1221.
The speed of a D.C. series motor is
A
proportional to the armature current
B
proportional to the square of the armature current
C
proportional to field current
D
inversely proportional to the armature current
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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
The condition for maximum power in case of D.C. motor is
back e.m.f. = 2 x supply voltage
back e.m.f. = | x supply voltage
supply voltage = | x back e.m.f.
supply voltage = back e.m.f.
If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed willbe
small
the same as rated voltage
high
zero
The e.m.f. induced in the armature of a shunt generator is 600 V. The armatureresistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be
620 V
600 V
580 V
640 V
A 220 V D.C. generator is run at full speed without any excitation. The open circuitvoltage will be
about 50 V
220 V
zero
about 2 V
A sinusoidal voltage of 5 Hz is applied to the field of a shunt generator. The armaturevoltage wave
willbeof5xiVHz
will be of v Hz 5
will be zero
will be of 5 Hz
The open circuit voltage of a compound generator is 250 V. At full load the terminalvoltage
may be greater or less than 250 V
none of the above
will be less than 250 V
will always be 250 V
Copper brushes in D.C. machine are used
in both of the above cases
in none of the above cases
where low voltage and high currents are involved
where high voltage and small cur-rents are involved
Equilizer rings are required in case armature is
delta wound
duplex wound
wave wound
lap wound