Exploration 12: Reverse Engineering of a DC Motor

3.4

Exploration 12: Reverse Engineering of a DC Motor

In Exploration 12 and Extension 6 you saw that if a loop carrying current was placed in a magnetic field, the resulting force could cause a rotation. In this Exploration you will look at a device that uses this effect in a practical manner. Throughout this Exploration take careful notes about your discoveries.

Equipment:

·  Small 1.5-3 V DC motor

·  Compass

·  Battery pack (1.5-3 V)

·  Connecting wires

·  Small screwdriver

·  Magnifying glass

1.  Connect the motor to the battery pack and observe its motion. In particular, note the direction of rotation as compared to the direction of current flow. Disconnect the motor from the battery pack.

2.  Carefully dismantle the motor. Ask your instructor if you are unsure of how to remove the case of the motor. Take care as you are working not to damage any of the parts of the motor – you want to be able to reassemble it.

3.  Locate the magnets in the motor and determine their polarity.

4.  Locate the windings (coils) and their electrical connections.

5.  You will also find a circular device with splits, or grooves in it. This is called the commutator. If you are unsure of where this is located, ask your instructor.

6.  Carefully draw a diagram showing the basic structure of the motor.

7.  Look carefully at the motor and try to determine the path of the current through the motor. You will want to spend some time on this step, since the circuit is probably more complicated than you think at first glance. Look at the current flow for different positions of the motor windings. Is the current flow always the same? What do you think is the role of the commutator in the motor?

8.  Once you have figured out the current flow, check with your instructor to see if you have it right. Then consider how the current interacts with the magnetic fields of the magnets to make the motor turn. Remember to apply the right-hand rule as necessary. When you think you have figured out the interaction between the current and the magnetic fields, check with your instructor to see if you are right.

9.  When you have completed your investigation and verified with your instructor that you have correctly identified the function of each part of the motor, carefully reassemble the motor.

10.  Using what you have learned, write a detailed summary of the structure and operation of the electric motor.

3.5

Dialog 13: The DC Motor

In Exploration 13 you identified the parts of a DC motor. You may have been surprised at the simplicity of its construction. It is a common misconception that electric motors are very complicated devices, but in reality they have only a few components. While dismantling your motor you should have been able to identify the magnets, the coil (current loop), and the commutator. How does each of these components contribute to the operation of the motor?

The main purpose of the electric motor is to convert electrical energy into useful mechanical work. The concept necessary to create the mechanical work is to create a torque on a coil in an external magnetic field. The torque occurs when forces are applied to the coil at a distance away from the axis of rotation, or shaft. This causes the coil to rotate. There are two ways to model this rotation. One model considers the magnetic poles created by the current in the coil, which is an idea we discussed in section 2.17 of this module. The other model considers the forces on the currents in the coil, as we discusses in section 2.8 of this module. They are both excellent models, but they view what is occurring in a different manner. We will investigate both models.

The Model Using Magnetic Poles

In this model the electric current going through the coil sets up a magnetic field. The right hand rule, which we discussed in section 2.17 of this module, can be used to determine which side of the coil is the north pole and which side of the coil is south pole. The north and south poles will be attracted to or repelled by the external permanent magnets.

To see this in more detail, let us start with the coil in the position of maximum torque. A coil in the position of maximum torque between two permanent magnets is shown in Position 1 and the graph below. Note the direction of the current in the coil. It enters the coil in the wire that has an

identifying dot on it. The south pole of the coil is attracted to the north pole on one of the permanent magnets, while the north end of the coil is attracted to the south end of the other permanent magnet. This causes torque on the coil and causes it to rotate.

The coil rotates approximately 45o and is at Position 2, shown below. The torque is still in the same direction but is now smaller (see Position 2 on the torque vs. time graph below). The torque is smaller because the perpendicular distance between the forces is smaller. The current is still entering the coil on the wire with the dot.

Another rotation of 45o brings the coil to Position 3. There is no torque at this position (see position 3 on the torque vs. time graph). The current in the coil is reversing direction at this position. The coil keeps turning because of inertia.

In Position 4 the current has already reversed its direction. The current now enters the coil on the wire without a dot and leaves on the dotted wire. This reversing of the current creates magnetic poles on the coil that keep the coil rotating in the same direction. The new north pole of the coil is repelled by the north end and attracted to the south end of the permanent magnets. The new south pole of the coil is repelled by the south and attracted to the north ends of the permanent magnets.

