Question 1

Why is the DC motor speed-torque characteristic linear?

Accepted Answer

Combining the armature circuit equation V_a = E + I_a×R_a, back-EMF E = K_e×ω, and torque T = K_T×I_a yields the linear relationship ω = V_a/K_e − (R_a/(K_e×K_T))×T. The no-load speed is ω_0 = V_a/K_e and the stall torque is T_s = K_T×V_a/R_a. This linear characteristic is the signature of PMDC and separately-excited DC motors.

Question 2

How does a series DC motor differ from a shunt motor?

Accepted Answer

In a series motor, the field current equals the armature current, so flux φ ∝ I_a and torque T ∝ I_a². This gives very high starting torque at low speed but theoretically infinite no-load speed (dangerous — never run unloaded). Speed varies greatly with load. Series motors are used in cranes, traction, and high-torque starting applications.

Question 3

What is field weakening in DC motor drives?

Accepted Answer

Above rated speed, the armature voltage is at its ceiling. By reducing field flux φ, the back-EMF E = K_e×φ×ω is lowered, allowing higher speed. Torque capacity drops proportionally to flux, but output power remains approximately constant. This constant-power region (field weakening) is widely used in machine tool spindles and EV motor drives.

Question 4

Why is DC motor starting current so high?

Accepted Answer

At start (ω=0) there is no back-EMF, so I_a = V_a/R_a — typically 5–10 times rated current — which can damage the armature winding and commutator. Solutions include: inserting a starting resistor (switched out in steps), or a soft-start drive that ramps the armature voltage from zero using PWM or thyristor control.

DC Motor & Drive Speed-Torque Simulator

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