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DC Machines: Exercises and Questions for Electrical Engineering Students, Exams of Electrical Engineering

Technological University of the Philippines (TUP)Electrical Engineering

A set of exercises and questions related to dc machines, covering topics such as hysteresis loss, voltage regulation, eddy current loss, equivalent resistance, dc motor types, back emf, efficiency, and more. It provides a valuable resource for electrical engineering students to test their understanding of dc machine principles and applications.

Typology: Exams

2024/2025

Available from 04/05/2025

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jason-morales-5 🇵🇭

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TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES
938 AURORA BOULEVARD CUBAO QUEZON CITY
ELECTRICAL ENGINEERING DEPARTMENT
DC MACHINES
Name:____________________________________________Score____________________
Direction. Write the letter of the correct answer. Give your step by step solution on the
provided solution pad. No solution is considered wrong.
1. The hysteresis loss in a 6,600 Volts, 60 Hz transformer is 480 watts. What will be the loss when the
transformer is connected to a 6, 900 V, 40 Hz source?
a. 558 W b. 657 W c. 520 W d. 417 W
2. Calculate the percent voltage regulation for a transformer with a percent resistance of 2% and a reactance
of 4% of rating 550 KVA, when delivering 450 kVA at 0.6 pf lagging.
a. 5.5% b. 4.2% c. 4.4% d. 3.6%
3. The eddy current loss in a 2300 V, 60 Hz transformer is 280 watts. What is the eddy current loss at 2400 V,
50 Hz supply of electricity?
a. 280 W b. 300 W c. 305 W d. 257 W
4. A single phase, 100 kVA, 2400/240 Volt, 60 Hz transformer has a primary resistance of 0.828 Ω and a
secondary resistance of 0.009 Ω. Calculate the equivalent resistance referred to the primary.
a. 1.728 Ω b. 0.837 Ω c. 1.6 Ω d. 1.46 Ω
5. Which DC Motor cannot be used without a load connected to it?
a. Shunt b. Series c. Compound d. Universal
6. A shunt motor armature has a resistance of 0.07 ohms and a current of 50 Amps at 120 Volts. What is the
back emf or counter emf of the motor?
a. 108.5 V b. 116.5 V c. 112.2 V d. 118.3 V
7. A 440 V shunt motor has Ra = 0.8 Ω and RSH = 200 Ω. What is the back emf when giving an output of 7.46
KW at 85 % efficiency?
a. 465.8 V b. 425.8 V c. 400 V d. 440 V
8. A DC Shunt Motor develops 15 hp at 120 V. The armature efficiency is 95 %. What is the armature
resistance?
a. 0.10 Ω b. 0.06 Ω c. 0.08 Ω d. 0.60 Ω
9. A DC Shunt Motor develops 15 hp at 120 V. If Ra = 0.061 Ω and ISH = 2 A. What is the overall efficiency?
a. 90 % b. 93 % c. 95 % d. 94%
10. A 5 KW, 220 V DC Shunt Motor has a line current of 30 Amps. Ra = 0.3 Ω and RSH = 200 Ω. What is the
constant loss of this motor? (Core Loss)
a. 1600 W b. 1350 W c. 250 W d. 1500 W
11. The no load speed of the compound motor is 1650 rpm. How fast will it operate when it delivers rated
horsepower output, if the speed regulation is 12.5 %?
a. 1444 rpm b. 1856 rpm c. 1467 rpm d. 1500 rpm
12. The current flowing in the armature conductors of a DC motor is
a. DC b. AC c. AC and DC d. transients
13. If the current in the armature of a DC series motor is reduced to 50 %, the torque of the motor will be
a. 50 % of the previous value b. 25 % of the previous value
c. 150 % of the previous value d. 125 % of the previous value
14. Which of the following is a correct statement about series motor?
a. Field winding consist of few thicker wire b. run easily without a load
c. Almost constant speed d. poor torque
15. If the back emf in a DC motor is absent, then
a. Motor will run at very high speed b. motor will not run
c. Motor will run at low speed d. motor will burn
pf2

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TECHNOLOGICAL INSTITUTE OF THE PHILIPPINES 938 AURORA BOULEVARD CUBAO QUEZON CITY ELECTRICAL ENGINEERING DEPARTMENT DC MACHINES Name:____________________________________________Score____________________ Direction. Write the letter of the correct answer. Give your step by step solution on the provided solution pad. No solution is considered wrong.

