Construct Electric Motor

8/10/2019 Construct Electric Motor

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CONSTRUCT YOUR OWN ELECTRIC MOTOR

Have you ever pulled apart a motor and wondered how it works? Even understandingthe basic concepts of a motor, the small motor found in many electrical appliances suchas clocks and small toys, can have a completely different layout to how you imagined.

This activity not only aims to help you understand the concept of electric motors better,but to help you see how the three pole motor used in everyday circumstances works.

HOW DOES A MOTOR WORK?

If you place a magnet near a compass, the direction the compass points in will change.

Why is this so?



A compass contains a magnet that can rotate freely. If a magnet experiences a magneticfield other than its own, the magnet will rotate to align itself with the magnetic field. In thecase of a compass, it is usually the Earth’s magnetic field. The end that points in thedirection of the Earth’s Magnetic North Pole is called the north pole.

This turning force is known as a torque (T in the diagram above). The torque is createdfrom the opposite poles in each magnet attracting and the same poles repelling. Thiscauses the magnet in the compass to align itself with the magnetic field associated withit. By putting a magnet next to a compass, as the magnetic field of the magnet isstronger than the Earth’s, the magnet in the compass with align itself with the magnetnext to it.

Another interesting fact is that as electricity passes through a wire, a magnetic field isformed.

The direction of the field can be given by the ‘right hand rule’, that is:

If you coil the wire, it creates a magnetic field exactly like a bar magnet.

The direction of the magnetic field can also be found by the right hand rule, except thistime the fingers point in the direction that the electricity travels, and the thumb points inthe direction of the north pole.

What has all this got to do with motors?

From all these concepts is the basis of how a motor works.

Most books will discuss how a two pole motor works. A two pole motor will have a similarlayout to this:



(Modified from Cahill, Physics for Western Australia 12, figure 3.34)

The loop and the magnets will be represented like this:

When the current travels through the loop, a magnetic field will be formed in the loop likethe wire coil discussed earlier.

The torque above is the result of the attraction and repulsion of the two magnetic fields.

The commutator and brushes are used because when the loop rotates and passes thefollowing position without a commutator, it will stop rotating.

The motor stops because when the loop rotates past this point, the loop will start rotatingin the other direction until it passes this point again. Then it will rotate forwards again.This will continue to occur until the coil sits in this position, and will not move as thefields are aligned.



Therefore the commutator switches the direction of the current at this point exactly sothe coil will continue to rotate in the same direction. The brushes are a method ofcontacting the commutator so current can pass through it.

This two pole motor appears to work. Why is it not used in practical situations?

If the motor starts working in this position:

It will get stuck there and not work, as the field is aligned (see above).

Also, as the brushes are touching both pieces of the commutator as once, this will shortthe circuit and empty the battery.

This also happens momentarily whenever the loop reaches this position while rotating,which reduces the life of the battery, or is a waste of energy if electricity is taken from anexternal power source.

So what is the solution?

A three pole motor.

(Photos from electronics.howstuffworks.com)

The set up of the three pole motor can be represented as:

The loop starts at one of the commutator pieces, wraps around the ‘head’ (the T-Shape)between the commutator pieces and joins onto the next one. The current direction in theloops alternates depending on the orientation of the commutator and the effects areshown below:



Now we understand what we’re making, what are we waiting for?



EQUIPMENT

You will also need:

Phillips Head Screwdriver

2x Pliers

Wire Cutters

*The supervisor may decide to use a power supply instead of a battery. Instead usingthe battery and plastic coated wire, you will use:

1 Power supply

2 Banana Leads (wire with a clip at each end looking like a banana)

4 Alligator Clips

SAFETY

It is recommended that you will wear safety glasses and enclosed footwear for your ownsafety as anything can flick up into your eye or land on your foot. If your hair is longerthan shoulder length, tie it up as it can be caught in the motor while rotating. Do not playwith the magnets as well as they can be quite strong and if they come together tooquickly, it can pinch your fingers and cut you. Also, if you are using the lantern battery,do not try to short circuit it by placing a conductive material over the positive andnegative terminals, as it will get very hot and burn you, as well as emptying the battery.



METHOD

1 First, we will make the rotor (the rotating part of the motor).

Push the tacks into the crosses on the curved part of the cork. (Not on the flat part).Leave 0.5cm of the tack out of the cork. The cork should look like the diagram above.

