Building an inverter from scratch using a single Mosfet or IGBT inverter model!
What will you learn:
- Create a three phase inverter with RL load
- Add PWM modulation
- Simulate load currents
- Use the discrete inverter model with centerline modulation
- Use the averaged inverter model
Prerequisites:
- Basic knowledge of Caspoc
- Previous tutorial:
Building an inverter from scratch using discrete Mosfet or IGBT models
Inverter
How to build a three phase inverter using a single model with R-L load
We continue this series of tutorials on building a three phase inverter with creating a three phase inverter using a single model. These single models are found in the section Components/Library/PowerConverters/Inverter. We add a simple Pulse Width Modulation by simply adding a Centerline block from the Components/Library/PowerConverters/InverterControl/Modulation section and a three phase reference source from the Components/Library/PowerConverters/InverterControl/Source section. All required components can be found in the sections as shown below.
Single Inverter Component
We start with building the circuit by the placement of the DC source and adding an inverter model.
All components are from the Components/Circuit/RLC and Components/Circuit/Sources sections. The inverter model will be taken from the Components/Library/PowerConverters/Inverter section.
The inverter will be a Mosfet Inverter and is selected from the Components/Library/PowerConverters/Inverter section. On the left side is the connection to the DC link, on the right side is the three phase output of the inverter. The controlling signals to the Mosfet-gates are at the bottom of the inverter model.
First add a ground symbol (from the bottom button bar) and this is also the same button bar where you can select a scope. More on the scope later in this tutorial. Adding the ground symbol is of practical use when you begin building the circuit. First of all, you need a reference ground level as all voltage are referenced to the ground node. Second, by adding the ground symbol, Caspoc will not add the ground node by itself somewhere in the schematic.
After the ground symbol is added (1), we are going to add the voltage source(2), Inverter(3) and connect everything by wires(4).
- First add the ground symbol from the bottom button bar, by clicking the ground button on the bottom button bar. Please the ground symbol somewhere on the schematic, we can move it later.
- Add the voltage source from the Components/Circuit/Sources section. While dragging the component you can rotate it by clicking it with the right mouse button.
- Add the Mosfet inverter from the Components/PowerConverters/Inverter section. While dragging the component you will see many connections on either side. After placing the component only the active connections remain visible. You can rotate the component only after placement by clicking it with the right mouse button and selecting the rotation angle and Flip direction.
- Connect all components by wires. Left click on a node, drag and drop on the node where to connect to.
A passive load being a series connection of a resistor and inductor is selected from the Components/Circuit/RLC section. While dragging the component you can rotate it by clicking it with the right mouse button. Notice that there is a small dot displaying on the left side of the R and L. It shows the direction of the current measurement through the component and we already place them such, that the current is measured going from left to right.
Please R and L such that there is enough room between them for the wiring to the output.
Connect the load with the inverter in such a way that the phase do not connect. Start with drawing wires from a node. You can end the wire at any position on the schematic. First draw a wire from the left node of the inductor to just in front of the inverter, see the red and green arrows. Next we connect that wire to the inverter.
Finally connect the remaining wires to the inverter. You could also leave more space between the high side and low side mosfets, but this only enlarges the circuit.
The passive load will be parametrised such, that we will see a nice sinusoidal current. We choose a R=100Ω and L=100mH to get a nice meaningful result in the scope which is easy to interpret. The time constant τ=L/R is equal to τ=1ms and the current will be limited below 3 Amperes.
Right click each inductor and enter a value of 100mH.
Right click each resistor and enter a value of 100Ω.
Right click the voltage source and enter a value of 600 volt.
Now that the components are parametrised we can add the control blocks. We select a simple PWM regulation and therefore we take a Centerline block from the Components/Library/PowerConverters/InverterControl/Modulation section. The inputs for this block are the three phase modulation indices. We can give any signal here, as long as it is between -1 and +1. The output voltage will be modulated as Vout=((m+1)/2)*Vdc.
