Question

1. Name the various parts of a DC machine and give their function. 2. A four-pole de machine with a lap wound armature has a total of 652 armature conductors with an armature resistance of 0.1502. When run as a generator on open circuit at a speed of 400rpm, the armature terminal voltage is 262V. Each pole shoe arc has a diameter of 42cm, subtends an angle of 60° and has an axial length of 20cm. 2.1.Calculate the useful flux per-pole. 2.2.Calculate the air-gap flux density. 2.3.Calculate the armature terminal voltage if the generator supplies a load with a current of 20A. 3. A series-wound de motor has an armature winding resistance of 0.20 and a field winding resistance of 0.302. The motor has a rated speed of 750 rpm and a rated voltage of 460V and rated current of 50A. 3.1.Calculate the armature generated voltage and the developed torque when the motor runs at rated speed, drawing rated current from the supply. 3.2. Calculate the armature generated voltage, the speed and the developed torque when the machine draws 60% of rated current from the supply 3.3. With help of relevant curves and equations explain what would happen if the motor lost its mechanical load. 3.4.Calculate the value of starting resistor to be used to limit the starting current to twice the rated current and the resulting starting torque. 3.5. The motor is to be brought to rest electrically using plugging. Sketch a diagram to illustrate the use of plugging with series motors and derive the expression for the braking torque.

Answer 1

Soln Q.1 Parts of Doc. Machime :- o Stator Stator on Yoke on Frame ir - This part of Doc. machine is made from laminated steel sheet. Here, lamination is used for reducing iron losses. (Mainly eddy current loss) 2 Poles and field windings the staton.). Poles are placed inside stator (innen surface of Field windings are mounted on pole and supplied with. Doc. to produce magnetic field. source :- 3 Rotor with Armature windings: ~ Generally, a cylindrical roton lies in the central pant inside stator. It is also made from laminated stell stell to reduce iron losses. Rotor has slots in its penipheny ; through which armature winding is wounded. Ends of unmatune windings are connected to Commutaton, Rotor is notated using shaft, 4 Inter poles :- They ane placed between field poles and they solue commutation problem. to are used

5 Compensation winding :- → . They are placed in pole shoes; to reduce armature reaction. (6 -) Brushes :- They are made of conductive carbon material; and place on commutaton. * Simple diagram pole Field, winding Interpole -stator or Yoke Rotor Armature Conductors. DC Machine.