Introduction


Introduction

Review, Safety considerations when working in power electronics, before proceeding.

There are three major components in this electric-drives system which will be used to perform all the experiments in this course. These are as follows:

  1. Three-inverter power module with on-board controller and data logger.

  2. Coupled motor-generator system.

  3. Workbench simulation and rapid prototyping software platform.

As an introduction to the Workbench based electric drives system described above, a pre-built induction motor speed control model will be simulated and run in real-time. The following section, gives a brief overview of the various components, which will be used to run the induction motor speed control model. This will be followed by simulating the system in Workbench and a real-time implementation of the same on the inverter-motor drive platform.

Overall system

The overall system is shown below and consists of the following components:

Overall workbench-drives system

  1. Workbench software platform: A numerical simulation platform that supports model and code-based design. A toolbox within the platform consists of various pre-built commonly used arithmetic, logical, conditional, and storage operators that can be used to model complex systems by dragging and dropping these tools and interconnecting them. In addition to being able to simulate a system, the platform can generate digital controller specific code that can be programmed into a micro-controller/digital signal processor to do real-time control.

  2. Isolated DC power supply: This is used to power the inverter DC bus. In normal operation, the system is always run as motor-generator pair, where energy transferred to the motor is mostly recuperated by the generator. Thus, the DC power supply only needs to supply the losses and the initial charging current.

    please noteNote

    The communication channel between the computer and the three-inverter module is not isolated. Hence the DC power supply must be electrically isolated to avoid common mode current, which could otherwise potentially destroy the whole system.

  3. USB A-B COM channel: Code and data transfer happens via USB 2.0. Under ideal conditions, the maximum data transfer is 480 kbps, which translates to 15,000 samples per second of real-world variable such as motor currents, voltages, speed, etc.

  4. Three 3Φ-inverters: The hardware module contains all the power electronics, gate drivers, current and voltage sensors, digital signal processor, data-logger, and programmer necessary for design and deployment of common real-time motor control algorithms. There are three 3Φ-inverters, each rated 50 V, 7 A. Forced cooling using two frame-mounted fans allows for inverter to be switched safely up to 50 kHz at rated conditions. Each inverter is independently controlled by the on-board TI TMS320F28335 digital signal processor (DSP).

    In addition to PWM control of the inverters, the DSP reads feedback signals from current sensors A and C phase of each 3Φ-inverter, the DC bus voltage sensor, inverter over-current and over-temperature faults, and motor position encoder. The measured data as well as other model variables can be logged and viewed in real-time on Workbench software platform. This is made feasible by a dedicated on-board data transfer link between the COM and the DSP. The same channel is also used to transfer program from the COM to the DSP.

  5. Induction motor: The induction motor is rated 24 V, 2.8 A cont., and 1400 RPM at no load. In this experiment, the speed of this motor is maintained at a constant 100 rad/sec irrespective of load changes.

  6. DC generator: The DC generator is rated 60 V, 4 A cont., and 4700 RPM at no load. It has an encoder mounted on the rear end of the rotor shaft to measure the speed. Since the motor and generator are coupled, the magnitude of the rotational speed of the generator is same as that of the motor. The generator is used to emulate load changes.

  7. Motor generator coupler: The motor and generator are tightly coupled using an Oldham coupler. The coupling shaft is surrounded by a safety enclosure with viewing slots.

    warningWarning

    Do not leave any loose objects lying around as they can get pulled into the motor shaft while rotating and could be flung around at dangerously high speed. Always view from a safe distance.

  8. DSUB-15 encoder feedback: The motor/generator speed is measured via 2048-line magnetic rotary A-quad-B encoder attached to the generator. This encoder contains 2048 evenly spaced alternating north-south poles on a disc which is mounted on the motor frame. A hall-effect sensor is mounted on to the shaft and, as the rotor rotates, the alternating magnetic polarity generates alternating voltage pulses at the output of the hall-effect sensor. Counting the number of pulses per second yields the rotor speed. The rotor shaft has one more hall sensor that is mounted such that, the pulses generated are phase shifted by 90°. The direction of rotation is obtained based on the polarity of phase shift between the two signals.

Induction motor speed control simulation
  1. Copy the folder where pre-built example project for this experiment is present, usually in C:\Program Files (x86)\Sciamble\WorkBench v1\Examples\CUSPLab\AdvancedDrives\Experiment1 and paste it in a location where the user has permission to edit and save files, like the Desktop folder.

