we understand the failure mechanism,
and what effects any changes we make might have. Guessing, no matter how many
radios or amplifiers we have worked on, is never a real solution! An
authoritative sounding guess is really no better the guess of a totally
inexperienced person, and can often be worse. People listen more to those with
experience, even when they make no technical sense. An authoritative statement
such as “always do this….” always sounds “expert”, even when it is totally

Transformer Vacuum Tube Rectifiers and Transformer Heat Damage

Transformer heat comes
exclusively from transformer losses, not from power passing though the
transformer. This heat tends raises internal temperatures, based on how much
internal heat is produced and how quickly that heat “leaks off” to the
transformer outside. Naturally, a cooler transformer environment allows more
internal power dissipation before insulation is damaged. The first goal should
be removing heat, but we must understand what we are doing. If we don’t
understand what we are doing, we can easily make things worse.

Losses Cause Heat, Not Pass-through Power

there are several
sources of loss, the
predominant loss is
normally in winding resistance. This is often called “copper loss”. With
casual thought, we
might assume
removing 10 watts of
load power removes 10
watts of heat-stress on the
transformer. This
cannot be correct
because the
transformer loss
would always have to
equal load
power…which we should immediately recognize as an
situation. Yet we find repeated statements that removing rectifier filament load
removes ten watts (or some other number) of heat producing load power from the

Let’s consider a
typical old power
transformer. The
primary has to
supply power for all
the secondary
windings. The power
loss in the primary
is a combination of
the perfect power
factor load caused
by the filaments and
the sometimes very
high power factor of
a capacitor input
filtering system of
the high voltage
supply. The most
common modification
is to remove a
vacuum tube
rectifier and
substitute solid
state rectifiers.
Typical filament
power is 2 amps at 5
volts. We are often
told this removes 10
watts of power from
the transformer.

Since the primary
is sized to handle
all of the power
load any heating by
the rectifier
filament is very
low. The primary
current caused by
the rectifier
filament load is
only around 92
There is very little
change in line
current by removal
of the filament
load. We can
reasonably estimate
transformer heat by
measuring the 
filament winding
voltage change as
the rectifier tube
is switched to an
external 5 volt
supply. In the case
of a National NC-303, I
measured a filament voltage change
from 5.15 volts under rectifier load to 5.43
volts without rectifier filament loading. This means
the equivalent
secondary resistance
of the transformer
filament winding was
(5.43-5.15) / 
2 = 0.14 ohms. 
The 0.14 ohm
resistance would
drop .28 volts at 2
amperes.  The
actual loss
resistance is a
little less than
this…but it is
close enough.

We can now
transformer internal
heat caused by the
rectifier filament.
It is simple I^2 R
heating, so the heat
is 2^2*.14= 0.56
watts. Removing ten
watts of filament
load actually
removed only .56
watts of internal
transformer heat.

Another way to calculate this, since the load waveform is a sine wave, 
is with RMS voltage difference and average current. Filament current is about 2
amperes, and RMS voltage change caused by that current was 5.43 volts no load,
minus 5.15 volts full filament load, for a total of 0.28 volts drop. Since this
is a resistive load with a sine wave, transformer heating will always be LESS
than 0.28 * 2 = 0.56 watts.

It may seem logical we can get rid of 0.56 watts of heat by removing filament
load, but that really is not how it works!!! Let’s look at converting to solid
state rectifiers closely, and see what happens.  

Solid State Rectifier Conversion  

A little mentioned and not-well-understood factor

power transformer
heat when a vacuum tube rectifier is
replaced with solid
state diode rectifiers.
This heat increase occurs because
a solid state
rectifier is a much
harder switch than a vacuum tube. A high vacuum rectifier tube switches into and
out of conduction softly, over a range of many volts. A
solid state
rectifier diode, in stark contrast, either
fully conducts with
a minimal voltage
drop when forward
biased, or is
immediately fully off when
reverse biased.

A vacuum tube
rectifier waveform appears rather
soft when in
transition to conduction. The high vacuum tube can
drop 20-30 volts or
more, and does not
supply the extremes
of peak current
supplied by a solid
state rectifiers.
Substituting a solid
state rectifier for
a tube rectifier
always increases harmonics, and harmonics increase apparent power factor. This
greatly increases heating for a given total load power.

