Simple Electric Motor - Measure Impact of Additional Coil Turns

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swannyd
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Simple Electric Motor - Measure Impact of Additional Coil Turns

Post by swannyd »

Hi Science Buddies Community.

I'm working on a science fair project with my 7th grader. We are building the simple electric motor and want to study the impact of changing the number of copper coil turns. We are planning to use a C battery, small magnet, copper wire, and two safety pins to form the coil/battery connection. From what I can gather, increasing the number of copper coil turns from 1 to 3 to 6 to 9, etc should create a larger magnetic field and turn the "motor" faster. I'm trying to figure out if I can use a meter to measure this output numerically (voltage or amps??) or do we need to somehow count the motor RPM. Any ideas or suggestions?

thanks!
bfinio
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Re: Simple Electric Motor - Measure Impact of Additional Coil Turns

Post by bfinio »

Hi swannyd,

You can measure the motor RPM using a device called a tachometer, which measures a laser reflected off a bit of reflective tape. Assuming you are doing this project:

https://www.sciencebuddies.org/science- ... tric-motor

the difficulty might be that the wire coil is extremely light, so adding tape directly to the coil part might make it too heavy to rotate, and/or off-balance (although you could put one piece of tape on each side). You might be better off putting a single piece of tape on one end of the wire (the part sticking out on either the left or the right).

Here is an example of a similar project that uses a tachometer with a slightly different type of motor:

https://www.sciencebuddies.org/science- ... mple-motor

If you did want to use a multimeter, we have a very extensive multimeter tutorial here: https://www.sciencebuddies.org/science- ... multimeter

Hope that helps!

Ben
swannyd
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Joined: Wed Jan 12, 2022 11:07 pm
Occupation: Parent

Re: Simple Electric Motor - Measure Impact of Additional Coil Turns

Post by swannyd »

thanks bfinio. If I used a multimeter, what would I measure? if I just used 1 copper coil loop, I'm guessing the voltage would be similar to the battery voltage. with additional turns (thus length) this should add resistance but wouldn't really be a measure of motor output. I've seen this formula for HP of electric motor. so maybe I would measure the amps? other ideas?

Horsepower (hp) = (V x I x Eff)/746
Where,
V = Voltage of the Motor in Volts
I = Current in Amps
Eff = Efficiency of the motor in Percentage.
746 is a constant expressed in radians per seconds
bfinio
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Re: Simple Electric Motor - Measure Impact of Additional Coil Turns

Post by bfinio »

Hi swannyd,

Short story: in this case I think you are more interested in measuring the motor RPM with a tachometer, as that is the "output" variable you ultimately care about. The electrical variables are sort of "intermediate" variables if that makes sense, but ultimately what you care about is the mechanical output of the motor.

Longer story - this is kind of a lot to explain in a text forum post without giving you an introductory circuit theory lecture. This circuit only has two parts - the battery and the motor coil. The voltage across them, and the current through them, will always be the same for any given coil (this is not true for more complicated circuits with multiple parts). So in theory, yes, you could measure both of those (you'd need to measure them one at a time, or you'd need two multimeters to measure them simultaneously) and calculate the electrical power. However in this case you do not know the efficiency of the motor, so you can't really use the formula you found to calculate the actual *mechanical* output power of the motor. This starts to get into high school physics pretty quickly (circuits/Ohm's Law) and motor physics are really college level (I teach a junior-level mechatronics class at Cornell and this is something we get to later in the semester). Point being, for 7th grade, I think it's sufficient to use a tachometer to measure RPM and not worry about the electrical variables.

Hope that helps,

Ben
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