magnetically coupled circuits -...
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Magnetically Coupled Circuits
• Whenever a current flows through a conductor, a magnetic field is generated (magnetic flux)
• When time varying magnetic field generated by one loop penetrates a second loop, a voltage induced between the ends of the second wire
• New term is defined “mutual inductance” to differentiate from “inductance”
MUTUAL INDUCTANCE
Physical basis
• Production of magnetic flux by a current
• The production of voltage by time varying magnetic field
• Current flowing in one coil established flux about that coil and about second coil nearby
• The time varying flux surrounding second coil produces a voltage across the terminals of the second coil
• This voltage is proportional to time rate of change of current in first coil
• Define coefficient of mutual inductance or simply mutual inductance
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• M12=M21=M
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Dot Convention
• A current entering the dotted terminal of one coil produces an open circuit voltage with positive voltage reference at the dotted terminal of the second coil
• A current entering the undotted terminal of one coil provides a voltage that is positively sensed at the undotted terminal of the second coil
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Example
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Combined Mutual and Self‐Induction Voltage
For sinusoidal source
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Physical Basis of the Dot Convention
• The current i1 produces a flux which directed downward • Since i1 is increasing with time, the flux, which is proportional to
i1 is also increasing with time
• If i2 is a positive and increasing, i2 produces a magnetic flux which is directed downward and increasing
• The voltage across the terminals of any coil results from the time rate of change of the flux within the coil
• The voltage is greater with i2 flowing than it would be if i2 were zero
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Example
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Example
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LINEAR TRANSFORMER
Primary Secondary
Reflected Impedance
Define
• Presence of secondary increases
losses in primary
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T Equivalent Network
T equivalent
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Example
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IDEAL TRANSFORMER
Ideal transformer is a useful approximation of a very tightly coupled transformer in which coupling coefficient is unity and both secondary and primary inductive reactance are extremely large
Turns ratio
Sinusoidal Steady State
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As L1 ‐> infinity
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Use of Transformers for Impedance Matching
Example:
Match amplifier internal impedance of 4000Ω and a speaker impedance of 8Ω.
Primary and secondary current relationship
If L2 is very large and since (M=(L1L2)1/2)
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Use of Transformers for Voltage Level Adjustment