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SHIRDI SAI ENGINEERING COLLEGE
Anekal, Bangalore 562 106
Department of Electrical & Electronics Engineering
QUESTION BANK ( OLD SCHEME )
INDUSTRIAL DRIVES & APPLICATIONS(EE82 )
INTERNAL TEST NO:1
UNIT-I
1. What is an Electric Drive? Explain the classification of Electric Drives with merits anddemerits of each.
2. Discuss the various factors governing selection of motors for a drive system.
3. Distinguish between an electric motor and an electric drive. What are the methods by
which power is transmitted from the motor to the driven unit?
4. Compare the advantages of individual drive with group drive.
5. Differentiate between passive and active torques with illustration. Establish the criteria
for steady state stability of electric drives.
6. Explain the operation of a hoist motor driving a hoist motor with four quadrant
speed torque diagram..
7. Explain the block diagram of an Electric Drin? What are the advantages of an Electric
Drin?
8. Define natural characteristic related to DC Shunt Motor, draw natural rheostatic
speed - torque characteristic of dc shunt motor and explain.
9. Draw and Explain the modified speed - torque characteristics of DC Shunt Motor with
i) insertion of resistance in the armature circuit and
ii) variationof field circuit.10. With a neat sketch, explain the different methods of braking for a DC Shunt Motor.
What are the merits of electric braking? (or) With block schematics explain different
methods of electric braking bringing out the advantages and limitations of each.
11. Explain the different methods of speed control applied for DC Shunt Motor.
12. With a neat sketch, explain the different methods of braking for a 3 phase InductionMotor. What are the merits of electric braking?
13. Write short notes on rotor resistance variation in slip ring induction motor.
14. Draw speed - torque characteristics of a 3 phase Induction Motor and derive the
equation for the same.
15. Derive an expression for the ratio Ist/I for 3Phase induction motor in terms of S and
SmaxT. The stator resistance is considered to be negligible.
16. Derive an approximate expression considering the stator resistance to be negligible to
T/Tmax ratio in terms of slip S and slip corresponding to maximum torque SmaxT of
3 Phase induction motor.
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17. Explain the different methods of speed control applied for 3 induction motor.
18. The maximum torque of a 3 phase Induction motor is twice the full load torque and the
starting torque is equal to the full load torque. Calculate the full load slip. Neglect stator
impedance.
19. A motor costing Rs.80,000 /- (rupees eighty thousand only) is employed for group drive
for a certain installation. How will its total annual cost compare with the case where 5
individual motors each costing Rs.20,000 /- are used? The annual energy consumption
is
40,000 KWh with group drive whereas it is 30,000 KWh with individual drive. The cost
of energy is Rs. 5 /- per KWh. Assume depreciation, maintenance and other fixed
charges at 12% in case of group drive and 18% in case of individual drive.
ANS: Refer A first Course on Electrical Drives by S.K.Pillai
20. A 230V, 21A, 1000 RPM Dc Shunt Motor has an armature resistance of 0.05 and a
field resistance of 230. The magnetization characteristic of the machine is as follows:
Field current (A) 0.2 0.4 0.6 0.8 1.0 1.2 1.4
EMF at 1500 RPM (V) 50 100 150 190 219 235 245
Calculate the speed and torque developed by the motor with full load current under each
of the following conditions
i) No external resistance is included
ii) A resistance of 0.05 is connected in series with armature and
iii) A resistance of 110 is connected in series with field winding
ANS: Refer Electric Drives by N.K.De & P.K.Sen
21. A 200V, 10.5A, 2000 rpm DC Shunt Motor has the armature and field resistances of
0.5 and 400 respectively. It drives a load whose torque is constant at rated motor
torque. Calculate the motor speed of the source voltage drops to 175V.
ANS: Refer A first Course on Electrical Drives by S.K.Pillai
22. A 250V shunt motor giving 15KW at 1000 rpm, takes an armature current of 75A. The
armature resistance is 0.25 and the load torque remains constant. If the flux is reduced
by 20% of its normal value before the speed changes, find the instantaneous value of the
armature current and the torque. Determine the final value of the armature current and
speed.
