Alternator

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PHY 101 Lecture #11 Electromagnetic Generators and Transformers

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PHY 101 Lecture #11:Electromagnetic

Motors and GeneratorsProf. Peter R. Saulson

[email protected]://physics.syr.edu/courses/PHY101/

Off. Hrs: Tue 9:30 –11:00, Physics 263-4Prof. Schwarz’s Problem Sessions:

Mon and Tues, 5:15 – 6:15, Physics 202/204


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Outline

1. Electric Motor2. Magnetic Induction3. Electrical Generator

Direct Current or DC generator4. Alternating Current

AC generator


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1. Electric Motor

Force on current in B field means that electric energy can be turned into mechanical energy.

Current goes around coil, in B field of fixed magnet.

Right hand rule says one side of coil forced up, other side forced down.ROTATION!


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Real Electric Motor

Practical device: electric motor.Key parts:

– magnet (permanent or electromagnet)– rotatable coil (rotor)– connections to ensure current goes to coil always

in the “right” way– electrical energy input (battery)– mechanical energy output (shaft)


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Making current go the “right” way

If coil just hooked to battery, coil would just twist to “no-force” position.

Clever parts:BrushesSplit-ring commutator

Brushes make sliding contact (no twisting up!)Commutator changes direction of current just as

coil lines up in “happiest” orientation.Coil needs to flip, but then current reverses

again, ...PHY 101 Lecture #11 Electromagnetic Generators and

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Energy in, energy out

It takes electrical power to keep current flowing in coil.

Mechanical power can be extracted from rotating shaft.

Examples: – Let string wind up around shaft, lifting weight.– Turn the wheels of a car.

Motor is a device whose job is energy transformation.

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2. Magnetic induction

Electric motor converts electrical energy into mechanical energy.

Can we make electrical energy from mechanical energy, using magnets and wire? Yes!

Key idea: Induction of current from changing magnetic flux.

Governing equation:Faraday’s Law


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Observation of magnetic induction

Consider circuit with just coil of wire and ammeter. (Coil has some resistance.)

Move a magnet near the coil.Current flows whenever magnet moves.(No current when magnet is still.)Big current when:

Magnet moves fast.Magnet is close.Coil has many turns.


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Magnetic flux

Magnetic flux is the key quantity for induction.Consider coil with area A and N turns of wire.If magnetic field B fills the coil, then the

flux ΦΒ is

(Note: B must be perpendicular to coil. If not, use strength of B that is perpendicular to coil.)

.NBAΦB =


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Strength of voltage induced by moving magnet

Voltage in circuit comes from change in flux through the coil.

The strength of the voltage (“induced electromotive force” or “emf”) is given by Faraday’s Law:

or in words, “Induced emf equals rate of change of magnetic flux.”

,tΦV B

induced ΔΔ

=


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What has to move?

Induced emf depends only on the rate of change of magnetic flux.

Can equally wellmove magnet,move coil,turn magnet,or turn coil.


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3. Electrical Generators

Magnetic induction means that mechanical energy can be turned into electrical energy.

Practical device: electrical generator.Key parts:

– magnet (permanent or electromagnet)– rotatable coil (rotor)– connections to ensure current comes from coil

always in the “right” way– mechanical energy input– electrical energy output

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Generator is motor run backwards

Motor, generator parts are almost all the same.Input and output are reversed.

Apply mechanical energy to rotor (e.g. turn it by hand), and voltage is generated in coil.

Flux grows, then shrinks, as coil turns in B field.Just as flux would grow with opposite sign,

commutator switches contacts -- voltage out always has the same sign.

“Direct current”, or DC.


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Investigations with an oscilloscope

An oscilloscope is a way to study voltages that may change with time.

It plots a graph of voltage vs. time, V(t).The voltage scales and time scales are

adjustable, to enable the graph to be clearly read.

We can use it to look at V(t) out of a DC generator.


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“DC” from a generator isn’t steady!

As the coil turns between the poles, there are points in the cycle when flux is changing rapidly, other places where it doesn’t change at all.

Like a sine function, but “folded” so that it is always positive.

.tΦV B

induced ΔΔ

=


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Capacitor helps make V(t)more constant

Put a large capacitor between the output terminals of the DC generator.

The bumps in V(t) smooth out. Why?

Charge is stored on the plates of the capacitor.It doesn’t all get to leak off before another cycle

starts. Hence, variations get smoothed out.

This is an example of a filter.


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Generator transforms mechanical power into electrical power

Attach electrical load to generator, to use electrical energy.Light a bulb, turn a motor, …

Electrical energy comes from mechanical energy that turns the generator.

You can feel the power you supply.Compare how hard it is to turn generator,

with light bulb attached vs. without bulb.Generator needs mechanical power input.


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4. AC generator is even simpler

Split-ring commutator means voltage always has the same sign (current always same direction.)

We could replace commutator with slip rings. Each brush always touches same end of coil.

As coil turns, flux through coil changes sign.Induced voltage alternates sign.“Alternating current” or AC.

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