Motors convert electromagnetic energy into energy of motion or kinetic energy. Michael Faraday was the first person to create a device that used an electromagnet with a permanent magnets to apply or create a directed force. The motor principle may be stated follows:
when a current carrying conductor is located in an external magnetic field perpendicular to the conductor, the conductor experiences a force that is both perpendicular to both itself and the external magnetic field.
In class a demonstration set-up was constructed to show this principle. A current carrying wire was suspended in a magnetic field and when the circuit was closed the wire moved out of the field lines of the magnet. If initially the wire moved into the horsemagnet, when the current was reversed the wire moved out of the horseshoe magnet. See page 548 Fig 15.6 for a diagram or as viewed below.
Applying the right hand rule for electromagnetic force one may determine the direction of the motion or direction of the force. Definition and appropriate diagram (Fig 15.7) are on page 548.
In this project you are asked to desribe in detail the design and operation of an electric motor.
Terms that must be include in the discussion are
Key points to explain
Digram of a simple motor
Make sure you note the difference in the commutator rings.
The galvanometer is the backbone of all anolog meters, such as a voltmeter. A galvanometer consists of a permanent field magnet and a helix with a pointer attached to it. One design has a helix inserted into a magnetic field of the permanent magnet. When a current flows through the helix the current produced magnetic field react with the already present permanent field and will spin to follw the flux lines of the permantent field magnet. On spinning the pointer will be tracted on a calibrated scale and hence measurements can be made.
Two examples are the ammeter and the voltmeter. Diagrams for each are found on page 550 and 551 Fig 15.10 & Fig 15.11. An ammeter has a shunt resistor of low resistance connected in parallel to the galvanometer and the voltmeter has a high resistor connected in series with the galvanometer.
Two applications of the motor principle are shown in the diagrams below.
Michael Faraday succeeded in showing that a magnetic field could produce an electric current. This experiment will be demonstrated in class. A magnet will be inserted into a helix that is connected to a galvnometer. It will be left stationary for a moment, the magnet will then be removed. Observation indicates that only when the magnet is moving is a current produced.
On closing the switch a brief surge of current is detected by the galvanometer and then it returned to zero. On turning on the switch a magnetic field is building up in the ring. This changing magnetic field induces a current flow in the other coil. The direction of the current flow will be based on induce north pole of this secondary coil. The induced field will align itself so as to oppose the inducing or original field. This is known as Lenz's law (see below). Once the switch is closed the galvonometer will return to zero once the magnetic field has bulit up and is no longer changing.
On opening the switch the magnetic field on the left side will collapse. This moving or changing magnetic field will induce a magnetic field in the secondary coil, so as to oppose this collapsing. The induced field produces a current flow that is in the opposite direction to that of the "closing the switch" case.
The principle is rather simple; when the current is turned on the magnetic field lines build up in battery connect helix causing a magnetic flux build up in the other helix. When this happens a current is induced to flow in the other helix with the current being detected by a galvanometer.
The magnitude of induced current is proportional to the number of turns on each helix and the input voltage according to the formula:
V1 N1 ---- = ----- V2 N2
Heinrich Lenz discovered which way the current flows in a conductor when an induced current is producced.
A Statement of Lenz's Law:
An induced current will flow in such a direction that its own magnetic flux will be directed to oppose the change in flux that produced the induced current. Or in a simpler terms
The magnetic field of an induced current always opposes the change in magnetic field that is causing the induced current.
Transformers are Faraday rings connected to AC current. As such electrical energy is converted by the primary coil into magnetic energy and passed on to the second coil called the secondary coil. The building up and colapsing magnetic field of the primary coil causes the electrons to flow in the secondary coil.
Remember when a battery is connected to the primary coil; after connection, nothing happens, no current is flowing in the secondary coil.
Step-up transformers increase the voltage output in comparison to the input voltage. The relation is in direct proportion to the number of turns on the two coils of the transformer. If the input voltage is 6.0 V on a primary coil of 200 turns then the output voltage on a 600 turn secondary coil will be 3x or 18.0 V.
Since P = VI and assuming no energy lose in the transformer, P of the primary must equal P of the secondary so if the voltage of a step-up transformer increases the currnt (I) must dercrease in the same proportion. Using the same example as above if the input current is 1.2 amps then the output current will be 0.4 amps.
Step-down transformers have more turns on the primary than the secondary and reduce the voltage in the same ratio as the number of turns on wire on the primary coil to the secondary coil. Step-down transforms will therefore increase the current of the output curcuit.
Transformers are used to produce required voltages of electrical devices. If 12 volts are need to run a TV circiut board, then a step-down transformer will be used to drop 120 volts to the required 12 volts.
Formulae are listed on pages 572 and 573 of the text book.
Generators are used to convert kinetic, mechanical energy into elecrical energy. A generator in a sence is a motor in reverse; the energy conversions are the opposite and instead of a split ring parallel to the armature the ring (called a slip ring) is perpendicular to the axis.
Diagrams are found on page 568 Fig 15.38
The generator produed current flow that for half the cycle flows in one direction theen reverses direction for the second half of the cycle. This current is called alternating current.
DC current when plotted against time yields a straight line with zero slope parallel to the time axis. AC current will follow a sine function versus time.
In the province of Ontario and all of North America, generators producing power, rotate at 60 cycles per second producing a 60 Hz frequency for the AC current.