2 To make the coils we will use the 0.25mm enameled copper wire. The ‘front’ ofthe cork will be the end with the markings. That is:

Leave about 3-4cm of wire overhanging the edge of the front of the cork, and startwrapping the wire about one of the lines of tacks in the direction of the arrows.

You will need to wrap the wire around the tacks tightly or it will come loose. Wrap thewire around the tacks 80 times, and then leave another 3-4cm of wire overhanging thefront of the cork before cutting the wire. (Note: One loop around the tacks is shown in thediagram above where the wire completes an oval shape around the tacks.) That is:

3 Repeat step 2 for the other 2 sets of tacks.



4 The copper wire on the loops is coated with enamel that insulates the wire. If wewant to connect the wire to other conductive materials, we must remove the enamel toexpose the copper metal that is underneath it. For our rotor, we will need to remove theenamel off the overhanging wires. That is:

We will do this by using a nail file. If you hold the copper wire against the nail file, andslide the nail file along the copper, the enamel is stripped off the wire that is in contactwith the nail file.

We need all of the enamel to be stripped off the wire, so ensure that you hold the nail fileagainst the wire on different angles when removing the enamel.

You can see when all of the enamel has been taken off by looking at the wire. When theenamel has been taken off, the copper is visible (a light pink metal). Sections where theenamel has been missed still has the colour of the enamel on them. When you havestripped all of the enamel, check each wire from all angles to make sure the enamel hasbeen taken off, or it will not conduct the electricity properly.

5 We will now make the commutator. Most commutators are setup like this:

Ours will be positioned at the end of the cork, and will be flat.

Why? It is easier to make, and has a similar effect to the other style of commutator.

Our first step in making this commutator is to attach the wire to the thumb tacks. First,look at two loops next to each other, and find where the wire enters and exits each loop.We need to join the wire exiting from a loop to the wire entering the loop next to it.

Get these two wires and twist them together.



Do this for all the loops.

CHECK: Have you connected the right wires together? Check each loop that the wireexiting the loop is attached to the wire entering the next loop. Fix it if a mistake has beenmade. This is very important as the motor will not work if this is not correct.

6 Now wrap one of these twisted wires tightly around the base of one of the thumbtacks.

Tear a small piece of aluminum foil, about the size of 1.5cmx3cm, and twist it into a longsolid cylinder (about 1cm long and 2-3mm thick).

Wrap this foil shape around the base of the thumbtack on top of where the wire is. Itdoes not need to be wound as tightly as the wire.

Now push this into where the cross is between the two loops that the wire came from.Push the thumbtack in as far as possible. Push it in straight or the brushes will catchonto it when it rotates.

7 Now repeat step 6 for the other thumb tacks and twisted wires.

If the thumb tacks are touching, take them out of the cork, and put them back in closer tothe edge of the cork (but still between the two coils). Otherwise, if the thumb tacks aretouching, the motor will not work.

8 We will now start making the Stator (the stationary part of the motor). Nowposition the brackets onto the wooden block as shown and screw them into place usingthe large screws (2 per bracket).



9 With the other two brackets, put them onto the wooden block forming a U-shapewhere the holes align. Screw this in using the last 2 large screws. That is:

The shape of the brackets should be:

10 The trapezoidal shapes that are marked on the wood indicate where to place thebrass brushes. The brass brushes are already bent into shape, and the base of the

brass brushes looks like this:

These shapes match up to the shapes on the wood. Place the brass brushes with theorientation shown below and screw them into place with the smaller screws. It may helpif someone holds the base for you as it can slip.

The final layout should look like this:



11 The bearings we will use for the motor are a cheap low-friction bearing that lookslike this:

Our bearing will consist of a nail and a bolt with the end drilled out.

First we need to insert the nail with the end cut off into the cork rotor. Each end of the

cork has a dot marked on it.

Holding the nail using pliers, push the pointed end of the nail into the cork. Leave about1 to 1.5cm of nail out, and then pull it out.

This makes a hole to insert the nail into. Take the nail again in the pliers and push theflat end into the cork (the pointed end faces out of the cork) and leave it in there with 1 to1.5cm of nail remaining.

Repeat this for the end with the thumbtacks.

CHECK: The nail must not come into contact with any of the thumb tacks as this willshort the motor.



