1. In the following applications of the dc machines, but all are not matched correctly. Choose the wrong marking.
I. DC shunt motor - variable speed application
II. DC compound generator - constant supply voltage
III. Differentially compounded motor - welding purposes
a) III
b) II
c) I, III
d) III, II
Explanation: Differentially compounded generators are used for welding purposes, not motors.
2. In the following applications of the dc machines, but all are not matched correctly. Choose the wrong marking.
I. DC shunt motor - Lathe machine
II. DC compound generator - Drilling machine
III. Differentially compounded generator - welding purposes
a) II
b) III
c) I, III
d) III, II
Explanation: DC compound generators are used for maintaining constant voltage across the terminals.
3. Mark the order for the decreasing order of the voltage regulation for dc generators.
a) DC shunt generator > level compound generator > Differential compound generator
b) DC shunt generator < level compound generator < Differential compound generator
c) Level compound generator > DC shunt generator > Differential compound generator
d) Level compound generator < DC shunt generator < Differential compound generator
Explanation: Dc shunt motor is almost constant speed generator and thus its voltage regulation is very good than all and differential compound generator has most poor v.r.
4. Mark the correct option which affects the terminal voltage of a dc shunt motor.
a) Armature reaction
b) Source voltage variations
c) Compensating winding
d) Any of the mentioned
Explanation: If the dc motor has compensating winding, flux will remain constant regardless of load.
5. For a dc shunt motor of 5 kW, running at 1000 rpm, the induced torque will be ____________
a) 47.76 N
b) 57.76 N
c) 35.76 N
d) 37.76 N
Explanation: Torque = Power/(speed in rev/s)
= 5000/(2*pi*1000/60)
= 47.76 N.
6. The flux and the internally generated voltage of a dc machine is a ________ function of its magneto-motive force.
a) non-linear
b) linear
c) constant
d) inverse
Explanation: The flux and induced emf are non linear function of its mmf.
7. It is advised not to run dc series motor with no load. Why?
a) Because zero torque at no load will make speed infinite
b) Because zero torque as no load will not let machine start
c) Because infinite torque will be produced
d) None of the mentioned
Explanation: If the torque is zero in dc series motor then the speed will be infinite causing the damage of the machine permanently.
8. It is impossible to start a differential compounded dc motor.
a) True
b) False
Explanation: It is so because of the unstable speed and torque characteristics. At starting, armature current and series field current are high. Since the series flux subtracts from shunt flux, series flux actually reverses the magnetic polarity. This motor will typically be still or turn slowly in wrong direction while burning up because of excessive current.
9. A student is given a differential compound motor and he has been asked to make it start. How will he try?
a) By shorting series field at start
b) To run as shunt motor at start
c) By making rated current at start
d) All of the mentioned
Explanation: At starting, the flux should be rated so that there is no abnormal situation arise.
10. For a 100 hp 250 V, compound dc motor with compensating winding has a field current of 5 A to produce a voltage of 250 V at 1200 rpm. What will be the shunt field current of this machine at no load?
a) 5 A
b) 5.6 A
c) 4 A
d) 0 A
Explanation: At no load, armature current is zero, so the internal generated voltage is 250 V. So 5 A will be the shunt field current at no load.