This was preparation for a motors and generators practical test.
Physics: Motors and Generators Assessment
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Station 1: Wires in Fields
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Consider a current carrying conductor in a magnetic field:
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The stronger the field, the stronger the force acting on it.
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The stronger the current in the conductor, the stronger the force
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The longer the length of conductor in a magnetic field, the stronger the force
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If the conductor length is at right angles to the magnetic field, the force will be greatest. When it is parallel, it will be the least (none)
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F = BILsinø
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There is an equation to use when considering two parallel wires (Syllabus 9.3.1.2)
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When magnetic fields travel the opposite direction, they are attracted
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If the currents in parallel wires are traveling the same way, the wires will be attracted
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The stronger the current in each wire, the stronger each wire will be attracted to each other
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The further away from each other, the less the attraction is
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The longer the parallel section of the wires, the more attracted they will be.
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Station 2: DC Motors
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Parts of a DC motor
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Stator: Stationary part of the motor, contains the field magnet and encases the armature
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Armature: A material block that the wire coil is wrapped around. It rotates.
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Split Ring Commutator: The thing that makes this motor DC; a ring that the armature is connected to and rotates with. Brushes conduct a current to and from the commutator. It switches the current direction at the right time to make it continue spinning.
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Torque on a DC motor = nBIAcosø
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n is the number of loops in the coil
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B is the magnetic field strength of the field magnet
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I is the current running through the coil
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A is the total area of the coil
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ø is the angle made by the coil and the magnetic field lines
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You can increase the speed of a DC motor by:
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Motors have curved field magnets that kind of wrap around the armature. This means that the field lines almost alway go straight through, parallel to the coil: increasing the torque on the coil.
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Increasing the magnetic field
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bigger coil area
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having multiple coils
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increasing the loops in the coil
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more current
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You can use an electromagnet to make the field magnet and it can be powered by the same current that goes through the coil! Isn't that amazing?!
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Station 3: DC Motor and Back EMF
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Lenz's Law
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You move the north pole of a magnet towards the end of a coil. When a conductor is in a field of changing flux, a current is induced in it.
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The current in this coil induces its own magnetic field. This magnetic field REPELS the magnet that you are moving towards it.
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So after learning about Lenz's law:
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You can say that in MOTORS(!) when you put the current through the wire, the coil moves.
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This means a CONDUCTOR IS MOVING IN A MAGNETIC FIELD.
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When a conductor moves in a magnetic field, a current is induced in it
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This induced current is OPPOSING THE MOTION OF THE COIL so the induced current is going the opposite way to the current you are putting through it.
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This is back emf
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When a motor starts up:
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There is a lot of current being drawn to begin the movement of the coil
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The coil is not moving so there is no back emf
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Therefore there is lots of current being drawn and the motor can get really hot or burn out
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Startup resistors are used to reduce the amount of current drawn. The motors start moving slower but they don't catch fire.
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After the coil starts moving, there is some back emf that reduces the current being drawn to move the coil. The startup resistor stops being used (it is switched off)
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Station 4: Lenz's Law and EM Induction
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Change in flux
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When a conductor is in a changing magnetic field, it is cutting a lot of flux and in the conductor an electric current is induced.
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Lenz's Law
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When you move a magnet towards the end of the coil, an electric current is set up so that the magnetic field induced by the current opposes the movement of the magnet.
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So you move the north end of a magnet to one end of the coil, the coil sets up a north at that end to repel the north you are moving towards it.
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Eddy Currents
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They are like induced currents but they are induced in lumps of metal instead of coils. Usually they are completely useless and they annoy engineers by causing heat and loss of energy.
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When a magnetic field changes around a lump of metal, and eddy current is caused that opposes the magnetic field change.
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Station 5: Applications of Eddy currents.
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EM Braking
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Pendulum
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A copper pendulum swings between the bars in a U magnet.
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As it swings into it it slows down
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This is because a conductor is cutting flux lines that pass from one bar to the other
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The copper produces an eddy current that repels the magnetic field change
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On the north side of the U magnet, the copper will produce an north pole to repel the north pole it is moving towards and on the swing upwards it will produce a south pole to attract itself to the ople it is swinging waya from
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This is the exact opposite on the south side of the U magnet.
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Wheels
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A wheel in a trailer is rolling along. The electromagnetic brakes are activated.
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The wheel is a metallic disc with two electromagnets on either side of it. The disc rolls and the electromagnet causes a magnetic field. So the metal is moving through a magnetic field, cutting flux.
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In the metal, eddy currents are set up to repel the motion of the metal.
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Induction cookers
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You have a hotplate kind of thing called an induction cooker.
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It consists of an electromagnet under a ceramic plate.
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You put a metal pot on the ceramic plate and turn on the electromagnet
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The electromagnet sets up a rapidly changing magnetic field
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This magnetic field induces a current in the pot. Eddy currents cause heat in the metal so it cooks whatever is in the pot.
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They are much more efficient than gas cookers
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Induction furnaces work in the same way except that the electromagnet wraps around an insulator pot and you put a metal in the pot. The metal has eddy currents induced, heats up, and melts.
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Station 6: Transformers and domestic power supply
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Transformers
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Transformers transform electrical energy.
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There are step up and step down transformers and they refer to the stepping up and down of voltage
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A primary coil (current in) and a secondary coil (current out) are wrapped around an iron core. The core threads all the flux generated in the first coil so it can take most of the flux to the second coil and induce a current in it.
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The coils have different amounts of turns in them. If the primary has more than the secondary, then it is a step down transformer. If it is a step up transformer, the secondary coil has more turns.
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Step up: higher output voltage, lower output current. Step down: lower output voltage, higher output current
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Primary voltage on secondary voltage = primary turns on secondary turns = secondary current on primary current
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Transformers lose energy because of the eddy currents in the iron core cause heat. This is reduced by making lots of thing iron cores with insulation between each layer.
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Transformers only work with AC
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Edison vs Westinghouse
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Back in the day (1887) there was a race to supply America with electricity. It was either DC or AC. Thomas Edison and Edison General Electric backed DC while Westinghouse backed AC.
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DC was the official electricity distribution form in America at the time.
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George Westinghouse went overseas (Italy?) and saw AC in action. He liked it. He brought the idea to America and patented it.
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Westinghouse and General Electric went into rivalry over supplying electricity.
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Transformers only work with AC. AC could be transfered long ways by stepping down the current. This would reduce energy loss. DC could not be stepped down and lost heaps of energy in transit.
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In 1887 Niagara Falls needed a way of transferring the energy they were generating around. A competition was held where a grant would be given to the best system.
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Edison and Westinghouse proposed ideas (DC and AC respectively). Westinghouse won lots of money and stomped all over Edison, so AC became the official form of power distribution.
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