If this does happen, please remove the thumb tacks, and replace them closer to theedge of the cork (still between the coils) so they no longer make contact with the nail.

12 Now screw one of the nuts onto each bolt, about one third from the end.

Get one of the bolts, and put it through the highest hole in one of the brackets that arenot in the U-Shape.

Screw the other nut onto the bolt and tighten with two pliers (one holding each nut,turning in opposite directions) to keep this bolt in place.

Do the same with the other set of 2 nuts and 1 bolt, except this time put the second nuton end of the bolt. That is:

This allows you to be able to move the bolt around so the rotor can fit in.

13 It is now time to position the rotor between the two bolts. The layout for this isshown below:

It will be easier to do this with two or more people. As the brushes are bend to makecontact with the commutator, they will have to be pulled back to make room for the rotorto fit in.



Adjust the loose bolt back and forth, and tighten it when:

(a) The bearings hold the rotor in place so it doesn’t fall out.(b) If you spin the rotor with the brushes pulled back, it should spin for a fairamount of time before stopping.(c) The thumbtacks should be just touching the brushes. (They shouldn’t bepressed too hard against it.)

You may need to readjust the bolts a couple of times to do this.

Remember that to tighten the bolts; you will need two sets of pliers.

14 The brushes should make contact with the commutator when they are not heldback. If they do not, or if it looks like they are pressing against the commutator

too hard, bend them in and out from the base.

15 Now we have to position the permanent magnets into place in the U-shapedbrackets as shown below.

Ensure that the faces of the magnets are attracted to each other. ie:

If they repulse each other, the field will be wrong, and the motor will not work.

16 Now it is time to get your motor to work. If you are going to use the battery andplastic coated wire, then follow part (a). If you are going to use the power supplyand terminal, then follow part (b).

(a) Wrap one end of the red plastic coated wire on the positive terminal of the lanternbattery, and wrap the black plastic coated wire on the negative terminal. (Thenegative terminal is sometimes protected with a plastic cap. It is easier to removethis first.)



NOTE: DO NOT TOUCH THESE WIRES TOGETHER, OR IT WILL SHORT THEBATTERY AND THERE WILL BE NO POWER LEFT IN THE BATTERY TO DO

ANYTHING, INCLUDING ALLOWING YOUR MOTOR TO WORK.

Now touch the end of the red wire on one of the brushes, and the black wire onthe other brush, and the motor should spin!

(b) Plug the power supply in, but make sure the switch is off.

Insert one end of the banana leads into the DC part of the power supply. Onelead goes into the positive terminal, and one goes into the negative terminal.

Put the alligator clips on the other end of the banana leads.

Clip this onto the brushes: One clip goes on one of the brushes and the other clipgoes onto the other brush.

Now set the values on the power supply. This will vary depending on what yourpower supply box will deliver. In the table below, I have some values to set thevoltage and current to. Try to use the highest values that your box will allow. Forexample, if your box will not supply more than 6V, then use the 5V values.

Voltage (V) Current (A)

6V 0.3A5V 0.35A

4V 0.4A3V 0.45V

Check that you have followed these instructions properly.

Now turn on the power supply.

CONGRATULATIONS!

What if my motor doesn’t work?

Check the following:

Try pushing the rotor to ‘kick start’ it, as sometimes there is too much friction andresistance to initially get the rotor turning.



Are the brushes pressing too hard against the commutator? Adjust the pressurea little by bending them outwards.

Are the bearings too tight? Try and loosen then slightly.

Are opposite poles of the magnet facing each other? ie:

Change them if they are not.

If you are using the battery, are the red and black wires wrapped onto the batterycoils properly? Also, did you short circuit your battery by connecting eachterminal? If you did, there may be no power left.

If you are using the power supply, turn off the power, and then try using a highervalue for current or voltage than that was given in the table. This may mean thatthe start up power may need to be higher for your motor, as the resistances ineach motor vary due to small differences during construction.

Did you wrap all the coils in the same direction? Check, and if not you will need

to remove the rotor from the bearings and redo it. Did you attach the correct wires to the thumb tacks? Check, and if you have not,

remove the rotor and fix it.

If you are still stuck, ask someone’s whose motor works, check each part of the methodagain, then ask the supervisor.

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Construct Electric Motor