LECTURE 32 GENERATORS & MOTORS & TRANSFORMERS

Instructor: Kazumi Tolich Lecture 32

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¨ Reading chapter 22-8 & 23-5 to 23-7. ¤ Motors ¤ Generators ¤ Transformers Motors

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¨ An electric motor is exactly the opposite of a generator – it uses the torque on a current loop to create mechanical energy. Generators

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¨ An electric generator converts mechanical energy into electric energy.

¨ An outside source of energy is used to turn the coil, thereby generating electricity.

¨ The induced emf in a rotating coil varies sinusoidally.

¨ In circuit diagrams, an ac generator is represented by the symbol: Demo: 1

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¨ AC and DC generator ¤ AC: ¤ DC: with commutator, the direction of the current is changed every half cycle.

¨ DC motor ¤ A large open coil powered by DC current through a split-ring commutator spins between the poles of a set of permanent magnets. https://www.youtube.com/watch?v=Xi7o8cMPI0E https://www.youtube.com/watch?v=Jg4BkhyNbRg Hybrid car brake

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¨ When a hybrid car brakes, its motor acts like a generator and returns some of the car’s kinetic energy back to the battery as stored chemical energy.

¨ In a normal car, kinetic energy is not recovered because it is transformed to thermal energy through friction. Does a car use more gasoline with the lights on or off?

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¨ The battery must provide power to the lights when the lights are on, and the battery must be charged to provide the power, so the answer must be that the car uses more energy with the lights on.

¨ A student points out that, the battery is charged from the alternator, and that the alternator is turning no matter if the lights are on or off. So he argues that the car is always using the same amount of energy, therefore the same amount of gasoline.

¨ However, the energy required to turn the alternator depends on how much current it provides. With the lights on, it is providing more current, so the car uses more gasoline. Demo: 2

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¨ Hand crank generator ¤ A small crank generator lights lamp - easier to turn with the bulb out of the socket. Example: 1

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¨ The maximum induced emf in a generator rotating at ω = 210 rpm is E = 45 V. How fast must the generator rotate if it is to generate a maximum induced emf of Emax = 55 V? Transformers

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¨ A transformer is a device used to increase or decrease the voltage in a circuit without a loss of power. ¨ The voltages across the primary and secondary coils are related by the numbers of turns of the coils. ¨ Since the power in both circuits must be the same, if the voltage is lower, the current must be higher. I V N s = p = p I V N p s s Clicker question: 1 & 2

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¨ Transformers ¤ Turn ratios of 1:1, 1:2, and 2:1. V N p = p V N s s Example: 2

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¨ A step-down transformer produces a voltage of Vs1 = 6.0 V across the secondary coil when the voltage across the primary coil is Vp1 = 120 V. What voltage appears across the primary coil of this transformer if Vs2 = 120 V is applied to the secondary coil? Transformers & electric power transmission

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2 ¨ To minimize the P = I R heat loss in electric transmission lines, we use a high voltage and a low current.

¨ For safety, power is delivered to consumers at lower voltages and therefore with higher currents.

¨ Because of this ease of stepping the voltage up or down with transformers, AC rather than DC is in common use. Demo: 4

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¨ Vertical primary coil and secondary coils with lamps ¤ A primary coil that carries AC has an iron core which extends out of the top. ¤ A secondary coil lights up a large light bulb. Wireless charger

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¨ The base of an electric toothbrush has a cylindrical hole surrounded by a solenoid.

¨ The charger has a matching protruding cylinder containing a solenoid.

¨ When the toothbrush is placed on the charger, an AC current in the charger solenoid produces an alternating B field.

¨ Due to the changing B flux through the toothbrush solenoid, a current is induced in it, charging the battery in the toothbrush. Clicker question: 3

17 Example: 3

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¨ The primary coil of a step-down transformer has Np = 250 turns and is connected to an outlet of Vp = 120 V. The secondary coil is to supply Is = 20 A at Vs = 9.0 V. a) Find the current in the primary coil. b) Find the number of turns in the secondary coil.