  2. Click on the Workbench icon icon on desktop to launch the application.

  3. Workbench has tabs on the left, right and bottom that represents docked windows, each of which convey or control different piece of information to and from the user.

    Workbench - Tabs

    Clicking on these tabs reveal their respective docked windows. The content within these docks change based on the context as will be seen later.

  4. Click on the Toolbox tab on the left tabs section and pin the window by clicking on the Dock hide icon icon on the top right corner. Similarly pin the Explorer dock on the right. The tools in the Toolbox are grouped into categories based on their operation. Click on the drop-down list: Toolbox drop-down list to navigate between the tool classes.

  5. Click on the Dock pin icon icon in the Toolbox dock to hide the toolbox for now.

  6. The Explorer/Solution Explorer dock acts as both file browser to navigate to and, open, add, or remove files from project, as well as, a container to display the project structure. The toolbar within the Explorer dock: Workbench solution explorer toolbar consists of the following buttons:

    1. New simulation/real-time control project button : Creates a new project.

    2. Open simulation/real-time control project button : Opens an existing project.

    3. New model/script file button : Creates a new model or script file and adds it to the selected project.

    4. Open model/script file button : Adds an existing model or script file to the selected project.

    5. Remove project or model or script file button : Removes selected project, model, or script file. This does not delete it from the physical folder, simply removes it from the current project.

    6. Permanently delete model or script file button : Permanently deletes (sends to Recycle Bin) the selected model or script file.

    7. Download project file from server : Download project files.

    Click the second icon, Open simulation/real-time control project button to open an example project. This opens the file bowser within the Explorer dock.

  7. Navigate to the folder where the pre-built example project was pasted in Step 1. Double click the IMVectorControl.project node within that folder to open the project.

    Workbench navigate to project in file browser

  8. Click the Project expand button icon to explore the files within the project. To display the model file, double click the Workbench model logo ModelFile node in the Explorer dock.

    Simulation and real-time Induction motor speed control model

  9. The details of this model will be explored in greater depth in later experiments. This experiment is solely meant to be an introduction to the system that will be used extensively in the following experiments. This system consists of a mathematical model of an induction motor subsystem labelled IMMotor. The motor voltage is modulated using Sine-PWM, the reference signal for which is derived from the inner current and outer speed loop PI controllers preceding it. The reference speed is stepped from 0 to 100 rads/s at time t = 1s. In the bottom section of the model is the mathematical model of a DC motor. The motor is operated in torque control mode and its torque is stepped at time t = 3 s, to emulate a sudden change in load torque experienced by the Induction motor. The aim of this experiment is to verify that the controller has been designed properly to rapidly compensate for the increased load demand to actively maintain the motor speed same as the desired reference speed.

  10. Run the model by clicking on the Numerical simulation and real-time prototyping Run button button in the top dock. Double click on the Scope labelled SpeedScope to view the motor speed, which closely tracks the reference speed.

    Induction motor speed control speed response to step in reference

    To focus the data, double click anywhere on the plot or click on the Model display scope autofocus button in the Scope's toolbar. Also, view the results in all the other scopes present in the model.

  11. As seen above, the motor rotor speed closely matches the reference speed and the speed settles quickly. Change this to a more under-damped response by changing the Integrator gain in the PI block. To do this, double click on PIIq subsystem in the MotorModel model file. Double click the gain block labelled kI to open its properties in Property dock on the left. Multiply the Gain value by a factor of 10.

    Induction motor speed control PI Integrator gain property

    Re-run the simulation and observe the speed result.

    Induction motor speed control speed response to step in reference

  12. Change the kI gain to 0.1 times that of the original value and re-run the simulation and observe the speed result.

  13. Change the kI gain to 0.0001 times that of the original value and re-run the simulation and observe the speed result.

This concludes the section on getting started with Workbench simulation. In the following section, the same model will be run in real-time to control an actual Induction motor.

Induction motor real-time speed control
  1. Assemble the Induction motor - DC generator set as shown below:

    Motor, generator set with coupler and safety enclosure assembly

    1. DC Generator (one with A-quad-B encoder mounted on its back).

    2. 4 pole Induction motor.

