Step 1

Collecting the Components

Your kit of parts should include a stepper motor. This is the kind of motor that is used in printers and other devices that must position an object precisely. A stepper motor usually consists of 2 coils of wire and a permanent magnet attached to a rotating shaft. Sound familiar? The stepper motor also qualifies as an Alternating Current generator (Alternator).

Figure. Stepper motor parts

Here is what you need to do this activity.

• 4, 5, or 6 wire stepper motor
• Cordless drill with variable speed control
• PC Oscilloscope and test leads
• PC Oscilloscope software
• A power diode (also called a rectifier)

Step 2

Setting up the generator

Because there are 2 separate coils of wire in the stepper motor, you will need to identify which wires attach to a single coil (we will use only one coil in this activity).

To do this, configure your multi-meter to measure resistance. Set the dial on the lowest resistance range, usually 200 ohms. Test to see if the meter is working by touching the probes together. You should see a resistance reading of less than 1 ohm.

Next touch one probe to one of the stepper motors wires and touch each of the other wires until one gives a reading. Mark these 2 wires using a marker, tape, or by recording the wire colors. Make sure the remaining wires are taped up so that any bare leads are not touching or simply cut of any bare wire.

Figure. Using the Ohmmeter function to find the leads for one of the coils

Next clamp the motor to a table or a wooden block.

Figure. Stepper motor clamped to a wood block

Attach the motor shaft securely to the drill's chuck. At this point you will need to support the drill. (If the motor's shaft won't fit into the drill's chuck it's because it has a gear or other attachment. make sure these are removed before continuing.)

Next attach the 2 coil wires you identified earlier to the 2 probes of the oscilloscope.

Figure. Stepper motor attached to Oscilloscope (drill not shown)

Step 3

Setting up the Oscilloscope

Connect your PC Scope to a USB port on your computer and fire up the software. Set each of the oscilloscope controls as shown below.

• Channel. Again we will use channel A. Make sure it is selected by clicking the ChA Button and that your probes are connected to the channel A jack (see image above).
• Voltage Range. Use your mouse to set the voltage selector to 2.0 volts (per division, that is each grid line on the y-axis represents 2.0 volts).

Figure. Voltage Range

• Time Base. Set the time base selector to 20ms (milliseconds).

Figure. Time Base

• Trigger. There is no need to trigger the oscilloscope as you will be producing a continuous current of electricity. Set the trigger to Auto as in the following illustration.

Figure. Trigger

Step 4

Run the First Trial

If your drill has a low/high speed switch set it to the low speed. Run the drill at about half speed by depressing the trigger switch. Make sure you don't tangle the wires in the drill chuck. The video clip that follows shows the method.

• Click the Run button on the oscilloscope software. Adjust the drill's speed to get a trace similar to the following then press the Stop button to freeze the trace on the scope display. If the trace goes off screen, increase the voltage range to 5 volts per division. Note: If you can't get a suitable waveform try adjusting the TimeBase.

Figure. Click the Run Button

• When you get a trace similar to the following, press the Stop button to freeze the trace on the scope display. Save the graph image for your report. Remember Screen, Save screen Image to file.

Figure. Waveform Trace

Make appropriate entries in you module portfolio.

Step 5

Run the second trial

• Repeat the first trial only this time run the drill at a faster speed. Keep the oscilloscope settings the same. You should see a trace similar to the following.

Figure. Waveform Trace

You should see two very obvious differences between the two traces. Answer these questions

1. What are the differences?
    
2. How can you explain these two differences?

Make appropriate entries in your module portfolio.

Step 6

Using the Oscilloscope Software to Analyze the Trace

The Easyscope software has a built in set of analysis tools that can be used to measure parameters of the trace. Notice we did not see a couple of single pulses but instead a series of pulses. This is usually called a wave train and we refer to its particular shape as a waveform. One single wave should have a positive and a negative pulse as in the following image.

Figure. A Single Waveform

You can find the time it takes to generate one complete wave, also called the Time Period of the wave by using a set of cursors built into the software. Click the CursorX button.

Figure. the CursorX Button

When the cursors show up on the oscilloscope display, drag them by clicking on the blue squares (numbered 1 and 2) to enclose exactly one complete wave.

Figure. Cursors Define 1 Wave

Look at the bottom of the display. You will see the time (remember the x-axis measures time) for the position of cursor 1 and 2 (C1 and C2). The number highlighted in blue is the Time Period of the wave, that is the time difference between the positions of C1 and C2. (The word Delta used in science and technology means "a change in"). In the example above, it is 28 milliseconds or 28/1000 of a second.

• Record your Time Period (Delta) for your report and save the image to a file.
• Finally, click on the Blue Delta display. It will change to Freq (Frequency).

Figure. Frequency

The frequency is how many full waves are produced in one second. The unit of frequency is named after Heinrich Hertz, a German scientist who made early discoveries on what we today call Radio Waves. The abbreviation is Hz. In the example above, the frequency shown is 35.71 Hz or 35.71 waves per second.

• Record your frequency for your report and save the image to a file.
• Find the electrical information tags / stickers on some pieces of equipment that plug into an electrical outlet. Can you find the symbol Hz and if so, what is the number associated with it? Is it the same for each piece of equipment?
• What determines the frequency of the alternating current? That is how, can the frequency be changed?

Make appropriate entries in your module portfolio.

Step 7

Changing AC to DC

Locate the power diode (rectifier) from your parts list. Disconnect one coil from the stepper motor and insert the diode between the stepper motor coil and the oscilloscope probe.

Figure. Diode (rectifier) inserted in the circuit which blocks current in one direction

• Repeat Step 4.

Did you get a trace that looks like the following?

Figure. Halfwave Trace

or, did it look like this?

Figure. Halfwave Trace, Diode in opposite direction

• Which ever trace you got the first time, reverse the diode, and do the trial again. You should get the other trace of the two shown above.

Make appropriate entries in your module portfolio.

Step 8

What is Going on Here?

See if you can answer the following

• How are these traces different than the traces in Step 4?
• What is the function of the diode in this circuit?
• Are the traces in Step 7 an example of Alternating Current or Direct Current?