Electric Power Distribution, Generators and Motors

New ideas for today Magnetic induction Lenz’s law Transformers and power transmission Motors and Generators

Why such high voltage?

Transformers!

Observations about Power Distribution Household power is AC (alternating current) Power comes in voltages like 120V & 240V Power is transmitted at “high voltage” Power transformers are everywhere

Power Consumption in wires Reminder: power consumption = current × voltage drop voltage = resistance × current power consumption = resistance × current 2 So what? Wires waste power as heat Doubling current quadruples wasted power Better not transmit high current!

DCAC Edison Tesla Westinghouse vs.

AC = alternating current Current switches direction 60 times per second (in N. America) DC= “direct current” AC

Power Transmission Power delivered to a city is: power delivered = current × voltage drop Power wasted in transmission wires is: power wasted = resistance × current 2 For efficient power transmission: Use low-resistance wires (thick, short copper) Use low current and high voltage drop Can accomplish this with AC (alternating current) power transmission.

neutral hot ground 7000 V 120 / 240 V kV

Voltage Hierarchy High voltage is dangerous High current is wasteful Use the following scheme: low voltage circuits in neighborhoods (120/240 V) medium voltage circuits in cities (7000 V) high voltage circuits across the countryside (155, ,000 V) Use transformers to change voltage

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Electromagnetic Induction Changing magnetic field  electric field Electric field in conductor  current Current  magnetic field Induced magnetic field opposes the original magnetic field change (Lenz’s law)

inductivecharging B B

Lenz’s Law

That “emf” can be useful…

Transformer Alternating current in one circuit induces an alternating current in a second circuit Transfers power between the two circuits Doesn’t transfer charge between the two circuits

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Current and Voltage Power arriving in the primary circuit must equal power leaving the secondary circuit Power = current × voltage A transformer can change the voltage and current while keeping the power unchanged! Secondary voltage = Primary voltage Secondary turns Primary turns

Step Down Transformer Fewer turns in secondary circuit so charge is pushed a shorter distance Smaller voltage rise A larger current at low voltage flows in the secondary circuit

Step Up Transformer More turns in secondary circuit so charge is pushed a longer distance Larger voltage rise A smaller current at high voltage flows in the secondary circuit

Transformers are often filled with nasty stuff filled with nasty stuff

You decide to use a transformer to increase the voltage from a battery, and hook it up in the circuit shown below. When you close the switch, 1.5 the voltage across the lightbulb is: (A) bigger than 1.5 V (B) smaller than 1.5 V (C) zero Clicker question

Electric Generators and Motors A generator provides electric power A generator requires a mechanical power A motor provides mechanical power A motor requires electric power Alternator

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Electric Generator Rotating magnet makes changing magnetic field induces AC current in the loop Converts mechanical power into electrical power

Electric Motor Input AC power AC current makes changing magnetic field causes magnet to turn Converts electrical power into mechanical power A motor is a generator run backwards !