# Dc Machines

Tutorial 4 – DC Machine

1. Describe operation of DC machine (motor and generator) using illustrations. Specifically explain the purpose of: a. brushes d. armature winding b. split ring commutators e. effect of back emf (in operation) c. field winding 2. Draw the different configurations of DC machines a. shunt c. separately excited b. series 3. Develop, using Kirchhoff’s laws, equation for Ea (armature voltage) in both motoring and generating mode. 4. Describe three (3) methods of speed control in DC machines. 5. A shunt dc motor has P poles and is rotating at speed N rpm. The machine windings are lap wound. There are z armature conductors and a parallel paths. The flux developed is ɸ webers per pole. Derive an expression for the armature voltage developed in terms of ɸ, N, z, p and a. 6. A 30 hp, 230 V, 1150 rpm shunt motor has four poles, four parallel paths and 882 armature conductors. The armature circuit resistance is 0.170 . At rated speed and rated output the armature current is 110 A and the field current is 2.2 A. Calculate: a. the electromagnetic torque b. the flux per pole c. the rotational losses d. the efficiency e. the shaft load f. the field flux is reduced by 15% of its original value by means of a field rheostat. Determine the new operating speed. 7. A separately excited dc motor when operated at constant flux has an armature resistance of 1.03 Ω. When the motor is operated at no load from a 50 V supply, it develops a speed of 2100 rpm and draws a supply current of 1.2 A. Determine: a. the numerical value of Ka b. the no load rotational losses c. the electromagnetic torque developed by the motor d. the output power of the motor when operating at 1700 rpm from a 48 V supply 8. A 30 HP, 250 V, 1150 rpm shunt motor has four poles. There are 1200 armature conductors. The armature is lap wounded and has a circuit resistance of 0.18 . At rated speed and rated output the armature current is 100A and...

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