Electrical Engineering is a free introductory textbook to the basics of electrical engineering. See the editorial for more information....


Author: E.E. Kimberly

  Problem Answer
1 Construct accurately the circle diagram for Example 18-2. Calculate the torque and the power output at one-half of rated speed. Calculate the starting torque with 200 volts applied (line-to-line).  
2 At what continuous power output may a 50-hp, 3-phase, 60-cycle, 220-volt , polyphase, open-type, 40-deg C induction motor be operated if it be totally enclosed and connected to a single-phase, 208-volt, 50-cycle line?  
3 The full-load speed of an induction motor is given as 960 rpm. What is the frequency for which it was probably designed? If the windings are open for inspection, how may the number of poles be determined?  
4 A 3-phase, 220-volt, 6-pole, 25-hp, 25-cycle induction motor has a power input of 575 watts with 19.5 amp at no load. With the rotor blocked and 51.5 volts applied to the stator, the power input is 2050 watts with a line current of 66 amp. The effective resistance of the motor is 0.085 ohm per phase. Draw the approximate circle diagram, and determine the starting torque and the maximum running torque in pound-feet.  
5 A 3-phase, 4-pole, 60-cycle, 220-volt, polyphase induction motor is to be used as a frequency converter by driving the open-circuited rotor backward against the direction of the rotating field. If the rotor has half as many conductors in series per phase as the stator, what will be the voltage and frequency available at the slip-rings when the rotor is driven backward at 900 rpm?  
6 A 6-pole, 50-hp, 25-cycle induction motor runs at 5% slip at full load. What is the frequency of the rotor currents?  
7 At full load, what is the speed of the motor of Problem 6?  
8 A 40-hp, 440-volt, 1200-rpm induction motor with 5% slip at full load has a starting torque 1.5 times its full-load running torque. How many pound-feet of starting torque would it develop with 230 volts applied?  
9 How many pound-feet of starting torque would the 40-hp motor of Problem 8 develop if started on the 80% tap of its starting auto-transformer?  
10 When tested for the purpose of finding the characteristics by circle diagram, a certain two-phase induction motor yields the following data:
                  Phase 1                     Phase 2
            Volts   Amp    Watts       Volts   Amp    Watts       Speed
No load      440    5.5     440         440    5.5     440      1800 rpm
Blocked      113   11.4     575         113   11.4     575          0
Resistance = 1.56 ohms per phase.

Draw the circle diagram and calculate:

  • Starting torque, in pound-feet, considering starting torque as 1.5 times full-load torque.
  • Torque at half of rated speed.
  • Efficiency at 1.5 times full-load hp.

Last Update: 2011-02-25