Q21Basic Electrical Engineering

The performance and industrial application of a DC motor are strictly determined by three fundamental operating characteristics. These characteristics describe how the motor reacts dynamically to changes in electrical input and mechanical load.

1. Torque vs. Armature Current Characteristic (Electrical Characteristic): This curve plots the mechanical torque developed ($T_a$) against the armature current ($I_a$) drawn from the supply. Torque is governed by the equation $T\propto \Phi I_a$. - DC Shunt Motor: The field flux ($\Phi$) is virtually constant. Therefore, $T_a\propto I_a$. The characteristic is a straight line passing through the origin. It provides a moderate starting torque. - DC Series Motor: The field windings are in series with the armature, so $\Phi \propto I_a$. Therefore, before magnetic saturation, $T_a\propto I_a^2$. This parabolic relationship means series motors provide massive, unmatched starting torque, ideal for heavy electric trains and hoists.

2. Speed vs. Armature Current Characteristic: This curve plots the speed of rotation ($N$) against the armature current ($I_a$). Speed is governed by the equation $N\propto \frac{E_b}{\Phi }$. - DC Shunt Motor: As load ($I_a$) increases, the back EMF ($E_b$) drops slightly due to armature resistance drop ($I_a{R}_a$). Consequently, the speed drops only very slightly. It is considered a constant-speed motor, excellent for lathes and machine tools. - DC Series Motor: As load increases, $I_a$ increases, meaning flux ($\Phi$) also rapidly increases. Since $N\propto 1/\Phi$, the speed drops drastically (hyperbolically). Conversely, at no-load ($I_a\approx 0$), the flux is nearly zero, causing the speed to approach dangerously high, destructive levels. Series motors must NEVER be started without a mechanical load.

3. Speed vs. Torque Characteristic (Mechanical Characteristic): Derived from the first two, it plots Speed ($N$) against Torque ($T_a$). - DC Shunt Motor: The curve is almost perfectly flat, showing constant speed across varying torques. - DC Series Motor: Exhibits a highly drooping curve. High torque is only available at low speeds (starting), and high speed is achieved at low torque (running light).