Dc Motors Objectives Part 05
81 . A dc shunt motor has external resistance R1 in the field circuit and R2 in the armature circuit. The starting armature current for the motor will be minimum when R1 is minimum and R2 is maximum.
82 . In a dc motor, speed control by varying the armature circuit resistance provides a constant torque drive .
83 . A dc shunt motor draws an armature current of 40A under normal rated conditions. If the field flux and the armature terminal voltage are reduced to half, then for constant power output, the armature current will be 80A.
84 . A differentially compounded dc motor runs at a full load speed of480rpm. Now if the series field of the motor is short circuited, then full load speed will be less than 480rpm.
85 . A cumulatively compounded dc motor runs at a full load speed of 480rpm. Now if the series field of the motor is short circuited, then full load speed will be more than 480rpm.
86 . In a dc shunt motor, speed control by the variation of the field flux will be constant power drive
87 . A dc shunt motor is started on no load and runs at 400rpm. The motor is made to run on no load for ten hours. The speed of motor after ten hours will be more than 400rpm.
88 . Which of the following is likely to be the shunt field winding resistance for a 5KW shunt motor 200 ohms.
89 . Two dc series motors are coupled. One motor is run as generator and other run as motor. The friction losses of the two machines will be equal when both have same speed,
90 . Two series motors are coupled. One motor is run as generator and other run as motor. The iron and friction losses of the two machines will be identical when their speeds and excitations are identical.
91 . In series parallel control method, when two dc series motors are connected in series, the speed of combined set is one fourth of the speed of the motors when connected in parallel.
92 . A dc motor is provided with interpoles. Which of the following represents the correct sequence of poles? (capital letter indicates main pole and small letter indicate interpole) N-n-S-s-N-n-S-s
93 . In series-parallel control of dc series motors, the torque in series arrangement is four times the torque in parallel arrangement.
94 . Speed control of a cumulatively compounded dc motor can be affected through
a) change of armature resistance (b)change of armature voltage (c) change of field resistance
95 . The starter for a dc motor also provides protection to the motor against damage (a) due to short circuit in the equipment (b)from long term overloads from excessive starting current
96 . The brush voltage drop in dc motor is usually of the order of 2 V.
97 . The characteristics of a cumulatively compounded dc motor is represented by the curve OE
98 . The characteristics of a differentially compounded dc motor is represented by the curve OB.
99 . In case starting resistance of a series motor is short-circuited, the motor will run at very high rpm.
