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Third Period Lab Supplement to ILM 030303c—Transformer Loading The ILM’s Explanation of Transformer Action Transformer action is a fundamental electrical concept like Ohm’s Law, Kirchhoff’s Voltage and Current Laws, motor action, and electromagnetic induction. It explains how transformers work and helps explain how induction motors work. In many respects transformer action is analogous to motor action and generator action. The explanation of transformer action in ILM 030303c considers three or four different fluxes: (1) the exciting flux, (2) the secondary current flux, (3) the resultant flux, the difference between the exciting flux and the secondary current flux, and (4) the primary current flux. The ILM’s explanation asks you to overlook that lines of magnetic flux do not intersect, asks you to overlook that flux lines of opposing polarity avoid “running” in parallel, and is problematic about the effect of these fluxes on the iron’s permeability and saturation.

An Alternate Explanation of Transformer Action An alternate explanation, which respects the characteristics of magnetic lines of flux, considers the transformer to have only one flux instead of three or four. The alternate explanation considers that the primary and secondary currents are magnetomotive forces of opposing polarity that produce a single flux. Note that the term current is better for applying Ohm’s and Kirchhoff’s Laws but the term magnetomotive force is better for considering magnetism. However, current and magnetomotive force are essentially the same thing: the rate of movement of electric charges.

No Load Conditions At no load, primary current consists of only the exciting current, which is the current needed to establish the transformer’s flux and to supply the iron losses. The exciting current will continue to be present at all load levels. Referring to the illustration of transformer action, notice that induced primary voltage EP opposes the polarity of primary current IP and of applied voltage VP, in accordance with Lenz’s Law. H1 and X1 have the same instantaneous polarity, i.e., X1 has the same polarity with respect to X2 as H1 has with respect to H2. The exciting current has two components: • the IXL component, which creates the magnetic field • the IR component, which supplies the iron losses (hysteresis and eddy current losses) In mid-sized and large transformers, the IXL component is much larger than the IR component, so that the exciting current is largely inductive reactive.

Supplement to ILM 030303c, 9/1/2010


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Some writers, such as Theodore Wildi, use the term magnetization current as a synonym for exciting current. Other writers use the term only for the IXL component, the component that creates the magnetic field.

Conditions When Load Is Added—Transformer Action The transformer’s primary winding acts as a load to its voltage source (VP) whereas the secondary winding acts as a source to the load it will supply. Once the secondary load is connected, secondary current in the illustration flows out of X1 whereas primary current flows into H1. The polarity of the secondary current or magnetomotive force opposes the polarity of the primary current or magnetomotive force. Use the coil hand rule to confirm this relationship for a quadrant I (0–90°) reference current. Because the secondary magnetomotive force opposes the primary magnetomotive force, an increase in secondary current reduces the net magnetomotive force of the transformer and therefore the transformer’s flux, i.e., the transformer is partially demagnetized. With less flux, lower primary and secondary voltages are induced. A lower induced primary voltage EP, which has a polarity opposite to applied primary voltage VP, imposes less subtractive effect on the applied primary voltage. For example, if the applied primary voltage VP is 120 V and the induced primary voltage EP was 119 V but now drops to 118 V, the difference or net voltage doubles from 1 V to 2 V. This small change in induced primary voltage EP has a greatly amplified effect on the current. In fact primary current increases enough to restore the flux to almost its former level. With increased primary current, the induced voltages are restored to almost what they were before load was added to the transformer. Therefore, slight changes in induced primary voltage EP have an amplified effect on primary current and magnetomotive force IP. The previous paragraph describes the most important aspects of transformer action. In summary, an increase in secondary current demagnetizes the iron slightly, which reduces the induced voltages slightly. A lower primary induced voltage permits the primary current to increase enough to match the increase of secondary current that occurred when the load connected to the secondary winding increased.

Transformer Flux is Proportional to the Resultant of the Transformer’s Magnetomotive Forces A transformer’s flux is the resultant of two magnetomotive forces in the transformer: the primary current (which has two components: the exciting current component and the load current component) and the secondary current (which is the same as the load current).

