1. ECE 3301 General Electrical Engineering
Presentation 23
Inductance

3. Inductance
An electrical current in any conductor causes a magnetic field to exist around the conductor.
The magnetic field forms a closed loop around the conductor as illustrated below.

4. Inductance The magnetic field obeys the right-hand rule.
If the current is in the direction of the thumb of the right hand, the magnetic field is pointed in the direction the fingers form around the conductor.

5. Inductance
If the conductor is formed into a coil, the magnetic field is reinforced by adjacent conductors.

6. Inductance The magnetic field is said to “link” the turns of the coil.
Energy is stored in the magnetic field.

7. Inductance
The effect of the magnetic field is to maintain the current in the conductor.
This effect is called inductance.

8. Inductance
An inductance in a circuit is represented by the symbol shown below.

9. Inductance
An inductance in a circuit is represented by the symbol shown below.
Inductance is measured in henries (H).

10. Inductance
The current-voltage relationship for inductance is given by the equation

11. Inductance
The voltage across an inductance is proportional to the time-rate-of-change of the current through the inductance.
The constant of proportionality is called the inductance.

12. Drives a time-varying current through the inductor
The current source
Drives a time-varying current through the inductor

13. The voltage across the inductance
Is proportional to the time rate-of-change of the current

14. The voltage across the inductance
Is proportional to the slope of this waveform

15. Slope = 0
Voltage = 0

16. Slope = 1
Voltage = L(1)

17. Slope = 0
Voltage = 0

18. Slope = -1
Voltage = L(-1)

19. Slope = 0
Voltage = 0

20. Drives a time-varying, sinusoidal current through the inductor
The current source
Drives a time-varying, sinusoidal current through the inductor

21. Period = T0
Amplitude

22. The voltage across the inductance
Is proportional to the time rate-of-change of the current

25. The current and voltage are 90 degrees “out of phase” with each other.
The voltage leads the current by 90 degrees.

26. Drives a time-varying current through the inductor
The current source
Drives a time-varying current through the inductor

27. The voltage across the inductance
Is proportional to the slope of this waveform

28. The instantaneous change in current
Causes an infinite voltage pulse across the inductor

29. An infinite pulse cannot be achieved !

30. Rule 1 of Inductance
Since this infinite voltage impulse cannot be physically realized, we conclude the first rule-of-thumb about inductance.
One cannot instantaneously change the current through an inductance.

31. Rule 2 of Inductance
Since the voltage across the inductance is proportional to the time rate-of-change of the current through the inductance, when the current is constant (DC), the voltage is zero.
A voltage of zero across a circuit element is the definition of a short circuit. This leads to the second rule-of-thumb about inductances.
An inductance is a short circuit to Steady State DC.

32. Inductance Consider an inductance driven by a voltage source.
The inductance has an initial current of i0 amps.

33. Inductance
The voltage-current relationship is given by:

34. Inductance
Performing a bit of calculus:

35. Inductance
Integrating both sides:

36. Inductance
Completing the integration:

37. Inductance
Solving for i(t):

38. Inductance
The current through an inductance is proportional to the integral of the voltage across the inductance plus the initial current through the inductance.

39. Places a time-varying voltage across the inductance
The voltage source
Places a time-varying voltage across the inductance

40. The current through the inductance
Is proportional to the area under the voltage waveform

41. Final current
Accumulated area
Initial current

42. Places a sinusoidal voltage across the inductance
The voltage source
Places a sinusoidal voltage across the inductance

43. Is proportional to the integral of the voltage
The current
Is proportional to the integral of the voltage

46. The current and voltage are 90 degrees “out of phase” with each other.
The voltage leads the current by 90 degrees.

47. Power and Energy in an Inductance
The instantaneous power absorbed by any circuit element is given by:
Using the voltage-current relationship for an inductance:

48. Power and Energy in an Inductance
Leads to the instantaneous power in an inductance:
By definition, power is given by:

49. Power and Energy in an Inductance
Consequently :
and :

50. Power and Energy in an Inductance
The energy stored in an inductance may be found by integration.
Assume an initial energy of zero and an initial current of zero.

51. Power and Energy in an Inductance
The energy stored in an inductance is:

55. Inductors in Series

56. Inductors in Parallel

57. Inductors in Parallel

58. Inductors in Parallel

59. Inductors in Parallel