Kinetic to Electric Lab

In this lab, I tested 4 different stepper motors to determine how each converts the kinetic energy of cranking their shafts into electricity. The process also involved building a rectifier circuit to deal with the alternating current they produced and measuring short circuit current and open circuit voltage with a multimeter and an oscilloscope.

This phenomenon is also possible with regular DC motors, but I concentrated on several types of steppers because of the advice I got from this article that Jeff suggest we read. (Mainly the fact that steppers generate electricity better at low rotation was what attracted me to them. )

First, I had to determine which wire leads were paired with one another on my motors. I accomplished this by measuring the resistance across the wires. On the 4 wire motors, any pair of wires showed a resistance while wires that did not match to the same coil showed no resistance. On the 5 wire Mototech stepper, I determined the 5th common ground wire by finding the wire that produced half the resistance that other connections produced. (One weird thing here though, I thought that only the pairs should produce a resistance like the 4-wire steppers did. This mototech stepper gave me a resistance reading for every connection that I tried, however, so I had to just try out the various options to see which wires matched. )

Simple LED light

Once I had determined all the wire assignments, I verified for myself that I could in fact get an LED to light off of one of the sets of stepper coils.

With the LED crossing two matching ends of my EM-319 stepper, when I turned the wheel of this EM-319 stepper, the LED would flicker. This would work in both directions.

As I understand it, each time a magnet passes a coil in the motor it produces an alternating current. A positive voltage as it moves toward the coil and a negative voltage as it moves past. ( Or maybe its the opposite order, but the basic notion is the same. ) To see the unrectified alternating current of my Kollmorgen stepper, watch this video.

Rectifier circuit connected to 4-wire stepper

In order to invert the negative voltage of AC to constant positive DC, I built a rectifying circuit based on the diagram above. This circuit captures the current coming off of both of the stepper's coils and makes any negative pulses positive. ( See the rectified Kollmorgen oscilloscope video to compare it to the alternating current you saw above. ) I made this oscilloscope reading by connecting its probes across the DC out wires that you can see in the diagram above.

I watched the voltage from all of the motors on the oscilloscope...
and here are their respective rectified output: EM-319, 21-84, Mototech

Being the largest, the Kollmorgen reached the highest voltage ( at around 12.6V from my cranking ). The 21-84 made it to about 5V, the Mototech to 3.7V, and the EM-319 to 2.1V. Note that these aren't definitive values by any stretch of the imagination. I just tried to crank at about the same speed for each. I think the motors could definitely get higher voltage output if I had a faster cranking mechanism.

Finally, I measured the current coming off of each of the motors with a multimeter. I had the dial set to 10A on the multimeter, and I'm not entirely sure that was the correct way to go about it. I placed the unit's leads basically the same way that I had the oscilloscope's probes set, and this placement gave me a reading in milliamps (whereas when I tried putting the multimeter's leads in what I thought to be in series with the circuit, I got no results. ) Here's a video cranking the 21-84 and the 0.080 reading I got. If I'm reading the multimeter correctly, the Kollmorgen gave me around 600 milliamps, the EM-319 produced around 100 mA, the 21-84 spat out around 80 mA, and the Mototech put out around 22 mA. Again these aren't definitive measurements ( especially since I'm not entirely sure that I was reading the multimeter correctly.

Questions:
1) Why are the waves that the different steppers make so varied on the oscilloscope? Does it map to the size of the coils / magnets? Is it indicative of the quality of the stepper? Which wave is most ideal? ->( I'm assuming the flattest? )
2) I'm not exactly sure how to read the amps on the multimeter. For instance, in the video I have here, the multimeter was set to 10A, and the readout was saying around 0.080. So does that mean I have 80 milliamps? Or 8 milliamps? How do the notches on this dial work exactly...20mA, 200mA, 10A?
3 I need some clarification on measuring the short circuit current. I was getting current readings when I connected the multimeter to the same leads that I attached the oscilloscope to. The motor also became more difficult to turn when I did this, as expected. But, I thought that I would need to put the multimeter in series with the circuit, interrupting it in the middle somehow. This confused me though since it seemed like the stepper circuit has two sides (I think I'm confused by the double diode bridges...)