Download UNIT-IV 1 Mark questions

UNIT-IV
1 Mark questions
1. In a d.c. motor, unidirectional torque is produced with the help of
(a) brushes
(b) commutator
(c) end-plates
(d) both (a) and (b)
2. The mechanical power developed by the armature of a d.c. motor is equal to
(a) armature current multiplied by back e.m.f.
(b) power input minus losses
(c) power output multiplied by efficiency
(d) power output plus iron losses
3. A d.c. motor can be looked upon as d.c. generator with the power flow
(a) reduced
(b) reversed
(c) increased
(d) modified
4. Write the torque equation of DC motor?
5. The current drawn by a 120 V d.c. motor of armature resistance 0.5 Ω and back
e.m.f. 110 V is .......... ampere.
(a) 20
(b) 240
(c) 220
(d) 5
6. Write the basic principle is used in dc motors?
7. If the pole flux of a d.c. motor approaches zero, its speed will
(a) approach zero
(b) approach infinity
(c) no change due to corresponding change in back e.m.f.
(d) approach a stable value somewhere between zero and infinity.
8. Which of the following d.c. motor would be suitable for drives requiring high
starting torque such as cranes ?
(a) shunt
(b) series
(c) compound
(d) permanent magnet
9. As the load is increased, the speed of a d.c. shunt motor
(a) increases proportionately (b) remains constant
(c) increases slightly
(d) reduces slightly
10. The Ta/Ia graph of a d.c. series motor is a
(a) parabola from no-load to overload
(c) parabola throughout
(b) straight line throughout
(d) parabola upto full-load and a straight line at overloads.
11. Draw the Torque verses armature current characteristic for DC series motor?
12. When load is removed, which motor will run at the highest speed?
(a) shunt
(b) cumulative-compound
(c) differential compound
(d) series
13. A series motor is best suited for driving
(a) lathes
(b) cranes and hoists (c) shears and punches
(d) machine tools
14. A 220 V shunt motor develops a torque of 54 N-m at armature current of 10 A.The
torque produced when the armature current is 20 A, is
(a) 54 N-m (b) 81 N-m (c) 108 N-m (d) None of the above
15. The d.c. series motor should never be switched on at no load because
(a) the field current is zero
(b) The machine does not pick up
(c) The speed becomes dangerously high
(d) It will take too long to accelerate.
16. The main disadvantage of the Ward-Leonard control method is
(a) high initial cost
(b) high maintenance cost
(c) low efficiency at High loads
(d) all of the above
17. The direction of rotation of a D.C. series motor can be changed by
(a) interchanging supply terminals
(c) either of (a) and (b) above
(b) interchanging field terminals
(d) None of the above
18. Which D.C. motor will be preferred for machine tools?
(a) Series motor
(b) Shunt motor
(c) Cumulative compound motor
(d) Differential compound motor
19. Differentially compound D.C. motors can find applications requiring
(a) high starting torque
(b) low starting torque
(c) variable speed
(d) frequent on-off cycles
20. According to Fleming's left-hand rule, when the forefinger points in the direction of
the field or flux, the middle finger will point in the direction of
(a) current in the conductor (b) resultant force on conductor
(c) torque
(d) none of the above
21. Mention the types of starters used in dc machines?
22. The type of D.C. motor used for shears and punches is
(a) shunt motor
(b) series motor
(c) differential compoutid D.C. motor
(d) cumulative compound D.C. motor
23. When two D.C. series motors are connected in parallel, the resultant speed is
(a) more than the normal speed
(b) loss than the normal speed
(c) normal speed
(d) zero
24. For constant torque drive which speed control method is preferred?
(a) Field control
(b) Armature voltage control
(c) Shunt armature control
(d) Mechanical loading system
25. Write the advantage of four-point starter?
26. Which of the following law/rule can he used to determine the direction of rotation of
D.C. motor ?
(a) Lenz's law
(b) Faraday's law
(c) Coloumb's law (d) Fleming's left-hand rule
27. The condition for maximum power in case of D.C. motor is
(a) back e.m.f. = 2 x supply voltage
(b) back e.m.f. = .5 x supply voltage
(c) supply voltage = .5 x back e.m.f.
(d) supply voltage = back e.m.f.
28. Which of the following motor has the poorest speed regulation?
(a) Shunt motor
(b) Series motor
(c) Differential compound motor
(d) Cumulative compound motor
29. 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
30. 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%
31. Ward-Leonard control is basically a
(a) voltage control method
(b) field divertor method
(c) field control method
(d) armature resistance control method
2Mark Questions
1. Mention the types of speed control methods for DC motors?
2. Draw the torque verses speed characteristic for DC series motor?
3. Write the advantages of armature resistance control of DC series motos?
4. Write the formula for back emf and Torque equation of DC motor?
5. Write the advantage of four point starters?
6. Why Series motor should not start at no-load condition?
7. Why starter required for starting the DC motor?
8. Draw the speed verses armature current characteristic for DC shunt motor?
9. Write the any two applications of DC series and shunt motors?
10. Write the any two application of cumulative compound motor?
5 Mark questions
1. A 220V DC machine has an armature resistance of 0.5Ω. If the full load armature current
is 20A, find the induced emf when the machine acts as
i) generator ii) motor
2. Derive an expression for the torque developed in a dc machine
3. A 230V D.C. shunt motor runs at 800 r.p.m and takes armature current of 50A. Find
4.
5.
6.
7.
8.
resistance to be added to the field circuit to increase speed to 1000 r.p.m at an
armature current of 80A. Assume flux proportional to field current. Armature
resistance=0.15 ohms and field winding resistance=250 ohms.
A DC shunt motor supplied at 230V runs at 990 rpm. Calculate the resistance
required in series with the armature circuit to reduce the speed to 500rpm assuming
that armature current is 25A.
Derive the condition for maximum power output of DC motor?
Write the applications of DC motors used for various purposes?
Explain the significance of back EMF in a DC motor and explain how it controls the
value of armature current in DC motor?
A 6 pole lap wound, 240V motor has the following particulars; No. of armature
conductors is 740, useful flux/pole is 0.035wb, armature resistance is 0.3ohms. If the
total torque developed by the motor is 140 N-M, find the armature current taken and the
speed of operation.
10 Mark Questions
1. Explain the principle and working of DC motors?
2. Explain speed control technique by Ward-Leonard system & give advantage and dis
advantages of this mothod?
3. Explain the characteristics for different types of DC motors?
4. Explain flux control methods for different typef of DC motors briefly?
5. Explain armature voltage control methods for different typef of DC motors briefly?
6. Explain the speed control of DC motors briefly?
7. Explain the about the 3-point and 4-point starters?