ANS: Refer A first Course on Electrical Drives by S.K.Pillai
23. A 400V, 15KW DC Shunt Motor takes 40A and runs at a speed of 1200RPM. The shunt
field resistance is 200. Assume that the load torque values as the square of the speed,
neglect iron and friction losses and also the armature reaction effect. Calculate theresistance to be connected in series with the armature to reduce the speed to 1000 rpm,
take armature resistance as 0.55.
ANS: Refer Electric Drives by N.K.De & P.K.Sen
24. A 220V, 21A, 1000 rpm DC Shunt Motor has an armature resistance of 0.05 and a
field resistance of 220. The armature current of the motor on full load is 21A and it
runs at 1000 rpm, if a resistance of 0.5 is inserted in series with the armature,
determine
a. The speed of the motor at full load torque
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b. The speed of the motor at 60% of full load torque.
ANS: Refer A first Course on Electrical Drives by S.K.Pillai
25. A 250V Dc Shunt Motor has armature resistance of 0.25 and shunt field resistance of
250 . When driving a load of constant torque at 600 rpm, the armature current is 20A.If
it is desired to raise the speed from 600 rpm to 800 rpm, what resistance should be
inserted in the shunt field circuit? Assume that the magnetic circuit is unsaturated.
ANS: Refer A first Course on Electrical Drives by S.K.Pillai
26. A 500V, 45KW, 600 rpm DC Shunt Motor has full load efficiency of 90%. The field
resistance is 200 and the armature resistance is 0.2. The field current is maintained
constant. Armature reaction and brush drop may be neglected. Calculate the rated
armature current and hence find the speed under regenerative braking with no external
resistance condition at which the machine develops an electromagnetic torque equal to
the rated value.
ANS: Refer Electric Drives by N.K.De & P.K.Sen
27. A 220V, 10KW, 1200 rpm DC Shunt Motor has full load efficiency of 85%. The field
losses and armature resistance an 110 and 0.25 respectively. Neglect rotationallosses and armature reaction. Calculate the value of the resistance required to be
inserted in series with armature to reduce the speed to 900 rpm when
a. The load torque is constant regardless of speed.
b. The load torque varies as the square of the speed.ANS: Refer Electric Drives by N.K.De & P.K.Sen
28. A 200V D.C. Shunt Motor takes 5A and runs at 1000rpm on No load. The armature &
field resistances are 0.2 & 220 respectively. Calculate the Speed, developed
torque and shaft torque when the motor input is 11kW.
ANS: Refer A first Course on Electrical Drives by S.K.Pillai
29. A 500V shunt motor, taking an armature current of 240A while running at 800 rpm is
braked by disconnecting the armature from the supply and closing it on a resistance of
2.02 , the field excitation remaining constant. Calculate
a. the initial braking current
b. the torque at 600 rpm as a fraction of that at 800 rpm. The armature resistance is
0.05 .
ANS: Refer Electric Drives by N.K.De & P.K.Sen
30. A 220V, 20KW DC Shunt Motor running at its rated speed of 1200rpm is to be braked
by reverse current braking. The armature resistance is 0.1 and rated efficiency of
motor is 88%. Calculate,a. The resistance to be connected in series with the armature to limit the initial
braking current to twice the rated current
b. The initial braking torque
c. The torque when the speed falls 400 rpm.ANS: Refer Electric Drives by N.K.De & P.K.Sen
31. A 400V, 750 rpm, 70A, DC shunt Motor has an armature resistance of 0.3 . When
running under rated conditions, the motor is to be braked by plugging with armature
current limited to 90A. What external resistance should be connected in series with the
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armature? Calculate the initial braking torque and its value when the sped has fallen to
30 rpm.
ANS: Refer Electric Drives by N.K.De & P.K.Sen
32. A 230V, 3 phase, 50Hz, 4 pole, 10A, 0.85 pf, squirrel cage induction motor has a full
load rated speed of 1440 rpm. The stator losses amount to 86.16 watts. The total inertia
of the motor together with load is 0.0486 kg m2. Determine
i. The input power
ii. Power transferred to rotor
iii. Rotor copper loss
iv. Permissible energy in one minute
v. Total energy dissipated in rotor during starting and breaking
vi. Total number of starts and stops, by dol stop by plugging without
exceeding allowable temperature risk.
