1 ELECTRICAL TECHNOLOGY EET 103/4 Define and analyze the principle of transformer, its parameters and structure. Describe and analyze Ideal transformer, equivalent circuit, and phasor diagram Calculate and justify efficiency, losses, performance 2 (CHAPTER 22) TRANSFORMERS 3 FUNCTION OF A TRANSFORMER The main function of an electrical power transformer is to transfer electrical energy from one side (primary) to the other side (secondary). The secondary current and voltage may or may not be at the same level as that of the primary current and voltage. The energy is transferred by means of magnetic coupling. The magnetic flux produced by the current in primary winding links the secondary winding. Since the flux varies with time, this flux linkage results in an induced voltage in the secondary winding. If the secondary winding is terminated with a load, the induced voltage will drive a secondary current through the load. Introduction This chapter covers: Mutual inductance that exists between coils of the same or different dimensions. Basic to the operation of a transformer Three of the basic operations of a transformer. step up/down the voltage/current impedance matching device isolation The dot convention. Mutual Inductance Transformers are constructed of two coils placed so that the changing flux developed by one will link the other. The coil to which the source is applied is called the primary coil. The coil to which the load is applied is called the secondary coil. Mutual Inductance 7 Primary transformer formula for voltage using Faradays Law: Voltage induced across the primary is directly related to the number of turns in the primary coil and the rate of change of magnetic flux linking the primary coil 8 Voltage induced across the primary is also directly related to the self-inductance of the primary and the rate of change of current through the primary winding: Mutual Inductance The magnitude of e S, the voltage inducted across the secondary is determined by; Where N S : the number of turns in the secondary winding m : the portion of the primary flux P that links the secondary winding Mutual Inductance If all of the flux linking the primary links the secondary: The coefficient of coupling (k) between two coils is determined by; Mutual Inductance Since the maximum level of m is P, the coefficient of coupling between two coils can never be greater that 1. Coils with low coefficients of coupling are termed loosely coupled. Mutual Inductance The ferromagnetic steel core ensures that most of the flux linking the primary also links the secondary >> establishing a coupling coefficient very close to 1 Mutual Inductance The both coils are overlapping results in the flux of one coil linking the other coil The absence of a ferromagnetic core results in low levels of flux linkage between the coils.The closer the two coils are, the greater the flux linkage, and the higher the value of k Mutual Inductance Mutual inductance between two coils is proportional to the instantaneous change in flux linking one coil due to an instantaneous change in the current through the other coil. In terms of inductance of each coil and the coefficient of coupling, the mutual inductance is: The greater the coefficient of coupling (greater flux linkages), or the greater the inductance of either coil, the higher the mutual inductance between the coils. The Iron-core Transformer An iron-core transformer under loaded conditions The Iron-core Transformer The iron core will serve to increase the coefficient of coupling between the coils by increasing the mutual flux m.. The magnetic flux lines will always take the path of least reluctance, which in this case is the iron core. When the current I p through the primary circuit of the ion-core transformer is a maximum, the flux m linking both coils is also a maximum. The flux is directly proportional to the current through the winding. The Iron-core Transformer If; then; The induced voltage across the primary due to a sinusoidal input can be determined by Faradays law; Or; Indicates that the induced voltage e p leads the current through the primary coil by 90 o The Iron-core Transformer The effective value of e p is; Since the flux linking the secondary flux equals that of the primary; The Iron-core Transformer Dividing; For and ideal transformer; and Hence; The Iron-core Transformer The ratio of the magnitudes of the induced voltages is the same as the ratio of the corresponding turns: The ratio N p /N s, usually represented by the lowercase letter a, is referred to as the transformation ratio. If a < 1, the transformer is a step-up transformer. If a > 1, the transformer is a step-down transformer. The Iron-core Transformer Example22.2 (a) Find the maximum flux m. (b) Find the secondary turns N s. The Iron-core Transformer Example22.2 Solution (a) The Iron-core Transformer Example22.2 Solution (contd) (b) 24 The primary and secondary current of a transformer are related by the inverse ratio of the turns: 22.3 The Iron-core Transformer Reflected Impedance and Power The impedance of the primary circuit of an ideal transformer is related to the impedance load by the transformation ratio If the load is capacitive or inductive, the reflected impedance will also be capacitive or inductive. For an ideal iron-core transformer; 26 Example22.3 (a) Find I p and V g.