Featured Project
# Controllable, User-Friendly 3-Phase Inverter
Team Members:
- Johnathan Vogt (jsvogt2)
- Shyam Peden (speden2)
- Alex Chirita (chirita2)
# Problem
While normal 3-phase AC power systems operate with consistent phase differences of 120 degrees, these systems are not always perfect. There may be occasions (fault conditions) where the power system becomes unbalanced. In order to test small machines under these conditions, one might want to create controllable AC waveforms with adjustable phase angles.
# Solution
We will create an inverter system that is capable of creating three AC waveforms with controllable phase angles. Phase A will serve as the reference 0 degree phase, while the B and C phases will be controllable with respect to this reference phase. This will be achieved using analog control, likely via potentiometers. The PCB will function as a normal 3-phase switching inverter, with switching control handled by the microcontroller, which takes input from analog signals to control the output AC waveforms.
There will be 5 main subsystems : input stage with a boost topology, 3 MOSFET H-bridges, Encoder, CONFIRM button and small OLED Display as the user interface, and a TI-C2000 microcontroller as it has enough PWM channels and high resolution timers, whose firmware will include the user input control, and the power control for the bridge
# Solution Components
- Microcontroller TI-C2000 F2800157SPN
- Rotary encoder PEC11R-4215F-S0024
- Button 320E11BLK
- Display NHD-0420CW-AB3
- Power FETS G18N20T
- Low side FET Drivers DGD0211CWT-7
- High side FET Drivers 1EDN7550BXTSA1
- Power capacitors (for bridges)
- Low resistance resistors for shunts (Voltage, current sensing)
- Connectors
- Potentiometers for precision voltage division
- Inductor cores
- Copper wire
- Linear voltage regulators for low-power IC’s (78xx Series)
## Subsystem 1: Boost Stage
The main purpose of this project is the inverter stage, while the input is some sort of DC that mimics a solar panel. We will not focus on it being powered from a solar panel during the semester and leave it as a modification/addition to the inverter part for further development. The input voltage needs to be converted to a setpoint and fed into a common DC bus. This will be done with a half bridge boost converter. A good quality of this system is the freedom to choose which variables to control. The circuit will be able to respond to quick changes in input voltage, but this semester we will be using a constant DC power supply instead of a solar panel to reduce cost and complexity. Therefore the boost converter will be run by a switching algorithm with a fixed input voltage. Later the algorithm could be changed to an MPPT conversion if needed.
## Subsystem 2: H-bridges
The bridge subsystem will contain three MOSFET H-bridges, each corresponding to one of the phases. Each of the phases will have a similar layout since the control is only achieved by the gate signal which is fully generated by the microcontroller. We decided to go with an H-bridge because it’s a good middle ground between multi-level bridges and a half-bridge. The H-bridge will allow us to generate a good quality sine wave when averaged out and filtered.The sine wave will be generated from -Vmax to +Vmax, and the sign will be decided by using a proper pair of MOSFETS from the 4 available. Each mosfet will have a corresponding low side or high side gate driver, which will receive their PWM control signals from the microcontroller. Each phase will have an LC low-pass filter at the end to reduce switching harmonics.
## Subsystem 3: User interface
The user interface will consist of a display, encoder, and a confirm button. The user will use the encoder and confirm button to navigate the user interface where the phase angle will be set for phase B and C in relation to phase A, which is static. Users can also choose to use an autoset where the microcontroller will default to 120 degrees between each of the phases.
## Subsystem 4: Input control
The microcontroller will run polling input from the button and encoders, run a loop which checks whether the phase is within bounds (-180 to +180 degrees with respect to phase A) and override the proper variables variables, which will be used by the switching subsystem as target phase angle.
## Subsystem 5: Switching control
The constant loop will check the phase angle variables and calculate the expected voltage for each phase. It will then generate the PWM for each phase that matches the needed Vrms. The output voltage which first went into the resistor divider to fit within maximum operating voltage of the microcontroller will be collected as samples over the wave period, Vrms calculated and compared to the set Vrms after which the switching signals for each phase will be adjusted.
# Criterion for success
Our device will be considered successful if we can accurately display a 3-phase network on the oscilloscope. Each phase has to have the same amplitude and frequency as well as have the same phase angle between each phase as set by the user. Across all 3 phases the inverter should be able to output 0.83A of current (~100W) and each phase should be able to handle 0.33A (~40W). The output RMS voltage is 120V.
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