ANS: Refer Electric Drives by N.K.De & P.K.Sen
33. A 3 phase 415V, 50Hz, 74.6KW, 6 pole slip ring induction motor drives a fan load
whose torque varies as the square of the speed. The resistance of each rotor phase is
0.2 . At full load the motor runs at a speed of 970 rpm with slip rings short-circuited.The torque speed relation of the motor may be assumed to be linear from no load to full
load. Determine the additional resistance required into each rotor phase to reduce the
speed of the fan to 600 rpm.
ANS: Refer Electric Drives by N.K.De & P.K.Sen
34. A 400V. 3 phase, 50Hz, 4 pole squirrel cage induction motor has the following
equivalent circuit parameters.
R1 = 0.1 , X1 = 0.4 , R2 = 0.1 , X2 = 0.4 , Xm = 14.0 . The motor was
operating on full load at slip = 0.05, when the two stator terminal were suddenly
interchanged. Calculate the primary current and the braking torque immediately after
application of plugging.ANS: Refer Electric Drives by N.K.De & P.K.Sen
35. The rotor of a 6 pole, 50 Hz, slip ring induction motor has a resistance of 0.2 per
phase and runs at 960 rpm on full load. Calculate the resistance per phase to be inserted
in the motor circuit such that speed is reduced to 800 rpm. The torque being
i. The same as before and
ii. Proportional to square of the speed.
ANS: Refer Electric Drives by N.K.De & P.K.Sen
36. A 3 Phase, 440V, 50Hz, 10Pole star connected induction motor has the following
parameters.R1 = 0.5 Ohm. R2 = 0.45 Ohm. X1 = 0.6 Ohm. X2 = 1.8 Ohm. Sf= 0.05 .
Ratio of effective stator to rotor turns is 1/3 . The motor is to be braked at rated speed
and external resistance of 1.75 Ohm per phase has been inserted into the rotor circuits
( resistance value referred to stator ). Calculate the initial braking torque with D.C.
Rheostatic braking.
ANS: Refer Electric Drives by N.K.De & P.K.Sen
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37. A 400V. 3 phase, 50Hz, 6 pole, star connected induction motor has the following
equivalent circuit parameters referred to stator.
R1 = R2 = 1 , X1 = X2 = 2 . The motor is to be braked by plugging from its initial
full load speed of 950 rpm. Stator to rotor turns ratio 2.3.
i) Calculate the initial braking current and torque as a ratio of their full load values.
ii) What resistance must be inserted in the rotor circuit in order to reduce the
maximum braking current to 1.5 times full load current? What will be the initial
braking torque now?
ANS: Refer Electric Drives by N.K.De & P.K.Sen
38. A 440V, 3 phase , 50 Hz, 10 pole squirrel cage induction motor has the following data:
Stator resistance per phase = 1.2
Stator leakage reactance per phase = 2.4
Rotor resistance per phase = 0.006 Rotor standstill reactance per phase = 0.022
Effective turns ratio = 10 : 1
Full load slip = 0.04.
The stator is star connected. The motor was operating on full load when plugging is
executed. Calculate the braking torque immediately after plugging. Neglect magnetizing
current.
ANS: Refer Electric Drives by N.K.De & P.K.Sen
NOTE: I prepared the Question Bank for this subject from previous VTU Exam QPsand also from standard Reference and Text Books as mentioned in our VTU
Syllabus.All above mentioned problems were solved in the class and also gave
notes and solutions of all problems (Hard copy) to the students.
SHIRDI SAI ENGINEERING COLLEGE
Anekal, Bangalore 562 106
Department of Electrical & Electronics Engineering
QUESTION BANK
INDUSTRIAL DRIVES & APPLICATIONS(EE82 )
UNIT-II
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1. Define i) Continuous rating ii) Short time rating iii) Intermittent rating
iv) Continuous maximum rating of an electric drive motor.
