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At steady-state characteristics of a DC series motor, which of the following statements are correct? I) The speed-torque relationship of a DC series motor is non-linear. II) Both the armature current and the field current of a DC series motor decrease with increasing load torque. III) The efficiency of a DC series motor is generally higher compared to DC shunt motor. IV) The efficiency of a DC series motor is generally lower compared to DC shunt motor.
Statements I, II and IV are correct
Only statements I and IV are correct
Only statements I and III are correct
Statements I, II and III correct
Only statements I and IV are correct
Statement I is correct because the flux in a DC series motor is directly proportional to armature current IaтАЛ, leading to a speed-torque characteristic defined by NтИЭT1тАЛ, which is non-linear. Statement IV is correct because DC series motors exhibit higher copper losses at variable loads and lack the high-efficiency constant-speed characteristics of shunt motors, making them generally less efficient in steady-state operations compared to the shunt design.
Statement I is correct because the flux in a DC series motor is directly proportional to armature current IaтАЛ, leading to a speed-torque characteristic defined by NтИЭT1тАЛ, which is non-linear. Statement IV is correct because DC series motors exhibit higher copper losses at variable loads and lack the high-efficiency constant-speed characteristics of shunt motors, making them generally less efficient in steady-state operations compared to the shunt design.
TaтАЛтИЭ╬жIaтАЛ тАФ Torque equation where ╬жтИЭIaтАЛ before magnetic saturation
NтИЭ╬жVтИТIaтАЛ(RaтАЛ+RseтАЛ)тАЛ тАФ Speed equation for DC series motor
In a DC series motor, the field winding is connected in series with the armature, meaning IfтАЛ=IaтАЛ. As load torque increases, IaтАЛ increases, causing magnetic flux ╬ж to increase (until saturation), which forces speed N to decrease sharply. This relationship is hyperbola-like, resulting in the non-linear curve.
Speed is inversely proportional to the square of armature current at light loads.
DC series motors should never be started at no-load due to dangerously high speeds.
Efficiency is impacted by the significant I2R losses in the series field winding.
Shunt motors provide superior speed regulation, leading to better overall efficiency in varying industrial load conditions.
High starting torque
Ability to handle heavy sudden loads
Poor speed regulation
Cannot run at no-load (danger of runaway speed)
Electric traction (locomotives)
Hoists and cranes
| Feature | DC Series Motor | DC Shunt Motor |
|---|---|---|
Speed Regulation | Poor (Highly variable) | Good (Relatively constant) |
Starting Torque | Very High | Moderate |
Statement II is incorrect because armature and field current increase (not decrease) with increasing load torque in a series motor.
Statement III is incorrect; DC shunt motors are generally more efficient for constant load applications due to their stable flux and lower winding losses.
The official answer B correctly identifies only I and IV as accurate.
B is correct тАФ Statements I and IV accurately describe the non-linear speed-torque nature and the comparative efficiency disadvantage of DC series motors.
Always remember that in series motors, NтИЭIaтАЛ1тАЛ (for high loads) or Ia2тАЛ1тАЛ (for low loads); use this to quickly eliminate options claiming constant speed.
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