Parallel Inverter: It’s Basics, Operation and waveform

parallel inverter

Inverter is an electronic circuit which converts DC power into AC power. The inverter circuit in which the commutating component C (capacitor) is connected in parallel with the load via transformer called a parallel inverter. This circuit is also called Push-pull inverter.

Parallel Inverter working is similar to the class B commutation. Parallel inverter has important role in Uninterrupted Power Supply (UPS). 

Parallel inverter circuit consist of two thyristor T1 and T2, a transformer, inductor L and a commutating component C. Capacitor (C) is connected in parallel with the load via transformer therefore it is called a parallel inverter. And inductor (L) is connected in series with supply to make the source current constant. Here we also use a center -tapped transformer. Centre tapping is done in the primary winding of transformer so, primary winding is divided into two equal halves ao and ob.

Circuit Diagram of Parallel Inverter

Operation of Parallel Inverter:

The operation is divided into four modes:

Mode I (0< t < t1): In this mode we give firing pulse to thyristor T1 and T1 get turned on and T2 is turned off.  Current flow from Supply Vs …. T1…. ao (upper half of primary winding) …. back to Vs. As a result, Vs voltage is induced across upper as well as lower half of the primary winding of transformer. And Vs voltage is induced in secondary winding.

Mode I – Operation where T1 ON and T2 OFF

So, output voltage across load is Vs.

So, the total voltage across primary winding is 2Vs. Here capacitor is connected in parallel with primary winding therefore capacitor charge with 2Vs voltage with upper plate is positive and lower plate is negative.

Mode II (t1< t < t3): In this duration we give firing pulse to thyristor T2 and T2 get turned on. At this time capacitor start discharging through T1 therefore T1 turned OFF. This time current flow from supply Vs …. T2…. bo (lower half of primary winding) …. back to Vs.

Mode II – Operation

Now this time capacitor charged with upper plate is negative, from +2Vs at t=t1 to -2Vs at t=t2.  Load voltage also changes from Vs at t=t1 to -Vs at t=t2. After t=t2 voltage across capacitor is maintain constant -2Vs between t= t2 to t3.

So, load voltage is also constant -Vs.

Mode III (t3< t < t4): In this mode again, we give firing pulse to thyristor T1 and T1 get turned on. At this time capacitor start discharging through T2 therefore T2 turned OFF. This time current flow from supply Vs …. T1…. ao (upper half of primary winding) …. back to Vs. So, the total voltage across primary winding is 2Vs.

Mode III – Operation

Now this time capacitor charged with upper plate is positive, from -2Vs at t=t3 to +2Vs at t=t4.  Load voltage also changes from Vs at t=t3 to -Vs at t=t4.

So, output voltage across load is Vs.

Now draw the waveform

Waveform of parallel Inverter 1) Ig1 is the gate current given to T1 2) Ig2 is the gate current given to T2. 3) Vc capacitor voltage 4) Ic current across capacitor 5) Vo output voltage waveform

Frequently Asked Questions (FAQ’s)

Why this circuit is called parallel inverter?

The inverter circuit in which the commutating component C (capacitor) is connected in parallel with the load via transformer called a parallel inverter. This circuit is also called Push-pull inverter.

What is the main difference between parallel inverter and series inverter?

One main difference between a series and a parallel inverter is that series inverters are connected one after another. Whereas, parallel converters are only connected individually. Second main difference between the two is that series inverters are used in small sub servers, whereas, parallel inverters are used in main servers.

what is series inverter?

The inverter circuit in which the commutating elements L and C are connected in series with the load to form an under damped circuit is called a series inverter. This circuit is also called load commutated or self-commutated inverter.

Series Inverter: It’s working, Operation and Waveform

Inverter is an electronic circuit which converts DC power into AC power. The inverter circuit in which the commutating elements L and C are connected in series with the load to form an under damped circuit is called a series inverter. This circuit is also called load commutated or self-commutated inverter.

This circuit is called load commutated inverter because the load component (L and C) is responsible to turned off the thyristor. It is called self-commutated inverter because in this circuit anode current itself become zero resulting the thyristor turned off.

The operation of series inverter is similar to Class A commutation.

Series inverter is operating at high frequency 200 Hz to 100 KHz. Therefore the size of the commutated component is small. Here, the value of L and C is choose in such a way the R, L & C forma underdamped circuit.

Circuit Diagram of Series Inverter

The Circuit Diagram of Series Inverter is shown in the figure. It consists of two thyristors (TI and T2). The thyristor T1 and T2 are turn on appropriately to get the output voltage of desired frequency. This circuit consist of L and C connected in series with load (R).

 Initially we considered that thyristor T2 is turned off and the polarity across capacitor is shown in figure.

Series Inverter Circuit

Operation of Series inverter

The whole operation is divide into three modes:

Mode-I ( T1 on and T2 off): In this mode we give firing pulse to thyristor T1 so, T1 get turned on and T2 thyristor is turned off initially. So, current flow from supply Vs…..T1……load……back to Vs.

The nature of the load current  is alternating due to under damped circuit. So, this time capacitor (C) start charging gradually from -Vs to its max voltage. This time inductor (L) also get charge.  When the load current becomes maximum the voltage across capacitor becomes + Vs. When the load current becomes zero at point a the voltage across capacitor becomes +2Vs. Then the load current becomes zero the thyristor T1 automatically turns off at point a. 

Mode- II (T1 and T2 both off): This time thyristor T1 turns off because the load current becomes zero from point a to b. In this time duration The thyristor T1 and  T2 are turned off and voltage across capacitor becomes equal to +2 Vs.

Mode III (T1 off and T2 on): In this mode we give firing pulse to thyristor T2. So, T2 get turned on. In this time capacitor start discharging its energy from +2Vs to – Vs through thyristor T2 and R – L circuit . Due to capacitor discharging reverse current flow across the load. Now at point C thyristor T2 turns off automatically due to load current  becomes zero. The thyristor T2 turns off during point C to D and thyristor T1 again turns on. In this way cycle repeat.

Now, we see in the waveform the time duration ab and cd is called as dead zone

Voltage and current Wave form of Series Inverter 1) Ig1 is the gate pulse of thyristor T1 2) Ig2 is the gate pulse of thyristor T2 3) Io (output current) 4) Vc (capacitor voltage) 5) VL (Load voltage)

Application of Series Inverter

Series Inverter is basically used in high frequency applications (200 Hz to 100 KHz) because it generate high frequency sinusoidal waveform.

  • It is used in Induction heating.
  • For Fluorescent lighting.
  • Used in Sonar transmitter.
  • Used in Ultrasonic generator.