DC motors can be controlled using various methods, and one of those methods is field control. Field control involves changing the strength of the magnetic field within the motor to control its speed and torque. This is usually achieved by controlling the current flowing through the field winding of the motor.
There are two main types of field control methods for DC motors: armature resistance control and field rheostat control.
Armature Resistance Control:
In this method, the armature resistance of the DC motor is changed to control its speed. By adding external resistance in series with the armature circuit, the effective voltage across the armature terminals is reduced, leading to a decrease in speed. Conversely, reducing the external resistance increases the effective voltage and increases the speed of the motor. This method is simple and widely used, but it's not very efficient as it causes energy losses in the form of heat in the resistors.
Field Rheostat Control:
This method involves changing the field current of the DC motor using a field rheostat (variable resistor). By adjusting the field current, the magnetic field strength can be controlled, which in turn affects the motor's speed and torque. Increasing the field current strengthens the magnetic field, leading to higher torque and lower speed. Decreasing the field current weakens the field, resulting in higher speed and lower torque. This method is more efficient than armature resistance control as it doesn't waste as much energy in resistive losses.
It's important to note that both of these field control methods have their limitations. They are primarily used in applications where smooth speed control over a wide range isn't critical, and where the motor operates at relatively constant loads.
Modern motor control techniques have advanced significantly with the advent of electronic devices and microcontrollers. Pulse-width modulation (PWM) control and various closed-loop control strategies like PID control are commonly used to achieve more precise and efficient speed control of DC motors. These methods allow for smooth and accurate control, better energy efficiency, and reduced wear and tear on the motor components.