Parallel Inverter

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This blog post summarize Parallel Inverter

Parallel Inverter using SCR


The inverter plays a vital role in Uninterrupted Power Supply (UPS). It is used to convert the direct current (DC) to alternating Current (AC) of required voltage.

ac-to-dc.jpg (338×174)

The single phase parallel inverter circuit consists of two SCRs T1 and T2, an inductor L, an output transformer and a commutating capacitor C. The output voltage and current are Vo and Io respectively. The function of L is to make the source current constant. During the working of this inverter, capacitor C comes in parallel with the load via the transformer. So it is called a parallel inverter. The operation of this inverter can be explained in the following modes.

Mode I

In this mode, SCR T1 is conducting and a current flow in the upper half of primary winding. SCR T2 is OFF. As a result an emf Vs is induced across upper as well as lower half of the primary winding.

In other words total voltage across primary winding is 2 Vs. Now the capacitor C charges to a voltage of 2Vs with upper plate as positive.

mode-1-parrallel-scr.jpg (537×440)

Mode II

At time to, T2 is turned ON by applying a trigger pulse to its gate. At this time t=0, capacitor voltage 2Vs appears as a reverse bias across T1, it is therefore turned OFF. A current Io begins to flow through T2 and lower half of primary winding. Now the capacitor has charged (upper plate as negative) from +2Vs to -2Vs at time t=t1. Load voltage also changes from Vs at t=0 to –Vs at t=t1.

Mode III

When capacitor has charged to –Vs, T1 may be tuned ON at any time . When T1 is triggered, capacitor voltage 2Vs applies a reverse bias across T2, it is therefore turned OFF. After T2 is OFF, capacitor starts discharging, and charged to the opposite direction, the upper plate as positive.

Lab Experiments


To construct and test the performance of single phase parallel inverter using SCR.

Apparatus Required

☞Trainer kit- Prallel inverter -1 No

☞Patch Cords – Sufficient Nos

☞CRO – 1 No

☞10:1 CRO Probe – 1 No

Circuit diagram for Parallel Inverter


Procedure of Parallel Inverter

CRO Settings

☞Time (X) Axis : 5ms/div

☞Voltage (Y) Axis : 20V/div

☞Probe : 1:10 CRO Probe is suitable one.

Initial Settings

☞Ensure the 230V Power Supply is in proper with Tester.

☞Ensure the CRO is working properly with probe checking and proper ground line axis.

Experiment Steps

☞The connections are made as per the circuit diagram given above.

☞When the load is Rheostat, then it must be in maximum position.

☞Turn on the SW2 and observe the waveform across between G1&K1 and G2&K2. And ensure it is like triggering pulse given in model graph.

☞Turn ON SW1 and observe the waveform across the R load.

☞Measure the X-axis time interval of ON time of Load Voltage and OFF Time of Load Voltage.

☞Adjust the value of Firing angle and note down the load voltage and frequency

☞Repeat the step 5.

☞Repeat the experiment for different values of α and note down Vo.

Note: If the connections are wrong the kit will get damage. Please switch off kit when not in use.

Model waveform

parallel-inverter-for-scr.jpg (600×287)

LOAD Output




Above experiment are done and results are verified.

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