Spin Rite Simple Electric Motor Project

[ChristianS]

I am trying to help my grandson with this project and we are confused. I did register him on the sciencebuddies web site yesterday and posted our question out there but have not heard back from anyone so I am reaching out to you to maybe answer our question.
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> We have put everything together and the motor works fine with all three of the different number windings. In the “Testing Your Electric Motor” section it tells to measure the voltage across the axle supports and record the measurement. We always get 1.5 volts (which is the voltage of the battery) and the only difference we see is that the 10 windings runs the fastest and then the 30 and 50 each run slower. Am I trying to make more out of this then there is or are we correct that we will always have the some voltage? What makes it more confusing is that the next section “Average Voltage Data Table” would indicate they should be different depending on the windings. HELP!!!!!
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> Your prompt response would be greatly appreciated as we are running out of time for our project.
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> Thank you,
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> Ken Schaff

[Moderator note: URL for Project Idea: http://www.sciencebuddies.org/science-f ... 51_B.shtml ]

[NancyRoberts]

Your measurements should certainly be different. Ensure the coil axles are in solid (good constant) contact with the axel supports. Placing this coil across the poles of the battery should most certainly drop the potential across the poles. The only reason I would think you end up just measuring the battery is if one of the axels lost contact with its support. Then you would have an open circuit, and measuring across the axel supports is no different then measuring the battery directly. The greater the number of coils, the lower the voltage reading should be.

Although I don't think this is the problem here, check to ensure your meter is set to the porper fidelity so changes in voltage that aren't significant leaps can be read on the display. So for example, sometimes one can set the meter to display only the ones place and we wouldn't then see any fractional changes.

Good luck!

-Nancy

[wendellwiggins]

I haven't tried this project, so my answer relies just on general experience, but here goes.
As you have noticed, the voltage measurement is made directly across the battery. That's why you always see roughly 1.5v. However, when the motor is running, the current flowing through it causes a small drop in the battery voltage. The more current that flows through the battery and motor, the more the voltage will drop. The exact drop will depend on how fresh your battery is and several details of the motor construction.
Try the measurements again, and look at your meter very carefully to detect very small changes in the voltage. Try reading the voltage with the rotor removed from the supports. While one of you watches the meter, have the other one place the rotor on the supports, and look for a small drop as it begins to run.

[Craig_Bridge]

If you picked a 1.5v battery with a low internal resistance and a high current capability your meter may not show any difference in voltage. For the voltage measurement in this experiment to work, you do not want one of the high performance batteries for use in radio controlled motorized toys! Try using the lowest DC voltage scale and hope that the jitter filtering in your meter doesn't average out the reading. The tape on the contacts means that the current is interrupted. During the time the current is interrupted, the voltage will read the battery voltage with no load. During the time the current is flowing, the instantaneous voltage will be changing so the average should be less than the open circuit battery voltage. Whether your meter is capable of measuring this small difference is still in doubt.

A better experimental electrical measurement design would be to measure DC current which requires the meter to be in series with the battery and armature (coil winding of the motor). Unfortunately, that complicates the mechanical design.

Your finding that the armature with fewer turns rotated faster is surprising. More turns of the same diameter typically means a more powerful magnetic field which translates into more torque which usually means more acceleration which results in a higher speed of rotation. Do you have a means of weighing the armatures? The heavier (more mass) the armature, the more force it takes, so you might have a situation where the mass is increasing faster than the force from the magnetic field with more turns.

I may have used some words that you may not be familiar with. On the [url]http://www.sciencebuddies.org/science-fair-projects/project_guide_index.shtml
[/url] there is a section under Electricity & Electronics that has some reference general materials that maybe helpful.

[ChristianS]

Can you explain what exactly happens when the number of coils is increased. Does this cause more resistance?

[kgudger]

Hi:

This is an interesting question, and one that trips up a lot of people when they experiment with electromagnets (which is what the rotor of the motor is.)

The simple answer is yes, more coils increases the resistance of the electromagnet.

Because this experiment uses a (roughly) fixed voltage on the rotor, the current through the rotor decreases as you add more coils. Important variables for an electromagnet's magnetic field strength are the number of coils and the current through the coils. As you can see, with a fixed voltage, changing the number of coils affects TWO variables. A good science experiment tries to control as many variables as possible, so that you're only measuring one effect at a time. The equation for the magnetic strength of the coil:

B is practically constant and is given by
B = (u0 * N * i ) / l
where μ0 is the magnetic constant, N the number of turns, i the current and l the length of the coil.

Keith

[Craig_Bridge]

with a fixed voltage, changing the number of coils affects TWO variables

If you are using the same guage wire and the same diameter, there are at least THREE variables affected! The mass of the rotor also increases.

Any mechanical change and most electrical changes in any electric motor experiment usually affects multiple variables.

[NWhitten]

My son and his friend are doing this experiment as well. They have been unable to get a voltage reading aside from 1.5 volts. They've only been able to measure the safety pins that hold up the coiled copper wire. If they touch the copper axles, the rotor stops spinning and there is no voltage measured at all. Also, the insulation of the copper wire has been removed from only 1/2 of the wire per the instructions, so wouldn't that impact the voltage reading? They've had a similar experience with 10 coils spinning the fastest. The 30 coil and 50 coil motors are heavy, unbalanced and wobbly. This has been an interesting experiment, but the voltage measurements have so far been impossible! If anyone has any suggestions on how to get measurable results out of this experiment, they would greatly appreciate it!

[Craig_Bridge]

IMO: The measurements called for in this Science Buddies experiment are FLAWWED.
While it is possible to measure extremely small voltages differences with expensive lab quality meters with more than three significant digits of precision.
It is not reasonable to attempt to make these measurements with the affordable and commonly available test equipment most investigators will be using.

Instead of attempting to measure small voltage differences based on differences in current differences flowing through the very low resistance leads, simply measuring the current in the circuit with a typical commonly available DVM is more likely to show differences.

Start by putting the leads in the proper DVM locations to measure DC current and start on the highest current scale and put the meter in series with the battery and armature. You may have to switch to a lower current scale to see differences.

NOTE: You MUST use the same current scale for ALL the various armatures you test. Each DVM current scale typically introduces a different series resistance into the circuit. If you measure all armatures on the same DVM DC current scale, then you are using the same circuit and comparisons are fair. If you use different current scales, then your measurements are affected by two changes, 1) internal meter series resistance, and 2) difference in armatures.

Additional potential for measurement failures:
Your DVM current scale may introduce too much resistance for the motor to turn with some/all armatures you intend to test. If this happens, then you can divide the 1.5v open circuit battery voltage by the "locked armature" current reading to calculate an inoperable series resistance. You can then procure a resistor that is 1/10th or 1/20th of that value to use as a series resistor in place of the meter and then attempt to measure the voltage drop across that resistor and back calculate the current.