Generators, Motors, Electromagnets

• Topic 6 covers generators and motors
• Includes information about electromagnets
• Includes information about the factors affecting their strength
• Explains AC vs. DC generators
• Explains AC vs. DC motors including parts and function

Magnetism

• Every object has magnetic poles
• The poles are usually randomly aligned
• Magnets have most or all of their poles aligned in the same direction

Magnetism and Electricity

• Moving a magnet past a wire produces electric current through induction
• Moving wire perpendicularly to a magnetic field induces a current
• Electric currents also can create magnetic fields

Electromagnets

• Convert electricity to strong magnetic forces
• The components are a soft iron core, a coil of copper wire and a power source

Advantages of Electromagnets

• Poles are reversed if current flow is reversed
• Can turn on or off, unlike permanent magnets
• Can be made very strong for powerful jobs

Changing Electromagnet Strength

• Strength can be improved by increasing the number of coils of wire
• Increasing voltage of the source can improve strength
• Increasing the size of the core used can improve strength
• Changing the diameter of the wire can improve strength
• Wires in an electromagnet must be insulated

Generators

• Simplest form is a magnet moving inside a wire coil or a wire coil moving inside a magnet
• There are two types of generators: AC (alternating current) and DC (direct current)
• Discovered by Hans Christian Oersted and Andre-Marie Ampere in 1820

AC Generators

• Produces AC or Alternating Current from a wire coil rotating inside a stationary magnet
• AC allows electricity to travel without losing much energy
• AC generators are usually called Alternators
• An Armature is a rotating wire that is often copper wire around an iron core
• Slip rings connect to the armature to deliver current to its destination

AC Generators: Function

• When the wire moves up, current flows one way in the wire
• When the wire moves down, current moves in the other direction
• The loop must be connected to something to spin for example a hand crank, water turbine, wind turbine, coal or hydro

AC Generator: Parts

• A field magnet creates a magnetic field between a north and south pole
• The Armature is a rotating shaft with a coil wrapped around it
• When the wire moves up and down between the magnets it creates a current flowing in two directions
• Slip rings are attached to the armature and conduct current to the circuit through brushes
• Brushes conduct electricity creates by the generator out to the load

Alternating Current

• AC is very easy to step-up, to make stronger, or step-down
• Electric power plants need to generate large voltages to transfer over distance
• Resistance in wires cases electrons to slow down over distances
• It is not safe to use voltages produced in a power plant in a home
• A 'step-down' transformer reduces the voltage before the wires reach a home

DC Generators

• Produce direct current that pulses in one direction
• Uses a split ring commutator which causes the pulses
• Brushes connect and conduct electricity
• When the split is encountered, the circuit 'breaks' until the metal is touched again

Split Ring Commutator

• A split ring is required; otherwise, the armature would only go half a turn because of the magnetic fields
• The split allows the electron flow to be changed back instead of reversing direction like in an AC generator

AC versus DC Generators

• AC- current alternates direction, it has two slip rings always attached to the same wire
• DC- current moves in the same direction, split ring momentarily disconnects from one wire and connects to the other

Strengthening a Generator

• More coils of wire
• Speeding up the rotation
• Using stronger magnets
• Having the magnets closer to the coil of wire

Motors

• Motors are the reverse of generators
• Electrical energy supplied by a source as converted to mechanical energy
• When a current runs through a coil of wire (armature), it becomes magnetized
• Just like generators, motors can be AC and DC

DC Motor: "St, Louis Motor"

• Components include the energy source to provide electrons
• Additional component are leads directing current from the source to the motor
• Brushes connect the leads to the armature
• The armature is a coil of wire often around an iron core
• Field magnets are permanent magnets that help direct the polarity of the electromagnet
• Split-ring commutator turns the current on and off so the motion is in one direction

DC Motor

• Current moves through the leads and into the armature to induce magnetism
• The armature is attracted to the opposite pole of the permanent magnet
• As the armature passes through the horizontal position the position of the poles in the electromagnet reverse
• Splits allow brushes to come in contact with the 'opposite' electrode which changes the polarity, allowing constant motion in one direction

AC Motors

• Rotating coil of wire, or armature, inside a steel cylinder
• Are surrounded by permanent magnets called stators
• Contains no split ring commutator
• Must run on AC current
• Used in larger appliances

AC Motors: Main Parts

• AC motors have two primary parts that push and pull on each other from magnetic field attraction and repulsion
• The two parts are rotors, for rotation, and stators, which are stationary
• Stators are two fixed magnets surrounding a shaft
• A rotor is a coil of wire
• When electric charge is applied to the coil of wires, it becomes an electromagnet
• The two magnets interact, and the shaft and coil of wires begin to rotate

Applications

• AC motors are used in clocks, power tools, washing machines and fans