Download E701 ELECTRICAL MACHINES III

Rajagiri School of Engineering and Technology
E701
ELECTRICAL MACHINES III
Tutorials
1. A 3 phase, 4 pole, 50 Hz Induction Motor takes 40A at full load of 1440 rpm, and develops a torque of
100Nm at Full load. The starting current at rated voltage is 200A. What is the starting torque? If a stardelta starter is used, what is the starting torque and starting current? Neglect magnetizing current.
2. Determine the suitable tapping on an auto transformer starter for an Induction motor required to start the
motor with 36% of the full load torque. The short circuit current of the motor is 5 times the full load
current and full load slip is 4%. Also determine the current in the supply leads as a percentage of full
load current.
3. Determine the starting torque of a three phase IM in terms of full load torque when started by means (i)
star delta starter (ii) auto transformer with 50% tapping. Ignore magnetizing current. The short circuit
current of the motor at normal voltage is 5 times the full load current and full load slip is 5%.
4. A 3 phase squirrel cage Induction motor has a starting current 175% of full load line current and
develops 35% of full load torque when operated by a star-delta starter. What should be the starting
torque and current if an auto transformer starter with 80% tapping is employed?
5. A 3 phase squirrel cage Induction motor takes 150% of full load line current and develops 30% of full
load torque at starting, when operated by a star-delta starter. What should be the starting torque and
current if an auto transformer starter with 80% tapping is employed?
6. A slip ring Induction motor has a rotor resistance of 0.03 Ω and a standstill reactance of 0.12 Ω. Find
approximately the value of external resistance to be added to the rotor resistance starter in order to
develop maximum torque at starting.
7. A 6 pole, 3 phase, 50Hz, slip ring Induction motor has a rotor winding resistance of 0.08 Ω per phase. If
its stalling speed is 800 rpm, find approximately the value of external resistance to be added in the rotor
resistance starter to obtain maximum torque at starting.
8. The rotor of a 4 pole, 50 Hz, slip ring Induction Motor has a resistance of 0.3 Ω per phase and runs at
1440 rpm at full load. Calculate the value of external resistance per phase which must be added to lower
the speed to 1320 rpm, the torque being the same as before.
9. Calculate the steps in a 5 section rotor starter of a 3 phase Slip ring Induction Motor, for which the
starting current should not exceed the full load current, the full load slip is 1.8% and rotor resistance is
0.015 Ω per phase.
10. Calculate the steps in a 4 section rotor starter of a 3 phase Slip ring Induction Motor, from the following
data:
Max. starting current = FL current; FL slip = 0.04; Rotor resistance per phase = 0.075 Ω.
11. A 5 step starter for a slip ring IM is to be designed. The resistance per phase of the rotor is 0.05 Ω and
the slip on full load is 3%. The motor is to be started with maximum current equal to full load current.
Calculate the resistance in each of the 5 steps of the starter.
12. Design the 5 sections of a 6 stud starter for a three phase wound rotor IM. The slip at full load is 2% and
the starting current is 1.5 times the full load current. The rotor resistance is 0.2 Ω per phase.
13. A 3 phase, 4 pole, 50 Hz, Slip ring Induction Motor has its rotor winding resistance as 0.22Ω / phase and
runs at 1440 rpm on full load. Calculate the approximate value of resistance to be added to the rotor
circuit / phase so as to reduce the speed by 15% with the same torque developed.
14. A 3 phase, 6 pole, 50 Hz, Induction Motor when fully loaded, runs with a slip of 3%. Find the value of
resistance necessary in series per phase of the motor to reduce the speed by 10%. Assume that the
resistance of the rotor per phase is 0.2 Ω. (Assume same torque developed)
15. The rotor resistance and standstill reactance of three phase IM are respectively 0.015 Ω and 0.09 Ω per
phase. At normal voltage, full load slip is 3 %. Estimate the percentage reduction in stator voltage to
develop full load torque at one half of full load speed. What is then the p.f.?
© DEE_JRN_2009
Rajagiri School of Engineering and Technology
16. The rotor resistance and standstill reactance per phase of a three phase IM are respectively 0.02 Ω and
0.11 Ω per phase. At normal voltage, full load slip is 4 %. Estimate the percentage reduction in stator
voltage to develop full load torque at one half of full load normal speed. Also calculate the rotor p.f.
17. A 6 pole, 3 phase, 50Hz, slip ring Induction motor is running at 3% slip when developing full load
torque. Its rotor winding resistance and standstill reactance are 0.12 Ω and 0.6 Ω per phase respectively.
For the same torque developed, calculate the speed of the motor if an external resistance of 0.5 Ω per
phase is added in the rotor circuit.
18. A 6 pole, 3 phase, 50Hz, slip ring Induction motor is cumulatively cascaded with a 4 pole motor. The
rotor circuit frequency of the 4 pole motor is found to be 2 Hz. Determine the slip in each motor and the
combined set speed.
© DEE_JRN_2009