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WorksheetsEE8353-ELECTRIC DRIVES AND CONTROLS-MODEL EXAMINATION NOV 20
Total questions: 45
Worksheet time: 3600secs
Which duty cycle has on load and off-loads period?
a) Intermittent duty
b) Short time duty
c) Continuous duty with constant load
d) Continuous duty with variable load
Electric drives can operate in all the which quadrants of speed-torque plane
a) four
b) one
c) Two
d) Three
The curve obtained between temperature rise Vs. time, when drive is loaded is called
a) Heating curves
b) Cooling curves
c) Linear curve
d) Parabolic curve
The time taken by the machine to reach a temperature rise of maximum temperature rise (θm) is called heating time constant. The value is
a) 63.2%
b) 69.2%
c) 85.3%
d) 95%
The loads are too short to allow the motor to reach its final steady value while periods of rest are too small to allow the motor to cool down to the which temperature.
a) ambient temperature
b) Room temperature
c) Nominal temperature
d) None of the above
Continuous duty with starting and braking duty factor for this duty cycle is
a) 2
b) 4
c) 8
d) 1
In dc series motor, which is directly proportional to armature current
a)flux
b)voltage
c)current
d)None of the above
DC series motor should be started with
a) Heavy load
b) No load
c) Light load
d) some load
Rotating magnetic field always rotates at a speed equal to
Rotor speed
synchronous speed
Both a) and b)
None of the above
Due to the capacitor connected in series with the auxiliary winding the capacitive circuit draws a leading current which increases the phase angle between
a) Im
b) Ist
c) Im and Ist
None of the above
In plugging method the back emf and voltage act in same direction so current flowing in the voltage is equal to
4v
8v
6v
2v
In regenerative braking of dc shunt motor
Eb=V
Eb<V
Eb>V
None of the above
Why starters are required in a DC motor?
a) Back emf of these motors is zero initially
b) These motors are not self-starting
c) These motors have high starting torque
d) To restrict armature current as there is no back emf at starting
For which motors DOL starter can be used?
a) Up to 5 H.P
b) Up to 10 H.P
c) Up to 15 H.P
d) Up to 20 H.P
In a manual shunt motor starter relay positions are ________________________
a) Over load relay is connected in series and no volt relay in parallel with the load
b) Over load relay is connected in parallel and no volt relay in series with the load
c) Over load relay and no volt relay are both connected in series with the load
d) Over load relay and no volt relay are both connected in parallel with the load
The purpose of blades in a squirrel cage induction motor is
a) To reduce the magnetic resistance of the rotor
b) To cool the rotor
c) To reduce the electrical resistance of rotor cage
d) None of these
The drawback of speed control of a slip ring induction motor with the help of resistance in the circuit is that
a) It is applicable only to motors having power of more than 100 kW
b) It results in high losses
c) With reduction in speed, the torque decreases significantly
d) The speed can be controlled only very broadly
The field of an induction motor rotor rotates relative to the stator at
a) Rotor speed
b) Synchronous speed
c) Slip speed
d) Very low speed
Ward-Leonard system of system of speed control is not recommended for _________
a) constant speed operation
b) Wide speed
c) frequent-motor reversed
d) very slow speed
Small DC motors have best speed control by _________
a) armature voltage control
b) field resistance control
c) any of the methods
d) none of the mentioned
Armature voltage control works for speeds _____ base speed and field resistance control works well for speed _____ base speed.
a) below, above
b) above, below
c) above, above
d) below, below
Ward Leonard method is ___________
a) Armature control method
b) Field control method
c) Combination of armature control method and field control method
d) Totally different from armature and field control method
In potential divider arrangement the variable rheostat position is at start point then voltage across armature is zero and hence speed is
a) Minimum
b) Maximum
zero
None of the above
Mark the wrong option. Which of the following cause and effect behaviour in speed control is correct when field resistance is increased.
a) Decrease in flux
b) Increase in armature current
c) Increase in EMF
d) Decrease in speed
In the rotor voltage injection method, when an external voltage source is in phase with the main voltage then speed will ___________
a) Increase
b) Decrease
c) Remain unchanged
d) First increases then decrease
. In the rotor voltage injection method, when an external voltage source is in opposite phase with the main voltage then speed will _________
a) Increase
b) Decrease
c) Remain unchanged
d) First increases then decrease
In consequent poles method the number of stator poles get changed in the ratio
a) 1:2
b) 2:1
c) 4:2
d) 1:4
In cascade control method of two induction motor if the torques produced both act in the same direction cascading is called as
a) Differential cascading
b) single cascading
c) Cumulative cascading
None of the above
In kramer system when the field resistance is changed the back e.m.f of motor
remains constant
zero
changes
None of the above
AC voltage regulators convert fixed a.c voltage to variable one without changes in
voltage
current
frequency
None of the above
The 10 minutes rating of a motor used in domestic mixer is 200 watts.The heating time constant is 40 minutes and the maximum efficiency occurs at full load ( continuous).Determine the continuous rating.
