1. Mark the correct answer(s) (20 %) 1.1 The armature of an electrical motor is laminated to reduce 1.2 What is the advantage of DC motors over AC motors? 1.3 Induction motors have the advantage of 1.4 On what factors the speed of dc motor depends? 1.5 If a DC motor is connected across the AC supply it will 1.6 Slip exists in which machines? 1.7 At slip s = 1 torque developed in machine is 1.8 Slip speed is the 1.9 The speed of an induction motor will be increased by 1.10 When turning, the rotor conductor should be _________________ in induction motor. 2. Answer / Calculate the following (40 %) 2.1 Why is the number of poles of electromagnets and permanent magnets in DC motor not the same? 2.2 What are OCC and SCC, and what are they used for? A rectangular armature of 50 turns is shown below, detail a. a) b) c) The armature is placed in the field of a two pole magnet (detail b). This is basically an electromagnet producing a field. The field goes from N to S. The armature windings and the field coils are connected in series. When the motor is operating, the current flowing in the armature is 1.5 A. 2.3 In which direction must the current flow in the field coils to produce the field direction indicated in detail c? 2.4 What is the magnitude of the force on the side LM of the armature when it is placed in the field of the two pole magnet, as shown in detail b? The side LM is 8 cm long and has a current of 1.65 A flowing in it. The magnetic field in the region is 0.10 T. 2.5 The armature is at rest in the position shown in detail b. When the current starts flowing as indicated in the sketch, it will… 2.6 The current in the motor is doubled to 3.0 A. This increases the magnetic field by a factor of 2. What effect will this have on the force on the side LM? A DC electric motor is shown below. The permanent magnet produces a uniform magnetic field of 0.03 T between the poles. The coil JKLM, wound on a square armature of side 50cm, consists of 20 turns of wire. There is a current of 2 A in each turn of the coil. The armature can rotate about the XY axis. 2.7 For the coil oriented as shown, calculate the magnitude of the total force exerted on the side JK. 2.8 For the coil oriented as shown, calculate the magnitude of the total force exerted on the side KL.
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