GTHS KUMBO_Electrical Department_Third sequence exam……………………………Jan.20121 PART ONE : TECHNOLOGY 1. Explain why the series field DC mo tor rota tes at hig h spee d at no load when it is supplied at his nominal voltage. 2. For eac h of the follow ing electrical machines, cite a magn etic mat erial wh ich is used in the manufacturing of the magnetic circuit: a) transformer; b) DC machine stator, c) permanent magnet DC motor. 3. Give the role of the “isopther me” in an AC electrical machine. 4. Give the rol e of the exci tation resis tanc e in a D C motor . 5. Give the rol e of auxiliary commutation poles in a DC motor. 6. Give the d iffere nces be twee n an asyn chro nous sq uirrel ca ge motor an d an asynchronous wound winding motor. 7. Name the test which is conducted in order to de termi ne iron an d frictio n losses in a DC motor. PART TWO: ELCTROTECHN OLOGY Exercise 1: Shunt generator The open circuit characteristics at 1000rpm of a 4 pole, 220V shunt generator with 72 slots and 8 conductors per slot with armature conductors lap connected is as f ollows: Field current (A) 0.25 0.5 1 2 3 4 5 emf (V) 25 50 100 175 220 245 255 The field circuit resistance is 75Ω . 1. Ex plain ho w thi s test was carr ied o ut REPUBLIC OF CAMEROON Pea ce – Work – Fath erl and …………… GTHS KUMBO/ ELECT DPT THIRD SEQUENCE EXAM Class: F 3 7 Option: Electrotechnology Duration: 03H Coe ffic ien t: 3 Written paperELECTRICAL MACHINES No document is allowed except the one given to the c andidates b the examiners.
