1、 英文资料 TRANSFORMER 1. INTRODUCTION The high-voltage transmission was need for the case electrical power is to be provided at considerable distance from a generating station. At some point this high voltage must be reduced, because ultimately is must supply a load. The transformer makes it possible fo
2、r various parts of a power system to operate at different voltage levels. In 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 b
3、e 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
4、 when transforming power from one voltage level to another. Typical efficiencies are in the range 92 to 99%, the higher values applying to the larger power transformers. The current flowing in the coil connected to the ac source is called the primary winding or simply the primary. It sets up the flu
5、x in the core, which varies periodically both in magnitude and direction. The flux links the second coil, called the secondary winding or simply secondary. The flux is changing； therefore, it induces a voltage in the secondary by electromagnetic induction in accordance with Lenzs law. Thus the prima
6、ry 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 v
7、oltage causes a small current I to flow in the primary winding. This no-load current has two functions: (1) it produces the magnetic flux in the core, which varies sinusoidally between zero and m, where m is the maximum value of the core flux； and (2) it provides a component to account for the hyste
8、resis and eddy current losses in the core. There combined losses are normally referred to as the core losses. The no-load current I 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
9、 core, the no-load current will lag the primary voltage by nearly 90. It is readily seen that the current component Im= I0sin0, called the magnetizing current, is 90 in phase behind the primary voltage VP. It is this component that sets up the flux in the core； is therefore in phase with Im. The sec
10、ond component, Ie=I0sin0, 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 I0 = Im+ Ie It should be noted that the no-load current is distortes and nonsinusoidal. This is the res
11、ult 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 winding， there will be an induced EMF E in the seco
12、ndary winding in accordance with Faradays law, namely, E=N/t. 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 90, therefore, they are 180 out of phase with the applied voltage. Since no current flows in the secon
13、dary 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 avera
14、ge value of the induced voltage given by Eavg = turnsg iv e n ti m e g iv e n ti m e ain f lu x in c h a n g e which is Faradays law applied to a finite time interval. It follows that mm 4 f N1 /( 2 f )2N E a v g which N is the number of turns on the winding. Form ac circuit theory, the effective or
15、 root-mean-square (rms) voltage for a sine wave is 1.11 times the average voltage； thus m4.44fNE Since the same flux links with the primary and secondary windings, the voltage per turn in each winding is the same. Hence mPP 4.44fNE and mSS 4.44fNE where Ep and Es are the number of turn on the primar
16、y 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 SPSP NNEEa Assume that the output power of a transformer equals its input power, not a bad sumption in practice considering the high
17、 efficiencies. What we really are saying is that we are dealing with an ideal transformer； that is, it has no losses. Thus Pm = Pout or VpIp primary PF = VsIs secondary PF where PF is the power factor. For the above-stated assumption it means that the power factor on primary and secondary sides are
18、equal； therefore VpIp = VsIs from which is obtained SPSP IIVV SPEE a It shows that as an approximation the terminal voltage ratio equals the turns ratio. The primary and secondary current, on the other hand, are inversely related to the turns ratio. The turns ratio gives a measure of how much the se
19、condary voltage is raised or lowered in 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 d
20、ata, 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 t
21、ransformer the transformation ratio a is greater than unity (a1.0), while for a step-up transformer it is smaller than unity (a1.0). 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
22、 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 se
23、ction 4 it is shown how the primary winding current adjusts itself to the secondary load current when the transformer supplies a load. Looking into the transformer terminals from the source, an impedance is seen which by definition equals Vp / Ip. FromSPSP IIVV SPEE a, we have Vp = aVs and Ip = Is/a
24、.In terms of Vs and Is the ratio of Vp to Ip is ss2SSPP I Va/aIaVIV But Vs / Is is the load impedance ZL thus we can say that Zm (primary) = a2ZL This equation tells us that when an impedance is connected to the secondary side, it appears from the source as an impedance having a magnitude that is a2
25、 times its actual value. We say that the load impedance is reflected or referred to the primary. It is this property of transformers that is used in impedance-matching applications. 4. TRANSFORMERS UNDER LOAD The primary and secondary voltages shown have similar polarities, as indicated by the “dot-
26、making” convention. The dots near the upper ends of the windings have the same meaning as in circuit theory； the marked terminals have the same polarity. Thus when a load is connected to the secondary, the instantaneous load current is in the direction shown. In other words, the polarity markings signify that when positive current enters both windings at the marked terminals, the MMFs of the two windings add.
