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Single Phase Inverter



An inverter is an electrical device that transforms a DC input to an AC output at a selected voltage and frequency, a process called DC to AC conversion. For example, inverters are heavily used in the interface between solar cells and the electrical grid, where DC power generated from the solar cell must be converted to AC in order to be compatible with the grid. They are also essential in uninterruptible power supplies which store energy in a battery, but must produce 120 Volt 60 hertz power for computers. An inverter operates by chopping its DC input into a series of pulses to create an oscillating wave. Depending on the amount of filtering, the output may be a square wave, a pseudo-sine wave, or a sine wave. This video will introduce the basic principals of a simple inverter and demonstrate its operation in a simple circuit.

The input of an inverter is a constant DC voltage. An inverter circuit includes electronic switches such as metal oxide field effect transistors, insulated gate bipolar transistors, or silicon controlled rectifiers under the control of a clock or frequency generator. When the clock signal turns on a switch, the DC input is chopped, or its polarity is flipped. This process is called commutation. Repeated chopping creates a series of pulses or square waves. Because the clock period determines the pulse rate, changing the inverter's control frequency changes the output frequency accordingly. A type of switching called pulse width modulation produces a stream of pulses with varying widths that can be filtered to approximate a sine wave. Pulse width modulation is desirable because machines and electrical equipment often require power with sinusoidally varying voltage to operate properly. For the many inverter topologies, such as H-bridge, three phase and multi-level inverters, the half-bridge inverter is a fundamental building block. The half-bridge inverter in this simplified diagram applies its DC supply V in across two identical capacitors in series, which act as a voltage divider. Because the capacitors have the same value, they have the same voltage across their terminals and the node between them is at V in/2. This point is the AC ground for the load. The half-bridge inverter uses two switches in series and two non-overlapping or out-of-phase clocks to alternately connect the node between them to V in and zero Volts. To avoid a short circuit of the DC power one switch must turn off before the other one turns on. The load is connected from the point between the two switches to the point between the two capacitors. When switch A is on and switch B is off, the load is connected to V in and has a positive voltage of 1/2 V in across it, relative to the AC ground. When switch A is off and switch B is on, the load is connected to zero Volts and has a negative voltage of 1/2 V in across it relative to the AC ground. As this switching process repeats the load alternately has positive and negative voltage across it with amplitude of 1/2 V in. In this simple case, the AC power is a square wave. Now that the basics of a single-phase inverter have been explained, let's demonstrate the device by building a DC to AC half-bridge inverter with square wave switching, and then observe its operation.

First, configure two-function generators to produce 10 kilohertz square waves oscillating from 0 to 10 Volts with a 48% duty cycle. Synchronize the outputs to be 180 degrees out of phase with each other. Each function generator independently controls one of the two field effect transistor switches of the half-bridge inverter. The square wave turns the transistor on when the output is high and turns it off when the output is low or zero Volts. Because the duty cycle is 48%, the remaining 2% of the period is dead time between the on states of the two transistors. During this time the outputs of both signal generators are low, preventing the transistors from conducting simultaneously and avoiding a short circuit of the DC supply. Connect one channel of an oscilloscope to the output of each function generator. Then confirm that the square waves have the expected amplitude, frequency and duty cycle. The two square waves must also have opposite phases so one is high while the other is low. Capture the scope screen for later reference. Turn off the function generator outputs but leave the generators on. Finally, set the DC power supply to positive 15 Volts but do not connect it to any circuitry, then turn it off.

Build the half-bridge inverter circuit and use a 51 ohm resistor for the load resistance, R load. With the DC power supply turned off, connect its output to inverter input VDC. Connect a differential probe across R load to measure V out, then connect a regular scope probe between high out, which is pin seven, and ground. Set the scope scaling to 10x and the probe scaling to 20x. Scale all measurements accordingly. Record the scaling from the probe and oscilloscope in order to account for missing factors later on. Connect one function generator's output to High in, which is pin 10, and controls switching of the upper transistor. Connect the function generator's ground to the common ground of the circuit. Connect the other function generator's output to Low in, which is pin 12, and controls switching of the lower transistor. Connect the other function generator's ground to the common ground of the circuit. Capture the wave forms at High out and V out and measure the output voltage, amplitude and frequency. Record the current and voltage readings on the DC power supply. Repeat the measurements with an input frequency of five kilohertz and observe the difference in the output AC wave form. Finally, turn off the DC power supply and disconnect the function generators from the circuit.

The output voltage of this half-bridge inverter is a square-wave with an amplitude of 1/2 VDC and some dead time causing the output voltage to be zero for around 4% of the switching period. Square-wave inverters have high total harmonic distortion and are rarely used in real applications. However, they are the building blocks of many more advanced inverters with better switching schemes, such as sinusoidal pulse width modulation. These more sophisticated methods not only reduce the total harmonic distortion, but also ease filtering requirements for undesired harmonics in the AC output voltage.

Inverters are commonly used in the interface between available DC power and AC applications equipment and machinery. Large rays of solar cells are now producing power in many areas and contribute to the local electrical grid. Solar cells produce DC power however, and inverters are used to transform it to AC power with the proper voltage and frequency for the grid. Many machines use AC power, but not at the fixed 120 Volt RMS and 60 hertz frequency of the main supply. The rotor speed of an induction motor, for example, depends on the frequency of the current driving it. Variable frequency drives use AC to DC conversion to generate internal DC power. Inverters in turn use this DC power to generate AC power with adjustable voltage and frequency, which enables control of the induction motor's speed and torque.

You've just watched Jove's introduction to single-phase inverters. You should now understand the basics of DC to AC conversion and how the frequency of the AC output can be adjusted by changing the switching frequency. Thanks for watching.

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