This Lab Will Be Completed in Room 335 CTB (Doorcode 26951)

Lab #7 Help Document

This help document will be structured as a walk-through of the lab. We will include instructions about how to write the report throughout this help document.

This lab will be completed in room 335 CTB (doorcode 26951).

You will need to partner up for this lab in groups of two.

This lab will require the use of the following pieces of equipment:

DC power supply

DMM

Variac Transformer

integrated brushless DC motor

DC motor on stand

AC shaded-pole induction wall-plugged motor

Clamp Meter

Weights

String with adjustable loops

There are 3 individual parts of this lab that you will need to do. The order that you do them in is not important but we do ask that you record your results on the lab report in the order that they are presented here and on the lab document.

DC Motor

For this part of the lab, you will need the DC motor on stand, DC power supply, DMM, string with adjustable loops, and weights.

1. With the motor disconnected, turn the shaft at various speeds with your fingers and feel the way it rotates. Record your observations. For your lab report, record how the motor feels when it rotates. Does the motor resist being turned or is it continuous? Does it do a sort of stop-go-stop-go gyration as you turn it?

For the next part, you will need to connect this motor to the DC power supply and the DMM. To do so, first make sure that the DC power supply is properly configured and that there is a wire between the black and green leads for the side of the power supply you plan to use.

Next, configure the DMM to measure the current. Unlike in past measurements, you will want to place the red lead in the 10A circle (upper right hand side) instead of the 100mA circle (lower right hand side). Black lead stays in COM circle (lower left hand side). Also, remember to push the button with the A and bar with dots for DC current measurement.

Now, connect the DMM, DC power supply, and DC motor in series by connecting the two black leads of the DMM and DC power supply together. Then connect the red lead from the DC power supply to the red wire for the DC motor. Then connect the red lead from the DMM to the blue wire from the DC motor.

You will need this setup for parts 2-4 for the DC motor. Now proceed with part 2.

2. Connect the motor to +6 Volts with no load and measure the armature current. For your lab report, measure and record the no-load current(this means the current that the motor draws when it is not mechanically loaded) using the DMM, not the reading from the DC power supply. Also describe (with pictures or words) how the leads of the power supply and DMM are connected to the motor to make this measurement. Also specify that the DMM is set to measure DC current.

3. Briefly stall the motor with your fingers and measure the armature current. Do not leave the motor stalled for more than a few seconds. Record this value. For your lab report, record the current you read from the DMM when you stall the motor.

For the next part, you will need to add the string with an adjustable loop to the setup. Loop one loop over the wheel on the DC motor so that it is inside of the groove.

At the other end, you will need to attach denominations of weight starting at 10g and moving up 10g at a time until you get to 100g. This end should be adjustable, so you will be able to tighten the loop around the weight to keep it from slipping out.

Now continue to part 4.

4. Vary the load on the armature; observe the resulting armature current. Do this by applying a 10g weight to a cotton string draped over the flywheel. Then increase to 20g, 30g, 40g and so on up to 100g. Plot the resulting relationship. For your lab report, record your readings and plot the values on a graph showing the relationship between the increasing weight and the resulting current.

TIP: If your motor stalls before you reach 100g, record up to the highest value it reaches and include a note stating the weight it stalled at. You only need to include values up to the stall point in your graph, so if your motor stalls at 90g, include all the points up to 80g on the graph.

5. Continue incrementing the load on the motor by increasing the weights in 10g increments (or more if necessary); observe the weight required to stall your motor. Remember to not leave the motor stalled for more than a few seconds. For your lab report, record the weight that was required to stall your motor. If your motor was stalled before reaching 100g back in part 4, restate that your motor stalled at that weight for this part of the lab.

For the next part of the lab, you will need to use the DMM to measure the frequency of the motor as it rotates. To set the DMM up to do this, leave the black lead in the COM hole (lower left-hand side) and move the red lead to the V/ohm hole (upper left-hand side). Also, push the button labeled FREQ to make frequency measurements.

To set the motor up for a frequency measurement, first connect the power supply to the motor (red lead to red wire and black lead to blue wire). Then connect the DMM leads to the other two green and yellow wires that are available (the order for this connection does not matter).