Transformer Action and the Polarities of Voltage, Current, and Flux Refer to the illustration for the following review of transformer action with an emphasis on polarity: •

Applied primary voltage VP drives primary current IP in the counterclockwise direction around its circuit and upward through the primary winding.

Primary current IP creates the magnetic flux polarity shown in the illustration (clockwise flux for a quadrant I conventional current, i.e., an alternating current

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Supplement to ILM 030303c, 9/1/2010

during the interval 0–90°). The current is alternating, but we will consider a quadrant I current, which is increasingly positive, as the reference current. •

Primary current IP creates induced primary voltage EP with its polarity arrow pointing downward. In accordance with Lenz’s Law, the polarity of the induced primary voltage opposes the polarity of the applied primary voltage and therefore restricts the primary current.











Transformer Action •

Primary current IP also induces secondary voltage ES. As with any transformer, X1 has the same instantaneous polarity with respect to X2 as H1 has with respect to H2. Because the transformer illustrated is subtractive, the polarity arrow for induced secondary voltage ES points downward.

With the ES polarity arrow pointing downward, secondary current IS also flows downward through the secondary winding and counterclockwise around the secondary circuit.

Using conventional current and the coil hand rule for a quadrant I reference current, counterclockwise flux would tend to be produced by the secondary current. Therefore, the secondary magnetomotive force opposes the primary magnetomotive force (which created a clockwise flux) and partially demagnetizes the core.

As more secondary current flows and the transformer is partially demagnetized, less voltage is induced in the primary (less EP).

With less primary induced voltage, applied primary voltage VP is opposed less and more primary current flows (because the difference between applied primary voltage VP and induced primary voltage EP increases—this difference may be called the net voltage).

With more primary current, transformer flux is partially restored. In fact flux is restored almost to the value it had before load was added to the secondary winding.

In the end, the amount of demagnetization caused by secondary current turns out to be quite small. As secondary current increases, the primary current increases proportionally to almost completely match the increased secondary current.

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Secondary terminal voltage VS has a polarity opposite to the polarity of the secondary source voltage, which is secondary induced voltage ES. Therefore the polarity arrows for both VS and ES point downward. These polarities are consistent with Kirchhoff’s Voltage Law.

When load is removed from the secondary winding, the secondary current decreases to cause the primary current to decrease also, following a sequence similar to that described above but with opposite effects.

(Percent) Voltage Regulation As current increases from no load to full load, the core’s flux is diminished only slightly— one percent or so. This flux decrease causes a decrease in the primary and secondary induced voltages and in the secondary terminal voltage. However, a greater decrease is caused by resistive and reactive voltage drops in the primary and secondary windings. These drops are called the resistive and reactive voltages and are abbreviated IR and IX. The resistive and reactive voltages produce resultant impedance voltages (abbreviated IZ) in the windings. Impedance voltages are internal voltage drops causing the transformer’s secondary terminal voltage (VS) to decrease as transformer load increases (just as in a battery or DC generator, increased load causes the battery or generator’s terminal voltage to decrease). The standard ratio used to express terminal voltage variations as load on a transformer, generator, or alternator increases is called the percent voltage regulation and is calculated using the following formula.

Percent Voltage Regulation =

V NL − VFL × 100 VFL

Inductive reactive loads cause the transformer to have lower secondary terminal voltages (VS) and greater percent voltage regulations than resistive loads of the same current magnitude, i.e., the change of secondary voltage from no load to full load is greater with inductive loads. Leading loads (i.e., loads with resistance and some capacitive reactance) reduce the percent voltage regulation, compared to resistive loads of the same current magnitude. Therefore, resistive loads with some capacitive reactance will cause the transformer supplying them to have better (lower) percent voltage regulation. A purely capacitive load actually gives a negative percent voltage regulation, i.e., the secondary voltage increases as load is added.

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Transformer Action