Given data:
- Three-phase three pulse converter
- Input: Three-phase 400 V, 50Hz supply
- Load: R = 2 Ω, E = 200 V
- Load current: 20 A
- Source inductance: 2 mH

To find: The value of the overlap angle for inverter operation.

Explanation:
1. Converter Operation:
- The given three-phase three-pulse converter is a type of inverter circuit used for converting DC power into AC power.
- It consists of six thyristors connected in a three-phase bridge configuration.
- The converter operates by controlling the firing angle of the thyristors to regulate the output voltage and frequency.

2. Load Current:
- The load current is constant at 20 A, which means the converter needs to supply a constant current to the load.

3. Source Inductance:
- The source inductance of 2 mH is present to limit the rate of change of current and reduce harmonics.
- It helps in maintaining a more sinusoidal current waveform.

4. Overlap Angle:
- The overlap angle is the angle between the conduction period of one thyristor and the beginning of the conduction period of the next thyristor.
- It determines the amount of overlap between the conduction periods of the thyristors and affects the output waveform and distortion.

5. Calculation of Overlap Angle:
- To calculate the overlap angle, we need to consider the load current waveform and the firing angles of the thyristors.
- Since the load current is constant at 20 A, the waveform will be a square wave.
- The firing angles of the thyristors determine the conduction periods and the overlap angle.

6. Firing Angles:
- In a three-pulse converter, three thyristors conduct at a time, and the other three are non-conducting.
- Let the firing angles of the thyristors be α, β, and γ.
- The conduction periods of the thyristors can be represented as:
- Thyristor 1: 0 to α
- Thyristor 2: α to β
- Thyristor 3: β to γ
- Thyristor 4: γ to π
- Thyristor 5: π to π+α
- Thyristor 6: π+α to π+β

7. Calculation of Overlap Angle (Contd.):
- The overlap angle can be calculated by subtracting the ending firing angle of one thyristor from the starting firing angle of the next thyristor.
- In this case, we need to calculate the overlap angle between Thyristor 3 and Thyristor 4.
- The starting firing angle of Thyristor 3 is β, and the ending firing angle of Thyristor 4 is γ.
- Therefore, the overlap angle can be calculated as: γ - β.

8. Finding the Overlap Angle:
- To find the overlap angle, we need to consider the load voltage and the converter operation.
- The load voltage is given as E = 200 V.
- The converter operation is in inverter mode