101.
These days D.C. motors are widely used in
A
electric traction
B
machine shops
C
pumping sets
D
air compressors
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121 practice sets · Page 6 of 7
These days D.C. motors are widely used in
electric traction
machine shops
pumping sets
air compressors
The type of D.C. motor used for shears and punches is
differential compoutid D.C. motor
cumulative compound D.C. motor
shunt motor
series motor
The total losses in a well designed D.C. generator of 10 kW will be nearly
1000 W
1500 W
100 W
500 W
The starting resistance of a D.C. motor is generally
1000 Q
infinitely large
low
around 500 Q
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
6.77%
8.84%
2.36%
4.76%
The speed of a D.C. shunt motor more than its full-load speed can be obtained by
decreasing the armature current
increasing the armature current
decreasing the field current
increasing the field current
The speed of a D.C. shunt motor is required to be more than full load speed. This is possible by
reducing the field current
decreasing the armature current
increasing the armature current
increasing the excitation current
The speed of a D.C. shunt motor can be increased by
increasing the resistance in armature circuit
increasing the resistance in field circuit
reducing the resistance in the field circuit
reducing the resistance in the armature circuit
The speed of a D.C. series motor is
proportional to the armature current
proportional to the square of the armature current
proportional to field current
inversely proportional to the armature current
The speed of a D.C. motor can be varied by varying
field current
applied voltage
resistance in series with armature
any of the above
The ratio of starting torque to full-load torque is least in case of
series motors
shunt motors
compound motors
none of the above
The purpose of retardation test on D.C. shunt machines is to find out
stray losses
eddy current losses
field copper losses
windage losses
The power mentioned on the name plate of an electric motor indicates
the power drawn in kW
the power drawn in kVA
the gross power
the output power available at the shaft
The plugging gives the
zero torque braking
smallest torque braking
highest torque braking
none of the above
The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is
4
2
1
0.5
The main disadvantage of the Ward-Leonard control method is
high initial cost
high maintenance cost
low efficiency at Hght loads
all of the above
The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ?
hysteresis loss
field copper loss
armature copper loss
eddy current loss
The hysteresis loss in a D.C. machine least depends on
Frequency of magnetic reversals
Maximum value of flux density
Volume and grade of iron
Rate of flow of ventilating air
The direction of rotation of a D.C. series motor can be changed by
interchanging supply terminals
interchanging field terminals
either of a and b above
None of the above
The current drawn by the armature of D.C. motor is directly proportional to
the torque required
the speed of the motor
the voltage across the terminals
none of the above