1、 外文资料翻译 Power Transformer Principles 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 m
2、akes it possible for 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
3、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 l
4、ittle power is lost 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.
5、 It sets up the flux 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
6、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 transf
7、er. The impressed voltage 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 ac
8、count for the hysteresis 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 reacta
9、nce due to the iron 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 pha
10、se with Im. The second 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 emo III It should be noted that the no-load current is distortes and nonsinusoidal.
11、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 winding， there will be an induced EMF
12、 E in the secondary winding in accordance with Faradays law, namely, tNE / . 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 flo
13、ws 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 func
14、tion. The average value of the induced voltage given by Eavg = turns c h a n g e i n f l u x i n a g i v e n t i m eg i v e n t i m e which is Faradays law applied to a finite time interval. It follows that Eavg = N 21/(2 )mf = 4fNm which N is the number of turns on the winding. Form ac circuit theo
15、ry, the effective or root-mean-square (rms) voltage for a sine wave is 1.11 times the average voltage； thus E = 4.44fNm Since the same flux links with the primary and secondary windings, the voltage per turn in each winding is the same. Hence Ep = 4.44fNpm and Es = 4.44fNsm whereEp and Es are the nu
16、mber 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 a = psEE = psNN Assume that the output power of a transformer equals its input power, not a bad sumption i
17、n practice considering the high 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 equal； therefore VpIp = VsIs from which is obtained
