Dc Motors Objectives Part 06
101 . The speed of a dc motor can be controlled by
a) changing the field resistance (b)changing the terminal voltage applied to the armature (c)inserting a resistor in series with armature circuit
102 . When the direction of power flow reverses, a differentially compounded motor becomes a cumulatively compounded motor.
103 . Speed torque characteristics of a dc shunt motor are shown in figure, RF is field resistance. It can be concluded that RF3>RF2>RF1
104 . In a permanent magnet dc motor which method of speed control cannot be used? Varying the field current.
105 . When the direction of power flow reverses a cumulatively compounded motor becomes a differentially compounded generator
Question #106-108 refers to data given below:
A duplex lap-wound armature is used in six-pole dc machine with six brush sets, each spanning two commutator segments. There are 72 coils on armature, each containing 12 turns. The flux per pole in machine is 0.039wb and the machine spins at 400rpm.
106 . The number of current paths in machine is 12
107 . The number of conductors in the machine is 1728.
108 . The induced emf is 224.6V
109 . The relation b/t electrical angle in degrees Øe, mechanical angle in dgress Øm when the number of poles is P, is given by 2Øe=PØm.
110 . Frog –leg winding is combined lap and wave winding on a single rotor.
111 . The function of a field regulator for a compound motor is to control the flux.
Question 112 to 115 refer to the data given below:
A 12-pole dc generator has a simplex wave wound generator containing 144 coils of 10 turns each. The resistance of each turn is 0.011ohm. Its flux per pole is 0.05wb and it is turning at a speed of 200rpm.
112 . The number of current paths is 2.
113 . The induced armature voltage is 2880V.
114 . The effective armature resistance of this machine is 7.92ohm.
115 . If a 1Kohm resistor is connected to the terminals of this generator what is the resulting induced counter-torque on the shaft of the machine 396N-m.
116 . The electromagnetic torque and speed are in same direction in case of dc motors.
117 . The generated emf and the current are in the opposite direction in the case of dc motors.
118 . Eddy current losses in dc machines are given by B3f2t2V where V stands for volume of iron part.
119 . The starting resistor is connected in series with the armature.
120 . In case of compound motor is started with an open series field the motor will not start.