Three Phase Inverter: 120 Degree Conduction Mode Explained

You will find the 120 degree conduction mode as a switching method used in a three phase inverter. In this mode, each switch conducts for 120 degrees of the cycle. This pattern shapes the output of the inverter and changes how the three phase ac supply behaves.

• Each switch works for only part of the cycle, which keeps two switches on at once.

• This strategy leads to a simple output pattern and helps reduce harmonic distortion in delta connected loads.

• The result is a smoother voltage waveform and better power quality.

Key Takeaways

• The 120 degree conduction mode lets each switch work for 120 degrees. This helps make the power better.

• Using this mode lowers harmonic distortion. This is good for delta-connected loads. It gives smoother voltage and helps equipment last longer.

• The switching pattern in 120 degree conduction mode is simple. This makes setup easy. It also keeps the output balanced for all phases.

• This mode works well for things like variable speed drives. It is also good for backup power systems. It gives steady performance for factories and other places.

• For more efficiency, you can use 180 degree conduction mode. It lets more switches work at the same time. This gives more power.

Three Phase Inverter Basics

Circuit Overview

A three phase inverter is used when you need to change DC into AC. It makes three different AC outputs. Each output is spaced 120 degrees from the others. This creates a three-phase AC supply. It works well for motors and machines in factories.

Tip: Three phase inverters work better than single-phase inverters. You can use them for motor drives, UPSs, and solar inverters that connect to the grid.

Here is a quick look at the main parts and what they do:

Component	Function
Three-phase inverter	Changes DC power into three AC waveforms. Each one is 120 degrees apart. This gives a three-phase AC output.

Most of these inverters use a full-bridge design. Freewheeling diodes are important when you use RL loads. These diodes help control the current and keep the switches safe.

• Often uses full-bridge design.

• Used a lot in motor drives, UPSs, and solar inverters for the grid.

• Freewheeling diodes are needed for RL loads.

Operating Modes

A three phase inverter can work in two main ways. Each way changes how the switches work and how the output looks.

• 120-Degree Conduction Mode: In this way, each switch is on for 120 degrees. Only two switches are on at the same time. The output pattern is simple. This way helps lower harmonics, especially with delta loads.

• 180-Degree Conduction Mode: In this way, each switch is on for 180 degrees. Three switches are on at once. This way uses the switches better. It is good for systems that need more power.

You can also use pulse width modulation to control the output voltage. This helps make the waveform better. It changes how long each switch stays on. This gives you more control over how the inverter works.

A three phase inverter is flexible and efficient. You can pick the mode that works best for your needs.

120 Degree Conduction Mode Operation

Switching Sequence

When you use a three phase inverter in 120 degree conduction mode, you follow a special switching pattern. Each switch in the inverter turns on for exactly 120 degrees of the electrical cycle. Only two switches conduct at any moment. This pattern repeats every cycle and creates the three-phase AC output.

You can break down the switching sequence into six clear steps. Each step lasts for 60 degrees. Here is how the sequence works:

1. Mode 1 (0° to 60°): Switches T1 and T6 turn on.

2. Mode 2 (60° to 120°): Switches T1 and T2 turn on.

3. Mode 3 (120° to 180°): Switches T2 and T3 turn on.

4. Mode 4 (180° to 240°): Switches T3 and T4 turn on.

5. Mode 5 (240° to 300°): Switches T4 and T5 turn on.

6. Mode 6 (300° to 360°): Switches T5 and T6 turn on.

Note: This six-step process repeats for every cycle. You always have two switches conducting, which keeps the output balanced.

This switching sequence helps you create three separate AC outputs, each spaced 120 degrees apart. The pattern also makes sure that the inverter works smoothly and delivers power to your load.

Conduction Intervals

In a three phase inverter, each switch conducts for 120 degrees out of the 360-degree cycle. You can see this by looking at the timing for each switch. For example, switch T1 turns on at 0 degrees and stays on until 120 degrees. After that, it turns off, and the next switch takes over. This pattern continues for all six switches.

You always have two switches on at the same time. This setup allows each switch to operate for one-third of the total period. During each interval, the inverter directs current through the load in a specific way. For example, from 0 to 60 degrees, switches T1 and T6 are active. This creates a certain voltage across the output terminals.

The six-step switching process shapes the output waveform. You get a stepped voltage pattern at the output. This pattern is sometimes called overmodulation. It can cause more distortion in the output waveform, especially if you use a simple switching method. However, the 120 degree conduction mode helps reduce some harmonics, which improves the power quality for many loads.

Tip: If you use a delta-connected load, this conduction mode can help you get smoother voltage and lower harmonic content.

The way you control the switches in the inverter affects the voltage and current that reach your load. By following the 120 degree conduction mode, you make sure that the three phase inverter delivers a balanced and reliable AC supply. This method works well for many industrial and commercial applications.

Waveforms in Three Phase Inverter Circuit

Output Voltage

When you check the output voltage in a three phase inverter using 120 degree conduction mode, you see a repeating pattern. Each phase-to-neutral voltage stays at one value for 120 degrees. Then, it drops to zero for the next 60 degrees. This happens for every phase, but each phase starts at a different time. The phases are spaced 120 degrees apart. The phase-to-phase voltages change in steps every 60 degrees. These steps make the three-phase AC output that machines need.