Start from the high voltages and slowly work down to 2V. The motor starts to have issues just below 2V which is why we recommend starting high and going slowly down.

Now continue to part 6.

6. Create a speed/voltage plot by measuring the RPM of the motor at +2 Volts, then again at +3 Volts, +4 Volts, +5 Volts, and +6 Volts. The RPM may be measured by measuring the frequency of the tachometer output wires, then dividing by 3 (the number of armature windings). For the lab report, record the frequencies that you measure and the voltages that they are measured at. Use this data to make a graph showing frequency vs. voltage. Also describe (using pictures or words) how you connected the DMM and DC power supply leads to the motor to make these measurements. Specify that the DMM was set to measure frequency.

For this next part, you need to reverse the black and red leads of the power supply so that the black lead is connected to the red wire of the motor and the red lead is connected to the blue wire of the motor.

7. Reverse the power supply leads to the motor, so that you are essentially applying -6 Volts to the motor. Record your observation of what happens. For the lab report, record what happens when you reverse the power supply leads going to the motor.

Brushless DC Motor

For this part of the lab you will need a Brushless DC Motor, the DC Power Supply, two sets of leads for the DC power supply, and the DMM.

NOTE: The motor for this part of the lab is expensive and is sensitive to voltage change. If improperly connected, it WILL fry. Read this part of the lab thoroughly and make all connections properly BEFORE turning on the power supply.

Here’s another warning from the lab document (we really want you to not fry these motors so please read this).

It is important to remember that all brushless DC motors require external commutation. This particular model has the commutation electronics built into the motor. Be sure to observe the proper connections for the motor, or it will not function as you would expect. Use the 24 Volt output of the power supply for wire #1 (red) and wire #2. Make sure you do not reverse these two wires. Use another variable output of the power supply for wire #4 (speed command), but make sure you do not exceed the 5V maximum for this input. Note: for this motor, it is essential that you never reverse the polarity of the applied voltage, or you will burn out the commutation electronics.

Now that you have been sufficiently warned, let’s proceed to step 1 which does not require that the motor be connected.

Now let’s go over how to connect the Brushless DC motor to the DC Power supply.

For this part of the lab, you will be using the two variable outputs of the DC Power Supply. This is different than previous labs where you have only ever used one output. To set the power supply up, first make sure that the tracking buttons are both set to INDEP and that the two variable outputs of the supply both have wires connected between the black and green leads. Make sure that both outputs have lead wires connected to them and that the red lead is connected to the red output while the black is connected to the green output. Also, make sure that both current knobs on both sides of the DC power supply are turned up to ½ or greater. If this is not done, the supplies may not output any electricity.

Now turn on the DC power supply. Set the left side to 24V using the left-most voltage knob and set the right side to 1V using the right-most voltage knob (the reason for this is so that when you test your connections to the motor, you run a lower chance of causing any damage if the wiring is wrong). Do not adjust the 24V side of the supply. It is supposed to remain constant throughout the lab. Any voltage adjustments will be made to the 1V side (right side). NEVER exceed 5V on the right side as this can fry the commutation circuitry.

The DC power supply is now ready but we will make the connections with it powered off, so turn it off before proceeding.

Look at the Brushless DC motor and you will see a green connection panel with 7 holes and 5 wires. The first wire (red) is the 24V power input of the motor. The second wire (black) is the ground for the motor and commutator. The third wire (orange) is another ground but will not be used for this lab, so ignore it. The fourth wire (purple) is the variable 0-5V input power for the commutation circuit. The fifth wire (white) is used to switch the direction that the motor rotates in (connect it to the common ground to achieve this).

The motor will need to be connected to the DMM for current measurements. Set the DMM to measure current by placing the black lead into the COM hole (lower left) and the red lead into the 100mA hole (lower right). Set the DMM to measure DC current by selecting the A button with the bar and 3 dots.

