The Starter motor. Student booklet

Transcription

1 The Starter motor Student booklet

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3 The Starter motor - INDEX :20 The Starter motor The starter motor is an electrical motor and the electric motor is all about magnets and magnetism: A motor uses magnets to create motion. Do you remember the law? Opposites attract and like repelling. Inside an electric motor, these attracting and repelling forces create rotational motion. To be able to understand how an electric motor works, you must first learn about the fundamental facts about electromagnetic. That's why we are going to start this module talking about that. Later, when we come to the start motor it will be easier for you to understand. Contents Permanent magnets The electromagnet Inside an electric motor Electromagnets and Motors Armature, Commutator and Brushes Putting it all together Electric toy motor Armature & brusches The stationary electromagnet Control relay & solenoid switch Excercise - Magnetic Field I Excercise - Magnetic Field II Excercise - Identify & measure Excercise - Measure & assemble Sum up Assessment INDEX -

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5 The Starter motor - ei116_2005.dcr :18 Before talking about electromagnets, let's talk about normal "permanent" magnets like the ones you have on your refrigerator and that you probably played with as a kid. You likely know that all magnets have two ends, usually marked "north" and "south," and that magnets attract things made of steel or iron. And you probably know the fundamental law of all magnets: Opposites attract and likes repel. _ So, if you have two bar magnets with their ends marked "north" and "south," the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south). An electromagnet is the same way, except it is "temporary" - the magnetic field only exists when electric current is flowing. 1

6 The Starter motor - BasEl_starter_02.swf :18 The electromagnet To be able to understand how an electric motor works, you must first learn about the fundamental facts about electromagnetic. That's why we are going to start this module talking about that. An electromagnet starts with a battery (or some other source of power) and a wire. The basic idea behind an electromagnet is extremely simple: By running electric current through a wire, you can create a magnetic field. 2

7 The Starter motor - BasEl_starter_03_02.swf :18 Inside an electric motor Let's start by looking at the overall plan of a simple two-pole DC electric motor. A simple motor has six parts: 1. Commutator 2. Brushes 3. Axle 4. Armature or rotor (Electromagnet) 5. Field magnet (permanent magnet or a stationary electromagnet) 6. DC power supply of some sort If you have ever played with magnets you know about the fundamental law of all magnets: Opposites attract and likes repel. So if you have two bar magnets with their ends marked "north" and "south," then the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south). Inside an electric motor, these attracting and repelling forces create rotational motion. The armature (or rotor) is an electromagnet, while the field magnet is a permanent magnet (the field magnet could be an electromagnet as well, but in most small motors it isn't in order to save power). 3

8 The Starter motor - BasEl_starter_04.swf :18 Electromagnets and Motors An electromagnet is the basis of an electric motor. You can understand how things work in the motor by imagining the following scenario. Say that you created a simple electromagnet by wrapping 100 loops of wire around a nail and connecting it to a battery. The nail would become a magnet and have a north and south pole while the battery is connected. Now say that you take your nail electromagnet, run an axle through the middle of it and suspend it in the middle of a horseshoe magnet as shown in the figure below. If you were to attach a battery to the electromagnet so that the north end of the nail appeared as shown, the basic law of magnetism tells you what would happen: The north end of the electromagnet would be repelled from the north end of the horseshoe magnet and attracted to the south end of the horseshoe magnet. The south end of the electromagnet would be repelled in a similar way. The nail would move about half a turn and then stop in the position shown. You can see that this half-turn of motion is simply due to the way magnets naturally attract and repel one another. The key to an electric motor is to then go one step further so that, at the moment that this half-turn of motion completes, the field of the electromagnet flips. The flip causes the electromagnet to complete another half-turn of motion. You flip the magnetic field just by changing the direction of the electrons flowing in the wire (you do that by flipping the battery over). If the field of the electromagnet were flipped at precisely the right moment at the end of each half-turn of motion, the electric motor would spin freely. 4

9 The Starter motor - BasEl_starter_05.swf :18 Armature, Commutator and Brushes Consider the image on the previous page. The armature takes the place of the nail in an electric motor. The armature is an electromagnet made by coiling thin wire around two or more poles of a metal core. The armature has an axle, and the commutator is attached to the axle. The commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet. The "flipping the electric field" part of an electric motor is accomplished by two parts: the commutator and the brushes. This picture shows how commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment. The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator. 5

10 The Starter motor - BasEl_altern_01.swf :19 Putting it all together When you put all of these parts together, what you have is a complete electric motor. The key thing to notice is that as the armature passes through the horizontal position, the poles of the electromagnet flip. Because of the flip, the north pole of the electromagnet is always above the axle so it can repel the field magnet's north pole and attract the field magnet's south pole. 6

