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Chapter 20

Chapter 20. Induced Voltages and Inductance. Motors. Motors are devices that convert electrical energy into mechanical energy A motor is a generator run in reverse A motor can perform useful mechanical work when a shaft connected to its rotating coil is attached to some external device.

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Chapter 20

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  1. Chapter 20 Induced Voltages and Inductance

  2. Motors • Motors are devices that convertelectrical energy into mechanical energy • A motor is a generator run in reverse • A motor can perform useful mechanical work when a shaft connected to its rotating coil is attached to some external device

  3. Motors and Back emf • The phrase back emf is used for an emf that tends to reduce the applied current • When a motor is turned on, there is no back emf initially • The current is very large because it is limited only by the resistance of the coil

  4. Motors and Back emf, cont • As the coil begins to rotate, the induced back emfopposes the applied voltage • The current in the coil is reduced • The power requirements for starting a motor and for running it under heavy loads are greater than those for running the motor under average loads

  5. Example 20.6 – page 680 • A motor has coils with a resistance of 10Ω and is supplied by a voltage of 120V. When the motor is running at its maximum speed, the back emf is 70V. Find the current in the coils • A) when the motor is first turned on; • B) when the motor has reached its maximum rotation rate.

  6. Example 20.6 – cont. A) Find the current in the coil when the motor is first turned on. B) Find the current in the coil when the motor is rotating at its maximum rate.

  7. Self-inductance • Self-inductance occurs when the changing flux through a circuit arises from the circuit itself • As the current increases, the magnetic flux through a loop due to this current also increases • The increasing flux induces an emf that opposes the change in magnetic flux • As the magnitude of the current increases, the rate of increase lessens and the induced emf decreases • This opposing emf results in a gradual increaseof the current

  8. Self-inductance cont • The self-induced emf must be proportional to the time rate of change of the current • L is a proportionality constant called the inductance of the device • The negative sign indicates that a changing current induces an emf in opposition to that change

  9. Self-inductance, final • The inductance of a coil depends on geometric factors • The SI unit of self-inductance is the Henry • 1 H = 1 (V · s) / A • You can determine an expression for L

  10. Joseph Henry • 1797 – 1878 • First director of the Smithsonian • First president of the Academy of Natural Science • First to produce an electric current with a magnetic field • Improved the design of the electro-magnet and constructed a motor • Discovered self-inductance

  11. Inductor in a Circuit • Inductance can be interpreted as a measure of opposition to the rate of change in the current • Remember resistance R is a measure of opposition to the current • As a circuit is completed, the current begins to increase, but the inductor produces an emf that opposes the increasing current • Therefore, the current doesn’t change from 0 to its maximum instantaneously

  12. Example 20.7 – page 682 • A) calculate the inductance of a solenoid containing 300 turns if the length of the solenoid is 25cm and its cross-sectional area is 4x10-4m2. • B) calculate the self-induced emf in the solenoid described in (a) if the current in the solenoid decreases at the rate of 50A/s?

  13. Example 20.7 – cont. A) calculate the inductance of the solenoid. • µo = 4  x 10-7 T.m / A • µo is called the permeability of free space

  14. Example 20.7 – cont. B) calculate the self-induced emf in the solenoid.

  15. RL Circuit • When the current reaches its maximum, the rate of change and the back emf are zero • The time constant, , for an RL circuit is the time required for the current in the circuit to reach 63.2% of its final value

  16. RL Circuit, cont • The time constant depends on R and L • The current at any time can be found by

  17. Energy Stored in a Magnetic Field • The emf induced by an inductor prevents a battery from establishing an instantaneous current in a circuit • The battery has to do work to produce a current • This work can be thought of as energy stored by the inductor in its magnetic field • PEL = ½ L I2

  18. Example 20.8 – page 684 • A 12.6V battery is in a circuit with a 30mH inductor and a 0.15Ω resistor, as in Fig. 20.27. The switch is closed at t=0. • A) find the time constant of the circuit; • B) find the current after one time constant has elapsed; • C)find the voltage drops across the resistance when t=0 and t=0ne time constant. • D) what’s the rate of change of the current after one time constant?

  19. Example 20.8 – cont. • A) find the time constant of the circuit; • B) find the current after one time constant has elapsed;

  20. Example 20.8 – cont. • C) find the voltage drops across the resistance when t=0 and t=one time constant. • D) what is the rate of change of the current after one time constant?

  21. Example 20.8 – cont.

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