Step-up and step-down transformers

Chapter 9.2 Step-up and step-down transformers

Lessons In Electric Circuits Volume II – AC Book
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Lessons In Electric Circuits Volume II – AC Book

  • 9.2.STEP-UP AND STEP-DOWN TRANSFORMERS229but the primary and secondary currents are not. In this particular case, the primary current is28.35 mA while the secondary current is only 9.990 mA: almost three times as much current in theprimary as the secondary. Why is this? With less inductance in the primary winding, there is lessinductive reactance, and consequently a much larger magnetizing current. A substantial amount ofthe current through the primary winding merely works to magnetize the core rather than transferuseful energy to the secondary winding and load.An ideal transformer with identical primary and secondary windings would manifest equal voltageand current in both sets of windings for any load condition. In a perfect world, transformers wouldtransfer electrical power from primary to secondary as smoothly as though the load were directlyconnected to the primary power source, with no transformer there at all. However, you can see thisideal goal can only be met if there is perfect coupling of magnetic flux between primary and secondarywindings. Being that this is impossible to achieve, transformers must be designed to operate withincertain expected ranges of voltages and loads in order to perform as close to ideal as possible. Fornow, the most important thing to keep in mind is a transformer’s basic operating principle: thetransfer of power from the primary to the secondary circuit via electromagnetic coupling.• REVIEW:• Mutual inductance is where the magnetic flux of two or more inductors are “linked” so thatvoltage is induced in one coil proportional to the rate-of-change of current in another.• A transformer is a device made of two or more inductors, one of which is powered by AC,inducing an AC voltage across the second inductor. If the second inductor is connected to aload, power will be electromagnetically coupled from the first inductor’s power source to thatload.• The powered inductor in a transformer is called the primary winding. The unpowered inductorin a transformer is called the secondary winding.• Magnetic flux in the core (Φ) lags 90o behind the source voltage waveform. The current drawnby the primary coil from the source to produce this flux is called the magnetizing current, andit also lags the supply voltage by 90o.• Total primary current in an unloaded transformer is called the exciting current, and is com-prised of magnetizing current plus any additional current necessary to overcome core losses. Itis never perfectly sinusoidal in a real transformer, but may be made more so if the transformeris designed and operated so that magnetic flux density is kept to a minimum.• Core flux induces a voltage in any coil wrapped around the core. The induces voltage(s) areideally in- phase with the primary winding source voltage and share the same waveshape.• Any current drawn through the secondary winding by a load will be “reflected” to the primarywinding and drawn from the voltage source, as if the source were directly powering a similarload.9.2Step-up and step-down transformersSo far, we’ve observed simulations of transformers where the primary and secondary windings were ofidentical inductance, giving approximately equal voltage and current levels in both circuits. Equality