Physics 204, Spring 2011

 

Fun with Magnets and Motors

 

Like electrical phenomena, magnetic phenomena were not particularly familiar in the pre-technological world.  Our word “magnet” is derived from the place name Magnesia, a district in Greece where lodestone or magnetite occurs naturally.  But of course you don’t have to go to Magnesia to see magnetic effects – they are everywhere!  Even in our modern technological world, much of what magnets do is hidden from sight, in the sense that we don’t usually see the magnets. 

 

I.                    We have a few lumps of natural lodestone – play with them!

II.                 It is not known how the magnetic compass was invented, but it may have come about through observation of floating needles of steel or iron.  It is such a peculiar thing that very thin needles can float on the surface of water (really supported by surface tension, not buoyancy) that people may have done this essentially for fun.  Then they may have noticed that the needles tended to point the same direction.  This seems to me a plausible reason why people might have invented a delicate suspension for needles without knowing about the phenomenon of the compass ahead of time.  Try floating a needle – if we can get several floating, we can then see if they tend to point the same way.   To do it, rest the needle on a small piece of absorbent paper and carefully place it to float in a cup of water.  The paper will absorb the water, and slowly sink away, leaving the needle floating if you are lucky.  These will be needles as they came from the manufacturer, not deliberately magnetized.  (Steel typically has a slight magnetization, acquired by accident.)

III.                Use a compass to map the magnetic field of a permanent magnet, or of an electromagnet (coil of wire  with a current in it).

IV.              Build a motor!  We have little kits.  See the next page for instructions.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Construction of a DC motor:  

 

This is called a “universal motor”, and it is used in almost all portable electric tools because it requires only a DC current supply (such as a battery).  So, once you are done with this lab, you will know what’s inside your Black and Decker portable drill!

Figure 3

 

 

A motor is an electromechanical device that converts electrical energy into mechanical rotational energy.  This is done by placing a coil of wire carrying a small current, which is free to rotate about its center, in an external magnetic field.  The coil experiences a torque given by t = m x B, so that it will start to rotate to align itself with the field, as in figure 3.  When it is aligned with the field, it no longer experiences a torque.  At that point the electrical contacts for the current in the coil are broken, so that the coil no longer carries a current (nor experiences a torque), but its angular momentum carries it past the zero‑torque position by some angle less than 90 degrees.  At that point the electrical contacts are reestablished, but on the opposite side of the coil, so that the current flows in the opposite sense with respect to the coil.  But the coil itself has now rotated to a direction opposite its initial direction, so that the net effect is that the coil feels a torque in the same direction as originally, and the coil continues to rotate!  If the coil is connected to a shaft, the shaft can be used to turn something, thereby converting the electrical energy provided by the power supply (in this case a battery) into mechanical work.

 

Study the completed motor to see how all this is accomplished in practice. The rotating coil is called an armature, and the electrical contacts are called the brushes. The permanent magnets must be arranged so that one set has its north pole closest to the armature, while the other set has its south pole closest to the armature.

 

 

 

 

 

 

 

 

Motor Assembly

 

Assembling the armature:

a.  Place the two iron cores in the core wells of the plastic T-frame.

b.  Place the shaft between the core flanges.                                                               

c.       Put the other half of the T-frame in place.

 

 

Winding the armature:

 

  1. Unroll and smooth the copper wire to its full length and mark the middle by slightly bending the wire at the center.  Cut 6” from each end to be used later.

 

  1. Lay the center of the wire diagonally across the center of the T-frame and neatly wind half of the wire over one core and then the other half over the opposite core.  Be careful of the direction in which you wind the wire around the two sides of the armature.  IGNORE THE INSTRUCTIONS INCLUDED WITH THE MOTOR!!  The wire should be wrapped such that the current will flow in the same direction around the armature on both sides – this is more subtle than it seems, so think carefully about which way the current is flowing once you’ve wrapped the wire around the sides of the armature.  Why is the direction that you wind the wire important?

 

c.  Leave at least 2” of wire at each end and remove the enamel insulation using sandpaper.

d.  Bend the “T” ends of the brass commutator segments to a 90o angle; then slide the commutator into the notches of the plastic T-frame.

e.  Next, slide the plastic end cap over the commutator segments to hold them in place.

f.        Fasten the ends of the coiled wire to the “T” of the commutator by soldering them in place.

 

Preparing the motor base:

a.  Insert the copper brushes into the brush holes with a small screwdriver.

b.  Connect the battery terminals to the brushes by soldering the two 6” lengths of wire saved from the previous section.  Be sure to sand the insulation off the ends of the wire before soldering!

c.  Place a D cell battery into the battery well and magnets on either side of the armature.

d.  Give the rotor a nudge.  It should continue to rotate on its own!

 

Draw a picture of the armature in two orientations that you think illustrate how the motor works.  In these pictures, indicate the direction of the current in a loop of wire, the direction of the magnetic field due to the external magnets, the position of the external magnets and their poles, the direction of the forces on the wire loop, the direction in which the loop rotates, and anything else you think is helpful in illustrating to the instructor that you now know how to build a DC motor.  Then give yourself a big pat on the back because you now understand the inner workings of a vacuum cleaner, a fan, a power drill, and a whole slew of other everyday devices!