Without adding a
suitable series
resistance or inductance, there is a
significant increase in load power
factor in capacitor input supplies.  (It doesn’t
do much to a choke
input supply, since
the choke input supply already has a
very good power

Because so little is gained by
removal of rectifier filament
power, transformer internal heating is often significantly increased by substitution of
a high vacuum rectifier with solid state
rectifiers in a capacitor input supply. We can increase transformer internal
heat even when we add a voltage compensating resistance, because the resistance
does not emulate the soft switching of a high vacuum rectifier.

Where does most
of the power transformer’s heat come

If filament loads
produce little heat,
where does most of
the heat come from?
With capacitor input supplies, most power transformer heat comes from the high
apparent power factor. Short duration, but very large, load current-pulses cause much
I^2R heating of the
primary and
secondary windings then we would expect just from resistance and the same load current
and power.

compare models of
three loads using
transformers with the same equivalent secondary

A pure resistance
is easy. R1
represents the
transformer ESR
resistance), R2
represents the load.
This load is a
purely resistive load like
a filament winding
or good full-wave
choke-input filter:

Supply voltage is
100V peak, or 70.7
volts RMS. The
resulting RMS
current would be
70.7/110 = .643
amps. Dissipation is
I^R or .413 times
the resistor value.
This would give an
average dissipation
of 4.13 watts
(yellow line) in the
transformer, and
41.3 watts (blue
line) in the load. A
10 ohm ESR
transformer would be
very efficient. Only
9.1% of total system
power is lost in the
power transformer with a purely resistive load of unity power factor.

transformer with a
full-wave and
capacitor input

With no changes
except the addition
of a full wave
rectifier capacitor
input supply about
15 watts is lost in
the transformer with
54 watts delivered
to the load. Out of
69 watts total
power almost 22% is lost
in the transformer! Running the same
load power
more than
doubles transformer
heat when we change from a purely resistive load to a capacitor input supply!
Transformer heat went from 9% of load power to 22% of load power. This is why
transformers designed for resistive loads often make poor capacitor input power
supply transformers.

Now let’s change
to a half wave

We now have 18
watts of transformer
heat and 40.5 watts
of load power. The
transformer would
dissipate around 31%
of the total system
power of 58.5 watts.
We actually reduced
load power by 25%
while transformer heat
increased by

Comparison of
this typical ESR
small power
transformer example
for dissipation with
various loads:

Resistive (like a
filament circuit
load) or full-wave
choke input filter       

Capacitor input
with full-wave          

Capacitor input
with half-wave         

Clearly the
largest power
transformer heat
savings would come
from changing to a
choke input supply
if the equipment
will tolerate the
voltage being
reduced to
approximately 64%
of voltage
available with the
capacitor input
filter.  This
assumes no voltage
drop in rectifiers.
Voltage drop in high
vacuum rectifiers
would actually make
the high voltage
reduction less, so
you may wind up with
75% or more of the
original voltage.
This is just one of
those things we have
to try in a real
working system.

Also, removing
filament load does
considerably less
than we might
expect. This is
because tube
filaments present a
resistive load with
perfect power
factor. The
transformer isn’t
heated nearly as
much by that type of

You might wonder
why high power
amplifiers use
capacitor input
supplies. Slightly
transformer size is
significantly less
expensive than
adding a filter
choke, and the
overall increase in
size required by a
capacitor input
supply results in
much less weight and
size increase than a
filter choke would
add. What we are
talking about here
is the possibility
of reducing heat in
an existing power


Some useful
things to remember:

  • peak dc
    voltage from a
    capacitor input
    supply is 1.4
    times the RMS
  • loaded voltage
    from a choke input
    supply with
    inductance or
    larger is .9 times
    RMS transformer
  • Vacuum tube
    rectifiers will
    often add an
    additional voltage
    drop of somewhere
    around 30-50 volts
    when used in
    capacitor input
    supplies. They
    drop considerably
    less voltage in
    choke input


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