a) 80.567 W
63.225 W
70.527 W
32.665 W
At full load of a 10 h.p the temperature rise of a motor is 25
° c after one hour and 40 ° c after two hours.Find heating time constant of motor.2.365 hrs
1.9576 hrs
6.022 hrs
9.336 hrs
At full load of a 10 h.p the temperature rise of a motor is 25
° c after one hour and 40 °c after two hours.Find final temperature rise on full load.
56.2°c
93.5°c
44.5°c
62.5°c
At full load of 10 H.P temperature rise of a motoris 30°c after one hour and 40 °c after two hours.Find final temperature rise on full load.
a) 55.66 °c
44.77 °c
66.32°c
22.36°c
The temperature rise of motor after operating for 30 minutes on full load is 20 °c and after another 30 minutes it becomes 30°c on the same load.Find the time constant.
0.7214 hrs
1.235 hrs
2.556 hrs
4.235 hrs
A 220V dc motor has an armature resistance of 0.75 Ω it is drawing an armature current of 30 A,driving a certain load.calculate the induced e.m.f in the motor under this condition.
197.5 v
225 v
120.6 v
155 v
A 4 pole d.c motor has lap connected armature winding.The flux per pole is 30mWb.The number of armature conductors is 250.when connected to 230 v d.c supply it draws an armature current of 40 A.Calculate the back e.m.f and the speed with which motor is running.Assume armature resistance is 0.6 ohms.
200 v and 1238 r.p.m
206 v and 1648 r.p.m
152 v and 1028 r.p.m
100 v and 900 r.p.m
A three phase induction motor has a ratio of maximum torque to full load torque as 2.5:1.Determine the ratio of maximum torque to full load torque if star -delta starter is used.The rotor resitance and standstill reactance per phase are 0.4 ohms and 4 ohms respectively.
0.325
5.236
0.165
4.235
A 3 phase,400 V,50 Hz,1420 r.p.m, 100 A delta connected squirrel cage induction motor takes 8 times full load current and develops 2.2 times full load torque at stand still when started direct on line .What will be the motor current and starting torque as a ratio of full load torque when the motor is started by star delta starter?
0.256
0.733
2.365
5.258
A DC series motor having an armature resistance of 1 ohm,run at a speed of 800 r.p.m at 200 V with a current of 15 A.Find the speed at which it will run when a 5 ohm resistance is connected in series at the same supply and taking the same current.
436.98 r.p.m
569.225 r.p.m
736.252 r.p.m
475.675 r.p.m
Two induction motor are used for the cascade control.The main motor has 4 poles while the auxillary motor has 6 poles.Determine the various synchronous speeds possible for the set.Assume supply frequency 50 Hz.
1500,1000,600,3000 r.p.m
1000,1200,300,500 r.p.m
1600,1050,200,3250 r.p.m
1250,1640,600,3540 r.p.m
A 220 V,1200 r.p.m ,1 phase full converter fed separately excited DC motor having a armature resistance and current of 0.25 ohm and 40 A respectively.For the delay angle of 30 ° find the speed of the motor.consider motor constant ka ϕ =0.18 V/r.p.m
1035.646 r.p.m
2335.646 r.p.m
1895.646 r.p.m
1069.646 r.p.m
A single phase fully controlled bridge rectifier is fed from 230 V,50 Hz supply.The load is highly inductive. Find average load voltage and current if the load resistance is 10 ohms and firing angle is 45 ° .
156 V,10 A
236 V,20 A
98 V,10 A
146.422 V.14.6422 A
For a fully controlled bridge converter having resistive load of 100 ohms determine average output voltage.Assume that supply voltage is 230 V,50 Hz and SCRs are fired at 60 °
255 V
400 V
155.3 V
246.25 V
For a fully controlled bridge converter having resistive load of 100 ohms determine r.m.s value of output voltage.Assume that supply voltage is 230 V,50 Hz and SCRs are fired at 60
°
566.2 V
983.2 V
223.269 V
144.36 V
