1.
The condition for maximum efficiency for a D.C. generator is
A
eddy current losses = stray losses
B
hysteresis losses = eddy current losses
C
copper losses = 0
D
variable losses = constant losses
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The condition for maximum efficiency for a D.C. generator is
eddy current losses = stray losses
hysteresis losses = eddy current losses
copper losses = 0
variable losses = constant losses
The armature voltage control of D.C. motor provides
constant torque drive
constant voltage drive
constant current drive
none of the above
The armature torque of the D.C. shunt motor is proportional to
field flux only
armature current only
both a and b
none of the above
Starters are used with D.C. motors because
these motors have high starting torque
these motors are not self-starting
back e.m.f. of these motors is zero initially
to restrict armature current as there is no back e.m.f. while starting
Speed control by Ward Leonard method gives uniform speed variation
in one direction
in both directions
below normal speed only
above normal speed only.
Sparking, is discouraged in a D.C. motor because
it increases the input power con-sumption
commutator gets damaged
both a and b
none of the above
Sparking at the commutator of a D.C. motor may result in
damage to commutator segments
damage to commutator insulation
increased power consumption
all of the above
Regenerative method of braking is based on that
back e.m.f. is less than the applied voltage
back e.m.f. is equal to the applied voltage
back e.m.f. of rotor is more than the applied voltage
none of the above
One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven
runs as a generator
does not run as a generator
also runs as a motor comes to stop after sometime
No-load speed of which of the following motor will be highest ?
Shunt motor
Series motor
Cumulative compound motor
Differentiate compound motor
In which of the following applications D.C. series motor is invariably tried?
Starter for a car
Drive for a water pump
Fan motor
Motor operation in A.C. or D.C.
In Ward-Leonard control the lower limit of speed is imposed by
residual magnetism of the generator
core losses of motor
mechanical losses of motor and gen¬erator together
all of the above
In variable speed motor
a stronger commutating field is needed at low speed than at high speed
a weaker commutating field is needed at low speed than at high speed
same commutating field is needed at low speed than at high speed
none of the above is correct
In the D.C. motor the iron losses occur in
the field
the armature
the brushes
the commutator
In D.C. shunt motors as load is reduced
the speed will increase abruptly
the speed will increase in proportion to reduction in load
the speed will remain almost/constant
the speed will reduce
In D.C. motor which of the following can sustain the maximum temperature rise?
Slip rings
Commutator
Field winding
Armature winding
In D.C. machines the residual magnetism is of the order of
2 to 3 per cent
10 to 15 per cent
20 to 25 per cent
50 to 75 per cent
In D.C. machines fractional pitch winding is used
to improve cooling
to reduce copper losses
to increase the generated e.m.f.
to reduce the sparking
In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be
100%
around 90%
anywhere between 75% and 90%
less than 50%
In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will
remain unchanged
reduce to one-fourth value
increase four folds
be doubled