7 When the reference direction for a current enters the dotted terminal of a coil, the reference polarity of the voltage that it induces in the other coil is positive at its dotted terminal. Dot convention
SJTU8 Examples How could we determine dot markings if we don’t know?
SJTU9 Series connection 12 M (a)mutually coupled coils in series-aiding connection L T =L 1 +L 2 +2M 12 M (b)mutually coupled coils in series–opposing connection L T =L 1 +L 2 -2M Total inductance
SJTU10 Parallel connection L1L1 L2L2 I + V M L1L1 L2L2 I + V M (a)mutually coupled coils in parallel-aiding connection (b)mutually coupled coils in parallel–opposing connection Equivalent inductance
SJTU11 Coefficient of coupling The coupling coefficient k is a measure of the magnetic coupling between two coils k < 0.5 loosely coupled; k > 0.5 tightly coupled.
SJTU18 Z 22 I2I2 Equivalent secondary winding circuit
SJTU19 Ideal transformer + - + - 1: n three properties: 1.The coefficient of coupling is unity (k=1) 2.The self- and mutual inductance of each coil is infinite (L 1 =L 2 =M=∞), but is definite. 3.Primary and secondary coils are lossless.
SJTU21 Transformer as a matching device + - + - 1: n RLRL - + - + R L /n 2 + - + 1: n R - + - + R - n2Rn2R
SJTU22 Transformer as a matching device + - + - 1: n RLRL Zin Vs1 Z1Z1 Z 2 /n 2 Vs2/n Vs2 Vs1 Z1Z1 Z2Z2 1: n I1I1 I2I2 Thevenin equivalent
SJTU23 Vs2 Vs1 Z1Z1 Z2Z2 1: n I1I1 I2I2 nVs1 n 2 Z 1 Z2Z2 Vs2
SJTU24 Solving Ideal Transformer Problem Method 1: Write out equations first –Loop equations or Nodal equations –Two more transformer equations Method 2 : Form equivalent circuit first –Reflecting into secondary –Reflecting into primary V s1 V s2 Z1Z1 Z2Z2
SJTU26 General transformer model 1.Lossless, k=1, but L 1,L 2,M are not infinite + - + - L1L2 M + - + - 1: n L1
SJTU27 General transformer model 2. Lossless, k≠1, L 1,L 2,M are not infinite + - + - L1L2 M + - + - 1: n LMLM L S1 L S2
SJTU28 General transformer model 3. No restriction + - + - L1L2 M + - + - 1: n LMLM L S1 L S2 /n 2 R1 R 2 /n 2
SJTU29 SUMMARY Mutual inductance, M, is the circuit parameter relating the voltage induced in one circuit to a time-varying current in another circuit. The coefficient of coupling, k, is the measure of the degree of magnetic coupling. By definition, 0≤k≤1 The relationship between the self-inductance of each winding and the mutual inductance between the windings is The dot convention establishes the polarity of mutually induced voltage Reflected impedance is the impedance of the secondary circuit as seen from the terminals of the primary circuit, or vise versa.
SJTU30 SUMMARY The two-winding linear transformer is a coupling device made up of two coils wound on the same nonmagnetic core. An ideal transformer is a lossless transformer with unity coupling coefficient(k=1) and infinite inductance. An ideal transformer can be used to match the magnitude of the load impedance, Z L, to the magnitude of the source impedance, Z S, thus maximizing the amount of average power transferred.