Question

Describe with a suitable diagram speed control of a d.c shunt motor by field current control method.

Answer 1

Controlling the speed of a DC shunt motor can be effectively achieved through the field current control method. In this approach, the speed is regulated by adjusting the current flowing through the motor's field winding, which alters the magnetic field strength. Let’s break down the process and include a suitable diagram to enhance understanding.

### Concept of DC Shunt Motor

A DC shunt motor consists of two main windings:

1. **Armature Winding (A)**: Located on the rotor, responsible for producing torque.
2. **Field Winding (F)**: Connected in parallel (shunt) with the armature winding, creates a magnetic field.

### Working Principle

The speed of a DC motor is inversely proportional to the field flux (φ). The basic equation governing the speed (N) of a DC shunt motor is given by:

\[
N \propto \frac{V - I_aR_a}{φ}
\]

Where:
- \(N\) = Speed of the motor (RPM)
- \(V\) = Supply voltage
- \(I_a\) = Armature current
- \(R_a\) = Armature resistance
- \(φ\) = Field flux

By reducing the field current, we decrease the field flux \(φ\), which results in an increase in speed \(N\).

### Field Current Control Method

1. **Adjusting the Field Resistance**: By adding a variable resistor (rheostat) in series with the field winding, the field current can be controlled.
2. **Increasing Speed**: When the resistance is increased, the field current decreases, reducing the magnetic field strength (flux). This leads to an increase in motor speed.
3. **Decreasing Speed**: Conversely, reducing the resistance increases the field current, enhancing the magnetic field strength, and thus decreasing the motor speed.

### Diagram

Here’s a simplified diagram illustrating the speed control of a DC shunt motor through field current control:

```
          +-----------------------+
          |                       |
          |                       |
        ----                     ----
       |    |                   |    |
       | R  |                   | R  |
       |    |                   |    |
        ----                     ----
          |                       |
          |                       |
          +---- Field Winding ---+
          |                       |
          |                       |
          |                       |
          |                       |
          |                       |
        ----                     ----
       |    |                   |    |
       |    |                   |    |
       | A  |                   | A  |
       |    |                   |    |
        ----                     ----
          |                       |
          |                       |
          +---------- Supply ----+
```

### Explanation of Components

- **R (Variable Resistor/Rheostat)**: Used to adjust the field current. Increasing resistance reduces field current, decreasing magnetic flux.
- **Field Winding**: Connected in parallel with the armature winding, producing a magnetic field necessary for motor operation.
- **Armature (A)**: The rotating part of the motor where mechanical output is derived.

### Advantages of Field Current Control

- **Simplicity**: The method is straightforward and easy to implement.
- **Smooth Control**: Allows for smooth variation of speed over a wide range.
- **Efficiency**: Generally efficient at lower speeds.

### Disadvantages

- **Limited Speed Range**: Not suitable for extremely high-speed operations.
- **Heat Generation**: The variable resistor may dissipate heat, requiring careful management.

### Conclusion

The field current control method for speed control of a DC shunt motor is a practical and effective technique that balances simplicity and functionality. By varying the field current, one can achieve a wide range of motor speeds while understanding the fundamental relationship between field strength and motor performance.

Answer 2

To explain speed control of a DC shunt motor using the field current control method, let me start with a brief overview. In this method, the speed of the motor is controlled by varying the field current, which in turn affects the magnetic flux in the motor.

### Basic Concept
- **DC Shunt Motor**: A DC motor where the field windings are connected in parallel (shunt) with the armature winding.
- **Field Current Control**: Adjusting the field current changes the flux, which impacts the motor's speed.

### Diagram Description
1. **DC Shunt Motor**: Draw a motor with two windings: the armature winding and the field winding.
2. **Field Winding**: Connected in parallel with the armature winding.
3. **Power Supply**: A DC voltage source connected to both the armature and field windings.
4. **Field Current Controller**: A variable resistor (rheostat) placed in series with the field winding to adjust the field current.
5. **Speed Control**: The motor’s speed is inversely proportional to the field flux. By increasing the field resistance, the field current decreases, reducing the flux and increasing the speed. Conversely, decreasing the field resistance increases the flux and reduces the speed.

### Diagram
```
+V ---- (Armature) -----> |  Motor  | -----> -V
                            |
                          -----
                          |     |
                        (Field) (Rheostat)
                          |     |
                          -----
                           |
                          ---
                          ---
                           |
                          ----
                          GND
```

### Explanation
- **Increasing Field Resistance**: Reduces the field current, decreases the flux, and thus increases the speed.
- **Decreasing Field Resistance**: Increases the field current, enhances the flux, and thus decreases the speed.

This control method is straightforward but less efficient compared to armature control methods.