(b) Find Z p Reflected Impedance and Power 27 Example22.3 Solution 22.4 Reflected Impedance and Power (a) 28 Example22.3 Solution (contd) 22.4 Reflected Impedance and Power (b) Impedance Matching, Isolation, and Displacement Transformers can be particularly useful when you are trying to ensure that a load receives maximum power from a source. Maximum power is transferred to a load when its impedance is matched with the internal resistance of the supply. Impedance Matching, Isolation, and Displacement Transformers are used for impedance matching. Transformers are frequently used to isolate one portion of an electrical system from another. Isolation implies the absence of any direct physical connection. Impedance Matching, Isolation, and Displacement Example 22.5(a) An 8- speaker is directly connected across a source V g, whose internal resistance is 512 (i.e. unmatched condition). Determine the power to the speaker Impedance Matching, Isolation, and Displacement Example 22.5(a) Solution The source current; Impedance Matching, Isolation, and Displacement Example 22.5(b) The same speaker is directly connected to the same source through a matching transformer. (i.e. matched condition). Determine the power to the speaker Impedance Matching, Isolation, and Displacement Example 22.5(b) Solution The reflected impedance; Impedance Matching, Isolation, and Displacement Example 22.5(b) Solution (contd) The source current; Equivalent Circuit (Iron- core Transformer) For the non ideal or practical iron-core transformer, the equivalent circuit appears below. Part of the equivalent circuit includes an ideal transformer. Equivalent Circuit (Iron- core Transformer) Equivalent Circuit (Iron- core Transformer) All elements in the iron-core transformer other than the ideal transformer are the elements of the transformer that contribute to the nonideal characteristics of the device. Resistance R p and R s are simply the geometric resistance of the primary and secondary windings. Leakage flux L p and L s is a small amount of flux that links each coil but does not pass through the core. Equivalent Circuit (Iron- core Transformer) The resistance R C represents the hysteresis and eddy current losses (core losses) within the core due to an ac flux through the core. The inductance L m (magnetizing inductance) is the inductance associated with the magnetization of the core, that is, the establishing of the flux m in the core. The capacitances C P and C S and the lumped capacitances of the primary and secondary circuits, respectively, and C w represent the equivalent lumped capacitance between the windings of the transformer. Equivalent Circuit (Iron- core Transformer) For power transformer; Winding capacitances have negligible effects because of the low operating frequency. Under normal operating condition, normally i p >> i m. Hence, C p, C s, C w, R C and L m may be omitted. The approximate equivalent circuit becomes as follows; Equivalent Circuit (Iron- core Transformer) Reduced equivalent circuit for the nonideal iron- core transformer. Equivalent Circuit (Iron- core Transformer) Normally, L p and L s are represented by their equivalent reactances; and Equivalent Circuit (Iron- core Transformer) X s and R s may be reflected into the primary circuit using the relationship; and Equivalent Circuit (Iron- core Transformer) The equivalent circuit now becomes; Equivalent Circuit (Iron- core Transformer) Combining the series resistances and reactances, results in the following circuit; Equivalent Circuit (Iron- core Transformer) The load resistance R L may also be reflected into the primary circuit; Equivalent Circuit (Iron- core Transformer) By voltage divider rule; is the load resistance reflected into the primary circuit of the transformer Equivalent Circuit (Iron- core Transformer) Example 22.7 (a) Determine R e and X e. (b) Determine the magnitude V L and V g. Equivalent Circuit (Iron- core Transformer) Example 22.7 Solution (a) Equivalent Circuit (Iron- core Transformer) Example 22.7 Solution (b)