1 Introduction

In V/f control, the speed of induction motor is controlled by fixing the ratio of the stator voltages to stator frequency, usually defined by the rated values. This will maintain the air gap flux at approximately the desired value at steady-state. Sometimes this scheme is called the scalar control because it focuses only on the steady state dynamic.

1.1 Objectives

The main objective of this lab is to operate the AC drive as a simple constant ratio V/f source to supply the induction machine and see how the induction machine performs under different speeds and load levels.

2 Prelab

Each student must complete a prelab to hand in at the beginning of your laboratory section. You must have completed all actions of the prelab before being allowed to participate in the Lab. The laboratory is usually completed in pairs, so please try and find a partner in the same lab section as you. See the laboratory schedule on eClass to make sure you show up to the correct time and place.

2.1 Prelab Reading

Review the safety information as well and equipment specifications for this laboratory by viewing the following information:

  • Look at equipment pages to familiarize yourself with the equipment listed below that is used in Lab 2. Note that there are links to the pdf user manuals available.
  • Read over the entire lab manual so you understand what you will be undertaking during the lab.
  • Make sure you print the Results sheet to bring to lab.
  • Read the Safety Page to familarize yourself with the Laboratory Rules and Dangers.

2.2 Pre-lab Questions

  1. What is constant V/f operation for an Induction Machine Variable Speed Drive?
  2. Which parameters effect the V/Hz operation of the drive for Open Loop speed mode?
    1. At the low frequencies of the V/f curve? (4 parameters)
    2. At the high frequencies of the V/f curve? (4 parameters)
  3. using the name plate of the induction motor what should be the torque at rated values? Show your calculations.

3 Experimental Procedure

3.1 Setup

3.1.1 AC Drive/Motor Setup

Figure 1: AC Drive/Motor Block Diagram

  1. Wire the AC Drive/Machine setup the same way you did in lab 1 as shown in the diagram above.
    1. Use the shielded safety banana leads (available on the wall).
    2. Using the appropriate color banana leads, connect the supply terminals A, B, C and GND of the 3 Phase Power Supply to the input terminals R, S, T and GND of the Powerflex 70 AC Drive.
    3. Using the appropriate color banana leads, connect the output terminals U, V and W of the Powerflex 70 AC Drive to the input terminals X, Y and Z of the Allen-bradley Induction Machine. Also make sure you connect the Allen-bradley Induction Machine to earth ground.

Circuit Check

At this point please get a lab instructor (LI) or teaching assistant (TA) to double check your wiring.

  1. Go through the startup menu of the Powerflex 70 AC Drive and configure it in a similar manner as Lab 1 as shown below.
    1. Apply power to the Powerflex 70 AC Drive by pressing START on the 3 Phase Power Supply.
    2. Go through the the Start-Up on the Main Menu to configure the Powerflex 70 AC Drive as shown below.

3.1.2 Fluke 43B Setup

  1. Setup the Fluke 43B the same way you did in Lab 1.
    1. plug it in using the supplied adaptor.
    2. connect the voltage test leads with 4mm test probes.
    3. connect the current probe configured with 10mV/A making sure it is also zeroed.
  2. Connect the Fluke 43B to measure the DC component for the Baldor PMDC Machines armature voltage leaving the current probe disconnected for now.

DANGER

Check that the motors are free to rotate and are clear of any wires/leads/fingers or anything else that could potentially touch them. Especially the shaft as this rotates very fast and has a very significant amount of power. The machine can also get fairly hot when they are operating under load.

AC Drive and Fluke 43B Setup Check

At this point please get a lab instructor (LI) or teaching assistant (TA) to double check your AC Drive and Fluke 43B setup.

3.2 Open Circuit Test of Baldor PMDC Machine

  1. Using the AC Drive/Machine to rotate the shaft at the Command Frequencies listed on the Results Sheet in the DC Machine - Open Circuit Test table. Measure both the shaft speed and the Baldor PMDC Machines DC component of the armature voltage and record it in the appropriate spot on the results sheet.
  1. Once you have the results, press the red stop button on the Powerflex 70 AC Drive to stop the AC Machine from rotating. Turn off the 3 Phase Power Supply and leave the AC Drive/Machine connected for the next part.

3.2.1 DC Drive/Motor Setup

Warning

Make sure the 3 Phase Power Supply is off.

Figure 2: DC Drive and Motor Connections Diagram

  1. Wire the DC Drive/Machine setup the same way you did in lab 1 as shown in the diagram above.
    1. Use the shielded safety banana leads (available on the wall).
    2. Plug in the PLX15 DC Drive control power supply available on the right side of the unit to a standard 120V recepticle. (this supply needs to be maintained at all times during drive operation)
    3. Using the appropriate color banana leads, connect the supply terminals A, B, C and GND of the 3 Phase Power Supply to input terminals L1, L2, L3 and GND of the PLX15 DC Drive, respectively.
    4. Using the appropriate color banana leads, connect the armature output terminals A+ and A- of the PLX15 DC Drive to the armature terminals +A1 and -A2 of the Baldor PMDC Machine, respectively. Also make sure you connect the Baldor PMDC Machine to earth ground.

