DC Machines and Transformers Questions

REPUBLIC OF CAMEROON THE UNIVERSITY OF BAMENDA

Peace-Work-Fatherland P.0 BOX 39 Bambili

School/Faculty: COLTECH Department: EEE _______ Lecturer(s): Pr. NFAH MBAKA Eustace ____________

Course Code: EEEP2205 ____ Course Title: DC Machines and Transformers _________________________________

Date: 02/01/2018 _______________________ Hall: ________________________________ Time:2hrs ________________

Instructions:

Answer all questions _______________________________________________ __________

Second Semester Examinations 2016/2017

1. A short shunt compound dc generator has armature, shunt field and series field resistances of 0.8Q, 45 O and

0.6

Ω

respectively. The generator supplies a load of 5 kW at 250 V.

(a) Sketch the circuit of the short shunt compound dc generator (2mks)

(b) Calculate the armature terminal voltage. (3mks)

(c) Calculate the shunt field current. (3mks)

(d) Calculate the armature current. (3mks)

(e) Calculate the e.m.f. generated. (3mks)

(f) Calculate the electrical efficiency. (3mks)

2. A bipolar dc series motor has a total armature and field resistance R=0.1 Q and the magnetic circuit is

unsaturated. If the supply voltage of the motor is 750 V and the motor draws a full load current of 200 A at a

speed of 1Orev/s.

(a) Sketch the circuit of the bipolar dc series motor (2mks)

(b) Find the back e.m.f. of the motor. (3mks)

(c) Find the mechanical power developed by the motor. (3mks)

(d) Find the useful power output corresponding to a usefultorque is 2100 Nm. (3mks)

(e) Find the power corresponding to constant losses in the motor. (3mks)

(f) Find the electrical efficiency of the dc series motor (3mks)

3. A 100 kVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary impedance is

(0.3 + j1.1)Ω and the secondary impedance is (0.01 + j0.035)Ω. The supply voltage is 2200 V at 50 Hz.

(a) Sketch the equivalent circuit referred to the primary circuit (3mks)

(b) Calculate the equivalent impedance referred to the primary. (3mks)

(c) Draw the phasor diagram for lagging power factor. (3mks)

(d) Derive expression for voltage regulation from phasor diagram (3mks)

(e) Calculate the voltage regulation at full-load for a lagging power factorof 0.8. (3mks)

(f) Calculate the voltage regulation at full-load for a leading power factorof 0.8. (3mks)

4. The primary and secondary resistances of a 500 kVA transformer are 0.42Q and 0.0011Q respectively. The

primary voltage, secondary voltage and iron loss are 6600V, 400V and 2.9 kW respectively.

(a) Sketch the equivalent circuit referred to the secondary circuit (3mks)

(b) Calculate the equivalent resistance of the transformer referred to the secondary circuit (3mks)

(c) Calculate the full-load output of the transformer at a power factor of 0.8 (3mks)

(d) Calculate the copper losses at full-load (3mks)

(e) Calculate the efficiency of the transformer at full-load (3mks)

(f) Calculate the efficiency of the transformer at half-load (3mks)