2. With usual notations find an expression for the temperature rise of a machine. Sketch the
temperature rise Vs time curve.
3. Explain how the rating of a motor can be selected for a continuous duty variable load
based on the equivalent current method.
4. What is heating & cooling time constants? Explain how the rating of a motor is affected
by the temperature rise.
5. What is meant by rating of motors? Discuss how the type and size of motors for
intermittent loads is determined.
6. Derive the expression used for the short time rating of a machine. Sketch the temperature
rise Vs time curve.
7. Explain the classification of duty for motors with load diagrams. Mention one example
in each case.
8. Derive the expression used for the short time rating of a machine. Sketch the temperature
rise Vs time curve.
9. Derive the expression used for the intermittent rating of a machine. Sketch the
temperature rise Vs time curve.
10. Derive the expression used for the moment of inertia of a flywheel.11. Develop expression to determine power ratings of electric motors under the following
applications.
i) Continuous duty and constant load
ii) Continuous duty and variable load
12. Explain how the rating of a motor can be selected for a continuous duty variable load
based on the equivalent torque and power method.
13. Explain how the rating of a motor can be selected for a continuous duty variable load
based on the average losses method.
14. The heating and cooling time constants of a 100 KW motor are 90 and 120 minutes
respectively. Find the rating of the motor when subjected to a duty cycle of 18 minutes
on certain load and 30 minutes on no load. Assume the losses are proportional to thesquare of the load.
ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
15. Plot the load curves and select the proper motor for the intermittent duty:
i) P1 = 35 KW for t1 = 3 sec
ii) P2 = 17 KW for t2 = 20 sec
iii) P3 = 35 KW for t3 = 2 sec
iv) P4 = 13 KW for t4 = 15 sec
Between the operating periods (ii) and (iii), then is a pause of tc1 = 37 sec and at the
end
of the cycle then is another pause tc2 = 40 sec. ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
16. The motor has a heating time constant of 70 minutes and cooling time constant of 90
minutes. When run continuously on full load of 200 KW, the final temperature rise is
40 0C. The motor is driving an intermittent load having 10 minutes on load period,
followed by 15 minutes shut down. What is the maximum load it can supply during
the on load period? Assume the losses to be proportional to the square of the load.
ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
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17.The enclosure of a 10 KW motor is equivalent to cylinder of 70 cm dia and 100 cm
length. The motor weighs 500 Kg. Assuming that the specific heat is 700J / Kg / 0C
and that the peripheral surface of the enclosure of the motor is capable of heat
dissipation of 12.5 W / m2 / 0C, calculate the heating time constant of the motor and its
final temperature rise. Efficiency of the motor is 90%.
ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
18. On the basis of heating, select a suitable motor for the following intermittent duty:
i) Constant load of 85 KW for 3 sec
ii) Constant load of 15 KW for 20 sec
iii) Constant load of 35 KW for 2 sec
iv) Constant load of 10 KW for 15 sec
Between the operating periods (ii) and (iii), there is a period of rest for 37 sec and
after
(iv) there is a another rest period of 43 sec.
ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
19. Determine the half hour rating of a 25 KW motor having a time constant of 1.5 hours.
Assume that the motor cools down completely between each load period and that theiron losses which remain constant are 90% of copper losses at full load.ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
20. The 15 minutes rating of a motor used in domestic mixer is 400 watts. If the heating
time constant is 60 minutes, determine the continuous rating of the motor if the
maximum efficiency of the motor occurs at 80 % of full load.
ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
21. A motor has the following duty cycle:
Load rising from 200 to 400 H.P : 4 minutes
Uniform load of 300 H.P : 2 minutesRegenerative braking H.P
Returned to supply from 50 to zero : 1 minute
Decking or rest period : 1 minute
Estimate suitable H.P rating for the motor.ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
22. A motor with a thermal time constant of 45 min has a final temperature rise of 75 0C
on continuous rating.
i) What is the temperature rise after 1 hr at this load?
ii) If the temperature rise on one hour rating is 75 0C, find the maximum steady
temperature at this rating.iii) When working at its 1 hr rating, how long does it take the temperature to increase
from 60 0C to 75 0C.
ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
23. A motor on test gave the following mean temperature rise:
Time (Hrs) : 0 0.25 0.5 0.75 1.0 1.25 1.5
Temp. rise ( 0C ) : 42.5 45.0 47.4 49.5 51.4 53.0 54.4
Estimate the final temperature rise and the time constant.
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24. The load cycle of a motor is given below:
i) Uniform acceleration for 30s with load increasing from zero to 2000 KW.
ii) Steady full load of 1500 KW for 50s.
iii) Regenerative braking for 15s with the power returned to the supply
reducing uniformly from 500 KW to zero.
iv) Re-loading for 25s.
Estimate the suitable rating for the motor.ANS: Refer Electric drives by S.K.Pillai & N.K.De and P.K.Sen
25.A motor operates on a periodic duty cycle in which it is clutched to its load for 10 min
and declutched to run on no load for 20 min. Minimum temperature rise is 40 0C.
Heating and cooling time constants are equal and have a value of 60 min. When load is
declutched continuously the temperature rise is 15 0C. Determine
i) maximum temperature during the duty cycle and
ii) temperature when the load is clutched continuously.
ANS: Refer Fundamental of Electrical drives by G.K.Dubey
26. A rolling mill driven by thyristor converter fed dc operates on a speed reversing duty
cycle. Motor field current is maintained constant at the rated value. Moment of inertiareferred to the motor shaft is 10,000 Kg-m2. Duty cycle consists of the following
intervals:
i) Rolling at full speed (200 rpm) and at a constant torque of 25,000 N-m
for 10 sec.
ii) No load operation for 1 sec at full load.
iii) Speed reversal from 200 to 200 rpm in 5 sec.
iv) No load operation for 1 sec at full load.
v) Rolling at full speed and at a torque of 20,000 N-m for 15 sec.
vi) No load operation at full load speed for 1 sec.
vii) Speed reversal from 200 to 200 rpm in 5 sec.
viii) No load operation at full speed for 1 sec.Determine the torque and power ratings of the motor.
ANS: Refer Fundamental of Electrical drives by G.K.Dubey
27. A constant speed drive has the following duty cycle:
i) Load rising from 0 to 400 KW : 5 min
ii) Uniform load of 500 KW : 5 min
iii) Regenerative power of 400 KW returned to the supply : 4 min
iv) Remains idle fof : 2 min
Estimate power rating of the motor. Assume losses to be proportional to (power)2.
ANS: Refer Fundamental of Electrical drives by G.K.Dubey
28. A motor has a heating and cooling time constants of 60 min and 90 min respectively.
When run continuously on full load of 20 KW, the final temperature rise is 40 0C.
a. What load motor can deliver for 10 min if this is followed by a shut down period
long enough for its to cool?
b. If it is on an intermittent load of 10 min followed by 10 min shut down, what is the
maximum value of load it can supply during the on load perod?
ANS: Refer Fundamental of Electrical drives by G.K.Dubey
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29. Half hour rating of a motor is 100 KW. Heating time constant is 80 min and the
maximum efficiency occurs at 70% full load. Determine the continuous rating of the
motor.
ANS: Refer Fundamental of Electrical drives by G.K.Dubey
UNIT-III
16. Derive an equation for the load torque as referred to motor shaft and moment of inertia
as referred to motor shaft load system with gears.
17. Write the equation of motion for the system as shown in below.
18. What do you understand by referred torque and referred moment of inertia with respect
to motor shaft axis of a drive system of multistage-geared transmission system?
19. Explain clearly transients in separately excited dc motor with relevant equation, also
draw the characteristic of dynamic response to step change in supply voltage.
20. Write short notes on Steady state Stability of an Electric Drive.
21. Write short notes on equation of motion.22. Write short notes on equivalent system.
23. Explain clearly transients in three-phase induction motor with relevant equation, also
draw the characteristic of dynamic response to step change in supply voltage.
24. A weight of 500 Kg is being lifted up at a uniform speed of 1.5 m / sec by a winch,
driven by a motor running at a speed of 1000 rmp. The moments of inertia of the motor
and the winch are 0.5 and 0.3 Kg m2 respectively. Calculate the motor torque and the
equivalent moment of inertia referred to the motor shaft. In the absence of weight, motor
develops a torque of 100 Nm, when running at 1000 rpm.
