Question

2) (a) Describe the principle of operation of 3-phase induction motor. [8 Marks]

(b) Explain slip as applied to three phase induction motor. [2 Marks]

3) Derive an expression for the total armature torque of DC motor. [10 Marks]

5) List the starting methods used in starting 3-phase induction motors and explain any two of these methods. [10 Marks]

6) (a) A 60 kW, 400 V d.c. shunt motor has 4 poles a 2-circuit wave-connected armature winding with 450 conductors. The flux per pole is 45 mWb. The full-load efficiency of the motor is 90.5%. The armature and interpole windings have a total resistance of 0.1 Ω. The shunt field resistance is 200 Ω. Calculate (i) the line current (ii) full-load speed and (iii) armature torque. [6 Marks]

(b) Explain counter or back e.m.f. in d.c. motor and state its importance. [4 Marks]

7) (a) A 3-phase, 50 Hz induction motor has 8 poles. If the full-load slip is 2.5 %, determine

i. the synchronous speed, 

ii. rotor speed, and 

(iii) rotor frequency. [6 Marks]

(b) State four (4) possible causes each of the following troubles in dc motor and suggest two solutions to avert them:

(i) sparking at brushes, and (ii) overheating. [4 Marks]

8) (a) With the aid of diagrams, describe the three characteristics of d.c. series motor. [6 Marks]

(b) Briefly explain the difference between squirrel cage induction motor and wound rotor induction motor in terms of construction. [4 Marks]

9) (a) A 2-pole, 3-phase, 50 Hz induction motor is running on no-load with a slip of 4 %. Calculate:

i. the synchronous speed, and

ii. the speed of the motor. [5 Marks]

(b) With the aid of diagram, explain induction generator. [5 Marks]

10) Explain armature reaction in dc motor. [10 Marks]

Answer 1

6 a) ans:

Pont - 60 kw A = 2, De shurst motor - Supply voltage 4 = 400V. poles p=4, wave connected to Ra=0.12, Rsh=20on Number of Conductors z = 450 flux/pole o = 45 mwb Efficiency y = 90.5% Pont 600103 Input power Pina Pin 7 7 0.905 - 66.298 kw. But Pin Vt IL. IL Line Current. Pin I = 4 66.2988103 400 ) IL - 165.7464 IL Ish. Ta 400 ZDO ' 4 Rsh Ish.a Cb Rsh Ist Het · 24. . Ia Il-Ish 165.746–2 2 = 163.7464.

Back emf Erba Vt-IaRa. = 400 163-746 X0.1 A. 383.625 X60X2 N Eb 383.625 V But Ebé OzNx P. motor N- full load speed Ebx60x4. фZP 45x17x450X4 N=568 rpm power developed in armalure is Pa= Eb Iq = 383.6258 163.746 Pa = 62.817kW But Paa zanta Ta Armature Torque Pax 60 62.817X103x60 :! Tas 6.288566 60 aan Ta = 1056.625 N-m.