D.C MOTOR BITS

1) A electrical d.c series motor is accidentally connected to single - phase a.c supply.The torque produced will be a) of zero average value
b) oscillating
c) steady and unidirectional
d) pulsating and unidirectional

ans d

2) The direction of rotation of a d.c. series motor can be reversed
a) by interchanging supply terminals
b) by interchanging field terminals
c) either by interchanging supply terminals or by interchanging field terminals
d) by interchanging supply terminals as well as field terminals

ans c

3) The dummy coils in d.c.machines are useful to
    a) increase the efficiency
    b) improve the commutation
    c) reduce the cost of the machine
    d) maintain mechanical balance of armature

ans d

4) Wave winding is employed in a d.c. machine of
    a) high current and low voltage rating
    b) low current and high voltage rating
    c) high current and high voltage rating
    d) low current and low voltage rating

ans b

5) When is the mechanical power developed by a d.c motor maximum?
    a) back  e.m.f is equal to applied voltage
    b) back e.m.f is equal to zero
    c) back e.m.f is equal to half the applied voltage
    d) none of the above

ans c

6) In a d.c. compound generator,"flat-compound" characteristic , required for certain applications, may be obtained by connecting a variable resistance.
     a) across the series field
     b) in series with the series field
     c) in parallel with the shunt field
     d) in series with the shunt field

ans a

7) If the armature current is increased to double its previous value, and the time of commutation is halved , how will the reactance voltage vary?
     a) it will be halved
     b) it will remain the same
     c) it will be doubled
     d) it will become four times

ans d

8) A dc motor operated from a type A chopper is switched to type B chopper. How does the motor operate when type A and B are in operation respectively?
     a) Plugging and motoring
     b) Motoring and plugging
     c) Regenerative braking and motoring
     d) Motoring and regenerative braking

ans d

9) The field flux method of speed control of dc shunt motor is generally used for which one of the following ?
    a) constant hp device below the base speed
    b) constant torque drive above the base speed
    c) constant torque drive below the base speed
    d) constant hp drive above the base speed

ans d

10) What are slot wedges in a dc machine,made of?
    a) Mild steel
    b) Silicon steel
    c) Fibre
    d) Cast iron

ans b