Circuit Check

At this point please get a lab instructor (LI) or teaching assistant (TA) to double check your wiring.

  1. Move the Fluke 43B’s current probe and voltage leads to the output of the Baldor PMDC Machine as shown in Figure 3 so the DC Component of both the armature voltage and current can be measured.

Figure 3: Fluke 43B PMDC Armature Measurement

Warning

Check that the Machine Shaft is still clear of anything that can interfere with it.

DC Drive and Fluke 43B Setup Check

At this point please get a lab instructor (LI) or teaching assistant (TA) to double check your DC Drive and Fluke 43B setup.

3.3 Volts per Hertz Operation of the Induction Machine

  1. Use the AC Drive/Machine to rotate the shaft and the DC Drive/Machine to create a torque on the shaft when required at the Command Frequencies listed on the Results Sheet in the AC Machine - Volts per Hertz table. Make the measurements in Table 2 and record them in the appropriate spot on the results sheet.
    1. Use the Monitor file of the Powerflex 70 AC Drive to measure the outputs of the AC Drive which is also the Inputs to the Allen-bradly Induction Machine.
    2. Use the Pocket-tach to measure the shaft speed.
    3. Use the Fluke 43B to measure both the DC Component of the armature voltage and current.
    4. To obtain no-load for the Allen-bradley Induction Machine adjust the torque/current knob on the PLX15 DC Drive to minimum (fully counter clockwise).
    5. To obtain full-load for the Allen-bradley Induction Machine adjust the torque/current knob on the PLX15 DC Drive until the Powerflex 70 AC Drive displays the rated current of the Induction Machine (5.8 Amps).
    6. To obtain 1/2-load for the Allen-bradley Induction Machine take the PMDC machines armature current reading at full-load and divid it by 2. Take that new armature current value and adjust the torque/current knob of PLX15 DC Drive until this new value is obtained for the armature current.

Warning

This is a reminder to make sure the speed/voltage and torque/current knobs are fully counter-clockwise before you begin so that you start in the unloaded condition.

Warning

During the experiment make sure that you don’t exceed the Allen-bradley Induction Machines rated current of 5.8 Amps which is fairly easy to do if you increase the load to much.

  1. Once you have the results to turn off the setup do the following:
    1. Return the torque/current knob on the PLX15 DC Drive to minimum (counter clockwise).
    2. press the red stop button on the PLX15 DC Drive to discount it from the circuit.
    3. press the red stop button on the Powerflex 70 AC Drive to stop the AC Machine from rotating.
    4. Turn off the 3 Phase Power Supply.

Results Sign-off

Before you start to cleanup, if you are happy that you have completed all of the measurements required for the lab please get a LI or TA to double check results and sign-off on your results sheet.

Cleanup Sign-off

Please put everything back to where it was when you came into the lab.

  • Disconnect your circuit and leave all of the equipment neatly at your station.
  • Put the Fluke 43B back in the case and make sure the Current Probe is turned off.
  • Hang the Hookup Leads back on the wall neatly.
  • Please make sure you bring all your stuff with you.

Once you are finished cleaing up you need to get another sign-off from a LI or TA.

4 Lab Report

The following is what you are expected to hand-in one week (by 4:00pm) after completion of the lab. You only have to hand-in one copy per group. There is an assignment box in the ETLC atrium marked ECE432 Lab: H11, H31, H41. Please, staple everything together in the following order.

  • Use the first page of the results sheet as your cover page. Make sure your names, student ID’s, CCID’s and lab section are visible in the table above. You need to obtain the signatures below after completing the Laboratory exercises.
  • Include the results sheet that you made all of your measurements on.
  • The calculation below showing your work for only one setpoint.
  • The 4 graphs below.
  • The answers to the post lab questions below.

4.1 Calculations

For the following calculations you need to show your work for only one setpoint to show how you calculate the value.

  1. From the DC Machine - Open Circuit Test results calculate \(k\phi\) for the Baldor PMDC Machine for each command speed.
  2. Using the average of the \(k\phi\) values above calculate the Torque on the shaft for all setpoints in the AC Machine - Volts per Hertz table of the results.
  3. Calculate the slip speed values in rpm for all setpoints in the AC Machine - Volts per Hertz table of the results.

4.2 Graphs

Create the following Graphs to hand in with your Results.

  1. Voltage vs. Speed for the DC Machine - Open Circuit Test
  2. Voltage vs. Speed for the Induction Machine
    • include 3 series (make sure the speeds are all in the same units)
      1. voltage vs. synchronous speed
      2. voltage vs. no-load speed
      3. voltage vs. full-load speed
  3. Torque vs. Speed * include a separate series for each command frequency
  4. Torque vs. slip speed
    • include a separate series for each command frequency

4.3 Questions

Answer the following questions to hand in with your Results.

  1. Is the assumption that \(\tau \propto \Psi^2 sn_S\) reasonable for constant V/f operation? Explain your answer.
  2. Did the AC drive with constant V/f control provide constant flux at all speeds? If not, at what frequencies is it limited and why?
    1. what happens at low speeds
    2. what happens at high speeds (> 60 Hz)