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| 'Manufacturers of Digital Power Analyzers, Digital Wattmeters, Harmonic Power Analyzers, Wideband Spectrum Power Analyzers, ATC Ohmmeters, Micro-Ohmmeters, Milliohm-Meters, Digital Ohmmeters, Igniter Testers, Current Calibrators, AC-DC Active Current Shunts, Hipot Testers, Body Comp Scales and more…' |
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Transformer Applications Notes
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INTRODUCTION
One important variable that concerns transformer manufacturers is the efficiency. This may seem simple to measure (i.e., Power In vs. Power Out), but there are other factors which are involved. Although very small values, core losses, copper losses, and leakage reactance effect the efficiency and can provide valuable knowledge to the manufacturer. Figure 1 is a diagram of an equivalent circuit of a transformer including losses and leakage reactance. |
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The parallel
branch that is composed of a resistor and an inductor
does not really exist, but the core loss current and the
magnetizing current act as though it does. The resistance
represents the core losses, while magnetizing the core
requires the inductance. Next, the sources sees a
resistance which consists of the primary wire resistance
plus the equivalent secondary resistance. Since the
secondary resistance has been referred to the primary,
the source sees the secondary resistance times the turns
ratio squared. The inductance (reactance) shown is the
leakage inductance of the primary plus the referred
secondary leakage inductance. Finally, there is the load
which is also referred to the primary and is equal to the
load impedance times the turns ratio squared.
This application note covers some examples of typical tests performed by manufacturers to determine the values discussed above, and to evaluate the transformer performance. TURNS RATIO TEST The voltage applied to the primary coil of the transformer divided by the number of turns of the coil is equal to the volts per turn. The volts per turn of the secondary are equal to the volts per turn of the primary. The primary voltage divided by the secondary voltage is equal to the voltage ratio. This voltage ratio corresponds to the turns ratio in the unloaded condition. A similar relationship exists between the primary and secondary current, but inversely proportional. EFFECTS OF FREQUENCY ON EFFICIENCY Due to the reactance found in a transformer, the frequency of the signal input into a transformer can effect the efficiency. From the equations XC = 1 / C = ½ fC and XI = 2 fL, it can be seen that the frequency does effect the reactance of the transformer which can effect the losses associated with these values. To determine the effect, the user can measure the power in versus the power out at different frequencies. |
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