The voltage waveforms do not look smooth like sine waves. They have flat parts and sudden jumps. This happens because the inverter switches turn on and off in steps.

Here is a simple table to show the voltage pattern:

Phase	Voltage Duration	Zero Duration	Phase Shift
A	120°	60°	0°
B	120°	60°	120°
C	120°	60°	240°

This pattern helps the inverter give balanced power to your load.

Output Current

The output current in a three phase inverter also has a special pattern. Each switch conducts for 120 degrees. This makes a six-step current shape in each phase. Two switches are always on together. This keeps the output steady and balanced.

You get steady current because the phase voltages match the line voltages. This setup lowers total harmonic distortion compared to single-phase inverters. The current stays controlled, which helps loads like motors.

If you use a delta-connected load, the six-step current shape works even better. It helps cut down unwanted harmonics and keeps your machines running well.

Harmonic Distortion and Power Quality

The stepped voltage and current shapes in the inverter do make some harmonics. But the 120 degree conduction mode helps lower these effects, especially for delta-connected loads. You get better power quality and less chance of overheating or vibration in your machines.

• Lower total harmonic distortion means your equipment lasts longer.

• Smoother current flow protects sensitive devices.

• Balanced output keeps your system working well.

Using a three phase inverter with this conduction mode makes your power supply more reliable and better quality.

Features & Comparison

120 vs 180 Degree Modes

A three phase inverter can use two main ways to control switches. These are called 120 degree conduction mode and 180 degree conduction mode. Each way changes how the inverter works and how much power it gives.

In 120 degree conduction mode, only two load terminals connect to the input supply at a time. The third terminal is not connected during this time. This setup means the inverter does not use all its power. If you use the same load for both modes, the 180 degree conduction mode works better. It gives you more power because three switches are on at once. The current also flows more smoothly in this mode.

You can look at the table below to see the main differences:

Conduction Mode	Efficiency	Performance
120-degree	Less efficient	Underutilization of inverter capacity
180-degree	More efficient	Better performance and continuous conduction

The 180 degree conduction mode lets the inverter send more power to your load. The current moves through the circuit without stopping. This gives better results, especially for big machines or motors.

Note: If you want your inverter to work its best, 180 degree conduction mode is usually the better choice for saving energy.

Suitability for Delta Loads

You might wonder why people still use 120 degree conduction mode. The answer depends on the type of load you connect to the inverter. If you use a delta-connected load, the 120 degree conduction mode has some good points.

Delta loads work well with the stepped voltage from the inverter. The six-step output fits what many factory machines need. The 120 degree conduction mode helps lower some unwanted harmonics in the output. This means your motors and machines run more smoothly and stay cooler.

• You get less harmonic distortion with delta loads.

• The inverter gives balanced output for each phase.

• Your machines last longer because they get cleaner power.

If you use a three phase inverter for delta-connected motors or other machines, the 120 degree conduction mode can give you better power quality. You may not get the most efficiency, but your machines work smoother and last longer.

Tip: Always pick the conduction mode that matches your load. This helps you get the best results from your three phase inverter.

Applications & Pros/Cons

Advantages

There are many good things about using 120 degree conduction mode in a three phase inverter. This way is easy to use and control. You can set up the switches without much trouble. The output has less unwanted noise, so machines work better.

• Easy switching pattern makes setup simple.

• Less unwanted noise means better power quality.

• Balanced output helps keep equipment safe.

• Works well with delta-connected loads.

This conduction mode gives you cleaner power and helps machines last longer.

Disadvantages

But there are also some problems with this method. The inverter does not use all its power. Only two switches are on at once, so it is not very efficient. The output voltage changes in steps, not smooth waves. This can be bad for some sensitive devices.

Disadvantage	Impact on System
Not very efficient	Less power goes to the load
Stepped output voltage	Can bother sensitive equipment
Not good for every load	Some loads need smoother voltage

The way you pick the conduction mode can change how reliable the system is. If the resistance in the inverter changes, you might see uneven voltage and more unwanted noise. This can make the inverter less steady, especially if something goes wrong.

Typical Uses

You can use 120 degree conduction mode in many real-life cases. It works well in three phase inverters for variable speed drives. It is also used in big power jobs, like high voltage direct current transmission. Many backup power systems use this mode. Some cheap inverter charger circuits use it too.

• Variable speed drive systems

• Big power jobs like HVDC transmission

• Backup power systems (UPS)

• Cheap inverter charger circuits

If you want a simple and steady inverter for your factory or backup, this conduction mode is a good pick.

You can make your power system better with 120 degree conduction mode in a three phase inverter. This way gives smoother voltage and better power quality. It works best for delta-connected loads. Picking the right conduction mode helps your inverter work well. It also makes sure it fits what you need. Always check your load type before choosing which mode to use.

 

 

 

 

 

Written by Jack Elliott from AIChipLink.

 

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