In a DC shunt motor, the speed can be controlled using field rheostatic control by varying the field current. The field rheostat is a variable resistor connected in series with the field winding of the motor. By changing the resistance of the field rheostat, the field current can be adjusted, which in turn affects the motor's speed.
Here's how the process works:
Increasing Field Current (Decreasing Resistance): When the resistance of the field rheostat is decreased, more current flows through the field winding. This increases the strength of the magnetic field produced by the field winding. According to the motor's speed equation (N ∝ (V - IaRa) / Φ), where N is the speed, V is the supply voltage, Ia is the armature current, Ra is the armature resistance, and Φ is the flux produced by the field winding, an increase in field current (Φ) will result in a decrease in speed, assuming other parameters are constant.
Decreasing Field Current (Increasing Resistance): Conversely, when the resistance of the field rheostat is increased, less current flows through the field winding, reducing the strength of the magnetic field. As a result, the motor's speed increases.
It's important to note that field rheostatic control is not the most efficient method of controlling the speed of a DC motor, as it wastes energy in the rheostat resistor. Additionally, the torque capability of the motor might be affected at low speeds due to the weakening of the field.
Modern motor control techniques often utilize more efficient and precise methods, such as armature voltage control (also known as armature rheostatic control), pulse-width modulation (PWM) techniques, or even closed-loop control using microcontrollers and feedback mechanisms to achieve accurate speed control without significant energy losses.