Armature voltage control works for speeds below base speed and field resistance control works well for speeds above base speed.

Armature Voltage Control

Armature voltage control is a method used to control the speed of a DC motor by varying the voltage applied to the armature. This method is effective for speeds below the base speed of the motor. Here's how it works:

1. Armature voltage control is achieved by using a variable DC voltage source, such as a chopper or a thyristor-based controller.

2. By increasing or decreasing the armature voltage, the back EMF of the motor can be modified. The back EMF is directly proportional to the speed of the motor.

3. When the armature voltage is reduced, the back EMF decreases, allowing more current to flow through the armature. This increases the torque and speed of the motor.

4. Conversely, when the armature voltage is increased, the back EMF increases, reducing the armature current and thus decreasing the torque and speed of the motor.

5. Armature voltage control is suitable for speeds below the base speed because at lower speeds, the armature reaction and voltage drop across the armature resistance are relatively small. Therefore, the control is more effective in this range.

Field Resistance Control

Field resistance control is another method used to control the speed of a DC motor. It involves varying the resistance in the field circuit of the motor. This method is effective for speeds above the base speed of the motor. Here's how it works:

1. Field resistance control is achieved by using a variable resistor connected in series with the field winding of the motor.

2. By increasing or decreasing the field resistance, the flux produced by the field winding can be modified. The flux is directly proportional to the speed of the motor.

3. When the field resistance is increased, the field current and thus the flux decreases. This reduces the back EMF and allows more armature current to flow, increasing the torque and speed of the motor.

4. Conversely, when the field resistance is decreased, the field current and flux increase. This increases the back EMF and reduces the armature current, decreasing the torque and speed of the motor.

5. Field resistance control is suitable for speeds above the base speed because at higher speeds, the armature reaction and voltage drop across the armature resistance become significant. Therefore, controlling the field resistance becomes more effective in this range.

In conclusion, armature voltage control is effective for speeds below the base speed of the motor, while field resistance control works well for speeds above the base speed. These control methods allow for precise speed regulation and are commonly used in various industrial applications.