1161.
During rheostat braking of D.C. series motors
A
motor is run as a generator
B
motor is reversed in direction
C
motor is run at reduced speed
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During rheostat braking of D.C. series motors
motor is run as a generator
motor is reversed in direction
motor is run at reduced speed
Differentially compound D.C. motors can find applications requiring
high starting torque
low starting torque
variable speed
frequent on-off cycles
D.C. shunt motors are used for driving
trains
cranes
hoists
machine tools
D.C. series motors are used
where load is constant
where load changes frequently
where constant operating speed is needed
in none of the above situations.
D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the
series motor
shunt motor
compound motor
any of the above
D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this
series motor
shunt motor
compound motor
any of the above
D.C. generators are normally designed for maximum efficiency around
full-load
rated r.p.m.
rated voltage
all of the above
D.C. generators are installed near the load centres to reduce
iron losses
line losses
sparking
corona losses
By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?
Frame
Shaft
Commutator
Stator
Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of
D.C. series motor
D.C. shunt motor
induction motor
all of above motors
At the instant of starting when a D.C. motor is put on supply, it behaves like
a highly resistive circuit
a low resistance circuit
a capacitive circuit
none of the above
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 resistance is connected in series with armature
a resistance is connected parallel to the armature
armature is temporarily open circuited
a high value resistor is connected across the field winding
As -the load is increased the speed of D.C. shunt motor will
reduce slightly
increase slightly
increase proportionately
remains unchanged
According to Flemings left-hand rule, when the forefinger points in the direction of the field or flux, the middle finger will point in the direction of
current in the conductor aovtaat of conductor
resultant force on conductor
none of the above
A three point starter is considered suitable for
shunt motors
shunt as well as compound motors
shunt, compound and series motors
all D.C. motors
A direct on line starter is used: for starting motors
iip to 5 H.P.
up to 10 H.P.
up to 15 H.P.
up to 20 H.P.
A D.C. series motor is that which
has its field winding consisting of thick wire and less turns
has a poor torque
can be started easily without load
has almost constant speed
Why are the D.C. motors preferred for traction applications ?
Torque and speed are inversely proportional to armature current
Torque is proportional to armature current
Torque is proportional to square root of armature current
The speed is inversely proportional to the torque and the torque is proportional to square of armature current
Which one of the following is not the function of pole shoes in a D.C. machine ?
To reduce eddy current loss
To support the field coils
To spread out flux for better unifor-mity
To reduce the reluctance of the mag-netic path
Which one of the following is not necessarily the advantage of D.C. motors over A.C. motors ?
Low cost
Wide speed range
Stability
High starting torque.