11 The Starter motor - BasEl_starter_03.swf :19 Electric toy motor You can see that this is a small and simple electric motor. From the outside you can see the steel can that forms the body of the motor, an axle, a nylon end cap and two battery leads. If you hook the battery leads of the motor up to a flashlight battery, the axle will spin. If you reverse the leads, it will spin in the opposite direction. Inside the end cap are the motor's brushes. These brushes transfer power from the battery to the commutator as the motor spins. The axle holds the armature and the commutator. The armature is a set of electromagnets, in this case three. The armature in this motor is a set of thin metal plates stacked together, with thin copper wire coiled around each of the three poles of the armature. The two ends of each wire (one wire for each pole) are soldered onto a terminal, and then each of the three terminals is wired to one plate of the commutator. This picture makes it easy to see the armature, terminals and commutator. The final piece of any DC electric motor is the field magnet. The field magnet in this motor is formed by the can itself plus two curved permanent magnets. 7

12 The Starter motor - ei222.dcr :19 The armature The armature is the rotating part of the starter motor and includes the armature winding and the commutator. It consists basically of an armature core with many loops of winding, which are symmetrically arranged and fixed into slots in the armature. The loops are insulated from each other. The brushes The current from the batteries is transfered to the armature commutator by four carbon brushes; one pair connected to plus of the batteries and the other pair connected to the ground. 8

13 The Starter motor - ei223.dcr :19 The stationary electromagnet The stationary electromagnet consists of two windings - one main winding and one shunt winding. The shunt winding causes that the armature turns slowly until the pinion meshes smoothly with the flywheel ring gear in the initial stage of the starting process. The windings are fitted on pole-shoes in order to reinforce the magnetic field. 9

14 The Starter motor - ei224.dcr :19 The control relay The function of the built-in control relay is to use a relatively low current to switch a high current. It operates in two switching stages to energize the starter motor magnetic field. _ Solenoid switch The solenoid switch shifts the starter motor pinion forward, acting by an engagement rod leading through the hollow armature shaft. The starter motor pinion meshes with the flywheel and starts spinning it around. 10

15 The Starter motor - BasEl_starter_exer_08.swf :19 Excercise - Magnetic Field I The part about the magnetic field might be a surprise to you, yet this definitely happens in all wires carrying electricity. You can prove it to yourself with the following experiment. Put the compass on the table and, with the wire near the compass, connect the wire between the positive and negative ends of the battery for a few seconds. What you will notice is that the compass needle swings. Initially, the compass will be pointing toward the Earth's north pole (whatever direction that is for you), as shown in the figure on the right. When you connect the wire to the battery, the compass needle swings because the needle is itself a small magnet with a north and south end. Being small, it is sensitive to small magnetic fields. Therefore, the compass is affected by the magnetic field created in the wire by the flow of electrons. 11

16 The Starter motor - BasEl_starter_exer_09.swf :19 Excercise - Magnetic Field II This is a simple way of visualize a magnetic field. Pour some iron fillings on a A4 paper and put the magnet under the paper. Shake the paper a little bit and you will see that the magnetic field will appear... The phenomenon that we call magnetism can be useful in many ways as long as you know how to use it. 12

17 The Starter motor - BasEl_starter_exer_10.swf :19 Excercise - Identify & measure 1) Disassemble the starter motor. 2) Identify the components. 3) Check for a ground fault in the armature (rotor) windings with multimeter Connect the test probes according to the picture. 13

18 The Starter motor - BasEl_starter_exer_11.swf :19 Excercise - Measure & assemble Field winding 1) Check the ground: Connect the multimeter to the field winding and the stator shell. Turn the multimeter knob to buzzer mode. The buzzer should not sound at this stage. _ 2) Check the field winding cable for short circuit or open circuit. Use multimeter Check for open circuit: Connect the multimeter to the field winding cable and the brush. Turn the multimeter knob to buzzer mode. The buzzer should sound 3) Reassemble the starter motor. 14

19 The Starter motor - BasEl_starter_sumup_12.swf :19 Sum up The picture shows the shape of the magnetic field around the wire. In this figure, imagine that you have cut the wire and are looking at it end-on. The green circle in the figure is the cross-section of the wire itself. A circular magnetic field develops around the wire, as shown by the circular lines in the illustration below. The field weakens as you move away from the wire (so the lines are farther apart as they get farther from the wire). You can see that the field is perpendicular to the wire and that the field's direction depends on which direction the current is flowing in the wire. The compass needle aligns itself with this field (perpendicular to the wire). Using the contraption you created in the previous section, if you flip the battery around and repeat the experiment, you will see that the compass needle aligns itself in the opposite direction. 15

20 The Starter motor - BasEl_starter_assess_13.swf :19 Assessment 1) What does an electric motor use to create motion? 2) What six parts does a starter motor contain? Name four. 3) What happens when the starter motor is flipping the electric field? 4) What kind of a motor is the starter motor? 16

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