The switching frequency is set in the Centerline block and in this simulation it is chosen to be 20kHz. This is equal to a period of T=50μ seconds and therefore 50u is entered as parameter in the Centerline block.
For the modulation indices we will use a three phase signal with an amplitude of 1 and a frequency of 50Hz. From the Components/Library/PowerConverters/InverterControl/Source section we select the block 3phase. In this block we can define a three phase signal where we can specify frequency, amplitude and phase. Since we need a modulation signal we keep the amplitude equal to 1. This means that he signals will vary from -1 to +1 in a sinusoidal matter. The modulation Centerline block will follow this signals and produces a sinusoidal output voltage.
To see the currents in the simulation, we are adding a scope. Select the scope from the bottom button bar and place it next to the load. Drag the right-bottom corner of the scope to enlarge the scope and to add more inputs. The inputs will appear automatically when enlarging the scope.
To measure current in the load, click with he left mouse button on the input circle of the scope, keep the left mouse button pressed down and drag over the circuit component, where you release the left mouse button. Upon correct hovering above the circuit element a green flag will appear saying that releasing will create the current measurement.
Notice that all circuit components R and L have a small dot on the left side. This means that current is measured flowing from the dotted node through the component. You can rotate the component to change the current measurement direction by selecting the component by a left mouse click and then to press Ctrl-R on the keyboard.
Before we can start the simulation, we have to set the correct simulation parameters. We assume that we want to see a complete period.
Open the simulation paramters dialog box by clicking on the timing parameters in the bottom statusbar.
Change the parameter Tscreen to 20ms, to see a complete period. Notice that we have selected a switching frequency of 20kHz, so a simulation step size of maximum 1μs is needed to have a more or less correct simulation.
Start the simulation by pressing the enter key or select Simulation/Start Simulation from the menu.
Averaged Inverter models
In order to speed up the simulation, we can use an averaged model. The advantage is simply simulation speed.
First we are going to remove the existing inverter model. Since the input for modulation on the left side is lined up withthe connections for the DC link, we also move th source block to the left. The distance between the DC link and the inverter output is equal for the switched and averaged inverter model, so they can easily be exchanged. Only the source or modulation connection has to be replaced.
The averaged model does not require a modulation block, as the modulation is averaged. Therefore we can directly connect the source block to the inputs of the averaged inverter model. The averaged output voltage will follow the source input signals multiplied with the DC link voltage.
The averaged inverter will be an IGBT Inverter and is selected from the Components/Library/PowerConverters/Inverter section. On the left side is the connection to the DC link, on the right side is the three phase output of the inverter. The controlling signals to the Mosfet-gates are now inside the model and only on the left side you have to connect the averaged controlling signals.
The output voltage is simply a multiplication of the DC link voltage and the signals a,b and c at the input.
| \[ v_{out}= \begin{bmatrix} v_a\\v_b\\v_c \end{bmatrix} = V_{DC} \cdot \begin{bmatrix} a\\b\\c \end{bmatrix} \] |
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The parameters of the inverter include the DC link capacitor and its equivalent series resistance Esr. Also the initial capacitor voltage can be entered in this block. The IGBTs and Diodes inside the mode are parametrized via Vce and Rce for the IGBTs and Vd and Rd for the Diodes
Connect the source block to the input of the averaged inverter model and keep the amplitude of the sinusoidal signal equal to 1. In that way, the top value of the output voltage will equal \( V_{DC}/2\) and the top-top output voltage will equal \(V_{DC}\). The output voltage is relative to the DC link input voltage. Its amplitude will vary between \(+V_{DC} \) volt and the GND connection, mostly \( 0\) volt. Therefore the top-top value will be equal to \(V_{DC}\).
Since we are using an averaged model, we can increase the simulation step size dt. In the previous simulations we used 1μs but now it can be easily increased to 100μs.
Start the simulation by pressing the enter key or select Simulation/Start Simulation from the menu.
