EE 311: EE Junior Lab Experiment 5 - Single Phase Transformers
description
Transcript of EE 311: EE Junior Lab Experiment 5 - Single Phase Transformers
EE 311: EE Junior Lab Experiment 5 -
Single Phase TransformersJ. 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 ,
• 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
1i c
2i
c
1 2
1N 2N
Ideal Transformers
)2(
)1(
22
11
c
c
N
N
)4(
)3(
222
111
c
c
Nv
Nv
)5(22
2
1
1nvv
N
Nv
2
1
2
1
2
1ini
N
Ni
Equivalent Circuits
Hysteresis and Saturation• Ferromagnetics are permeable but exhibit
hysteresis and saturation, limiting operation– magnetic field intensity, H, versus flux density B
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 mX
mV mI
Effects on the Equivalent Circuit
• Magnetizing current, hysteresis effects and core losses are modeled by a shunt resistor
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
mI
mV
1R
1XR
mX
2R
2X
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
2I
Short Circuit Test • Rated current is applied to a winding from
variable source with other winding short-circuited– voltage, current and power are measured
Simplified Short-Circuit Eq. Circuit• Short circuit reflected across the ideal transformer
– and calculated from short circuit measurements
)10(2
)9(2
2
2
1
2
2
1
eq
eq
XXnX
RRnR
eqR eqX
Open Circuit Test • One winding is open circuited, and rated voltage is
applied to the other winding– current and power flow are measured
Simplified Open-Circuit Eq. Circuit • Neglecting and from the circuit allows
– and to be calculated from the measured values of , , and
1R 1X
ocV ocI ocPmXR
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
%100V load 2
load 2circuitopen 2
VV
VR
Procedures: Polarity Test• Apply a small voltage across P1 and P2 of test
transformer to determine polarity of secondary (S)
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
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
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
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!