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This section contains necessary information in order to modify the parameters of an electric drive. The method is based on an example which uses the AC3 drive model. In this example, the nominal power of the motor is changed from 200 hp to 5 hp. The complete procedure is described in order to:
Open the ac3_example example (type ac3_example in the MATLAB Command Window). The parameters are set for a 200 hp motor.
Simulate the model in accelerator mode and observe the results.
Double-click the Field-Oriented Control Induction Motor Drive block and select the Asynchronous Machine tab and copy into the drive's mask the 5 hp motor's parameters that are shown in the next figure.
Enter the New Motor Parameters
In this section we start with the tuning of the flux regulator's parameters. The parameters are empirically tuned until a satisfactory response is obtained. When you retune the regulator's parameters, it is of primary importance to visualize the reference signals and the variables of these two regulators.
In order to measure the signals associated to the flux regulator, add into the demux subsystem the blocks shown in the next figure:
Add Blocks to Measure Flux Regulator Signals
Select the Controller tab in the mask of Field-oriented Control Induction Motor Drive block and set the Regulation type to Torque regulation to access the controller parameters.
The torque regulation mode is required in order to bypass the speed regulator parameters and act directly on the FOC controller.
Remember that the current controlled by the FOC controller depends of the machine flux. The flux controller ensures that the required flux is correctly applied to the machine.
Set the Lowpass filter cutoff frequency to 10 kHz in order not to limit the flux frequency. The highest flux frequency depends of the switching frequency.
Set also the Flux output limits to 150% of the Nominal flux, the Proportional gain to 1, the Integral gain to 0, the hysteresis band to 1 and the Machine flux to 0.705. This last value is computed as follows:
$$\frac{{V}_{LL}\left(rms\right)}{\sqrt{3}\cdot 2\cdot \pi \cdot f}=\frac{460}{\sqrt{3}\cdot 2\cdot \pi \cdot 60}$$
Reset the Flux Regulator Parameters
To apply the nominal torque to the motor, modify the torque set point and the load torque blocks as shown in the next figure.
Set Points in Torque Regulation Mode
Set the sampling decimation of the scope block to 1, the Variable name to simout1 and check the Save data to workspace parameter with a Structure with time format.
Set the Scope Parameters
Simulate the system for 0.5s then open the powergui and click on FFT analysis.
Select the Stator current signal in the Input list and specify the Start time = 0.23, the Number of cycle = 1, a Fundamental frequency = 7.5, and a Max Frequency (Hz) = 20 000 Hz.
Click on the Display button to get the FFT graph shown on the next figure.
Observe the switching frequency of about 5 kHz. To attenuate this frequency, set the Flux controller Lowpass filter cutoff frequency parameter to 500 Hz.
Open the Scope block and observe the flux signal. Note that the steady state error is high and the time response is not really good:
Gradually increase the Proportional gain parameter of the controller and simulate until you obtain a satisfactory response. Increasing the gain above a certain value can cause a saturation of the Flux controller. The curve at the next figure is obtained with a proportional gain of 100.
Gradually increase the Integral gain and simulate until you obtain a satisfactory steady state result with minimal error. The next plot is obtained with a integral gain of 90.
Flux Regulator: Ki Tuning
In this section we are tuning the speed controller parameters. The regulator's parameters are empirically tuned until a satisfactory response is obtained.
Select the Controller tab in the mask of Field-oriented Control Induction Motor Drive block and set the Regulation type to Speed regulation to edit the controller parameters.
Set the Torque output limits to 150% of the nominal torque, the Proportional gain to 1, the Integral gain to 0, the Speed cutoff frequency to 500.
The highest speed frequency depends also on the switching frequency so take the same value as for the flux regulator lowpass filter cutoff frequency.
The speed ramp acceleration must be calculated not to exceed the torque output limit. The required torque to accelerate the motor at 1750 rpm/s is given by:
$$\begin{array}{l}{T}_{accel}=J\cdot \frac{Accel\left(\left(rpm\right)/s\right)}{30}\cdot \pi \\ {T}_{accel}=0.02\cdot \frac{1750}{30}\cdot \pi =3.67\text{Nm}\end{array}$$
In order to apply the nominal torque to the motor, modify the speed set point and the load torque, as shown in the following figure.
Set Points in Speed Regulation Mode
Set the scope decimation to 25 in order not to overload the memory. Start the simulation.
Observe the speed signal on the Scope block. The steady state error is high and the response time is not really good:
Poor Speed Response
Gradually increase the Proportional gain parameter of the controller and simulate until you obtain a satisfactory response time without overshoot. Note that if the gain is too high, the system will be oscillatory. The next plot is obtained with a proportional gain of 3.
Speed Regulator: Kp Tuning
Gradually increase the Integral gain and simulate until you obtain a satisfactory steady state value with a minimal steady state error. The curve at the next figure is obtained with a integral gain of 100.
Speed Regulator: Ki Tuning
The final drive regulators parameters are shown in the next figure.
Regulator Parameters
Select the Converter and DC bus tab in the mask of Field-oriented Control Induction Motor Drive block to tune the DC bus capacitor and the braking chopper parameters.
Set the DC Bus Capacitance parameter to 167e-6.
The DC bus capacitance is calculated in order to reduce the voltage ripple. It is calculated as follow:
$$C=\frac{{P}_{motor}}{12\cdot f\cdot {\Delta}_{V}\cdot {V}_{DC}}$$
where:
P_{motor} is the nominal power of the motor drive (W)
f is the frequency of the AC source (Hz)
Δ_{V} is the desired voltage ripple (V)
V_{DC} is the average DC Bus voltage (V)
This equation gives an approximate value of the capacitor required for a given voltage ripple level. Here the desired voltage ripple is 50V.
The motor drive of 5 hp (3728W) is fed by a 60Hz three-phase source. The average DC bus voltage is given by: V_{DC} = 1.35·V_{LL}, where V_{LL} represents the line to line rms voltage of the source. The source line to line voltage is 460 Vrms so the DC voltage is: V_{DC} = 621 V.
The required capacitor is then equal to:
$$C=\frac{3728}{12\cdot 60\cdot 50\cdot 621}=167\text{}\mu \text{F}$$
Set the Braking chopper Shutdown voltage to 660V and the Braking chopper Activation voltage to 700V.
In motor mode, the peak voltage of the DC bus is equal to:
$${V}_{peak}={V}_{LL}\cdot \sqrt{2}=460\cdot \sqrt{2}=650\text{V}$$
The shutdown voltage (V_{shut}) of the braking chopper should be a little bit higher than this value. The shutdown voltage is set to 660V and the activation voltage (V_{act}) is set to 700V in order to limit the voltage increase during regenerative braking.
Set the Braking chopper Resistance to 131 ohms.
The braking chopper resistance is calculated with the following relation:
$$R=\frac{{V}_{act}^{2}}{{P}_{motor}}=\frac{{700}^{2}}{3728}=131\text{}\Omega $$
The final DC bus parameters are shown in the next figure.
DC Bus Parameters