Q3Electric Drives
Question
Q.3. Draw and explain the torque-speed characteristics of separately excited dc motor.
Answer
A separately excited DC motor exhibits a torque-speed characteristic that is a straight line with a small negative slope, giving nearly constant speed with load torque, since torque is directly proportional to armature current while speed decreases only slightly (due to armature resistance drop) as load torque increases.
In a separately excited DC motor, the field winding is supplied from an independent (separate) source, so field flux φ remains constant regardless of armature current — the developed torque T = Kφ·Ia is therefore directly proportional to armature current Ia, and the speed N = (V-Ia·Ra)/(Kφ) decreases only slightly as Ia (and hence torque) increases, due to the small IaRa voltage drop across the armature resistance. This gives a torque-speed characteristic that is a nearly horizontal straight line with a small negative (drooping) slope — the motor speed remains close to its no-load value across a wide range of load torque, a highly desirable characteristic that makes the separately excited DC motor well suited to constant-speed industrial drive applications requiring good speed regulation under varying load conditions.