Position 5 is again at a position of maximum torque. The perpendicular distance between the forces is at a maximum. When comparing the figures for Position 1 and Position 5, they appear similar at first. However, there are subtle differences between the two positions. The first difference is the direction of current flow. In Position 1 the current flows into the coil on the dotted wire and out of the coil on the wire without a dot. The current direction was reversed in position 3 so now the current flows into the coil on the wire without a dot, and out of the coil on the dotted wire.

The reversal of the current also reverses which side of the coil is north and which is south. In Position 5 the south pole of the coil is still on top. In Position 1 the same physical side of the coil was on the bottom and was the north pole. Between Position 1 and Position 5 the coil has rotated 180 o or ½ rotation.

We have explained the first half of the rotation in detail. We will not explain the second half of the rotation in as much detail because the corresponding positions compared to those of Positions 1 through 4 would be very similar. Therefore, the only position shown for the second half of the rotation is position 6. In position 6 the coil has rotated 270o or ¾ of the rotation. Here the torque is

zero, which is similar to Position 3. Position 6 is important because the current reverses here. The reversal of the current creates the magnetic poles that will carry the coil back to Position 1 and from there the whole process will repeat.

Observe the coil positions again to see if you understand the model. Can you answer these questions?

1. From the direction of the current in the coil, can you use the right hand rule and determine the north and south poles of the coil?
2. Once the poles of the coil are determined, can you determine which way the coil will turn in the external field?
3. Where are the positions of maximum and minimum torque? Do you know why?
4. At what positions does the current reverse?
5. Using the six position diagrams, can you identify the two positions where the current reversed?
6. Can you explain why the current reversals are necessary keep the coil rotating in the same direction?

If you can answer these six questions, you are ready to explore an alternative model.

The Model Using Forces on Currents

This model considers the current that flows through the coil and its interaction with the external magnetic field caused by two permanent magnets. The sides of the coil will have forces on them given by F = ILB sin q (we discussed this force in section 2.8 of this module). Since the forces applied to the coil are at a distance away from the axis of rotation, or shaft, torque is applied to the coil, causing it to rotate. This basic process is illustrated in the following diagrams.

Our example will begin with Position 1, shown in the diagram and graph below. The positions shown in this model are the same as shown in the previous model. In Position 1 the torque is at a maximum. The wires at the opposite sides of the coil are denoted by a lighter and darker shade. The wire on the right side of the coil is darker and its current is coming toward us. Using the right hand rule shows us that the direction of the force is up. The wire on the left side of the coil is a lighter color and the current is going away from us. Using the right hand rule shows us that the force is down.

In Position 2 the coil has rotated 45o, as shown in the diagram below. The direction of the current and external magnetic field is the same. The only difference is that the perpendicular distance between the two forces is closer together so the torque is smaller (see the torque vs. time graph below).

In Position 3 the diagram shows that the coil has rotated 90o from Position 1. The direction of the current and external magnetic field is still the same. The two forces are along the same line so will cause zero torque (see the torque vs. time graph below). The coil will continue rotating by inertia. The current will reverse itself at this point. We will see this current reversal in the next position.

In Position 4 the current has already reversed direction. Now the darker wire, which used to have the current coming toward us, has reversed and is now going away from us. This reversal of current is necessary to reverse the direction of the forces so the torque on the coil will continue the rotation in the same direction. The distance between the two forces is similar to the second position, so the torque is smaller.

In Position 5 the coil has turned ½ rotation, or 180o from the Position 1, so you will notice that the diagram below looks very similar to the diagram for position 1. The only difference between these two diagrams is that the coil has turned ½ rotation in Position 5, causing the dark and light wires to switch positions. The two forces are at the maximum position apart so the torque is at a maximum.

The second half of the rotation will be very similar to the first half. Therefore, all the diagrams won’t be shown. The only diagram shown for the second half of the rotation is when the coil has rotated ¾ or 270o from the start.

At this position the forces are along a line so there is zero distance between them. The torque is zero. The coil continues to turn by inertia. The current will reverse a second time at this position. This reverses the forces and keeps the coil turning back to its original position. After one complete rotation, the process will continue to repeat itself.

Current Reversal

Current reversal is the role of the commutator. The simplest form of commutator is made of two curved strips of metal attached to the coil of the motor. These strips are in contact with two conducting brushes, which provide the electrical contact for current to flow to the coil, as shown in the diagram at right. The brushes are mounted to the frame so they stay stationary while the commutator rotates between them. The surface of the commutator and the surfaces of the brushes are in contact. As the coil turns, each metal strip first comes into contact with the positive terminal of the battery, then the negative. This alternating contact reverses the flow of current through the coil every one-half rotation, as required.