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+
+// ELECTRICAL MACHINES
+// R.K.Srivastava
+// First Impression 2011
+// CENGAGE LEARNING INDIA PVT. LTD 
+
+// CHAPTER : 4 : DIRECT CURRENT MACHINES 
+
+// EXAMPLE : 4.20
+
+clear ; clc ; close ; // Clear the work space and console
+
+
+// GIVEN DATA
+
+N = 1500;                   // Speed of the separately excited DC Motor in RPM
+Out_hp = 100;               // Output of the DC Motor in HP
+V = 500;                    // Motor operating Volatge in Volts
+VL = 440;                   // 3-phase Line-line Voltage in Volts
+f = 50;                     // Frequency in Hertz
+Ra = 0.0835;                // Armature Resistance in Ohms
+La = 5.7 * 10 ^ -3;         // Armature Inductance in Henry
+eta = 89/100;               // Operating Efficiency of the Motor 
+E_cons = 0.35;              // EMF constant in Volts per RPM
+
+
+// CALCULATIONS
+// For case (a)
+
+Out = Out_hp * 746;                 // Output of the DC Motor in Watts
+Ia = Out/(V*eta);                   // Rated Current in Amphere
+Vph = VL/sqrt(3);                   // Phase Voltage in Volts
+a = (3*Vph*sqrt(6)) / %pi;          // Constant
+E = N * E_cons;                     // Back EMF at Rated speed
+V = E + (Ia * Ra);                  // Terminal Volatge in Volts
+alpa = acosd(V/a);                  // Firing Angle
+
+// For case (b)
+// Assumed that No load current is about 12% of full load current
+
+Io = ( 0.12 * Ia );                     // No load current in Amphere
+V_b1 = a * cosd(0);                     // Terminal Voltage at Firing Angle 0 deg
+E_b1 = V_b1 + (Io * Ra);                // Back EMF at Firing Angle 0 deg
+N_b1 = E_b1/E_cons;                     // No load speed at Firing Angle 0 deg
+V_b2 = a * cosd(30);                    // Terminal Voltage at Firing Angle 30 deg
+E_b2 = V_b2 + (Io * Ra);                // Back EMF at Firing Angle 30 deg
+N_b2 = E_b2/E_cons;                     // No load speed at Firing Angle 30 deg
+
+
+// DISPLAY RESULTS
+
+disp("EXAMPLE : 4.20 : SOLUTION :-") ;
+printf("\n  (a)  Firing Angle at Rated speed and Rated Motor Current , alpa = %.2f deg \n",alpa);
+printf("\n (b.1) No load speed at Firing Angle 0 deg, N = %.1f RPM \n",N_b1);
+printf("\n (b.2) No load speed at Firing Angle 30 deg, N = %.1f RPM \n",N_b2);
+
+
+
+