Q15Basic Electrical Engineering

The performance and suitability of a DC motor for specific industrial applications are analyzed through three fundamental operating characteristics. These characteristics describe how the motor reacts to varying electrical and mechanical loads.

• 1. Torque vs. Armature Current ($T_a/I_a$): Also known as the electrical characteristic. It shows how the mechanical torque developed by the armature varies with the current it draws. For a DC Shunt motor, flux is constant, so torque is directly proportional to current (linear). For a DC Series motor, flux is proportional to current, making torque proportional to the square of the current ($T\propto I_a^2$), resulting in a parabolic curve and extremely high starting torque.
• 2. Speed vs. Armature Current ($N/I_a$): As the mechanical load on the motor shaft increases, the motor draws more armature current. This characteristic shows how the speed drops in response. A Shunt motor experiences a very slight drop, acting as a nearly constant-speed machine. A Series motor experiences a rapid, hyperbolic drop in speed as current increases; at no load (low current), its speed can become dangerously high.
• 3. Speed vs. Torque ($N/T_a$): Also known as the mechanical characteristic. Derived from the first two, it directly shows the speed at which the motor runs for a given required load torque. Shunt motors show a flat profile, making them ideal for lathes or machine tools. Series motors show a steep decline, making them ideal for heavy traction systems like trains or cranes where high torque at low speeds is essential.