D.C. Motors

As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit____________________?

A. a resistance is connected in series with armature
B. a resistance is connected parallel to the armature
C. armature is temporarily open circuited
D. a high value resistor is connected across the field winding

The armature voltage control of D.C. motor provides____________?

A. constant torque drive
B. constant voltage drive
C. constant current drive
D. none of the above

The speed of a motor falls from 1100 r.p.m. at no-load to 1050 r.p.m. at rated load. The speed regulation of the motor is______________?

A. 2.36%
B. 4.76%
C. 6.77%
D. 8.84%

Usually wide and sensitive speed control is desired in case of____________?

A. centrifugal pumps
B. elevators
C. steel rolling mills
D. colliery winders

Which of the following method of speed control of D.C. machine will offer minimum efficiency ?

A. Voltage control method
B. Field control method
C. Armature control method
D. All above methods

The armature torque of the D.C. shunt motor is proportional to____________?

A. field flux only
B. armature current only
C. both A. and B.
D. none of the above

As -the load is increased the speed of D.C. shunt motor will____________?

A. reduce slightly
B. increase slightly
C. increase proportionately
D. remains unchanged

Which of the following motor has the poorest speed regulation ?

A. Shunt motor
B. Series motor
C. Differential compound motor
D. Cumulative compound motor

Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of_____________?

A. D.C. series motor
B. D.C. shunt motor
C. induction motor
D. all of above motors

In a differentially compounded D.C. motor, if shunt field suddenly opens____________?

A. the motor will first stop and then run in opposite direction as series motor
B. the motor will work as series motor and run at slow speed in the same direction
C. the motor will work as series motor and run at high speed in the same direction
D. the motor will not work and come to stop