Now let’s connect the motor to the power supply. With the power supply off, connect the red lead from the 24V side of the power supply to the red wire of the motor. Now connect the red lead from the 1V side of the power supply to the purple wire of the motor. Connect the black lead from the 1V side of the power supply to the black wire of the motor. Now take the black lead from the 24V side of the power supply and connect it to the black lead from the DMM. Take the red lead of the DMM and connect it to the black wire of the motor and the 1V black lead from the power supply (make sure this connection is solid before turning on). The result should look like this:

READ THIS PARAGRAPH COMPLETELY BEFORE PROCEEDING! Now, with the DMM on and set to DC current and all other connections made, turn on the DC power supply. The motor should turn on but will not move very fast since its input to the commutator is only 1V. If your motor does not turn on, turn off the power supply immediately and check your connections. If it turns on, proceed to step 2.

2. With the motor wires properly connected, measure the no-load current with a speed voltage of 1.0 Volts. Repeat for speed voltages of 3.0 Volts, 4.0 Volts, and 5.0 Volts. For the lab report, record the no-load currents for the various voltages. Also, describe (using pictures or words) how you connected the power supply and DMM leads to the motor to make these measurements. Specify that the DMM was set to measure DC current.

This next part asks you to use a metal rod to apply a variable load to the motor. We don’t have any metal rods, so you will need to apply the load using something else. You could use the rope with a loop that is used for the DC brushed motor or you could use your fingers (loop is recommended. See below).

NOTE: If you use your fingers, set the input voltage to the commutator to 1V so that you don’t burn your skin from the friction of the metal turning.

3. Apply a variable load to the motor by placing a metal rod on and perpendicular to the shaft. Observe the current response as you change the frictional load using the metal rod. For the lab report, record what happens to the current when you apply a variable load to the motor.

First, take a look at the motor information provided on the IT318 website:

The information should be under the Aids section for lab 7. Pay close attention to page two where it shows diagrams as they will label the parts and show the circuit connections.

4. How would you reverse the direction of this type of motor? (study the spec sheet).

Once you’ve figured it out, try it and record your observations. The spec sheet can be found at under the Aids section of lab 7. Hint: Take a look at the circuit diagram that shows the switch connected to the white wire. Also, remember that the orange ground wire is not used in this lab because when we ran through this lab, it didn’t do anything if connected to something else. For the lab report, record your observations, which color wire you used, and what you connected it to in order to reverse the motor.

Shaded-Pole AC Induction Motor

For this part of the lab, you will need an AC shaded-pole induction wall-plugged motor, a Variac transformer, and an AEMC Clamp Meter.

Here’s some background information about AC induction motors:

AC induction motors use the commutation effect of the AC voltage; no brushes, commutator, nor external commutation is required. These are some of the simplest motors made, but the shaded-pole variety of AC induction motor suffers from extremely low starting torque.

1. With the motor disconnected, turn the shaft at various speeds with your fingers and feel the way it rotates. Record your observations. How freely does the shaft rotate? For your lab report, record how the motor feels when it rotates. Does the motor resist being turned or is it continuous? Does it do a sort of stop-go-stop-go gyration as you turn it?

The setup for this part of the lab is pretty simple. Just plug the Variac into an available power strip (make sure the strip is powered on). Then plug the motor into the front of the Variac. To make a measurement, take the Clamp Meter, set it to measure AC current, and measure the current. Do this by clamping around one of the wires going directly into the motor. If you clamp around both, you will not get a measurement because the cross currents will cancel each other out.

When done, proceed to part 2.

2. Connect the motor to the output of the variac. Start with the variac set to 0. Record the armature current and observe the rpm as you change the output of the variac to 20, 40, 60, 80, then 100. For your lab report, record your current measurements as the voltage increases. Use the voltage output reading from the analog screen on the variac to determine the voltage you are at, not the setting on the knob. Also, describe (with pictures or words) how you made this measurement.

3. Turn the variac back down to 20. Invert the plug from the motor into the variac; record your observations when you plug it back in with the leads thus reversed. What do you think it would take to reverse the direction of rotation of a shaded-pole AC induction motor? For the lab report, record your observations when you flip the plug and record how you think you would change the direction of the motor.