1. Which of the following motor has the poorest speed regulation ?
(a) Shunt motor
(b) Series motor
(c) Differential compound motor
(d) Cumulative compound motor
Ans: b
2. 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
Ans: a
3. 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
Ans: a
4. 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
Ans: b
5. 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
Ans: c
6. Usually wide and sensitive speed control is desired in case of
(a) centrifugal pumps
(b) elevators
(c) steel rolling mills
(d) colliery winders
Ans: d
7. 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%
(6) 4.76%
(c) 6.77%
(d) 8.84%
Ans: b
8. 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
Ans: a
9. 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
Ans: a
10. The speed of a D.C. shunt motor can be increased by
(a) increasing the resistance in armature circuit
(b) increasing the resistance in field circuit
(c) reducing the resistance in the field circuit
(d) reducing the resistance in the armature circuit
Ans: b
11. If I2 be the armature current, then speed of a D.C. shunt motor is
(a) independent of Ia
(b) proportional to la
(c) varies as (Ia)
(d) varies as la
Ans: a
12. In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will
(a) remain unchanged
(6) reduce to one-fourth value
(c) increase four folds
(d) be doubled
Ans: a
13. At the instant of starting when a D.C. motor is put on supply, it behaves like
(a) a highly resistive circuit
(6) a low resistance circuit
(c) a capacitive circuit
(d) none of the above
Ans: b
14. The speed of a D.C. motor can be varied by varying
(a) field current
(b) applied voltage
(c) resistance in series with armature
(d) any of the above
Ans: d
15. Which one of the following is not necessarily the advantage of D.C. motors over A.C. motors ?
(a) Low cost
(b) Wide speed range
(c) Stability
(d) High starting torque
. Ans: a
16. For a D.C. shunt motor if the excitation is changed
(a) torque will remain constant
(b) torque will change but power will remain constant
(c) torque and power both will change
(d) torque, power and speed, all will change
Ans: b
17. Which motor has the poorest speed control?
(a) Differentially compounded motor
(b) Cumulatively compounded motor
(c) Shunt motor
(d) Series motor
Ans: d
18. The plugging gives the
(a) zero torque braking
(b) smallest torque braking
(c) highest torque braking
(d) none of the above
Ans: c
19. The armature voltage control of D.C. motor provides
(a) constant voltage drive
(b) constant current drive
(c) constant torque drive
(d) none of the above
Ans: c
20. If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor
(a) of lower H.P. should be selected
(6) of higher H.P. should be selected
(c) can be used for 50°C ambient temperature also
(d) is to be derated by a factor recom-mended by manufacturer and select the next higher
H.P. moto
Ans: d
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