Question

Explain with sketch the operation of single phase fully controlled midpoint configuration with R load.

Answer 1

Sure! A single-phase fully controlled midpoint configuration is a type of converter circuit used to convert alternating current (AC) to direct current (DC). This circuit is particularly useful in applications where controlled rectification is needed. Let's break down how this circuit operates, especially when connected to a resistive (R) load.

### Components of the Circuit

1. **AC Supply**: A single-phase AC voltage source (usually sinusoidal).
2. **Thyristors**: Four thyristors (SCRs) connected in a bridge configuration. The midpoint configuration typically uses two thyristors in each half of the AC cycle.
3. **Load**: A resistive load (R) connected across the output.
4. **Diode**: Sometimes, diodes may also be included for protection against reverse voltage.

### Configuration Sketch

Here’s a simplified sketch of the circuit:

```
            +--------+
 AC Supply   |        |
  ----------|   SCR1  |----o---- R Load
   ~        |        |
            +--------+
            |        |
            |        |
  ----------|   SCR2  |----o
            |        |
            +--------+
            |        |
            |        |
  ----------|   SCR3  |----o
            |        |
            +--------+
            |        |
            |        |
  ----------|   SCR4  |----o
            |        |
            +--------+
```

### Operation of the Circuit

1. **Firing of Thyristors**:
   - Thyristors are semiconductor devices that can control the flow of current. They remain off until a gate signal is applied.
   - In a fully controlled midpoint configuration, you can control the firing angle of the thyristors (α), which determines when they are turned on in each AC cycle.

2. **AC Input Waveform**:
   - The input to the circuit is a sinusoidal AC waveform. The voltage alternates between positive and negative values.
   - The circuit turns the AC waveform into a pulsating DC output.

3. **Conduction of Thyristors**:
   - In the positive half cycle of the AC input (0 to π), SCR1 and SCR2 are triggered (fired) at an angle α.
   - Once fired, these SCRs conduct current, allowing the positive half of the AC waveform to pass through to the load.
   - In the negative half cycle (π to 2π), SCR3 and SCR4 are triggered at the same angle α.
   - These SCRs then conduct the negative half of the AC waveform to the load.

4. **Output Voltage**:
   - The output voltage across the load is determined by the firing angle α.
   - When α = 0° (immediate conduction), the output voltage is at its maximum, resembling a full-wave rectified output.
   - As α increases towards 90°, the output voltage decreases and the DC voltage becomes lower. This means that the average DC output voltage can be controlled by varying α.

### Average Output Voltage Calculation

The average output voltage (\( V_{avg} \)) across the load can be calculated as:

\[
V_{avg} = \frac{2V_m}{\pi} \cos(\alpha)
\]

Where:
- \( V_m \) = Peak voltage of the AC supply
- \( \alpha \) = Firing angle

### Summary of Operation

- **Control**: The key advantage of a fully controlled midpoint converter is its ability to control the average output voltage by varying the firing angle of the thyristors.
- **Pulsating Output**: The output is a pulsating DC voltage which can be further smoothed with filters if necessary.
- **R Load Behavior**: With a resistive load, the current through the load varies according to the output voltage, following Ohm’s Law (\( I = \frac{V_{avg}}{R} \)).

### Applications

- This type of converter is commonly used in applications requiring adjustable DC power, such as motor speed control, heating control, and various industrial power supplies.

By understanding this configuration, you can appreciate how power electronics manipulate AC voltage and current to achieve desired output characteristics efficiently. If you have any questions or need further details on specific aspects, feel free to ask!

Answer 2

A single-phase fully controlled midpoint configuration with an R (resistive) load is a type of phase-controlled rectifier circuit that uses controlled devices, typically thyristors, to convert AC power to DC power. This configuration is often used in applications requiring precise control of power delivery to a load.

### Operation of Single-Phase Fully Controlled Midpoint Configuration

#### Circuit Configuration

1. **Transformer**: The circuit begins with a single-phase transformer. It steps down or steps up the voltage as required and provides isolation between the AC source and the rectifier.

2. **Thyristors (SCRs)**: There are two thyristors (SCR1 and SCR2) used in this configuration. These are placed in series with the load across the secondary of the transformer.

3. **Load (R)**: The resistive load is connected across the midpoint of the transformer secondary and the junctions of the thyristors.

4. **Gate Control Circuit**: Each thyristor is controlled by a gate signal that determines when the thyristor will start conducting.

#### Working Principle

1. **AC Input**: The transformer provides an AC voltage to the midpoint configuration. The secondary of the transformer has a center tap, creating two equal halves of AC voltage.

2. **Thyristor Triggering**: The thyristors are triggered into conduction by applying a gate pulse. This pulse allows the thyristor to conduct when it is forward-biased.

3. **Conduction Phase**:
   - **Positive Half-Cycle**: During the positive half-cycle of the AC input, SCR1 conducts, allowing current to flow through the load and SCR1 to the midpoint of the transformer. SCR2 remains off.
   - **Negative Half-Cycle**: During the negative half-cycle, SCR2 conducts, allowing current to flow through the load and SCR2 to the midpoint. SCR1 remains off.

4. **Controlling Power**: The power delivered to the resistive load can be controlled by varying the firing angle of the thyristors. This is the phase angle at which the thyristor is triggered to start conduction. By adjusting the firing angle, the average DC output voltage to the load can be regulated.

5. **Load Voltage**: The output voltage across the load is a rectified version of the AC input voltage, with the waveform shape determined by the firing angle of the thyristors.

#### Sketch

Here is a simplified sketch of the single-phase fully controlled midpoint rectifier circuit:

```
        +-------SCR1-------+-----R Load-----+-------SCR2-------+
        |                   |                           |                     |
        |                   |                           |                     |
      (AC)              (Transformer)               (AC)             (Center Tap)
        |                   |                           |                     |
        |                  (Secondary)                |                     |
        |                   |                           |                     |
       +--|>|--+            +-------------+            +--|<|--+            |
        SCR1     |            |    Midpoint   |            |     SCR2  |            |
                |             +-------------+            |                    |
                |             |             |            |                    |
                |             |             |            |                    |
                |             |             |            |                    |
                |             |             |            |                    |
                |             |             |            |                    |
                +-------------+-------------+------------+-------------+
```

**Explanation of the Sketch**:
- The transformer has a center tap, providing two ends of the secondary winding.
- SCR1 is connected from one end of the secondary to the load.
- SCR2 is connected from the other end of the secondary to the load.
- The midpoint of the transformer secondary is connected to the other terminal of the load.
- The SCRs control the flow of current during different halves of the AC cycle.

#### Key Points
- The firing angle control allows for precise regulation of the output DC voltage.
- The use of thyristors enables full control of the rectification process, as opposed to uncontrolled rectifiers.
- The midpoint configuration ensures that the load receives a smooth DC output with controlled ripple.

This configuration is useful in various industrial applications where accurate control of power is essential, such as motor speed controls, light dimmers, and heating systems.