    3. Oldham coupler. Tighten if loose using M2.5 hex wrench.

    4. Oldham coupling disk. Attach it to the motor and generator coupling.

    5. Safety enclosure. The coupling unit resides inside the safety enclosure.

    6. NEMA 34 to NEMA 23 frame converter plate.

    7. M4 hex socket-head screws - x4.

    8. M4 hex flat-head/socket-head screws - x4.

    9. M5/M6 hex screws - x4.

    10. 2048-line A-quad-B encoder - Attach the encoder cable to the A-quad-B encoder.

  2. The wiring color code followed is:

    DC (Inverter input):

    1. DC +ve - (Red)

    2. Ground - (Green)

    3. DC -ve  -  (Black)

    AC (Inverter output):

    1. Phase A - (Black)

    2. Phase B - (Red)

    3. Phase C - (Blue)

    The inverter output terminals are located on the front panel of the three-inverter module. Connect the Induction motor terminals to the Inverter 1 terminals such that the color of both these terminals matches one another.

    please noteNote

    The motor cable color might be different from the color of the banana terminal. Henceforth when referred to the color of the motor terminal for making connections, it only refers to the color of the banana plug and not the color of the cable.

    Connect the DC generator negative (-) (black terminal) to Inverter 2 A phase (black terminal) and DC generator positive (+) (red terminal) to Inverter 2 B phase (red terminal). The DC generator might have a third cable with green terminal. This is the ground wire and must be left unconnected. Similarly connect the DC power supply to the inverter input, located on the back panel of the module.

    warningWarning

    Ensure that the DC power supply is turned off before making the connections. If the DC power supply does not have a dedicated ground terminal, that connection can be left floating. If present, it is strongly recommended that it be connected to the three-inverter module's ground terminal to mitigate any electrical hazard.

  3. Connect the encoder DSUB15 terminal, from the encoder cable connected to the generator, to the Encoder 1 terminal on the back panel of the three-inverter.

  4. Connect one end of the USB cable (USB-A) to the computer and the other end (USB-B) to the back panel of the three-inverter module.

  5. Turn ON the DC power supply and gradually ramp up the voltage from 0 to 40 V.

    warningWarning

    DC bus voltage must not exceed 42 V. A minimum of 20 V DC is needed to power the auxiliary power-converter inside the module. All the experiments have been designed with a DC bus voltage of 40 V unless mentioned otherwise.

  6. Click on the Numerical simulation to Real-time mode transition button icon in the top dock of Workbench to transition from the simulation mode, where the control algorithm was run on the computer using the linearized motor model, to the real-time mode, where the control algorithm is executed in real-time on a DSP and the generated signals are amplified to drive an actual motor. Workbench automatically identifies components that need to be evaluated, in real-time and auto-translates those to DSP specific code.

    please noteNote

    When in real-time mode the top dock changes from light-blue to light-salmon color. In this mode, pressing Run will immediately download the program to the connected real-time controller and begin execution. Any previously programmed code will be lost forever. All necessary laboratory safety precautions must be taken.

  7. Click Numerical simulation and real-time prototyping Run button to run the control algorithm in real-time. The following message will be displayed if code has been transferred successfully:

    Real-time mode output message

  8. Double click on the SpeedScope to view the real-time result. Click the Model display scope autofocus time axis alone button from the scope's toolbar to fit the whole Y-axis across full simulation time. Press this button whenever data goes out of focus. After about 5 s, stop data logging by clicking on the Numerical simulation and real-time prototyping Stop button in the top dock. This only stops the data logging. To power down the inverter, turn OFF the DC power supply and disconnect the USB to power down the controller.

    Make note of all the other Scope results as well. The 3Φ motor currents are not observed in real-time since the frequency is too high for the data logger which has limited bandwidth to capture sufficient data points.

This concludes the experiment. This experiment was a cursory introduction to various concepts that will be continuously re-visited in subsequent experiments.

Lab report and reading assignment
  1. Attach screenshot of results from all the scopes in the model from both simulation and real-time mode.

  2. Explain the reason for oscillatory behavior when kI value is increased in the speed PI loop. Also explain why there is a steady state error as kI → 0.

  3. How would the plots of Induction motor speed and current be different if the load torque applied by the DC motor at t = 3 s was negative in magnitude, as opposed to, what was done in this experiment where it was positive?

  4. Did the DC current reading on the power supply change when the induction motor was loaded by the DC motor? Explain why it did or did not.

  5. Go through further resource material on various features of Workbench available here