|
|
|
Many of us, restorers or users of old radios in particular, would like to extend the life of power transformers in old radios. One of the ways to do this is to reduce transformer heat. The heat in a transformer comes from losses. While there are several sources of loss, the predominant loss is normally in the resistance of windings. With only casual thought we might assume removing 10 watts of load removes 10 watts of heat or stress on the transformer. This cannot be correct because the transformer loss would always have to equal load power...an impossible situation. 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 milliamperes. 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 NC-303 I measured a change from 5.15 to 5.43 volts. This means the equivalent secondary resistance of the transformer filament winding is (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 determine 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. A second factor actually INCREASES power transformer heat when the rectifier is replaced with solid state rectifiers. This occurs because the solid state rectifier is a much harder switch. A solid state rectifier either fully conducts with a minimal voltage drop when forward biased or it is fully off when reverse biased. A vacuum tube rectifier is rather soft when in conduction. It can drop 20-30 volts or more and does not supply the extremes of peak current supplied by a solid state rectifier. Substituting a solid state rectifier for a tube rectifier without adding a suitable series resistance can significantly decrease power factor of the load presented by a capacitor input supply. (It doesn't do much to a choke input supply, since it already has a very good power factor.) Because we gain so little with removal of filament power we can actually increase transformer internal heating through substitution of solid state rectifiers...unless we add a suitable softening resistance or inductance. Where does most of the heat come from?If filament loads produce little heat, where does most of the heat come from? With capacitor input supplies most of the heat comes from the high peak currents causing excessive I^2R heating of the primary and secondary windings on peaks. Most of the stress on a power transformer comes from the high power factor of a capacitor input power supply. Let's compare models of three loads on a transformer of a given source resistance. A pure resistance is easy. R1 represents the transformer ESR (equivalent secondary 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. Now let's look at the same transformer with a full-wave and capacitor input filter:
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! If we ran the same load power we would more than double transformer heating!
Now let's change to a half wave rectifier:
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 20%! 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 9.1% Capacitor input with full-wave 22% Capacitor input with half-wave 31% 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 load. You might wonder why high power amplifiers use capacitor input supplies. Slightly increasing 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 transformer.
Some useful things to remember:
|
|
