Working of 3-Phase Inverter

The below circuit is a three phase inverter , designed to convert a direct current(DC) input into a three-phase alternating current(AC) output. In this configuration , three separate single-phase inverter switches are utilized , with each switch being connected to three load terminals simultaneously . To generate a three-phase AC supply , the inverter operates with a 120-degree phase shift between its three arms .This means that each switch in the circuit is turned on and off in a synchronized manner , creating a balanced AC output efficiency , the three-phase inverters are often connected to a single fuse and share the same DC power source .This arrangement simplifies the circuit’s control and protection mechanisms.

In a three-phase inverter , the pole voltage , which represents the voltage applied to the load , is equivalent to the pole voltage in a half-phase inverter used in single-phase applications . However in three-phase inverters , this voltage is distributed across three phases to create a balanced three-phase AC output . There are two primary conduction modes in both single-phase and three-phase inverters i.e.. 120-degree conduction mode and the 180-degree conduction mode. These modes refer to the timing and duration of the switching of the the inverter switches . in 120 degree mode , each switch conducts for 120 degrees of the electrical cycle , while in the 180-degree mode, conduction lasts for 180 degrees. These modes allow for different levels of control and output waveform quality in the inverter operation.

Modes of Conduction in 3-Phase Inverter

There are basically two modes of conduction :

• 180 degree conduction mode
• 120 degree conduction mode

180 Degree Conduction Mode

In this conduction mode each device conducts for 180 degrees with activation occurring at 60 degree intervals . The output terminals labeled D, E and F are connected in either star or a three phase delta configuration to the load . this balanced load arrangement is illustrated in the diagram.

During 0 to 60 degree phase angle range , switches S1 , S5 and S6 are in conduction mode . In this period , terminals D and F are connected to the positive point of the power source , while terminal E is linked to the source’s negative point . Additionally , there is an R/2 resistance between the two ends of the neutral and the positive terminals , while on R resistance is present between the neutral and the negative terminal.

This configuration ensures that the three phase inverter operates with a specific switching pattern , producing a balanced output across its terminals while controlling the flow of current through the load . Such precise control is essential for various applications , including motor drives and power systems , where maintaining balanced and controlled three phase ac power is crucial.

Load voltages are given as ,

VDN= V/3

VEN= -2V/3

VFN= V/3

Line voltages are ,

VDE = VDN – VEN = v

VEF = VEN – VFN= -v

VFD= VFN- VDN = 0

120 Degree Conduction Mode

In this conduction mode , each electronic device operates for 120 degrees , making it suitable for a delta connected load . This mode generates a six step waveform across one of its phases , where only specific devices conduct at any given instant , precisely at 120 degree intervals. In the load connection , the ‘D’ terminal connects to the positive end of the source , while the ‘E’ terminal links to the source negative terminal . The ‘F’ terminal remains in a floating state , meaning it is not connected to either the positive or negative side of the source .The phase voltages across the load correspond to the voltages , providing a controlled and balanced output across the phases.

phase voltages are equal to line voltages.

VDE = V

VEF = -V/2

VFD = – V

From the above waveform :

• From 0-60 : s1 and s6 are in conduction while remaining switches are opened.
• From 60 – 120 : s1 and s2 are in conduction while remaining switches are opened.
• From 120 – 180 : s3 and s2 are in conduction while remaining switches are opened .
• From 180 – 240 : s3 and s4 are in conduction while remaining switches are opened.
• From 240 – 300 : s5 and s4 are in conduction while remaining switches are opened.
• From 300 – 360 : s5 and s6 are in conduction while remaining switches are opened.

An inverter is a fundamental electrical device designed primarily for the conversion of direct current into alternating current . This versatile device , also known as a variable frequency drive , plays a vital role in a wide range of applications , including variable frequency drives and high power scenarios such as high voltage direct current (HVDC) power transmission. Its primary function is to control the torque and speed of electrical motors , making a vital role in many industrial and commercial settings. The versatility of inverters extends to their role in HVDC power transmission, where they are crucial in converting the DC power generated in power plants or offshore wind farms into AC power suitable for long transmission .

In this context , inverters help minimize energy losses and maximize the efficiency of electricity distribution over extensive distances. Here we will discuss about circuit design and working of inverter , types of inverters , advantages , limitations and applications of inverters .