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EE 311: EE Junior Lab Experiment 5 - Single Phase Transformers

EE 311: EE Junior Lab Experiment 5 - Single Phase Transformers. J. Carroll 9/25/06. Objective. The objective of this experiment is to examine the operating characteristics of a single phase two winding transformer Ideal Transformers - set of mutually coupled coils ,

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EE 311: EE Junior Lab Experiment 5 - Single Phase Transformers

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  1. EE 311: EE Junior Lab Experiment 5 -Single Phase Transformers J. Carroll 9/25/06

  2. Objective • The objective of this experiment is to examine the operating characteristics of a single phase two winding transformer • Ideal Transformers - set of mutually coupled coils , • Iron or ferromagnetic material core • Right-hand rule dictates • positive currents , produce positive core flux • coil flux linkages , are related to the core flux Background Theory

  3. Ideal Transformers

  4. Equivalent Circuits

  5. Hysteresis and Saturation • Ferromagnetics are permeable but exhibit hysteresis and saturation, limiting operation • magnetic field intensity, H, versus flux density B

  6. Steady-state AC Performance • AC saturation curve plots ac rms magnetizing voltage versus ac rms magnetizing current • the ratio provides a large signal approximation of the magnetizing reactance

  7. Effects on the Equivalent Circuit • Magnetizing current, hysteresis effects and core losses are modeled by a shunt resistor

  8. Symbols Used in Eq. Circuit • Definition of symbols in Figure 8 • magnetizing current • magnetizing voltage • primary winding resistance • primary winding leakage reactance • resistance representing core losses • magnetizing reactance • secondary winding resistance • secondary winding leakage reactance

  9. Remarks • This is a phasor equivalent circuit valid at a particular frequency • phasor quantities are required in the solution of this circuit, but all measurements are magnitudes • be careful to keep track of the difference • An ideal transformer appears as a part of this equivalent circuit • The direction of has been reversed for convenience • Short circuit and open circuit tests used to measure impedances of equivalent circuit

  10. Short Circuit Test • Rated current is applied to a winding from variable source with other winding short-circuited • voltage, current and power are measured

  11. Simplified Short-Circuit Eq. Circuit • Short circuit reflected across the ideal transformer • and calculated from short circuit measurements

  12. Open Circuit Test • One winding is open circuited, and rated voltage is applied to the other winding • current and power flow are measured

  13. Simplified Open-Circuit Eq. Circuit • Neglecting and from the circuit allows • and to be calculated from the measured values of , , and

  14. Voltage Regulation • An important consideration is ability to keep frequency and voltage in proper range • As load increases, voltage will go down • A measure of ``goodness'' is amount of voltage drop at full load, quantified by the voltage regulation

  15. Procedures: Polarity Test • Apply a small voltage across P1 and P2 of test transformer to determine polarity of secondary (S)

  16. Hysteresis and Magnetizing Current • Plot channel 1 input versus channel 2 input, or magnetizing current versus flux linkage, with one cycle of the input corresponding to one cycle around the B-H curve, observe proper scaling

  17. Final Transformer Comments • Transformers are extremely versatile • can change voltage and current levels • provide isolation between two windings, e.g., instrumentation amplifiers • pass AC signals from one winding to the other while ``filtering out'' the DC component • Transformer ratings range from • a fraction of a volt-amp in printed circuit boards • giga volt-amp range in power transmission systems • Used in constant frequency applications, such as in power systems, and also in variable frequency applications, such as audio or RF amplifiers

  18. SAFETY • This experiment involves medium-level voltages; you must know and follow the procedures in your Laboratory Safety Manual! • have TA check all circuits before energizing • lab partners share responsibility for lab safety • good procedure is for one partner to wire circuits, while the other partner verifies the wiring • be aware of the lab procedures at all times! • follow lab procedures for even minor circuit changes • protective eyewear is to be worn at all times

  19. SAFETY • predict signal levels and adjust meter ranges before energizing circuits to prevent meter overloads • energize circuits with zero variac output voltage, increase output slowly while monitoring circuit for over-currents • arrange experiments so meter leads do not need to be moved while circuit energized circuit • prevent energized leads from accidental contacts • do not touch energized circuits!

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