流I0产生的磁通φ0不同，它只停留在二次侧绕组中，因为这个磁通不流过一次侧绕组，所以它不是一个公共磁通。
另外，流过一次侧绕组的负载电流只在一次侧绕组中产生磁通，这个磁通被称为一次侧的漏磁。二次侧漏磁将使电压增大以保持两侧电压的平衡。一次侧漏磁也一样。因此，这两个增大的电压具有电压降的性质，总称为漏电抗电压降。另外，两侧绕组同样具有阻抗，这也将产生一个电阻压降。把这些附加的电压降也考虑在内，这样一个实际的变压器的等值电路图就完成了。由于分支励磁体现在电流里，为了分析我们可以将它忽略。这就符我们前面计算中可以忽略空载电流的假设。这证明了它对我们分析变压器时所产生的影响微乎其微。因为电压降与负载电流成比例关系，这就意味着空载情况下一次侧和二次侧绕组的电压降都为零。
TRANSFORMER
1) INTRODUCTION
The was need for the case electrical power is to be provided at considerable distance from a generating station. At some point this this paper we discuss power transformer principles and applications.
2) TOW-WINDING TRANSFORMERS
A transformer in its simplest form consists of two stationary coils coupled by a mutual magnetic flux. The coils are said to be mutually coupled because they link a common flux.
In power applications, laminated steel core transformers (to which this paper is restricted
are used. Transformers are efficient because the rotational losses normally associated with rotating machine are absent, so relatively little power is lost when transforming power from one voltage level to another. Typical efficiencies are in the range 92 to 99%, the the coil connected to the ac source is called the primary winding or simply the primary. It sets up the flux φ in the core, which varies periodically both in magnitude
and direction. The flux links the second coil, called the secondary winding or simpl
y secondary.The flux is changing; therefore, it induces a voltage in the secondary by electromagnetic induction in accordance with Lenz’s law. Thus the primary receives its power from the source while the secondary supplies this power to the load. This action is known as transformer action.
3) TRANSFORMER PRINCIPLES
When a sinusoidal voltage Vp is applied to the primary with the secondary open-circuited, there will be no energy transfer. The impressed voltage causes a small current Iθ to flow in the primary winding. This no-load current the core, which varies sinusoidally between zero and ? φm, where φm is the maximum value of the core flux; and (2) it provides a component to account for the the core. There combined losses are normally referred to as the core losses.
The no-load current Iθ is usually few percent of the rated full-load current of the transformer (about 2 to 5%). Since at no-load the primary winding acts as a large reactance due to the iron core, the no-load current will lag the primary voltage by nearly 90o. It is readily seen that the current component Im= I0sinθ0, called the magnetizing current, is 90o in phase behind the primary voltage VP. It is this component that sets up the flux in the core; φ is therefore in phase with Im.
The second component, Ie=I0sinθ0, is in phase with the primary voltage. It is the current component that supplies the core losses. The phasor sum of these two components represents the no-load current, or
It should be noted that the no-load current is distortes and nonsinusoidal. This is the result of the nonlinear behavior of the core material.
If it is assumed that there are no other losses in the transformer, the induced voltage In the primary, Ep and that in the secondary, Es can be shown. Since the magnetic flux set up by the primary windingthere will be an induced EMF E in the secondary winding in accordance with Faraday’s law, namely, E=NΔφΔt. This same flux also links the primary itself, inducing in it an EMF, Ep. As discussed earlier, the induced voltage must lag the flux by 90o, therefore, they are 180o out of phase with
the applied voltage. Since no current flows in the secondary winding, Es=Vs. The no-load primary current I0 is small, a few percent of full-load current. Thus the voltage in the primary is small and Vp is nearly equal to Ep. The primary voltage and the resulting flux are sinusoidal; thus the induced quantities Ep and Es vary as a sine function. The average value of the induced voltage given by
Eavg ?turns?change in flux in a given time
given timewhich is Faraday’s law applied to a finite time interval. It follows that
which N is the number of turns on the winding. Form ac circuit theory, the effective or root-mean-square (rms) voltage for a sine wave is 1.11 times the average voltage; thus
Since the same flux links with the primary and secondary windings, the voltage per turn in each winding is the same. Hence
And
where Ep and Es are the number of turn on the primary and secondary windings, respectively. The ratio of primary to secondary induced voltage is called the transformation ratio. Denoting this ratio by a, it is seen that
Assume that the output power of a transformer equals its input power, not a bad sumption in practice considering the ideal transformer; that is, it it means that the power factor on primary and secondary sides are equal; therefore
from which is obtained
It shows that as an approximation the terminal voltage ratio equals the turns r
atio. The primary and secondary current, on the other relation to the primary voltage. To calculate the voltage regulation, we need more information.
The ratio of the terminal voltage varies somewhat depending on the load and its power factor. In practice, the transformation ratio is obtained from the nameplate data, which list the primary and secondary voltage under full-load condition.
When the secondary voltage Vs is reduced compared to the primary voltage, the transformation is said to be a step-down transformer: conversely, if this voltage is raised, it is called a step-up transformer. In a step-down transformer the transformation ratio a is greater than unity (a>1.0), while for a step-up transformer it is smaller than unity (a<1.0). In the event that a=1, the transformer secondary voltage equals the primary voltage. This is a special type of transformer used in instances where electrical isolation is required between the primary and secondary circuit while maintaining the same voltage level. Therefore, this transformer is generally knows as an isolation transformer.
As is apparent, it is the magnetic flux in the core that forms the connecting link between primary and secondary circuit. In section 4 it is shown the transformer supplies a load.
Looking into the transformer terminals from the source, an impedance is seen which by definition equals Vp Ip. From
,
we terms of Vs and Is the ratio of Vp to Ip is
But Vs Is is the load impedance ZL thus we can say that
This equation tells us that when an impedance is connected to the secondary side, it appears from the source as an impedance impedance-matching applications.
4) TRANSFORMERS UNDER LOAD
The primary and secondary voltages shown . The dots near the upper ends of the windings circuit theory; the marked terminals a load is connected to the secondary, the instantaneous load current is in the direction shown. In other words, the
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