Electric Motors

Foundations of Physical ScienceWorkshop: Electric Motor

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Electric MotorCPO Science


Key QuestionsHow are electricity and magnetism related?How does an electric motor work?


Overview


Fun with MagnetsTake your motor completely apart to expose the little ceramic magnets that are in the rotor pocketsWhat stupid magnet tricks can you come up with?Compare force of attraction with force of repulsion


Fun with Magnets


How does a motor work?


ElectromagnetsCoil of wire wrapped around a steel pinYou can reverse polarity by switching direction of electricity


Electromagnets


Switching electromagnet polarityMechanism that makes electric current switch direction is called a commutatorClear = northBlack = south


Switching electromagnet polarity


Build a 4-pole electric motor


Design other motorsUsing other colored disks, decide how to build a working motorSave the green disk for last its a bit tricky!Use a photogate to measure frequency, and from that calculate the speed in rev per sec (frequency divided by # black areas)Who can build the fastest motor?


These are the key questions we will be investigating in this workshop. The investigations we will perform in this workshop are actually excerpts from investigation number 10.3 (Electric Motors and Generators). Key questions already investigated in:Electricity unitElectromagnets investigation where student make their own electromagnet

1. We are going to talk about magnets first because electric motors really ought to be called electromagnetic motors since the magnetism is what makes the force that makes a motor spin.(a)Kids love playing with magnets, and these are pretty strong ones (watch out for credit cards!)(b)The difference with these are in the north and south poles, they are on the faces, not the ends (point to slide, hold up magnet and point to it)These are made of ceramic, which means they are strong and won't demagnetize if dropped or forced together (show two magnets holding through your finger)2. We are then going to build a working electric motor (hold up the motor) from a design we know will work. (a) We will get to the idea of electromagnets along the way3. Next you are going to try and figure out how to make other designs for a motor that work.(a) Some designs will work, others won't, its a fun puzzle.(b) There is a trick built in with one of the designs (more on that later)

Most kids already know that magnets have north and south poles (a) Do magnets attract stronger than they repel? (b) How far can the magnetic force reach?(c) Does the magnetic force pass through objects unchanged?Take two magnets with their poles facing and put one on the black square at the start of the ruler. Now, take a second magnet and slowly slide it towards the first one. Record the distance where the magnet at the end first moves. This distance is a measure of the relative strength of the magnetic force.Try it with north facing south (attract), south facing south (repel) and north facing north (repel). Do you see any large differences? They should find that the distances are almost the same, the attractive force is just at strong as the repulsive force, only the direction changes.If we had more time we could put different materials between the magnets and test how the force was affected by comparing the distance with what we just observed with nothing but air in-between.

Take the motor apart and put six magnets in the rotor so they alternate north, south, north, south. Make the magnets evenly spaced (every other hole)Now bring a magnet close with your fingers (go to a table and do this in front of everybody).The new magnet (you are holding) will push and pull on the magnets in the rotor causing it to turn.NOW, if you can flip the magnet in your finger at the right time it will first pull and then push, making the rotor spin.Imagine flipping the magnet on the outside north-south-north-south, so it pushed and pulled on each successive magnet in the rotor. THE ROTOR WILL SPIN and this is the principle behind the electric motor.There are magnets in the rotor that alternate polarity (north/south)There are magnets outside (stator magnets) that reverse north and south so they push and pull on the magnets in the rotor, making the rotor spin.There is some mechanism (called a commutator) that keeps the switching of the outside (stator) magnets synchronized to the passing of the inside (rotor) magnets.

It would probably not be the worlds greatest invention if you had to stand there twiddling a magnet to make the motor spin. What we need is some way to switch the outside magnet (other than with fingers)The secret is to use electromagnets.There are two kinds of magnets ordinarily found: permanent magnets stay magnetic when they are not plugged in. (like these little ones). Electromagnets only become magnetic when you make electric current flow through the wires.

Electromagnets are useful because you can reverse the north and south poles by switching they direction of the electricity. (see slide) Or, you can turn them off by stopping the flow of electricity. Electromagnets can be very strong. Metal recycling companies use big electromagnets to lift cars then drop them into the car-crushing machine.For the electric motor we need to use electricity to reverse the poles of the electromagnet. If the electric current flows one way the top is north. If we reverse the current the bottom becomes north and the top becomes south.Take a look at the electromagnet, you can see the coil of wire wrapped around a steel pin. Those are the basic parts of most electromagnets, a coil of wire and a steel (magnetic) core. The steel is there to strengthen and focus the magnetic field created by the electricity in the coil. It would still be an electromagnet even without the steel but it would not be as strong.There is a neat way to figure out where the north pole will be if you know the direction of the current. If you wrap you right hand around the coil with you fingers curling in the direction of the current, your thumb will point to the north pole of the magnet. This is called the right hand rule (slide). Be careful not to use your left hand or you will get the opposite (wrong) answer. Remember one of the things we needed was a way to make the north-south switch happen at the right time. We want the electromagnet to switch just as a rotor magnet passes by, so it first pulls then pushes.Magnet 'A' is being pushed away while magnet 'B' is being pulled. As soon as magnet 'B' passes we need the electromagnet to switch so magnet 'B' gets pushed away and magnet 'C' get pulled.The mechanism that makes the electric current switch is called a commutator. All motors have some kind of commutator.This electric motor uses light beams to switch the electromagnet (hold up an electromagnet and show how the disc blocks the light beam)When there is clear under the light sensor the north pole faces the rotor.When there is black under the sensor the south pole faces the rotor.The disc is the commutator, and this allows us to make many different discs. You should have five discs, each with a different number of clear and black spaces around the rim.Find the pink disc. Put four alternating magnets in the rotor and orient the disc so the edges (clear to black) align with the centers of the four magnets. Tighten down the big nut then press and hold the button (it saves batteries if the kids have to press and hold to the button)Give it a little push and the motor should spin. You should see the little green lights blinking back and forth showing where the north pole is. Notice that the pole switch happens when the rim of the disc goes from clear to black, or black to clear.Stop the motor and rotate the disc with your finger, with the button pressed. You should see the switching and feel the little kick the electromagnet makes when a rotor magnet passes by.

Take a look at the other colored disks. How many magnets would you need for each one? The green disk is tricky. Once you know how to build a particular motor, hook up a photogate and find the frequency (breaks per second in this case). Divide the frequency by the number of black areas, and you will have the rev per sec. Multiply by 60 to get rpms. Who can build the fastest motor?

Note: Stop the groups when there is 20 minutes left in the workshop. There is time needed to summarize, answer questions, clean up, and get ready for next group!

Electric Motors

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