Physics 1051 Laboratory #5 The DC Motor. The DC Motor

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Victor Brooks
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1 The DC Motor

2 Contents Part I: Predictions Questions Part II: Building the Motor Materials Making the Support Arms Making the Coil Making the Armature Putting it Together The Completed DC Motor Energizing Your Motor You re on a Roll! Which End is Up? Observations The Commutator Wrap it Up!

3 Part I: Predictions Force on a Current Carrying Wire 1 All throughout the boxed region below, there is a uniform magnetic field pointing into the page (as indicated by the cross). A wire segment carrying a current in the direction shown is placed inside the region. B I Indicate the direction of the force on the wire segment, using either arrows or the dot or cross symbols. If the force is zero, write zero.

4 Force on a Current Carrying Loop 1 A square wire loop carrying a steady clockwise current is placed in the region. (Current in each of the four sides is equal.) B I On each of the four sides of the loop, indicate the direction of the magnetic force (if there is one) or write zero. QUESTION 1: Is there a net force acting on the loop as a whole? If so, state its direction. If not, explain.

5 Force on a Current Carrying Wire 2 In this region, a uniform magnetic field is present that points toward the bottom of the page. A wire segment carrying a current in the direction shown is placed in the region. B I Indicate the direction of the force on the wire segment, using either arrows or the dot or cross symbols. If the force is zero, write zero.

6 Force on a Current Carrying Loop 2 A square wire loop carrying a clockwise current is placed in the region. B I On each of the four sides of the loop, indicate the direction of the magnetic force (if there is one.) QUESTION 2: Is there a net force acting on the loop as a whole? If so, state its direction. If not, explain how you can tell.

7 Is There Motion? QUESTION 3: For the two cases of the square loops (i.e., in Questions #1 and #2), assume that the loop is free to move or twist. State whether you would expect there to be any motion of the loop that would move it away from its original position or orientation. Explain your answer.

8 Torque Consider the case of a metre stick lying on a low-friction surface, with equal forces applied to the two ends as shown. Top View meter stick QUESTION 4:a) Is there a net force on the metre stick? Explain. b) Will the stick move in any way? Explain. c) Check that your answers to #3 are consistent with your answer to this question about the metre stick and comment on the consistency.

9 The Magnetic Moment Vector This is a side view of a square current-carrying loop that is free to move or rotate. The four boxes show four different orientations of the loop with respect to the external magnetic field. The direction of the external uniform magnetic field is indicated. B B B B In your Activity Log draw the magnetic moment vector for all four cases. Also show the direction of the torque (if any) on all four loops. An example of finding the torque using cross products is given in the introduction to the lab!

10 Part II: Building the Motor Materials Wooden base DC power supply Wires and connectors 2 Magnets Wooden stick 2-3 m of thin copper wire (insulated) ~20 cm of heavy non-insulated wire or paper clips Some masking tape Sandpaper 200 g mass

11 Making the Support Arms Begin by shaping the two non-insulated wires or paperclips to resemble the photo to the right. Insert the longer ends of the newly formed support arms into the holes in the wooden base.

12 Making your Coil Form a coil by wrapping the thin copper wire around the base of a 200 g mass. Your coil should contain at least 25 turns. Secure the windings by wrapping the loose ends around the coil a few times on each side. Your finished coil should resemble the one pictured above.

13 Making the Armature Use the sandpaper to carefully remove the insulation from the loose ends of the coil. Remove all the insulation between the red arrows Insert the wooden stick between the turns of the coil just below the secured section on one side of the coil. Slide the stick through so that it passes through the center of the turns on the opposite side of the coil and on top of the secured section. It is important that your coil is balanced. Check this by spinning the armature in your fingers. Continued

14 Making the Armature cont. Be sure to position both wires on the same side of the stick as the top of the coil (see diagram). Use small pieces of masking tape to secure the loose ends of the coil to the wooden stick.

15 Putting it all Together! Place your completed armature in the cradle formed by the two support arms in the wooden base. Adjust the arms and armature so that the wires on the stick will contact the supports when it is rotated. Position the magnet directly under the coil in the indent in the base. Check that the armature spins freely and is balanced. If not adjust your setup.

16 The Completed DC Motor

Energizing your Motor Plug the cables into the output ports of the power supply and attach the alligator clips to the ends. Then clip the alligators clips to the support arms.

17 Energizing your Motor Plug the cables into the output ports of the power supply and attach the alligator clips to the ends. Then clip the alligators clips to the support arms. Make sure the current and voltage knobs are turned all the way to the left. Switch on the power supply. Have your partner hold the armature so that the wires are contacting the support arms. First slightly increase the voltage knob, then adjust the current until you observe a voltage and current output. Adjust both the voltage and current knobs so that there is at least 1.0 V and 1.0 A flowing through your circuit. If there is no voltage or current output on the power supply, check that you have removed the insulation of the wire enough and that there is good electrical contact.

18 You re on a Roll! Give the loop a slight flick with your finger to get it spinning. If it doesn t continue to turn, try spinning it in the other direction. Adjust the support arms and the armature until you get the motor spinning steadily. Note the rotational direction of your motor. You will use this information to determine the orientation of your magnet. Don t run your motor for more than a minute straight. The coil can get very HOT!

19 Which End is Up? QUESTION 5: Based on the direction of current and rotation of your motor, determine which pole of your magnet is facing up. Explain your answer and use a labeled bar magnet to see if you are right. Comment on your agreement with the bar magnet. QUESTION 6: What happens if you switch the alligator clips? Why?

20 Observations With your motor running, observe the effect of varying the current and voltage. QUESTION 7: Based on your observations how does the operation of a DC motor depend on the magnitude of the current? Is this behaviour predicted by theory? Explain.

21 The Commutator QUESTION 8: Why is it so important to have the two wires on the top side of the stick, and why must the coil be upright?

22 Wrap it up! Make sure that you have answered all the Questions completely.

The DC Motor. Physics 1051 Laboratory #5 The DC Motor

The DC Motor. Physics 1051 Laboratory #5 The DC Motor The DC Motor Physics 1051 Laboratory #5 The DC Motor Contents Part I: Objective Part II: Introduction Magnetic Force Right Hand Rule Force on a Loop Magnetic Dipole Moment Torque Part II: Predictions Force