Question

Problem 2 A separately excited DC motor is characterized by the followng intormation Armature resistance Ra-5 2 armature inductance La . very large, Field resistance RS200 Ω, ted voltage Vt 200 V, and an inertia constant J-12 kg m Neglecting friction losses and assuming no-saturation in the machine magnetic core system, answer the folowing questions: Part 1: (6 marks) he motor a as shown in figure 2a. The motor drives a constant power load of vaue 1 40375 hp (ie 1 hp 746 W). The ac vollage source is vs 24012 cos(100) vois ) Draw the waveform of the voltage across, and the current through the armature inding b) Calculate the motor efficiency if the motor runs at speeds 1000 rpm c) ช a 300 Ω resistance is connected across the fed winding terminals, calculate the motor speed D1 vi נס ure 2a

Answer 1

Part-1(a)

Anmotune cuwrunt:

Part-1(b)

Part-1(c) If a 300 ohm resistance is connected across the field winding terminal, then there will not be any change in motor speed. But if it would have been connected in series with the field winding then there would be change in speed.

Since 300 ohm resistance is connected across the field winding, the scenario is like a voltage source connected across two parallel resistance of 200 ohm and 300 ohm respectively, therefore field winding current would remain same and hence the flux will remain same and therefore speed will remain same.