Power System Blockset    

Building and Simulating the PWM Motor Drive

  1. Open a new window and save it as circuit5.
  2. Open the Power Electronics library and copy the Universal Bridge block into your circuit5 model.
  3. Open the Universal Bridge menu and set its parameters as follows:

    Power Electronic device =IGBT/Diodes; Port configuration= ABC as output terminals; Snubber Rs=1e5 W Cs=inf; Ron=1e-3 W; Tail: Tf=1e-6s; Tt=1e-6 s).

    Notice that the snubber circuit is integral to the Universal Bridge dialog box. As the Cs capacitor value of the snubber has been set to Inf (short-circuit), we are using a purely resistive snubber. Generally, IGBT bridges do not use snubbers, However, because each nonlinear element in the Power System Blockset is modeled as a current source, you have to provide a parallel path across each IGBT in order to allow connection to an inductive circuit (stator of the asynchronous machine). The high resistance value of the snubber will not affect the circuit performance.

  1. Open the Machines library. Copy the Asynchronous Machine SI Units block as well as the Machines Measurement Demux block into your circuit5 model.
  2. Open the Asynchronous Machine menu and look at its parameters. The parameters are set for a 3 HP, 220 V, 60 Hz, two pairs of poles machine. Its nominal speed is therefore slightly lower than the synchronous speed of 1800 rpm or ws= 188.5 rad/s.
  3. Notice that the three rotor terminals a, b and c are made accessible. During normal motor operation these terminals should be short-circuited together. Open the Connectors library. Copy the vertical Bus Bar block with two inputs and one output into your circuit5 model.
  4. Open the Bus Bar menu and change the number of inputs to three and the number of outputs to zero. Resize the block vertically and connect its three inputs to the three rotor terminals as shown on Figure 1-13.
  5. Open the Machines Measurement Demux block menu. When this block is connected at the ASM measurement output, it allows you to access specific internal signals of the ASM. Deselect all signals except the following three signals: is_abc (three stator currents), wm (rotor speed) and Te (electromagnetic torque).
  6. You will now implement the torque-speed characteristic of the motor load. We will assume a quadratic torque-speed characteristic (fan or pump type load). The torque T is then proportional to the square of the speed :

    The nominal torque of the motor is:

    Therefore, the constant k should be:

    Open the Functions & Tables library of Simulink and copy the Fcn block into your circuit5 model. Open the block menu and enter the expression of torque as function of speed:

  1. Connect the input of the Simulink Fcn block to the speed output of the Machines Measurement Demux block labeled wm and its output to the torque input of the motor labeled Tm.
  2. Open the Electrical Sources library and copy the DC Voltage Source block into your circuit5 model. Open the block menu and set the voltage to 400 V.
  3. Open the Measurement library and copy a Voltage Measurement block into your circuit5 model. Change the block name to Vab.
  4. Using ground blocks of the Connectors library, complete the power elements and voltage sensor interconnections as shown in Figure 1-13.
  5. In order to control your inverter bridge, you need a pulse generator. Such a generator is available in the Extras library of powerlib. Open the Extras/Control Blocks library and copy the Discrete 3-Phase PWM Pulse Generator block into your circuit5 model. Connect its Pulses output to the Pulses input of the Universal Bridge block.
  6. Open the Discrete 3-Phase PWM Pulse Generator block menu and set the parameters as follows:

    Modulation index m = 0.90; Frequency of output voltage = 60 Hz; Phase of output voltage = 0 degrees; Switching frequency Fs = 1080 Hz; Time step =10e-6 s

  1. Use the Edit/Look Under Mask menu of your circuit5 window to see how the PWM is implemented. This control system is entirely made with Simulink blocks. The block has been discretized so that the pulses will change at multiples of the specified time step. A time step of 10 µs corresponds to +/- 0.54% of the switching period at 1080 Hz.
  2. One common method of generating the PWM pulses uses comparison of the output voltage to synthesize (60 Hz in our case) with a triangular wave at the switching frequency (1080 Hz in our case). This is the method that is implemented in the Discrete 3-Phase PWM Pulse Generator. The line-to-line rms output voltage is a function of the DC input voltage and of the modulation index m as given by the following equation:

    Therefore, a DC voltage of 400 V and a modulation factor of 0.90 yield the 220 Vrms output line-to-line voltage, which is the nominal voltage of the asynchronous motor.

  1. You will now add blocks measuring the fundamental component (60 Hz) embedded in the chopped Vab voltage and in the phase A current. Open the Extras/Measurements library of powerlib and copy the Fourier block into your circuit5 model.

    Open the Fourier block menus and check that the parameters are set as follows:

    Fundamental frequency f1= 60 Hz; Harmonic number= 1;

    Connect this block to the output of the Vab voltage sensor.

  1. Duplicate the Fourier block. In order to measure the phase A current, you need to select the first element of the is_abc output of the ASM Measurement Demux.

    Copy a Selector block from the Signal & Systems library of Simulink.

    Open its menu and set Element to 1. Connect the Selector output to the second Fourier block and its input to the is_abc output of the Machines Measurement Demux block as shown on Figure 1-13.

  1. Finally, add scopes to your model. Copy one Scope block in your circuit. This scope will be used to display the instantaneous motor voltage, currents, speed and electromagnetic torque.In the Scope Properties/General menu of the scope, set the following parameters:

    Number of axes=4; Time range =0.05 s; Tick labels: bottom axis only.

    Connect the four inputs and label the four connection lines as shown on Figure 1-13. When you start the simulation, these labels will be displayed on top of each trace.

  1. Duplicate the four- input Scope and change its number of inputs to 2. This scope will be used to display the fundamental component of Vab voltage and Ia current. Connect the two inputs to the outputs of the Fourier blocks. Label the two connection lines as shown on Figure 1-13.

You are now ready to simulate the motor starting.

Simulating the PWM Motor Drive with Continuous Integration Algorithm

Open the Simulation/Parameters menu. Select the ode23tb integration algorithm. Set the relative tolerance to 1e-4, the absolute tolerance and the Max step size to auto, and the stop time to 1 s. Start the simulation. The simulation results are shown on.

The motor starts and reaches its steady-state speed of 181 rad/s (1728 rpm) after 0.5 s. At starting, the magnitude of the 60 Hz current reaches 90 A peak (64 A rms) whereas its steady-state value is 10.5 A (7.4 A rms). As expected the magnitude of the 60 Hz voltage contained in the chopped wave stays at:

Also notice strong oscillations of the electromagnetic torque at starting. If you zoom on the torque in steady-state, you should observe a noisy signal with a mean value of 11.9 Nm corresponding to the load torque at nominal speed.

If you zoom on the three motor currents, you can notice that all the harmonics (multiples of the 1080 Hz switching frequency) are filtered by the stator inductance, so that the 60 Hz component is dominant.

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Figure 1-14: PWM Motor Drive; Simulation Results for Motor Starting at Full Voltage


 Session 5: Simulating Motor Drives Using the Multimeter Block