  1. The hysteresis loss in a 6,600 Volts, 60 Hz transformer is 480 watts. What will be the loss when the transformer is connected to a 6, 900 V, 40 Hz source? a. 558 W b. 657 W c. 520 W d. 417 W
  2. Calculate the percent voltage regulation for a transformer with a percent resistance of 2% and a reactance of 4% of rating 550 KVA, when delivering 450 kVA at 0.6 pf lagging. a. 5.5% b. 4.2% c. 4.4% d. 3.6%
  3. The eddy current loss in a 2300 V, 60 Hz transformer is 280 watts. What is the eddy current loss at 2400 V, 50 Hz supply of electricity? a. 280 W b. 300 W c. 305 W d. 257 W
  4. A single phase, 100 kVA, 2400/240 Volt, 60 Hz transformer has a primary resistance of 0.828 Ω and a secondary resistance of 0.009 Ω. Calculate the equivalent resistance referred to the primary. a. 1.728 Ω b. 0.837 Ω c. 1.6 Ω d. 1.46 Ω
  5. Which DC Motor cannot be used without a load connected to it? a. Shunt b. Series c. Compound d. Universal
  6. A shunt motor armature has a resistance of 0.07 ohms and a current of 50 Amps at 120 Volts. What is the back emf or counter emf of the motor? a. 108.5 V b. 116.5 V c. 112.2 V d. 118.3 V
  7. A 440 V shunt motor has Ra = 0.8 Ω and RSH = 200 Ω. What is the back emf when giving an output of 7. KW at 85 % efficiency? a. 465.8 V b. 425.8 V c. 400 V d. 440 V
  8. A DC Shunt Motor develops 15 hp at 120 V. The armature efficiency is 95 %. What is the armature resistance? a. 0.10 Ω b. 0.06 Ω c. 0.08 Ω d. 0.60 Ω
  9. A DC Shunt Motor develops 15 hp at 120 V. If Ra = 0.061 Ω and ISH = 2 A. What is the overall efficiency? a. 90 % b. 93 % c. 95 % d. 94%
  10. A 5 KW, 220 V DC Shunt Motor has a line current of 30 Amps. Ra = 0.3 Ω and RSH = 200 Ω. What is the constant loss of this motor? (Core Loss) a. 1600 W b. 1350 W c. 250 W d. 1500 W
  11. The no load speed of the compound motor is 1650 rpm. How fast will it operate when it delivers rated horsepower output, if the speed regulation is 12.5 %? a. 1444 rpm b. 1856 rpm c. 1467 rpm d. 1500 rpm
  12. The current flowing in the armature conductors of a DC motor is a. DC b. AC c. AC and DC d. transients
  13. If the current in the armature of a DC series motor is reduced to 50 %, the torque of the motor will be a. 50 % of the previous value b. 25 % of the previous value c. 150 % of the previous value d. 125 % of the previous value
  14. Which of the following is a correct statement about series motor? a. Field winding consist of few thicker wire b. run easily without a load c. Almost constant speed d. poor torque
  15. If the back emf in a DC motor is absent, then a. Motor will run at very high speed b. motor will not run c. Motor will run at low speed d. motor will burn
  1. The main function of a commutator in a DC motor is to a. Prevent sparking b. reduce friction c. convert AC to DC d. convert DC to AC
  2. A starter resistor is necessary to start a DC motor because a. it limits the speed c. it starts the motor b. it limits the back emf d. it limits the starting current to a safe value
  3. An electric motor drives a mechanical load, taking 18.8 A from a 230 V source. Calculate the power input of the motor. a. 4,363 W b. 4,536 W c. 5,825 W d. 4,
  4. What is the most common type of motor that can be used for either AC to DC? a. series motor b. repulsion motor c. shunt motor d. shaded pole motor
  5. A 25-kVA 2,400/240-V transformer has a primary current of 10-A. What is the secondary current? a.0.10 A b. 100 A c. 20 A d. 50 A
  6. A device used to protect the motors for overcurrent due to excessive load. a. fuse b. disconnect c. magnetic contactor d. overload relay
  7. At no load the terminal voltage of an alternator is 530-V. At rated load, the voltage drops to 480-V. Calculate the percentage voltage regulation of the machine. a.10.42 % b. 9.43 % c. 90.56 % d. 12.52 %
  8. At what speed must an 8-pole, AC generator runs so that its frequency shall be 40-Hz? a.750 rpm b.600 rpm c. 900 rpm d. 500 rpm
  9. The torque of a series motor is directly proportional to a. the counter emf c. the square of the counter emf b. the armature current d. the square of the armature current
  10. A 230-V DC electric motor is use to drive a mechanical load of 5 hp. How much current is drawn from the supply if the motor and the mechanical load has an overall efficiency of 78%? a.22.5 b. 20.8 c. 21.8 d. None of these
  11. A 50-kVA transformer has a primary voltage of 6,600 volts and a secondary voltage of 250 volts. It has 52 secondary turns. Find the number of primary. a.1337 turns b. 1373 turns c. 1713 turns d. 1733 turns
  12. What is the overall efficiency of a 5-hp that draws 20-A at 240-V? a.90 % b. 87.8 % c. 80 % d. 77.7 %
  13. A certain alternator has 8 poles. At what speed must the alternator runs in order to have generated emf whose frequency is 40 cps? a.580 rpm b. 750 rpm c. 700 rpm d. 600 rpm
  14. Which of the following DC motors has the best speed regulation? a.Series motor b. Shunt motor c. Cumulative compound motor d. Differential compound motor
  15. A shunt generator has an armature current of 400-A and a shunt field current of 5-A. What is its output in kW if the terminal voltage is 220-V? a. 89.1 b. 80.5 c. 86.9 d. 84.