ANS: Refer A first Course on Electrical Drives by S.K.Pillai
10. A load torque of 6000Nm is applied by a motor through gears of ratios 1:2:4:8:16 in
terms of teeth. Speed of the motor is 960 rpm. Find the load torque as referred to motor
shaft if efficiency of each gear is 85%.Find power required by the load and motor input
power if motor efficiency is 88%.ANS: Refer Electric Drives by N.K.De & P.K.Sen
11. The following parameters are given for a separately excited Dc Motor.
240V, 75KW, 1750 rpm, Ra = 15m, La = 1mH, C = 1.27V-sec / rad, J = 3.7 Kg m2,
F = 0. Assume that the load inertia is equal to the motor inertia. Find the transfer
function indicating the natural frequency and damping ratio. Also find the time equation
Prepared by: Mrs.R.GUNASEKARI
Dry Friction
Viscous
Friction
Fan
m Tm
L T
L
V
W
Motor
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of the speed change for a constant input voltage and step change in load torque by
200Nm.
ANS: Refer A first Course on Electrical Drives by S.K.Pillai
12. A three phase, 415V, 6 pole, 50 Hz star connected slip ring induction motor has a sum of
stator and rotor leakage reactance referred to the stator of 1.0.It is connected to a
balanced 415V supply and drives a pure inertia load. The moment of inertia of the rotor
including the load is 10 Kg m2 . The DOL starting is used and the rotor circuit resistance
is adjusted so that the motor brings its load from rest to 0.95 of the synchronous speed inthe shortest possible time. Calculate the minimum time to reach 0.95 of synchronous
speed and the value of the rotor resistance referred to the stator.
ANS: Refer A first Course on Electrical Drives by S.K.Pillai
13. A weight of 1000 Kg is being lifted up at a uniform speed of 1.5 m / sec by a winch,
driven by a motor running at a speed of 960 rpm. The moments of inertia of the motor
and the winch are 1.6 and 3.2 Kg m2 respectively. Calculate the total load torque of the
system referred to the motor shaft.ANS: Refer A first Course on Electrical Drives by S.K.Pillai
14. The following parameters are given for a separately excited Dc Motor.
220V, 1500 rpm and with a field current of 0.5A, Ra = 1m, La = 0, F = 0, the
combinedinertia of the motor with load is 2.0 Kg m2.The motor drives a constant load torque
TL = 20 Nm. The armature terminals are suddenly connected to 220V dc supply, when
the field current If is 0.5A. (a) Derive the expressions for the speed m and the armature
current i as a function of time. (b) Determine the steady state values of speed and
armature current.
ANS: Refer Electric Drives by N.K.De & P.K.Sen
15. A three phase, 440V, 40 KW, 50 Hz induction motor driving a fan, whose torque is
Proportional to the square of the speed, has the speed torque characteristic given
below:
Speed
(rpm)
295
0
275
0
250
0
240
0
225
0
200
0
175
0
150
0
100
0
500 0
Torque
(Nm)
120 288 360 369 360 297 252 207 162 144 150
Determine the run up time of the fan motor having rated torque of 120Nm to accelerate
to its rated speed of 2950 rpm. The total inertia of the motor and the fan is equal to
2.0 Kg m2.ANS: Refer Electric Drives by N.K.De & P.K.Sen
16. A 3.73 KW, 7.8A, three phase, 50 Hz, 1450 rpm, 0.84 pf, delta connected cage induction
motor is to undertake frequent starts. The moment of inertia of the drive is 0.5 Kg m2.
Assume the ratio of stator to rotor resistance to be unity. Neglect magnetizing current
and
rotational losses. (a) Determine the number of starts per minute that this drive can make
under no load conditions. (b) How many starts per minute can be made, if the rated
speed
were 960 rpm, all the other parameters remaining unaltered?
ANS: Refer Electric Drives by N.K.De & P.K.Sen
Prepared by: Mrs.R.GUNASEKARI