Physics Project – DC Motor

Physics Project – DC Motor

Abstract: For this project I decided to design and build a working AC motor, although this later had to be changed to a more simple DC motor. The project looks at how putting more voltage through the copper coils affects the motor. My theory was that the more voltage put through, the faster it will turn. To investigate this I constructed a working DC motor, put various voltages through it, and observed at what speed it turned. From this I saw that my hypothesis was correct, and that the more voltage put through the coil, the faster and easier it turned.

Introduction: Electric motors are part of every day life. They are crucial to almost everything electronic in today’s world, and are found in items like fans, fridges, or washing machines. They convert electrical energy into mechanical energy, based on the principles of magnets and their ability to create motion through attracting and/or repelling. There are several different ways to make motors, but each needs a magnet (permanent or electromagnet) which attracts something else metal, causing motion. This motion is then in turn used to power other appliances e.g. it turns fins to create a fan.

In this experiment, I originally set out to make an AC squirrel cage induction motor – (Being called an induction due to the iron and copper coils inducing a rotating magnetic field.) Four electromagnets (iron wrapped in copper wire with a current through it) are placed set up at right angles to each other – like the corner of a square, such that each electromagnet has the same polarity as that of the one opposite it. The alternating current creates a rotating magnetic field, in which a cage like structure with copper bars is attracted to. It follows the field around in one continuous direction, creating motion and an effective brushless induction motor. (Reasons for failure are explained in discussion.)

The advantages of this type of motor are that not as much energy is lost through sparks with brushes, they are generally faster than their DC counterparts and do not wear, and the lack of sparks means no ozone production.

In a DC motor, the electromagnet is made from the armature (rotor/spinning part) and it is attracted to two opposite permanent magnets. A circuit to the battery is set up, and is connected to the spinning electromagnet via 2 brushes. When the circuit is complete, the top of the armature becomes north, and is attracted to the south magnet (on the left.) it then starts moving towards the magnet. One it reaches there, the momentum of the armature keeps it spinning in the same direction. As the south side follows behind, it is repelled by the south magnet in the same direction, keeping the motion and the coil spinning. The coil returns to its original spot, and the process repeats itself.

I have chosen to this experiment because I am interested in building and finding out how motors work in general. I also want to find out how different kinds of motors work, which ones are better and for what reasons, and to experience the challenge of designing and constructing a motor. I also expect this will help in future physics lessons, and also help the possibility of becoming an engineer in the future.

Aim: To design and construct a motor (either AC or DC) and see what occurs when more voltage is put through it.

Hypothesis: Putting more voltage through either ran AC or DC motor will result in it turning faster.

Materials and Methods:

Perspex base (20cm by 20cm.)

Drill

Ferrite (iron) sticks – preferably with large flat surface area

Copper Wire

Plastic tube with lid

Bolt

Bearings from scooter

Washers

Pieces of metal (for use as terminals)

Superglue or Silicon

Procedure:

The Board:

  1. Cut out a piece of Perspex 20cm by 20cm.
  2. Drill a hole in the centre, and four others equidistant from the centre arranged like the corners of a square.
  3. Into these holes place the four sticks or rods, and tie the ferrite blocks to them..
  4. Wrap copper wire clockwise around one piece of ferrite (wrap it up and then back down – 2 layers – almost completely covering the side covered, leaving the side facing the cage open.
  5. Using the same wire, wrap the ferrite stick diagonally opposite the original one also clockwise up and down.
  6. Now repeat the process for the other two diagonally opposite sticks, only this time wrap the wire around anticlockwise.
  7. Join the wires up so that each opposing pair is connected, and both pairs are connected to the positive and negative terminals (pieces of metal glued onto the Perspex.)

The Cage:

  1. Cut a plastic tube (about 4.5cm in diameter) in half.
  2. Take the lid of one end, and superglue it into the end with the bottom.
  3. Drill 15 small holes into the top and bottom of the tube.
  4. Cut the copper wire into 15 even strips, each slightly longer than the length of the tube.
  5. Bend each of these wires so that the ends bend into the holes, with the wires traveling straight up and down the tube.
  6. Drill a 1cm hole through the middle of both the bottom and the lid.

Putting it together:

  1. Place the bolt through the bottom of the middle hole in the Perspex, so that the shaft sticks straight up along with the ferrite.
  2. Slide a bearing from a scooter wheel down to the bottom of the bolt.
  3. Place the squirrel cage on top of the bolt, with the shaft going through the hole in the centre.
  4. At the top of the bolt, although not directly on top of the cage, screw a nut on. (As the squirrel cage turns, this will prevent it from sliding up and down too much.)
  5. Attach the alligator clips leading to the transformer to the positive and negative terminals.
  6. The iron should become an electromagnet, with pieces of metal attracted to it – also a faint humming sound may be heard.)

In theory, the opposing electromagnets have now set up a magnetic field, and the copper bars and iron shaft will be attracted to this field as it spins around, thus in turn spinning the cage, creating motion and an effective AC motor.

Variables: In the AC motor, the variables would change how strong the magnetic field was, how fast the cage could spin or both. The controlled variables I kept the same included: the amount of copper I put around each piece of iron, as well as how soft the iron was (I chose ferrite because it has a low carbon content, making it a softer metal and therefore a better electromagnet.) the distance between the magnets and the cage would also affect the pulling power, as well as the weight of the cage to be pulled. The more copper bars I put in affected not only its weight, but also how strongly it was attracted to the magnetic field.

The independent variable, the aspect I changed, was the amount of power I put through the electromagnet.

This affected the dependant variable, the strength of the magnetic field in each electromagnet, and thus how fast the cage (should have) spun.

Results: As more current was put through the AC ‘electromagnet’, the magnetic field did get stronger (which in theory would have made the squirrel cage turn faster.”) However, due to factors explained in the discussion, this outcome did not occur.

Discussion: The AC motor failed due to unforeseen circumstances. The bought copper wire was coated in a layer of (non conductive) enamel, preventing the flow of electricity between the joins of wire to each magnet. The wire at the alligator clips has been grazed, and so electricity conducted here and around the first magnet, but unfortunately this in itself is not enough to create a working induction motor. Other problems I encountered whilst building this project including getting the Perspex to stand straight whilst the silicon dried, and fitting the rods into the drilled holes.

If I could improve on the AC motor, I would purchase larger, flatter pieces of iron, suitable for electromagnets attract the squirrel cage better and make the motor effective. I would also use a multimedia conductivity tester to make sure the circuit was conducting at each stage of construction, rather than having the end result fail.

Conclusion: The more current or voltage that is put through a motor, the greater the magnetic attraction, and the faster it will spin.

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